Insects in the
garden
It is the nature of gardening that
"pests" proliferate, we provide ideal conditions for
them. Like "weeds", "pest" insects are just
ones that we don't like and which exploit the conditions created
in gardens for their own benefit. There are ways to avoid the
build-up of any species so that it doesn't become a pest.
Critical elements are to avoid pesticides and to grow a good
mixture of plants. Click here for links
to some organic gardening sites.
We shouldn't really think of insects as "good guys" or
"bad guys". Some can be helpful, some can cause damage
but the majority have no effect whatsoever. Furthermore, if you
wish to keep the beneficial species in the garden, you must
always keep some of the pests to feed them! Some brief
recommendatons are made at the end.
Potential pests
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Some aphids such as greenfly (left) can breed very rapidly and harm garden plants and crops. As well as depleting plants' resources, they spread viruses when they pierce plants with their mouthpieces. They are a favoured food of ladybirds, lacewings and hoverflies but are also eaten by many other insects and spiders. | The Homoptera is a sub-order of the Hemiptera. It contains a lot of distinct sub-groups many of which are garden pests: Coccoidea (scale insects and mealy bugs), Aleyrodidae (whitefly), Adelgidae(conifer aphids) and various leaf hoppers, as well as the aphids. (Bugs page) |
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Mollusca: slugs and snails. These can reduce crops and decorative plants (especially Hosta) to nothing. However, they are extremely important for natural cycles in breaking down old vegetation. Most gardeners cannot see any value for them but compost heaps would not be so effective without mollusc activity! |
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Beneficial insects
There are pages devoted to ladybirds and to ground beetles. Two other groups of insects are particularly useful in the control of aphids but there are large numbers of others (spiders, true bugs, harvesters &c) which also feed on aphids and other pests.
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Diptera: Syrphidae Hoverflies These flies are often mimics of bees and wasps but they do not sting. The larvae of most species feed voraciously on aphids. They are true flies, having only one pair of wings for flight. |
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Probably the most common species in gardens is Episyrphus balteatus whose adults can be very numerous in summer, feeding on nectar. |
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The adults (imagines) of hoverflies live on nectar so a continuous variety of flowers will attract them. Some plants, such as Limnanthes douglasi seem particularly attractive to them. |  |
There is an extremely comprehensive web-site with keys and many species illustrated at Syrphidae - mainly in Dutch but easily understood. |
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Neuroptera Lacewings Like hoverflies, only the larvae of lacewings feed on aphids. This is a green hoverfly, Chrysoperla sp.. |
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Lepidoptera Butterflies and moths There are vast numbers of web-sites dealing with these. Butterflies are lovely to look at and easily attracted to most flowering plants. |
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Keeping them in a garden is more difficult - they need host plants for their larvae (caterpillars). Some Lepidoptera are pests but maintaining good populations of predators can limit the number of caterpillars. |
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Hemiptera: Heteroptera These, like the Homoptera (above), are bugs. Some of them are plant feeders but many of them are predators. Lots are distinctively patterned (as the American specimen to left: © Stephen Rigden Photography) |
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but many are unobtrusive dull browns. Their wings overlap and are partly hardened (as beetles) but partly membranous (as flies) giving them a crossed appearance. They have a different life cycle from beetles and flies and their early stages (nymphs) are not winged.(Bugs page) |
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Hymenoptera: Apidae Bees Many families are useful for pollination of crops. |
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As well as honey, bumble and solitary bees, the Hymenoptera includes wasps, ants and numerous parasitic species such as the ichneumons. |
Other invertebrates
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Spiders (Arachnida: Araneae) The Arachnida includes, in Britain, spiders, Opiliones (harvesters), scorpions (rarely), pseudoscorpions, ticks and mites. Most of the small members of the class provide food for larger predators but some, especially spiders, kill large numbers of pests: web-building spiders catch aphids and flies. It is therefore essential that a garden contains vertical as well as horizontal variety. Covering walls and fences with climbers such as ivy (Hedera) provides good shelter. The pictures show a brood web, with many spiderlings, on a variety of ivy. |
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Myriapoda: centipedes and
millipedes. These two groups are not very closely related but are easily confused since they both have many legs. Centipedes are good things to have in the garden since they are fierce predators and will eat many pests. Some millipedes may be pests since they will eat roots although most consume decomposing vegetation. (More) |
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Earthworms Are well known for their part in improving soil structure, aerating and draining the soil, and incorporating dead plant material. (Detritivory). |
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Vertebrates
| Birds, like other vertebrates, can
be a 'mixed blessing'. no matter how many peanuts or breadcrumbs you feed them, most would still prefer a juicy beetle larva or crunchy spider. On the other hand, many species of bird eat aphids or larger pests such as caterpillars. As with all animals it is probably best to make the garden amenable to birds by planting seed-bearing annuals or native trees and hedges (for nesting and roosting as well as feeding) rather than just providing food for a few species. |
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| Frogs and toads
will eat large numbers of slugs as well as any
wasps foolish enough to approach their pond for a drink.
They will also eat any ground beetles that they
encounter. Of course, they need moist situations and a pond - these are also valuable to many invertebrates and can be used to grow marsh plants. Try to avoid steep sides to your pond. Clear excess weed in the Autumn and do not disturb the pond during the winter since amphibians and invertebrates will be hibernating at the bottom! Do not put fish in it - they will eat everything else! |
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| Some mammals can be useful in the garden: hedgehogs will keep snail and slug populations down. Other species are less welcome. In a small, enclosed garden it is quite easy for hedgehogs to be trapped and run out of food (although they are quite good climbers); they may often drown in steep-sided pools. Don't forget that food put out for hedgehogs, or birds, is equally attractive to other mammals: rats and mice can live on it and may enter the house when the garden supply runs out! (More mammals) |  |
GARDENING WITH INSECTS
There is a page on biological
control - go
All beneficial
insects (and other invertebrates) need:
- food for their larvae
- food for their 'adults' (imagines)
- shelter - for their eggs, their pupae and for over-wintering.
To maintain a variety of insects, you will need to make
provisions for all of these. This is very complex and
therefore the more varied your garden is (the number of plants,
the number of micro-habitats, etc), the more species you will
maintain. There are some clear "do's" and
"don't's":
Do NOT use biocides (insecticides will kill the
beneficial insects as well as their food); do not
trim hedges and herbaceous plants excessively (there will be
nowhere for the beneficial insects to shelter); do not burn
or send to landfill any garden waste unless essential (it may
carry eggs and pupae of beneficial insects).
DO encourage ground cover (both plants and dead
leaf); cover walls and fences with ivy or other climbers; have a
variety of flowering plants; have a variety of habitats (pond,
raised bed, shingle bed, compost heap, bushes).
Many flowering plants will attract
flying insects: Buddleja and butterflies is the best
known example but it is not clear how beneficial such plants are
in maintaining insect populations: butterflies don't lay eggs on Buddleja
and caterpillars don't eat it. On the other hand many mature
insects (including hoverflies and sometimes ladybirds) will take
nectar from many flowering plants, so a variety of flowers is
useful.
Some plants are valuable because they harbour aphids
(perhaps distracting pests from other garden plants?) and thus
keep aphids, hoverflies and lacewings alive. The best example is
Nasturtium which is guaranteed to attract blackfly (and sometimes
greenfly). Many 'weed' species are especially attractive to both
aphids and beneficials (e.g. nettles and thistles).
Some plants seem to be natural attractants
for beneficial insects, aphids or not. For instance the
'poached egg' plant, Limnanthes douglasii, attracts
large numbers of hoverflies. Ladybirds will lay their eggs on
some plant species even in the absence of aphids. Recorders in
the London and Essex ladybird surveys (go
to) are noting which plants ladybirds are found on: at
present no trends are clear except that certain native (and
weedy!) plants are very good: to date stinging nettles, ivy,
thistle, gorse and hawthorn are the preferred hosts. Amongst more
usual garden plants the umbeliferous herbs (Angelica, Chervil,
Parsley, Coriander &c) seem to be preferred. More
unexpected hosts include various species of Euphorbia and
Berberis. So, grow as many different plants as possible,
observe which beneficials are seen on them, and let us know!
SOME
RECOMMENDATIONS
- cover walls and fences with ivy or other
climbing plants;
- incorporate hawthorn, beech (and gorse?) into your hedgerows;
- include native trees and shrubs in your planting (if your
garden is large enough);
- leave dead leaves, plants and wood on the ground if possible;
compost them rather than burning or burying them;
- trim hedges, trees and shrubs only in late summer if feasible;
after cutting, leave the trimmings nearby for a few weeks (this
allows eggs and pupae to mature and return to their host plant);
- leave all dead wood in a pile in a corner of the garden (this
will house many animals, large and small);
- keep a small patch of weeds (preferably including nettles and
thistles) in a corner of the garden (perhaps behind the
woodpile?) - to avoid spreading, perhaps grow these in a
container;
- use mixed plantings: grow ground cover, annuals and herbs
amongst your choicer plants;
- a well-kept compost heap will house millions of invertebrates.
It's also the best means of recycling your household and garden
waste.
- grass with a variety of species and textures can be a good
haven for invertebrate life: keep part of your lawn partly mowed
or introduce native species (More).
It's a
hard life .....
Don't forget,
or be appalled by, the fact that beneficial animals in your
garden will prey on each other! Hedgehogs, toads and frogs will
eat slugs and wasps but also ground beetles. Ground beetles will
eat many pests but also worms, spiders and each other! Spiders
will eat many pestiferous flies and aphids but also take
ladybirds and lacewings. And so on .........
It's probably best not to interfere with these interactions -
they usually end in a good balance of insects which, overall, is
beneficial to you as a gardener. All of the predators that remain
will eat the smaller beasts, such as aphids, at the lower end of
the food chain!
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Biological Control
Created 2001
Last modified 24/9/2005
