Garden Insect Pests – Gardener’s Friends or Foes?

The life stages of the green apple aphid (Aphi...

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Insects are the most numerous of land animals, both in regard to species and as individuals. More than 20,000 kinds are found in Britain alone, and of these a considerable number live in gardens. So not surprisingly they are regarded with suspicion by many gardeners. Some are harmful, of course, but most do no harm at all, while a few are actually beneficial.

Besides the large number of true insects that inhabit our gardens, there are other creatures which we often loosely associate with them. These include spiders, which prey on small flies, centipedes that hunt slugs and other pests, and the harmful millipedes. Slugs and woodlice are other examples of garden pests that are not insects.

A true insect has three pairs of legs, and characteristically either one or two pairs of wings. Many pass through four distinct stages in their life cycle before they are mature. As far as the gardener is concerned, the larval stage is usually the most significant, for it is then that most feeding takes place. Insect larvae are known by various names, such as maggots, grubs, or caterpillars, and they feed mainly on vegetation. Some larvae reach maturity in a few days, while in others the period can be as long as two years.

The insect population of gardens is naturally varied, and is governed by both size and location. The type of soil is also influential; ants, for example, are usually common in sandy areas. And if weeds are allowed to grow unchecked they will attract more insects of various kinds; wild plants such as campions (Silene) and docks (Rumex) are often badly infested with aphids.

 

Beneficial Garden Insects

Before waging war on the garden insect pests that you find in your garden, it is as well to know how the beneficial kinds may be recognized. You do not want to harm bumblebees, for instance, as they visit the blossoms in the fruit garden, and in doing so carry pollen from flower to flower and so ensure a good crop. Honey bees perform the same useful service. Wasps are not so popular, but they do kill large numbers of harmful larvae.

Beetles are also unpopular in a general way, but of the nearly 4,000 species inhabiting Britain, some are definitely useful. The principal ones are described below.

Violet ground beetles are often unearthed by the spade. These lively beetles do not harm plants but prey on other insects, so should be allowed to go free. Their larvae, which live in the soil, are also useful.

Ladybirds are recognized by most people as friends of the gardener. They help by preying on aphids, and their larvae also consume countless numbers of these sap-sucking pests. Ladybird larvae are mainly bluish in colour, with black and yellow markings. They can be seen in summer on the foliage of plants and trees. The ladybird population is subject to considerable variation in numbers from year to year. Vast hordes were seen in the hot summer of 1976, and this was partly due to immigration, but they were far less common the following year.

Green lacewings or lacewing flies are equally useful, because of their destruction of aphids. There are several species of these very delicate-looking insects.

Ichneumons, or ichneumon wasps, are more subtle in behaviour. The females inject eggs into living caterpillars and other larvae. The larvae from these eggs feed inside their hosts and eventually kill them.

Among the more familiar of the ichneumons are the small Apanteles species that victimize the caterpillars of cabbage white butterflies. A single caterpillar can contain upwards of 20 of the larvae, which, on becoming fully grown, leave their victim and spin a small, oblong, silken cocoon in which to pupate. The cocoons are often seen on garden walls and fences, and they should be left to produce more of their useful kind.

 

Neutral Garden Insects

Apart from the various beneficial insects described, a number of species found in gardens might be termed ‘neutrals’, because although they do little good they are not harmful. Many of these are attractive in the garden, especially the brightly coloured butterflies such as the handsome red admiral, the peacock and the small tortoiseshell. The caterpillars of all three live on nettles and never harm garden plants. The only harmful butterflies found in Britain are the cabbage whites. Their caterpillars eat plants of the cabbage family and also nasturtiums. Many of the other caterpillars found eating plants belong to moths.

The true flies of the order Diptera include more than 5,000 species, and some are serious pests. A few are useful because their larvae are parasitic, like those of the ichneumons. Garden ponds are sometimes visited by dragonflies, and several of the larger ones have a wing span of 10cm (4in). They are certainly imposing, and some people call them ‘horse-stingers’, but they do not sting and are harmless. They prey on gnats and other small flies, and their larvae are aquatic.

 

Harmful Garden Insects

We are confronted with a depressingly large variety of harmful garden insect pests. Some of these are conspicuous, but with others only the symptoms of the attack are evident. The key to control is to act at the first signs.

Aphids are perhaps the best known pests. In addition to weakening plants by sucking their sap, they also secrete a sticky liquid known as honey-dew. This is unsightly on foliage, and it attracts ants.

Aphids increase in numbers at an alarming rate, for the females can give birth to living young without first associating with the males. They also pair in the conventional way and lay fertile eggs which overwinter.

There are many kinds of aphids (commonly known as greenfly and blackfly), some having a marked preference for a particular type of plant.

Cockchafers or May-bugs are very destructive to foliage, and their fleshy, pale larvae live underground and feed on roots.

Wireworms are tough-skinned, long, thin, yellow-brown larvae of the click beetles or skipjacks. They are often found in old grassland and in newly-cultivated ground. They can do much damage to the roots of cultivated plants.

Weevils are beetles of modest size, and are easily recognized by the beak-like projection from the head. Some eat holes in leaves, and these are not easily controlled. The notorious vine weevil often attacks rhododendron leaves, and its whitish larva feeds on the roots of some houseplants.

Cabbage root fly maggots are very destructive. The adult fly looks like a small housefly, and the maggot-like larvae feed on roots of young cabbage.

Narcissus flies and bulb flies also originate from pale-looking maggots, which tunnel into the bulbs of daffodils and related plants.

Leatherjackets are the larvae of crane-flies or daddy-long-legs. They feed on plant roots and can be especially troublesome on lawns.

Moths Numerous different moths are found in gardens, and although many are casual visitors, and do no harm, others are pests because their caterpillars are voracious feeders. The so-called cutworms, which can be so destructive to seedlings and other young plants in spring, are not really worms, but the caterpillars of various night-moths, including those of the large yellow underwing. They feed after dark, and can be hunted down with the help of a torch. The long-winged ghost swift moth is also a root-feeder in the caterpillar stage, and it can be a troublesome pest.

Several moths appear in late autumn and winter, and the wingless females lay their many eggs on the twigs of different trees. The caterpillars hatch out in early spring, and are of the ‘looper’ type. They arch the body into a loop when walking, and feed on most woodland and orchard trees. The species include the hated winter moth and the mottled umber. Because the females of these moths are wingless they have to climb the trees on foot, so they can be trapped by fixing sticky bands on the tree trunks. Another unpopular looping caterpillar is the offspring of the magpie moth, or currant moth. This has creamy-white wings liberally spotted with black and yellow. The caterpillars feed on the leaves of currants and gooseberries in early summer. These caterpillars should not be confused with the larvae of gooseberry saw-flies which also feed on gooseberries. If neglected the larvae may strip all the leaves from a bush.

Other garden insect pests such as caterpillars, often found in gardens, are those of the grey and black peppered moth. They are very twig-like. In appearance, and can sometimes be seen on roses in late summer, but they are not usually common enough to be very harmful. A smaller, but far more destructive, caterpillar belongs to the codling moth which lives inside apples and to a lesser extent in pears. It destroys much of the pulp, and on becoming fully grown tunnels through the skin of the fruit to find a place in which to spin its tough silken cocoon, and change into a pupa. The codling moth appears in spring, and can be controlled by spraying immediately after a blossom-fall. The cocoons are often attached to bark, and can be removed by scrubbing the trunks with a stiff brush.

 

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05. September 2010 by admin
Categories: Garden Management, Pest Control | Tags: , , , , , , , , | Comments Off on Garden Insect Pests – Gardener’s Friends or Foes?

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