Garden Pests – Understanding the Problem
Understanding the problem
Despite our best endeavours, plants may suffer from a pest, disease or virus. Adverse environmental conditions and shortages of plant foods, known as physiological disorders and mineral deficiencies respectively, can also cause unwanted symptoms.
“Pest” is the term given to any creature that affects a plant in a way we do not approve of. But a creature that is a pest in one situation may not be seen as such in another, for the aim in organic gardening is to keep pests at a manageable level (that is to say, at a level where they are no longer regarded as a pest) rather than to eradicate them completely.
Whatever the problem, it is important to identify the cause correctly. Similar symptoms can have very different causes. For example, the red blisters that appear on the leaves of red and white currants in early summer are caused by a pest, known as the redcurrant blister aphid. These pale yellow aphids can be seen initially on the underside of affected leaves, but symptoms will remain after they have moved on. Similar red blisters on peach leaves in the spring, on the other hand, are caused by a disease — peach leaf curl.
Having identified the cause of a problem, now find out more about it: whether the problem is a pest or disease, it is useful to know when it first appears, when it leaves, how it spreads, the range of plants it will attack, how and where it survives the winter and what level of infestation or infection can be tolerated.
This information will help in planning a control strategy. Some pests and diseases, for example, are devastating and may spread to many plants in the garden; others may not be nearly as bad as they look and may restrict their activities to a few specific plants. In the case of mineral deficiency, knowing the underlying cause can help correct the problem in the long term.
Creatures that can be pests
Many different creatures such as birds, mammals, insects, molluscs, mites and eelworms can act as pests. They vary considerably in habits, habitats, lifecycles and appearance.
Some pests (slugs, for example) look more or less the same from birth to death — they just get larger. Others, insects in particular, go through several very different stages of growth, so it is not always obvious that the different stages are related. They may have different common names and the different stages are not all necessarily capable of causing damage. For example, the leatherjacket is a garden pest, but its adult form, the crane fly (daddy-longlegs) does not harm plants. On the other hand, both vine weevil larvae and adults are pests.
Some pests, such as slugs and certain aphids, attack many plants. Others, such as the lily beetle and potato eelworm, restrict their activities to one or a few plants. Successive generations may stay on the same plant or move to more of the same species or to different plants. The move may be governed by the growth stage of the pest or the time of year.
Although we tend only to notice pests when they are damaging our garden plants, they may of course also be living on other garden plants, weeds and wild plants. These colonies can act as a useful food reserve for beneficial creatures, keeping their numbers up when other food supplies have been removed.
Spread and survival
Pests may walk, crawl or fly to find another host plant; others are transported in infectedand in or on plants. Some come from a great distance, blown on wind, transported in soil on our footwear or on purchased plants.
In the absence of a suitable host, some pests will leave the garden; others, such as the potato cyst eelworm, can survive in the form of tough “resting bodies” which can live for 20 years in the soil waiting for a suitable host plant.
Pests may survive the winter on plants, in the soil, in greenhouse staging and many other nooks and crannies, indoors and out.
Periods of activity
Some pests are active all year, except in cold weather. Others are active in certain seasons or during the “pest” stage of their life cycle.
Symptoms and identification
If the pest is visible, identification is relatively easy — but remember that the presence of a creature does not mean that it is necessarily the guilty party. Often, symptoms are all we have to go on because the pest is too small to see or it has already moved on.
Holes in foliage, stems or roots, or plants disappearing completely, are caused by pests with biting or rasping mouth parts: caterpillars, beetles, earwigs, fly maggots, sawfly larvae, midge larvae, woodlice, slugs and snails, rabbits and other mammals. Curled leaves and distorted growth are caused by creatures that feed on plant sap, either by piercing the plant tissue (for example aphids, capsid bugs, leaf hoppers, mites, whitefly, scale insects) or by living within the plant (eelworms), for instance.
THE BLACK BEAN APHID (BLACKFLY)
In late spring, female blackfly appear on the shoots of young broad beans. They feed and produce wingless females. If left unchecked, they spread down the stem and on to pods. The plant matures, producing less nutritious sap. This triggers the production of young aphids with wings, which move on to other beans or to their summer hosts, French and runner beans, dahlias, poppies and nasturtiums. The process is repeated.
Blackfly will continue to colonize these plants throughout the summer. In early autumn, when the food supply is finished, young aphids, known as “autumn migrants”, are born. These fly off to over-wintering sites such as, and Viburnum opulus. At the same time, winged males are born. These fly to the overwintering sites to mate with the females, who then lay eggs on stems and winter buds. The shiny black eggs will survive the winter.
In the spring, the eggs hatch. The young, wingless aphids feed and multiply until late spring, when winged offspring are produced. These fly to broad bean plants to repeat the cycle.