This group of serious pests belongs to the phylum Mollusca, a group
including the octopus and whelk and the slug’s close relatives, the snails,
which cause some damage to plants in greenhouses and private gardens.
|Figure 14.3 Slug damage on carrot
The slug lacks a shell and this permits movement into the soil
in search of its food source: seedlings, roots, tubers and bulbs. It feeds
by means of a file-like tongue (radula
), which cuts
through plant tissue held by the soft mouth, and
scoops out cavities in affected plants (see Figure 14.3).
In moist, warm weather it may cause above-ground
damage to leaves of plants such as border plants,
establishing turf, lettuce and Brussels sprouts. Slugs
move slowly by means of an undulating foot, the slime
trails from which may indicate the slug’s presence.
Horticultural areas commonly support populations of
50 000 slugs per hectare.
Slugs are hermaphrodite
their bodies both male and female organs), mate
in spring and summer, and lay clusters of up to 50
round, white eggs in rotting vegetation, the warmth
from which protects this sensitive stage during cold
periods. Slugs range in size from the black keeled slug (Milax
) 3 cm
long, to the garden slug (Arion
) which reaches 10 cm in length. The
mottled carnivorous slug (Testacella
) is occasionally found feeding on
gardeners use many non-chemical forms of control,
ranging from baits of grapefruit skins and stale beer to soot sprinkled
around larger plants. A nematode (Phasmarhabditis hermaphrodita)
is increasingly being used to limit slug numbers. The most effective
methods available to both amateur
the moment are three chemicals, aluminium sulphate
(an irritant), metaldehyde
(which dehydrates the slug) and methiocarb
acts as a stomach poison). The chemicals are most commonly used as
(which include food attractants such as bran and
sugar), but metaldehyde may also be applied as a drench. Some growers
estimate the slug population using small heaps of pellets covered with a
tile or flat stone (to prevent bird poisoning) before deciding on general
control. Use of metaldehyde
in gardens has recently
been claimed as a major contribution to the decline of the thrush
numbers. A simple device such as that seen in Figure 16.2, using a
modified plastic milk carton (containing slug pellets) prevents the entry
of mammals and birds.
Belonging to the large group of Arthropods,
which include also the woodlice, mites, millipedes
(see Table 14.1), insects
are horticulturally the most important arthropod group, both as pests,
and also as beneficial soil animals.
Structure and biology
The body of the adult insect is made up of segments, and is divided into three main parts: the head, thorax and abdomen (see Figure
14.4). The head
bears three pairs of moving mouthparts.
The first, the mandibles in insects (such as in caterpillars and beetles) have a biting
action (see Figure 14.5). The second and third pairs, the maxillae and labia in these insects help in pushing food into the mouth.
In the aphids, the mandibles and maxillae are fused to form a delicate tubular stylet, which sucks
up liquids from the plant phloem
tissues. Insects remain aware of their environment by means of compound eyes which are sensitive to movement (of predators) and to
colour (of flowers). Their antennae may have a touching and smelling function.
bears three pairs of legs and, in most insects, two pairs of wings. The abdomen
bears breathing holes (spiracles) along its
length, which lead to a respiratory system of tracheae. The blood is colourless, circulates digested food and has no breathing function.
The digestive system, in addition to its food absorbing role, removes waste cell products from the body by means of fine, hair-like
growths (malpighian tubules) located near the end of the gut.
|Figure 14.4 External appearance of an insect . Note the
spiracles and cuticle,
the three main entry points
Since the animal has an external skeleton
made of tough chitin, it must shed and replace its ‘skin’ (cuticle
, see Figure 14.4)
periodically by a process called ecdysis
, in order to increase in size.
The two main groups of insect develop from egg to adult in different
ways. In the first group (Endopterygeta), typified by the aphids, thrips
and earwigs, the egg hatches to form a first stage, or instar, called a nymph,
which resembles the adult in all but size, wing development
and possession of sexual organs. Successive nymph instars more closely
resemble the adult. Two to seven instars (growth stages) occur before
the adult emerges (see Figure 14.6). This development method is called incomplete metamorphosis.
|Figure 14.5 Mouthparts of the caterpillar and aphid . Note the
methods of obtaining nutrients.
The aphid selectively sucks up
dilute sugar solution from the
In contrast, the second group of insects ‘Endopterygota’ including
the moths, butterflies, flies, beetles and sawflies undergo a complete
The egg hatches to form a first instar,
called a larva
which usually differs greatly in shape from the adult. For example
the larva (caterpillar) of the cabbage white bears little resemblance
to the adult butterfly. Some other dam aging larval stages are shown
in Figure 14.7 and these can be compared with the often more
familiar adult stage. The great change (metamorphosis
) necessary to
achieve this transformation occurs inside the pupa stage (see
|Figure 14.6 Life cycle stages of a butterfly and an aphid pest . Note that all four stages
of the butterfly’s life cycle are very different in
appearance. The nymph and adult of the
aphid are similar
|Figure 14.7 Insect larvae that damage crops. Identification into the groups above can be
achieved by observing the features of colour, shape,
legs and mouthparts
The method of overwintering
differs between insect groups.
The aphids survive mainly as the eggs, while most moths, butterflies
and flies survive as the pupa. The speed
of increase of insects varies
greatly between groups. Aphids may take as little as 20 days to complete
a life cycle in summer, often resulting in vast numbers in the period
June–September. On the other hand, the wireworm, the larva of
the click beetle, usually takes four years to complete its life
Insect groups are classified into their appropriate order (Table 14.1)
according to their general appearance and life cycle stages. There
follows now a selection of insect pests in which each species’ particular
features of life cycle are given. Whilst comments on control are
mentioned here, the reader should also refer to Plant protection
for details of specific types of control (cultivations, chemicals, etc.) and for
explanations of terms used.
Aphids and their relatives ( Order Hemiptera )
|Table 14.1 Arthropod groups found in horticulture
This important group of insects has the egg–nymph–adult life cycle and
Peach-potato aphid (Myzus persicae)
This and similar species are often referred to by the name ‘greenfly’
It is common in market gardens and greenhouses. The nymph
and adult of this aphid may cause three types of damage. Using its sucking
stylet, it may inject a digestive juice into the plant phloem, which in young
organs may cause severe distortion.
Having sucked up sugary phloem
contents, the aphid excretes a sticky substance called honey-dew,
may block up leaf stomata and reduce photosynthesis, particularly when
dark-coloured fungi (sooty moulds) grow over the honeydew. Thirdly,
the aphid stylets may transmit viruses
such as virus Y on potatoes and
tomato aspermy virus on chrysanthemums.
The aphid varies in colour from light green to orange,
measures 3 mm in length (see Figure 14.9) and has a complex life cycle,
shown in Figure 14.8, alternating between the winter host (peach) and
the many summer hosts such as potato and bedding plants. In spring
and summer, the females give birth to nymphs directly without any egg
stage (a process called vivipary), and without fertilization by a male
(a process called parthenogenesis). Spread is by the summer flighted
females. Only in autumn, in response to decreasing daylight length and
outdoor temperatures, are both sexes produced, which having wings,
fly to the winter host, the peach. Here, the female is fertilized and lays thick-walled black eggs. In glasshouses,
the aphid may survive the winter as the
nymph and adult female on plants such
as begonias and chrysanthemums, or on
weeds such as fat hen.
|Figure 14.8 Peach-potato aphid life cycle throughout the year. Female aphids produce nymphs on both the peach and summer host. Winged
females develop from June to September. Males are produced only in autumn.
Eggs survive the winter. In greenhouses the life cycle may continue
throughout the year
|Figure 14.9 Rose aphid
occurs in early summer and
autumn by winged females.
The peach-potato aphid can be
controlled in several ways. In outdoor
crops, several organisms, e.g. ladybirds,
lacewings, hoverflies and parasitic fungi
(see biological control, Plant protection
naturally found in the environment,
may reduce the pest’s importance in
favourable seasons. In the greenhouse,
an introduced parasitic wasp (Aphidius
) is available to amateur
s and professional
gardener uses an aphicide containing pyrethrins.
Two other chemicals are used by both amateur
horticulturists: a contact chemical bifenthrin
and a soap concentrate
containing fatty acids
For the professional,
there is a biological control using the fungus
Verticillium lecanii, and many chemical controls, including pirimicarb
used outdoors and in glasshouses as a spray, and nicotine
used as a
smoke in glasshouses.
There are many other horticulturally important aphid species. The black
bean aphid (Aphis fabae
), which overwinters on Euonymus bushes,
may seriously damage broad beans, runner beans and red beet. The rose
aphid (Macrosiphum rosae
) attacks young shoots of rose.
Spruce-larch adelgid (Adelges viridis)
This relative of the aphid may cause serious damage on spruce grown
for Christmas trees.
Although nursery trees less than four years of age are rarely
badly damaged, early infestation in the young plant may result in
serious damage as it gets older. In June–September the adults move
to larch , acquire woolly white hairs and may cause defoliation of the
The green female adult develops from overwintering nymphs
on spruce, and in May (year 1) lays about 50 eggs on the dwarf shoots.
The emerging nymphs, injecting poisons into the shoots, cause abnormal
growth into pineapple galls, which spoil the tree’s appearance. After
a further year (year 2) on this host, the female adelges returns to the
Figure 14.9 R ose aphid spruce, where it lives for another year (year 3) before the gall-inducing
stages are produced.
is by flighted females.
In Christmas tree production, the adelges may be controlled
by sprays of deltamethrin in May, when the gall-inducing nymphs are
Glasshouse whitefly (Trialeurodes vaporariorum)
This small insect, looking like a tiny moth, was originally introduced
from the tropics, but now causes serious problems on a range of
glasshouse food and flower crops. It should not be confused with the
very similar, but slightly larger cabbage whitefly on brassicas.
All stages after the egg have sucking stylets, which extract
a sugary liquid from the phloem, often causing large amounts of
honeydew and sooty moulds on the leaf surface. Plants that are
seriously attacked include fuchsias, cucumbers, chrysanthemums and
pelargoniums. Chickweed, a common greenhouse weed, may harbour
the pest over winter in all stages of the pest’s life cycle.
|Figure 14.10 Adult glasshouse
actual size is about
1 mm long
The adult glasshouse whitefly ( Figure 14.10 ) is about 1 mm
long and is able to fly from plant to plant. The fertilized female lays
about 200 minute, white, elongated oval eggs in a circular pattern on the lower leaf surface
over a period of several weeks. After turning black,
the eggs hatch to produce nymphs (crawlers), which soon become flat
immobile scales. The last scale instar is thick-walled and called a ‘pupa’
from which the male or female adult emerges. Three days later, the
female starts to lay eggs again. The whole life cycle takes about 32 days
in spring, and about 23 days in the summer.
is mainly by introduced plants, or more rarely by chance arrivals
of adult through doors or vents.
of glasshouse whitefly is achieved in several ways.
horticulturalists should remove weeds (such as
chickweed or sowthistle) harbouring the pest from crop to crop. Careful
inspection of the lower leaves of introduced plants achieves a similar aim.
There is a reliable form of biological control. This involves a minute
wasp (Encarsia formosa
) which lays an egg inside the last scale stage of
the white-fly. The developing whitefly is eaten away by the wasp grub
and the scale turns black and soon releases the next generation of wasps
(see Plant protection
for more details). Control is usually most effective when
whitefly numbers are low.
gardeners can use spray products containing specially
formulated fatty acids
to control young and adult pest stages. Professional horticulturists
can choose between an insecticide that
physically blocks the insect’s breathing holes such as alginate/polysaccharide,
or a contact insecticide such as deltamethrin,
or the above-mentioned fatty acids
It is suggested that serious infestations of this pest receive a regular
weekly chemical spray to catch the more sensitive scale and adult stages.
Greenhouse mealy bug (Planococcus citri)
|Figure 14.11 (a) Mealy bug (b) Brown scale
This pest, a distant relative of the aphid, spoils the appearance
of glasshouse crops, particularly orchids, Coleus species, cacti and
Solanum species. All of the stages except the egg suck phloem juices
by means of a tubular mouthpart (stylet
), and when this pest is present
in dense masses it produces honey dew
and may cause leaf drop (see
Being a tropical species, it develops most quickly in high
temperatures and humidities, and at 30°C completes a life cycle within
about 22 days. The adult measures about 3 mm in length and produces
fine waxy threads.
Adults have wings, but the most important spread is by
introduction of plant material newly infested with small nymphs, or
from plant to plant when leaves are touching.
Mealy bugs are difficult pests to control, as the thick cuticle
resists chemical sprays, and the droplets fall off the waxy threads.
An introduced tropical ladybird, Cryptolaemus montrouzieri,
commercially available for controlling the pest and is most effective at
temperatures above 20°C. Spray products containing pure fatty acids
are effective against this pest and are available to both amateur
A recently reported related problem on beech has been the woolly beech
aphid (Phyllapis fagi).
Brown scale (Parthenolecanium corni)
The female scale, measuring up to 6 mm, is tortoise shaped (see
Figure 14.11) and has a very thick cuticle.
It may be a serious pest outdoors on vines, currants, top fruit and cotoneasters, sucking
phloem sugars out of the plants. It is also found in greenhouses
on vines, peaches and Amaryllis, causing stunted growth and
leaf defoliation. The nymphs are mobile, but do not spread far.
Transport of infested plant material is the main cause of outbreaks
in greenhouses. As with mealy-bug, control is made more difficult
because of the thick cuticle, but products containing fatty acids
give good control. A recently reported scale problem is soft brown
scale (Coccus hesperidum
) in greenhouses.
Leaf hoppers (Graphocephala fennahi)
|Figure 14.12 (a) Bud blast on rhododendron
a leafhopper (b) Capsid damage
(c) Capsid damage
on potato leaf
These slender, light green insects, about 3 mm long, well known
in their nymph stage as ‘cuckoo spit’, are found on a wide variety
of crops, e.g. potato, rose, Primula and Calceolaria. The adults
can fly from plant to plant. They live on the undersurface
leaves, causing a mottling of the upper surface. In strawberries,
they are vectors of the green-petal
disease, while in rhododendron
they carry the serious bud blast
disease that kills off the flower
buds (see Figure 14.12). August and September sprays of
products containing fatty acids
prevent egg laying inside buds
of rhododendron, and thus reduce the entry points for the fungus
Common green capsid (Lygocoris pabulinus)
This very active, light green pest measuring 5 mm in length and
resembling a large aphid, occurs on fruit trees and shrubs and
flower crops, most commonly outdoors. Owing to the poisonous
nature of its salivary juices, young foliage shows distorted growth
with small holes, even when relatively low insect numbers are
present and fruit is scarred (see Figure 14.12). The adult flies
from plant to plant. The chemicals used against aphids control
Thrips (Order Thysanoptera)
Whilst classified under the broad grouping of the Exopterygota
(like the aphids), thrips also produce a survival stage which is
often soil borne, and is loosely called a pupa (a life cycle stage
normally reserved for members of the other main insect group,
the Endopterygota). Due to their increased activity during warm
humid weather, thrips are sometimes called ‘thunder flies’ (and
are known for their ability to get into human’s hair in sultry
summer weather and cause itching).
Onion thrips (Thrips tabaci)
Thrips’ mouthparts are modified for piercing and sucking, and
the toxic salivary juices cause silvering in onion leaves, straw-brown
spots several mm in diameter on cucumber leaves, and white streaks on
The 1 mm long, narrow-bodied insect has feather-like wings.
The last instar of the life cycle, called the pupa,
descends to the soil, and
it is this stage which overwinters. In greenhouses there may be seven
generations per year, while outdoors one life cycle is common.
Adults may be blown considerable distances from nursery to
nursery in the wind. (The occurrence in Britain of Western flower thrip (Frankliniella occidentalis)
on both greenhouse and outdoor flower
and vegetable crops has created serious problems for the industry,
particularly because it transmits the serious tomato spotted wilt virus,
and is able to pupate on the plant, deep within dense flowers such as
Thrips infestations may be reduced in greenhouses by the use
of fine screens over vents, and by a double door system.
gardeners are able to use a recently introduced product for
thrip control containing natural plant extracts
which block the insect’s
use the predatory mite Neoseiulus cucumeris,
and the predatory bug Orius laevigatus
. Chemicals used include the
and the above named natural plant extract.
Western flower thrip has shown greater resistance to chemical control
than the other thrips and a careful rotation of chemical groups has
Earwigs (Forficula auricularia)
These pests belong to the order Dermaptera, and bear characteristic
‘pincers’ (cerci) at the rear of the 15 mm long body. They gnaw away
at leaves and petals of crops such as beans, beet, chrysanthemums and
dahlias, usually from July to September, when the nymphs emerge from
the parental underground nest. They usually spread
by crawling on the
surface of the soil, but they can also fly. Upturned flower pots containing
straw are sometimes used in greenhouses for trapping these shy
nocturnal insects. The professional
grower may use pirimiphos-methyl
as a spray or smoke.
Moths and butterflies
This group of insects belongs to the large grouping, the Endopterygota which has different life cycle details from the aphid group.
The order (Lepidoptera) characteristically contains adults with four large
wings and curled feeding tubes. The larva (caterpillar
), with six small
legs and eight false legs, is modified for a leaf-eating habit (see Figures
14.6 and 14.7). Some species are specialized for feeding inside fruit
(codling moth on apple, see Figure 14.15), underground (cutworms),
inside leaves (oak leaf miner), or inside stems (leopard moth). The
gardener may find large webbed caterpillar colonies of the lackey moth
) on fruit trees and hawthorns, or the juniper
webber (Dichomeris marginella
) causing webs and defoliating
Large cabbage-white butterfly (Pieris brassicae)
Leaves of cabbage, cauliflower, Brussels sprouts and
other hosts such as wallflowers and the shepherd’s-purse weed are
progressively eaten away. The defoliating damage of the larva may
result in skeletonized leaves.
|Figure 14.13 (a) Cabbage-white adult.
underwing moth with cutworm
This well-known pest on cruciferous plants emerges
from the overwintering pupa (chrysalis) in April and May and,
after mating, the females (see Figure 14.13) lay batches of 20 to
a 100 yellow eggs on the underside of leaves. Within a fortnight,
groups of first instar larvae emerge and soon moult to produce the
later instars, which reach 25 mm in length and are yellow or green
in colour, with clear black markings. They have well-developed
mandibles. Pupation occurs usually in June, in a crevice or woody
stem, the pupa (chrysalis) being held to its host by silk threads. A
second generation of the adult emerges in July, giving rise to more
damaging larval infestation than the first. The second pupa stage
The species is spread by the adults. (Care should be taken
not to confuse the cabbage-white larva with the large smooth
green or brown larva of the cabbage moth, or the smaller light
green larva of the diamond-backed moth, both of which may
enter the hearts of cabbages and cauliflowers, presenting greater
problems for control.)
There are several forms of control against the cabbagewhite
butterfly. A naturally occurring small wasp (Apantales
) lays its eggs inside the pest larva. A virus
disease may infect the pest, causing the larva to go grey and
die. Birds such as starlings eat the plump larvae. When damage
becomes severe, amateur
gardeners and professional growers
use spray products containing bifenthrin.
Winter moths (Operophthera brumata)
These are pests which may be serious on top fruit and
ornamentals, especially woody members of the Rosaceae family. The
caterpillars eat away leaves in spring and early summer and often form
other leaves into loose webs, reducing the plant’s photosynthesis. They
occasionally scar young apple fruit.
This pest’s timing of life cycle stages is unusual. The pest
emerges as the adult form from a soil-borne pupa in November and
December. The male is a greyish-brown moth, 2.5 cm across its wings,
while the female is wingless. The female crawls up the tree to lay 100–
200 light-green eggs around the buds. The eggs hatch in spring at bud
burst to produce green larvae with faint white stripes. These larvae
move in a characteristic looping fashion and when fully grown, descend
on silk threads at the end of May before pupating in the soil until
is slow because the females do not have wings.
A common control is a grease band
wound around the main
trunk of the tree in October which is effective in preventing the flightless
female moth’s progress up the tree. In large orchards, professional growers
use springtime sprays of an insecticide such as deltamethrin
kill the young caterpillars.
Cutworm (e.g. Noctua pronuba)
The larvae of the yellow underwing moth,
unlike most other
moth larvae, live in the soil, nipping off the stems of young plants and
eating holes in succulent crops, e.g. bedding plants, lawns, potatoes,
celery, turnips and conifer seedlings. The damage resembles that caused
(see Figure 14.13). The adult moth, 2 cm across, with brown
fore-wings and yellow or orange hind wings, emerges from the shiny
soil-borne chestnut brown pupa from June to July, and lays about 1000
eggs on the stems of a wide variety of weeds. The first instar caterpillars,
having fed on weeds, descend to the soil and in the later instars cause
the damage described above, eventually reaching about 3.5 cm in length.
They are grey to grey-brown in colour, with black spots along the sides.
Several other cutworm species such as heart and dart moth
) and turnip moth (Agrotis segetum)
may cause damage
similar to that of the yellow underwing. In all three species, their
typical caterpillar-shaped larvae should not be confused with the legless
leatherjacket which is also a common underground larva. The cutworm
species normally have two life cycles per year, but in hot summers this
may increase to three.
The larvae are able to crawl from plant to plant, but most spread
is by the actively flying adults.
gardeners remove a good proportion of the cutworm
larvae as they dig plots over. Good weed control reduces cutworm
damage. A soil-directed spray of bifenthrin
will achieve some control of
For the professional
horticulturist , soil drenches of residual insecticides
such as chlorpyrifos
have proved successful against the larva stage of
Leopard moth (Zeuzera pyrina)
The caterpillar of this species tunnels into the branches and
trunk of a wide range of tree species, such as apple, ash, birch, and lilac.
The tunnelling may weaken the branches of trees which in high winds
|Figure 14.14 Leopard moth larva emerging from
The Leopard moth has an unusual life cycle. The moth is
large, 5–6 cm across, and is white with black spots. In early summer
the female lays dark-yellow eggs on the bark of the tree. The emerging
caterpillar (see Figure 14.14) enters the stem by a bud, and then tunnels for 2–3 years in the heartwood. It has bacteria in
its gut which help to digest the xylem tissue that it eats. It
eventually reaches 5 cm in length, pupating in the tunnel,
and finally emerging from the branch the following summer
as the adult. Spread is only by adults.
Where tunnels are observed, a piece of wire may
be pushed along the tunnel to kill the larva.
Other moths worthy of mention here are the fruit-invading
species such as codling moth (Cydia pomonella
) on apple,
plum moth (Cydia funebrana
) and pea moth (Cydia
) each of which needs accurately timed insecticidal
control to avoid fruit damage by the pest (see also pheremone traps
because insecticidal control inside the fruit is not possible.
|Figure 14.15 Codling moth damage
Recently reported moth pests are the Holm oak leafmining
moth (Phyllonorycter messaniella
), the horse
chestnut leaf miner (Cameraria obridella
) and the
Leek moth (Acrolepiosis assectella
This group of insects, of the order Diptera, are
characterized by having only a single pair of
functioning wings. The hind wings are modified into
little stubs which act as balancing organs. The larvae
are legless, elongated, and their mouthparts, where
present, are simple hooks. The larvae are the only
stage causing crop damage.
Carrot fly (Psila rosae)
This is a widespread and serious pest on umbelliferous crops
(carrots, celery and parsnips). The grubs emerging from the eggs eat fine
roots and then enter the mature root using fine hooks in their mouths.
Damage is similar in all crops. In carrots, seedlings may be killed, while
in older plants the foliage may become red, and wilt in dry weather.
Stunting is often seen, and affected roots, when lifted, are riddled with small tunnels
(see Figure 14.16) that make the carrots unsaleable.
Damage should not be confused in carrots with cavity spot,
associated with a Pythium species of fungi which produces elongated
sunken spots partly circling the root.
|Figure 14.16 Carrot fly damage
The adult fly is 8 mm long, shiny black and with a red head. It
emerges from the rice-grain-sized overwintering pupa in the soil from late
May to early June. The small eggs laid on the soil near the host plant soon
hatch to give white larvae which damage the plant (see Figure 14.16).
The fully grown larva leaves the host to turn into a cylindrical pale yellow
pupa when a month old. A second generation of adults emerges in late
July, while in October a third emergence is seen in some areas.
The adult is the stage which spreads the pest.
gardener can use a variety of controls. Planting
carrots after the May emergence has occurred reduces infestation.
Covering carrot plots with horticultural fleece
(see Plant protection
16.2), prevents the adut from laying eggs next to carrots. Inter-planting
carrots with onions is said to prevent carrot fly from homing in on the
For the professional
grower, high levels of carrot fly can be prevented
by keeping hedges and nettle beds trimmed, thus reducing sheltering
sites for the flies. A seed treatment containing tefluthrin
larval infestation. A ground-directed spray at a high volume rate of lambda-cyhalothrin
reaches the larva in the soil.
Chrysanthemum leaf miner (Phytomyza syngenesiae)
The leaf miners are a group of small flies, the larvae of which
can do serious damage to horticultural crops by tunnelling through the
leaf. This species is found on members of the plant family Asteraceae.
Plants attacked include chrysanthemum, cineraria and
|Figure 14.17 Chrysanthemum leaf miner damage
The flies emerge at any time of the year in
greenhouses, but normally only between July and October
outdoors. These adults, which measure about 2 mm in
length and are grey-black with yellow underparts, fly
around with short hopping movements. The female lays
about 75 minute eggs singly inside the leaves, causing
white spot symptoms to appear on the upper leaf surface.
stage is greenish white in colour, and tunnels
into the pal-lisade mesophyll of the leaf, leaving behind the
characteristic mines seen in Figure 14.17 . On reaching its
final instar, the 3.5 mm long larva develops within the mine
into a brown pupa, from which the adult emerges. The total
life cycle period takes about three weeks during the summer
This pest is spread by the adult stage.
Weed hosts such as groundsel and sowthistle should be
controlled. Yellow sticky traps remove many of the adult flies. Certain
chrysanthemum cultivars show some resistance.
gardeners have no effective insecticide product to control the
larva inside the leaf.
use tiny wasps such as Diglyphus isaea
and Dacnusa sibirica
to parasitize the tunnelling leaf-miner larvae. Products
may be used outdoors and in greenhouses. (The
occurrence of South American leaf miner (Liriomyza huidobrensis)
American serpentine leaf miner (Liriomyza trifolii)
which are able to
damage a wide variety of greenhouse plants has, in recent years, created
many problems for horticulture).
Leatherjacket (Tipula paludosa)
This is an underground pest which is a natural inhabitant of
grassland and causes most problems on golf greens. After ploughing
up of grassland, leatherjackets may also cause damage to the crops
such as potatoes, cabbages, lettuce and strawberries. This pest is
particularly damaging in prolonged wet periods when the roots of young
or succulent crops may be killed off. Occasionally lower leaves may be
Life cycle. The adult of this species is the crane fly, or ‘daddy-longlegs’, commonly seen in late August. The females lay up to 300 small
eggs on the surface of the soil at this period, and the emerging larvae
feed on plant roots during the autumn, winter and spring months,
reaching a length of 4 cm by June. They are cylindrical, grey-brown in
and possess hooks in their mouths for feeding. During the
summer months, they survive as a thick-walled pupa.
is achieved by the adults.
gardeners remove this pest as they dig in autumn
and spring. Crops sown in autumn are rarely affected, as the larvae are
very small at this time. There are no pesticide products recommended to
control this pest.
and groundsmen use products containing the
which is drenched into soil to reduce the larval
Sciarid fly (Bradysia spp.)
The larvae of this pest (sometimes called fungus gnat
) feed on
fine roots of greenhouse pot plants such as cyclamen, orchid and freesia,
causing the plants to wilt. Fungal strands of mushrooms in commercial
houses may be attacked in the compost.
The slender black females, which are about 3 mm long, fly to
suitable sites (freshly steamed compost, moss on sand benches and wellfertilized
compost containing growing plants), where about 100 minute
eggs are laid. The emerging legless larvae are translucent-white with a
black head, and during the next month grow to a length of 3 mm before
briefly pupating and starting the next life cycle.
is achieved by the adults.
gardeners and Professional
yellow sticky traps to catch the flying adults in greenhouses. The
pest can be reduced by avoiding overwatering of plants. Biological
control by the tiny nematode Steinernema feltiae is now available. Professional
growers in mushroom houses attempt to exclude the
flies from mushroom houses by means of fine mesh screens placed
next to ventilator fans. A predatory mite (Hypoaspis mile
) is used to
control the larvae. The larvae also may be controlled by the insecticide, diflubenzuron,
incorporated into composts.
This group of insects in the order Coleoptera is characterized in the
adult by hard, horny forewings (elytrae) which, when folded, cover the
delicate hind wings used for flight. The meeting point of these hard wing
cases produces the characteristic straight line down the beetles back
over its abdomen. Most beetles are beneficial, helping in the breakdown
of humus, e.g. dung beetles, or feeding on pest species (see ground
). A few beetles, e.g. wireworm, raspberry beetle and vine weevil,
causes crop damage.
Vine weevil (Otiorhyncus sulcatus)
This species belongs to the beetle group but, as with all weevils,
possesses a longer snout on their heads than other beetles.
The larva stage is the most damaging, eating away roots
of crops such as cyclamen and begonias in greenhouses, primulas,
strawberries, young conifers and vines outdoors, causing above-ground
symptoms similar to root diseases such as vascular wilt. Close inspection
of the plant’s root zone will, however, quickly show the unmistakable
white grubs (see Figure 14.18). The adults may eat out neat holes or
leaf edges of the foliage of hosts such as rhododendron, raspberry and
grapes, and many herbaceous perennials (see Figure 14.18). Several
related species, e.g. the clay-coloured weevil (Otiorhyncus singularis
cause similar damage to that of the vine weevil.
|Figure 14.18 (a) Vine weevil larva and pupa (b) Adult Vine Weevil damage on Tellima
The adult is 9 mm long, black in colour, with a rough
textured cuticle (see Figure 14.19). The forewings are fused together,
the pest being incapable of flight. No males are known. The female
lays eggs (mainly in August and September) in soil or compost, next to
the roots of a preferred plant species. Over a period of a few years, she
may lay a thousand eggs as she visits many plants. The emerging larvae
are white, legless and with a characteristic chestnut-brown head. They
reach 1 cm in length in December when they pupate in the soil before
developing into the adult.
|Figure 14.19 Vine weevil adult-9 mm
is achieved by the female adult crawling around at night, or by
the movement of pots containing grubs.
gardeners sometimes use traps of corrugated paper
placed near infested crops. Inspection of plants at night by torchlight
may reveal the feeding adult.
s and Professional
s can use the nematode ( Steinemena
carpocapsae ) by incorporating it into compost or soil. Professional
growers have residual chemicals, imidacloprid or
chlorpyrifos incorporated or drenched into compost or soil.
Wireworm (Agriotes lineatus)
This beetle species is commonly found in grassland, but
will attack most crops. Turf grass may be eaten away by the larvae
) resulting in dry areas of grass. The pest also bores through
potatoes to produce characteristic narrow tunnels, while in onions,
brassicas and strawberries the roots are eaten. In tomatoes, the larvae
bore into the hollow stem.
The 1 cm long adult (click beetle) is brown-black and has the
unusual ability of flicking itself in the air when placed on its back. The
female lays eggs in weedy ground in May and June and the larvae, after
hatching, develop over a four year period. Fully grown wireworms are
about 2.5 cm long, shiny golden-brown in colour, and possess short legs
(see Figure 14.7). After a three week pupation period in the soil, usually
in summer, the adult emerges and in this stage survives the winter.
is by means of the flighted adults.
gardeners dig in green manure crops to lure
wireworms away from underground roots and tubers. Professional
growers may reduce serious damage to young crops by using a seed
dressing containing tefluthrin.
Garden Chafer (Phyllopertha horticola)
This pest is increasingly proving a problem on turf where the
large white grubs eat the roots. Small yellow patches appear in the lawn
or sports area, notably in summer when the grubs are becoming fully
grown. Further damage can occur when starlings, crows, moles, foxes
and even badgers dig up parts of the lawn to reach the succulent prey.
|Figure 14.20 Chafer grub-2 cm
The adult is a broad, 1 cm long, light-brown beetle with a
bottle-green upper thorax and head. Adults emerge from soil-borne
pupae in May and June. They feed on leaves of a variety of plants,
sometimes badly damaging fruit tree foliage. Eggs are laid near grasses
in early summer and emerging larvae develop during the next 10 months
into the characteristic stout white grub with a light-brown head. Their
body, which may reach nearly 2 cm in length, is curved and bears welldeveloped
legs (see Figure 14.20 ). The larvae are the winter- survival
stage. Pupation occurs in early spring.
is by means of the flighted adults.
For the Amateur
gardener, maintenance of a healthy, wellfertilized
lawn will lessen chafer attack.
Groundsmen may use a biological control, involving a nematode,
Heterorhabditis bacteriophora available for application during the
summer. The nematode enters the chafer grub through its spiracles, and
proceeds to release bacteria which digest the grub’s body. A soil-applied
insecticide containing imadocloprid is used against this pest.
Raspberry beetle (Byturus tomentosus)
The developing fruit of raspberry, loganberry and blackberry may be
eaten away by the 8 mm long, golden-brown larvae of this pest. Only
one life cycle per year occurs, the larva descending to the soil in July
and August, pupating in a cell from which the golden-brown adult
emerges to spend the winter in the soil. The adult female lays eggs in
the host flower the following June. Spread
is by means of the flighted
Since the destructive larval stage may enter the host fruit
and thus escape insecticidal control, the timing of the spray is vital. In
raspberries, a contact chemical such as bifenthrin,
applied when the
fruit is pink, achieves good control.
Other beetle pests
Springtime attack of flea beetle (Phyllotreta species
) on leaves of young
cruciferous plants (e.g. cabbages and stocks) is a serious problem to amateur
horticulturist alike (see Figure 14.21). In
recent years, four other increasingly common beetle problems have been
reported. These are viburnum beetle on Viburnum opulus, V. tinus and
V. lantana, rosemary leaf beetle (Chrysolina americana
) on lavender,
rosemary and thyme, red lily beetle (Lilioceris lilii
) on lilies, and
asparagus beetle (Crioceris asparagi
|Figure 14.21 Flea beetle damage
This group, together with bees (see Pollination and fertilization
), wasps and ants, are classified in the order
Hymenoptera, characterized by adults with two
pairs of translucent wings and with the fore- and
hind-wings being locked together by fine hooks.
The slender waist-like first segments
of the abdomen give these insects a characteristic
appearance. A sawfly larva is shown in
Figure 14.22 .
Gooseberry sawfly (Nematus
This is an important pest on
gooseberries, redcurrants and whitecurrants,
but not on blackcurrants. Extensive damage to
foliage may be caused by the caterpillars. In
some cases, the leaves of the whole bush may be
The adults emerge from their
overwintering soil-borne cocoons in late April.
Adults measure about 1 cm in length and
resemble flying ants. The male has a black
abdomen, the female a yellow abdomen. The
female, in early May, lays elongate 1 mm long,
light-green eggs in rows along the veins
of the leaves situated low down in the
host bush. Emerging young caterpillars
eat small holes in these leaves. Maturing
caterpillars (reaching up to 2 cm in
length and identifiable by their green
appearance with numerous black
and with a black head) consume
whole leaves and move outwards and
upwards from their original positions.
After 3–4 weeks of feeding, the mature
caterpillars drop to the soil and pupate.
There are 2–3 more lifecycle generations
of the sawfly from the second emergence
in early June to last soil pupation in late
|Figure 14.22 Viburnum sawfly damage
is by the flighted adults.
gardeners often pick off the first few young caterpillars
found on the base leaves of the bush at egg-germination times (in
late April, early June, early July and late August). In autumn and
winter periods, the removal of any mulch from around the bushes and disturbance of soil in the area encourage birds to seek out overwintering
pupae. Insecticide products containing pyrethrins
control this pest.
growers use two ingredients derived from plants. The
are effective against the young
caterpillars if directed on the under-leaf surface, and with special
attention being paid to the lower, central leaves.
No biological control is currently available for gooseberry sawfly, and
the insecticide formulated from the bacterium, Bacillus thuringiensis,
whilst effective on many caterpillars of moths, is ineffective against this
Rose leaf-rolling sawfly (Blennocampa pusilla)
The black shiny adults, resembling winged queen ants, emerge from the
soil-borne pupa in May and early June. Eggs are inserted into the leaf
lamina of roses, which, in responding to the pest, rolls up tightly. The
emerging larva, which is pale green with a white or brown head, feeds
on the rolled foliage. It reaches a length of 1 cm by August, when it
descends to the ground and forms an underground cocoon to survive the
winter until it pupates in March. All types of roses are affected, although
climbing roses are preferred. Damage caused by leaf-rolling tortrix
caterpillars, e.g. Cacoecia oparana,
may be confused with the sawfly,
although the leaves are less curled.
of the sawfly is achieved by means of the flighted adults.
gardeners may need to control this pest with an insecticide
Nursery stock growers
may use products
Springtails (order Collembola)
|Figure 14.23 Springtail – about 2 mm in size –
can jump by means of spring at the end of its
This group of primitive wingless insects, about 2 mm in length
( Figure 14.23 ) has a spring-like appendage at the base of the
abdomen. They are very common in soils, and normally aid in the
breakdown of soil organic matter.
Two genera, Bourletiella
however, may do serious damage to conifer seedlings
and cucumber roots respectively.
is slow since all stages are wingless.
The mites (Acarina) are classified with spiders and scorpions in the
Arachnida. Although similar to insects in many respects they are
distinguished from them by the possession of four pairs of legs, a
fused body structure and by the absence of wings
(see Figure 14.24).
Many of the tiny soil-inhabiting mites serve a useful purpose in breaking
down plant debris. Several above-ground species are serious pests on
plants. The life cycle is composed of egg, larva, nymph and adult
Glasshouse red spider mite (Tetranychus urticae and
|Figure 14.24 Glasshouse red spider mite. Note
that the red spider mite may be light green or red in
colour. Its extremely small size (0.8 mm) enables it to
The piercing mouthparts of the mites inject poisonous
secretions which cause localized death of leaf mesophyll cells.
This results in a fine mottling symptom on the leaf (see Figure
14.1), not to be confused with the larger spots caused by thrips.
In large numbers the mites can kill off leaves and eventually
whole plants. Fine silk strands are produced in severe infestations,
appearing as ‘ropes’ (see Figure 14.25) on which the mites move
down the plant. On flowering crops such as chrysanthemums,
these ropes make the plant unsaleable.
This pest is of tropical origin and thrives best in high
greenhouse temperatures. Both species are 1 mm in length. The first
species (T. urticae)
is yellowish in colour, with two black
spots (see Figure 14.25). The female lays about 100 tiny spherical
eggs on the underside of the leaf, and after a period of three days
the tiny six-legged larva moults to produce the nymph stage that
resembles the adult. The life cycle length varies markedly from
62 days at 10°C, to 6 days at 35°C when the pest’s multiplication
potential is extremely high. In autumn, when the daylight period
decreases to 14 h and temperatures fall, egg production ceases
and the fertilized females, which are now red in colour, move into the
greenhouse structures to hibernate (diapause), representing foci for the
next spring’s infestation.
|Figure 14.25 (a) Glasshouse red spider in the centre (b) webbing
occurs when adults and nymphs crawl from plant to plant when
leaves are touching. Wind currents can move mites attached to their silk
strands. The small size and under-leaf habitat of the pest combine to
keep its existence on introduced plants away from the gaze of growers,
especially older growers.
species (T. cinnabarinus),
which is dark reddish-brown, has
a similar life cycle to T. urticae,
but does not hibernate. The hibernation
habit of T. urticae
leads to it being a common pest on annual crops such as tomatoes, cucumbers and chrysanthemums, while T. cinnabarinus
is found more commonly on the perennial crops such as carnations,
arums and hothouse pot plants. The two species often occur together on
may be achieved in several ways. Amateur
gardeners and Professional
growers should carefully check incoming plants for
the presence of the mite, using a hand lens if necessary. A predatory
mite, Phytoseiulus persimilis is commonly introduced into cucumber,
chrysanthemum and tomato crops in spring. For the Amateur
are products containing fatty acids professional
, winter fumigation of greenhouse structures with chemicals
such as formalin
or burning sulphur
kills off many of the hibernating
females. A pesticide containing abamectin
is commonly used.
Gall mite of blackcurrant (Cecidophyopsis ribis)
Mites living inside the blackcurrant bud damage the meristem
and induce the bud to produce many scale leaves, which gives the bud its
unusual swollen appearance (see Figure 14.26). These buds either fail to
open or produce distorted leaves. In addition to the mechanical damage,
the mite carries the virus responsible for reversion disease
(see Horticultural diseases and disorders
), which stunts the plant and reduces fruit production.
|Figure 14.26 (a) Big bud symptoms on blackcurrant (b) Erineum mite damage on grape leaf
Unlike red spider mite, this species, sometimes called bigbud
is elongated in shape and is minute (0.25 mm) in size. It
spends most of the year living inside the buds of blackcurrants and, to a
lesser extent, other Ribes species. Breeding takes place inside the buds
from June to September, and January to April.
In May the mites emerge and are spread on silk threads and on
the bodies of aphids to infest newly emerging buds and plants.
The mite is controlled in three ways. Clean planting material
is essential for the establishment of a healthy crop. Pruning
out of stems
with big bud and destruction of reversion-infected plants slows down the
progress of the pest. Amateur
gardeners and Professional
spray a fine formulation of sulphur
during the May–June period when
the mites are migrating. There are no chemical ways to control the mite
in the bud. Recently there has been a reported problem on hazel in UK
caused by the hazel big bud mite (Phytopus avellanae
Tarsonemid mite (Tarsonemus pallidus)
. Distortion of developing leaves and flowers resulting from
small feeding holes and injected toxins are the main symptom of this
pest. This may happen to such an extent that leaves and petals are
stunted and misshapen, and flowers may not open properly. Plant species
affected are Amaranthus , Fuchsia , pelargonium and cyclamen (the
pest is sometimes referred to as ‘cyclamen mite’) A closely related but
distinct strain is found on strawberries.
This spherical mite, only 0.25 mm in length, lives in the
unex-panded buds of a wide variety of pot plants. In greenhouses, the
adults may lay eggs all the year round, and the 2 weeks life cycle
can cause a rapid increase in its numbers.
occurs mainly on transported bulbs, corms and plants.
Care should be taken to prevent introduction of infested plants
and propagation material into greenhouses. For the amateur
, there is no
recommended chemical product. For the professional
contact acaricide, abamectin,
is effective against the mite. Addition of a wetter/spreader
may help the spray penetrate the tight-knit scale leaves
Four other horticulturally important mites require a mention. The fruit
tree red spider mite (Panonychus ulmi
) causes serious leaf mottling
of ornamental Malus and apple. Conifer spinning mite (Oligonychus
) causes spruce leaves to yellow, and the mite spins a web of
silk threads. Bulb-scale mite (Steneotarsonemus laticeps
) causes internal
discoloration of forced narcissus bulbs. Bryobia mite (e.g. Bryobia
) attacks fruit trees, and may cause damage to greenhouse
crops, e.g. cucumbers, if blown in from neighbouring trees.
In addition to insects and mites, the phylum Arthropoda contains
three other horticulturally relevant classes, the Crustacea (woodlice),
Symphyla (symphilids) and Diplopoda (millipedes). The Chilopoda
(centipedes) superficially resemble millipedes, but are unrelated and are
useful general predators.
Woodlouse (Armadillidium nasutum)
The damage is confined mainly to stems and lower leaves of succulent
glasshouse crops such as cucumbers, but occasionally young transplants
may be nipped. A relative of marine crabs and lobsters, the woodlouse
has adapted for terrestrial life, but still requires damp conditions to
survive. In damp soils it may number over a million per hectare, and
greatly helps the breakdown of plant debris, as do earthworms. In
greenhouses, where plants are grown in hot, humid conditions, this
species may multiply rapidly, producing two batches of 50 eggs per year.
The adults roll into a ball when disturbed. Partial soil sterilization by
steam effectively controls woodlice.
Symphilid (Scutigerella immaculata)
In greenhouse crops, root hairs
are removed and may cause lettuce to
mature without a heart. Infectious fungi, e.g. Botrytis,
may enter the
roots after symphilid damage. These delicate white creatures, with 12
pairs of legs, resemble small millipedes. The adult female, 6 mm long,
lays eggs in the soil all the year round, and the development through
larvae to the adult takes about 3 months. Symphilids may migrate 2 m
down into soil during hot, dry weather. The recognition of this pest is
made easier by dipping a suspect root and its surrounding soil into a
bucket of water and searching for symphilids which float on the water
These elongated, slow-moving creatures are characterized by a thick
cuticle and the possession of many legs, two pairs to each body segment.
Many species are useful in breaking down soil organic matter, but two
pest species, the flat millipede (Brachydesmus superus
) and a tropical
species (Oxidus gracilus
), can cause damage to roots of strawberries and
These animals resemble millipedes, but are much more active. They
help control soil pests by searching for insects, mites and nematodes in