Non-toxic, no build-up of resistant pests and diseases.
Needs careful introduction and knowledge of life cycles.
Can easily be affected by pesticides.
There are two sources of ‘natural enemies’ to pests, the local species
and the exotic ones. Many pests of outdoor horticultural crops such as
peach-potato aphid are indigenous
(i.e. they are present in wild plant
communities in the UK
). Such pests are often reduced in nature by other
organisms which, as predators, eat the pest, or, as parasites, lay eggs
within the pest. These beneficial organisms, found also on horticultural
crops, are to be encouraged and in some cases are deliberately
introduced. A range of important organisms useful in horticulture is now
described in some detail.
Indigenous predators and parasites
It has been shown that a pair of blue tits can consume 10 000 caterpillars
and one million aphids in a 12 month period. The installation of tit
boxes is a worthwhile activity. Wrens, thrushes and blackbirds similarly
contribute to the control of garden insects.
Hedgehogs belong to the insectivore group of mammals, but are
omnivorous. Although their preferred diet is insects (up to 200 g per
day) they will eat slugs. Care must be taken that they are not exposed
to dead slugs which have consumed slug bait containing methiocarb or
methaldehyde, as these would be toxic to the hedgehog. Hedgehogs are
encouraged to enter gardens by means of small holes cut into the base of a fence panel. Within gardens, heaps of logs and piles of leaf litter in
a quiet location are suitable for their daytime and overwintering retreat.
Wooden hedgehog shelters are commercially available.
The black-kneed capsid
The black-kneed capsid
) is an insect
found on fruit trees alongside its pestilent relative, the common green
capsid. It eats more than 1000 fruit tree red spider mites per year. Its
eggs are laid in August and survive the winter. Winter washes used by
professional horticulturalists against apple pests and diseases often kill
off this useful insect. The closely related anthocorid bugs
, such as Anthocoris nemorum
, are predators on a wide range of pests, such as
aphids, thrips, caterpillars and mites, and have recently been used for
biological control in greenhouses.
Lacewings, such as Chrysopa carnea
, lay several hundred eggs per
year on the end of fine stalks, located on leaves. Several are useful
horticultural predators, their hairy larvae eating aphids and mite pests,
often reaching the prey in leaf folds where ladybirds cannot reach.
The 40 species of ladybird
beetle are a welcome sight to the
professional horticulturist and lay person alike. Almost all are predatory.
The red two-spot ladybird (Adalia bipunctata
) emerges from the soil in
spring, mates and lays about 1000 elongated yellow eggs on the leaves
of a range of weeds, such as nettles, and crops such as beans, throughout
the growing season. Both the emerging slate-grey and yellow larvae and
the adults feed on a range of aphid species. Wooden ladybird shelters
and towers are now available to encourage the overwintering of these
A worrying development in the last few years has been the rapid spread
and increase of the harlequin ladybird
from South-East Asia. This
species is larger (6–8 mm long) and rounder than the two-spot species
(4–5 mm). It has a wider food range than other ladybird species,
consuming other ladybird’s eggs and larvae, and eggs and caterpillars
of moths. Furthermore, it is able to bite humans and be a nuisance in
houses when it comes out of hibernation.
The ground beetle
(such as Bembidion lampros
), a 2 cm long black
species (see Figure 16.5), is one of many active carabid beetles that
actively predates on soil pests such as root fly eggs, greatly reducing
|Figure 16.5 (a) Predatory ground beetle
(b) I chneumon wasp parasitic on caterpillars
Superficially resembling wasps, these are commonly seen darting or
hovering above flowers in summer. Several of the 250 British species,
such as Syrphus ribesii
, lay eggs in the midst of aphid colonies, and the
legless light-green coloured grubs consume large numbers of aphids.
The flowers of some garden plants are especially useful in providing
pollen for the adults and therefore encouraging aphid control in the
garden. Summer flowering examples are poached-egg plant (Limnanthes
), baby-blue-eyes (Nemophila menziesii
) and Californian poppy
). Later summer and autumn examples are Phacelia
and ice plant (Sedum spectabile
Mites and spiders
such as Typhlodromus pyri
eat fruit tree red spider mite
and contribute importantly to its control. The numerous species of webforming
and hunting spiders
help in a very important but unspecific way
in the reduction of all forms of insects.
|Figure 16.6 Swollen aphid
The much maligned common wasp
) is a voracious
spring and summer predator on caterpillars, which are fed in a paralyzed
state to the developing wasp grubs. Digger wasps
also help control
caterpillar numbers and benefit from dead hollow stems of garden plant
which they use as nests all year round.
There are about 3000 parasitic wasp
species of the families
Ichneumonidae, Braconidae and Chalcidae found on other insects in
Britain. Ichneumons (Opion spp
. see Figure 16.5) lay eggs in many moth
caterpillars. The braconid wasp (Apanteles glomeratus
) lays about 150
eggs inside a cabbage white caterpillar and the parasites pupate outside
the pest’s dead body as yellow cocoon masses. The chalcid (Aphelinus
) parasitizes woolly aphid on apples.
|Figure 16.8 Glasshouse whitefly parasite.
(a) Encarsia wasp laying an egg into a
scale (b) Parasitized whitefly scale
black in colour (c) Application of
Encarsia to a
(as blackened whitefly scales)
The spiracles of insects provide access to specialized parasitic fungi
particularly under damp conditions. In some years, aphid numbers
are quickly reduced by the infection of the fungus Entomophthora
, while codling moth caterpillars on apple may be enveloped by Beauveria bassiana
. Cabbage white caterpillar populations are
occasionally much reduced by a virus
, which causes them to burst.
|Figure 16.7 Glasshouse red spider mite predator.
predator eating glasshouse red spider
mite (b) Eggs and young of
(c) Application of
Phytoseiulus to crop
Increased attention is being given by horticulturalists to the careful
selection of pesticides (if they are needed) to avoid unnecessary
destruction of indigenous predator and parasite numbers.
|Table 16.1 Biological control organisms reared commercially for
In recent years, commercial firms have begun to make available readyto-use products
containing indigenous predators or parasites to outdoor
growers. Examples are two-spot ladybird, lacewing larvae and three
nematode parasites (against slugs, vine weevil larvae and flea beetles
(see also Table 16.1).
Exotic predators and parasites
In greenhouses and polythene tunnels, high temperatures often all year
round and sub-tropical species of plants bring with them exotic pests
and diseases. Further, the increase of both pests and diseases is much
quicker than comparable pests or diseases growing outdoors. Also,
these greenhouse inhabiting organisms have, over the last half-century,
developed resistance to almost all available pesticides.
Biological control of exotic pests requires exotic predators and parasites.
And so the health of the major greenhouse crops is in large measure
due to two organisms: a South American mite which
eats all stages of the glasshouse red spider mite, and
a tiny South-East Asian wasp that parasitizes the
The conditions in a greenhouse have two advantages
for biological control. Firstly, the environment is
relatively isolated so that the controlling organisms
are not likely to disappear. Secondly, the glasshouse
environment is relatively stable and allows biological
control to be more measured (than in the outdoor
situation) with interactions between pests and their
predator or parasite being more predictable.
Almost all commercial production of glasshouse
crops in the UK
now uses biological control. The
two commonest biological control organisms
are described below in some detail. Much more
information is available from commercial companies
or from the Internet. A more extensive listing of
commercially available biological control species is
given in Table 16.1 .
Phytoseiulus persimilis (see Figure 16.7)
This is a 1 mm globular, deep orange, predatory
tropical mite used in greenhouse production to
control glasshouse red spider mite. It is
raised on spider mite-infected beans and then evenly
distributed throughout the crop, such as cucumbers,
at the rate of about one predator per plant. Some
growers who have suffered repeatedly from the pest
first introduce the red spider mite throughout the
crop at the rate of about five mites per plant a week
before predator application, thus maintaining even
levels of pest–predator interaction. The predator’s
short egg–adult development period (7 days), laying
potential (50 eggs per life cycle) and appetite (five
pest adults eaten per day), explain its extremely
Encarsia formosa (see Figure 16.8)
This is a small (2 mm) wasp which lays an egg into the
glasshouse whitefly scale
, causing it to turn
black and eventually to release another wasp. This parasite
is raised commercially on whitefly-infested tobacco
plants. It is introduced to the crop, such as tomato, at a
rate of about 100 blackened scales per 100 plants. The
parasite’s introduction to the crop is most successful
when the whitefly levels are low (recommended less than
one whitefly per 10 plants). Its mobility (about 5 m) and
successful parasitism are most effective at temperatures
greater than 22°C when its egg-laying ability exceeds that
of the whitefly.
The wasp lays most of its 60 or more eggs within a few
days of emergence from the black scale. Thus, a series of
weekly applications from late February onwards ensures
that viable eggs are laid whenever the susceptible whitefly
scale stage is present. The appearance of newly infected black scales
on leaves is often taken as an indication that
parasite introductions can be stopped.
An understanding of each pest’s and each biological control
organism’s life cycle is vital to ensure success in control.
A combination of biological methods may be used on
some crops, such as chrysanthemums, tomatoes, peppers,
aubergines and cucumbers in order to simultaneously
control a range of organisms occurring on the crop at
the same time (see Table 16.1, and integrated control
). Several specialist firms now have contracts to apply
biological control organisms to greenhouse units. There are
several practical points that confront growers in both the
outdoor and the glasshouse situation.
The main problems with biological control are:
- Unsuccessful application of biological control organisms
that lead to a severe pest problem.
- Introduction of a biological control organism that
subsequently kills desirable or beneficial organisms in
These can be minimized
- understanding both the pest’s and predator/parasites ’ life
cycles in order to achieve reliable control;
- carefully choosing the best predator or parasite for the
problem pest or disease concerned;
- taking care that environmentally useful species are not
subject to the attacks of the predators and parasites.
In most horticultural situations, there are important examples of natural balance
- With pests, their naturally occurring predators and parasites are an
important form of crop protection.
- With diseases, naturally occurring predators and parasites are less
common, but the nutritional condition of the plant and the resulting
naturally occurring bacterial and fungal populations on leaf, stem and
root surfaces (see phyllosphere and rhizosphere) often help
slow a disease’s progress.
- The garden represents a complex situation. There may be plant species
present from every continent, and any of these plant species
may be accompanied by a specific pest from its country of origin.
Plant species that have been present in the UK for many years (such
as apple) often have beneficial predators and parasites introduced
accidentally or deliberately, from their country of origin, that limit
pest numbers. It is quite likely, however, that for more recently
imported plant species, there may not be appropriate predators or
parasites to control an introduced pest occurring on the plant species
in the British Isles.
Some horticultural practices can disturb natural balances
- In a natural habitat such as woodland, a climax population of plants
and animals develops. Here, a complex balance exists
between indigenous pests and their predators/parasites. The food
webs include several types of predator/parasite found on
each plant species that limit (but do not eliminate) the pests. This
development of food webs is not achieved to such an extent in most
gardens since the natural succession of wild plant species mentioned
above is not desirable to gardeners as they aim for optimum
production of edible crops or for an aesthetic layout of decorative
plants free from weeds (see also Environment and ecology).
- Regular movement or removal of cultivated plants and weeds without
particular thought to the natural balance between predator/parasites
and pests will make pest attacks more likely in the garden/nursery
- The removal of the rotting hollow stems of herbaceous perennials and
branches of decaying wood which are common sheltering sites for
predatory beetles and centipedes reduces their control potential.
- In a similar way, removal of old plants such as brassicas or bedding
plants in autumn may take away the parasitized aphids or caterpillars
that would normally serve as the next year’s control measures.
- The absence in gardens of plant species acting as a pollen food
source to adults such as hoverfl ies may delay the emergence of their
predatory larvae amongst aphid populations.
- The lack of good soil structure resulting from poor cultivation
or inadequate incorporation of organic matter in a garden may hinder the
movement of predatory animals in their search for soil pests.
- A poor physical preparation of soil, and lack of attention to pH and
nutrient levels in soil may result in poor soil microbial action.
- The repeated planting of crops or annual bedding plants into the same
area of soil often leads to serious attacks of persistent soil-borne
pests or diseases. Notable examples are club root disease on brassicas and potato cyst nematode pest on potatoes. A
comparable situation is found when young trees and shrubs (such as
roses) are planted into a soil previously occupied by an old specimen
of the same plant species, with the resulting problem called ‘replant
disease’ caused by high level of Pythium fungus.
- The unconsidered use of pesticides may result in a rapid decrease in
predators and parasites and may considerably delay their appearance
and build-up the following growing season.
The natural balances of organisms can be maintained
in order to reduce pesticide use. At the private garden level, there are
an increasing number of practices being used that encourage natural
balances in order to reduce pesticide use. These physical and cultural
methods have been described earlier.