Plants are attacked by various diseases, and various small creatures, including a number of insects, feed on them. It is scarcely possible. And certainly not necessary, to have a garden from which pests and diseases are completely excluded, but it is essential to keep them within reasonable bounds. To do this successfully it is not necessary to have a detailed knowledge of pests and diseases but it is necessary to have a general idea of the main groups into which they fall, especially from the standpoint of treatment.
Consider the insect pests first. The two most familiar are the greenfly and the caterpillar. There are. In fact, a great many different kinds of greenflies and of caterpillars, which will be found described in detail in books devoted to garden pests, but since the methods of control are very similar for most kinds, the gardener can usually get along with no more knowledge than that which enables him to distinguish a greenfly from a caterpillar.
Greenflies are often referred to as plant lice and it is a good description. They are small and soft skinned and they breed rapidly so that, given favourable conditions, in a few days the young shoots and leaves of plants may become covered with them. They suck the sap, greatly weakening the plant in so doing, and often causing leaves to curl and become sticky. Moreover they act as carriers of disease, principally of virus diseases, and so it is very necessary to keep greenflies under control. They are most likely to be troublesome in spring and early summer, for it is the young shoots and leaves of plants that they like.
Greenflies are generally controlled by spraying and there are a number of good insecticides available for this purpose. Some of them, such as menazon, formothion and dimethoate. Are absorbed by the plant and are carried about inside it in the sap. This type of insecticide is known as systemic, and it is very useful because it cannot be washed off by rain nor is it likely to do a great deal of harm to those insects that do not damage plants.
The drawback to the systemic type of fungicide is that, if it is used on a food plant, a lettuce for example, or an apple, it cannot be removed by wiping or washing. Most systemic insecticides sold for garden use break down fairly quickly in the plant and food crops can be used safely a few weeks after application. However, some gardeners may prefer to reserve this kind of insecticide for the ornamental garden and use non-systemic insecticides for vegetables and fruit.
There are plenty of these capable of killing greenflies, including lindane, mala-thion, derris and pyrethrum. The last two are relatively harmless to all warm-blooded creatures which, of course, include human beings.
All greenflies are species of aphis and there are other aphis species which are not green but are similar in other respects and can be killed in the same way. Some are grey or blue-grey, some black, and these last are usually called blackflies.
Caterpillars are the grubs or larvae of moths and butterflies. Most people have a general idea of what a caterpillar looks like but they sometimes confuse them with large fly larvae, such as leatherjackets. The larvae of the daddy-long-legs. Obvious points of difference are that every caterpillar has short legs and a distinguishable head. Whereas the fly larva has not. Derris is a good caterpillar killer and various other chemicals are available including mala-thion. Lindane, carbaryl and trichlorphon.
Caterpillars do not suck sap but bite holes in leaves, fruits and even in stems and branches. So do the many weevils that attack plants and the damage they do may often be mistaken for the work of caterpillars. Weevils look like little beetles and they are so active that it is usually difficult to find them, particularly as most of them feed at night. However, the chemicals recommended for use against caterpillars are also poisonous to most weevils, so if you are doubtful whether it is caterpillars or weevils that are eating holes in the leaves of plants use a spray containing one or other of these chemicals.
As a class these are friends rather than foes but two exceptions are the flea beetle and the raspberry beetle. The former attacks the first young leaves of turnips. Cabbages, brussels sprouts and other allied crops, filling them with tiny circular holes. It is very small, lively and black and it is easily killed by dusting or spraying with lindane or carbaryl. The raspberry beetle feeds on the flowers of the raspberry, but it is its little creamy-white grubs that are most troublesome for they eat right into the raspberry fruits. Derris is the best chemical to kill them with and it must be applied at just the right time – as the first fruits turn pink.
Cabbage Root, Onion and Other Flies
Fly larvae often attack at or just below soil level. If young cabbage or brussels sprout plants start to flag and. When pulled up. It can be seen that the roots have been gnawed, it is very likely .that the cabbage root fly is responsible. Another fly attacks young onions and a third attacks the young roots of carrots. Lindane or tri-chlorphon can be used to kill them with no trouble.
The grubs of some flies tunnel into leaves and are called leaf-miners. Most insecticides do not gel at them because they are protected within the leaf, but again, tri-chlorphon and lindane will kill them.
Sawllies are not true flies and they produce larvae which look very much like the caterpillars of moths, having legs and a distinct head. Some eat into fruits, for example the apple sawfly, whose white grubs cause the maggoty apples one finds in the early summer. Some eat the surface of leaves, leaving a transparent skeleton, and some cause leaves to roll up. All can be destroyed by spraying with lindane, tri-chlorphon or derris.
Capsid bugs bring us back to the suckers as distinct from biters. They feed like the greenfly but they are larger and usually far less numerous. They do a quite disproportionate amount of damage, causing leaves to pucker and become stunted and flowers to be deformed. Since they are active and hide quickly when disturbed, it is often difficult to find them but the damage they do is very distinctive. Lindane and malathion are effective in killing them.
There are also leaf hoppers, little bug-like creatures which live on the undersides of leaves, sucking the sap and making the leaves become mottled and weak. They change their skins from time to time and the empty white skin-cases remain attached to the lower side of the leaf, providing a ready means of identification. Once again, lindane and malathion are effective.
Red Spider Mites
A rather similar grey or yellowish mottling is caused by red spider mites. These are not spiders and they are rust coloured rather than red. They are so tiny that a hand lens is required to see them clearly and they live on the underside of leaves, sucking the sap from them. They thrive in hot, dry weather and hate damp and cold, so that one way to keep them down is to spray frequently with water. Even more effective is derris.
Thrips also cause mottling but of a more streaky kind, usually with a good deal of brown marking. They are very small. Long in proportion to their breadth, and they run fast when disturbed. They often attack rose flowers, causing them to become deformed and develop dark markings. If such a bloom is tapped smartly over a piece of white paper the thrips will fall on to it and will be seen running away. They attack a great many other plants and the best way to deal with them is to spray with lindane or malathion.
Slugs and Snails
Slugs and snails can be very troublesome, particularly in wet mild weather. They feed largely at night, eating leaves and young stems, and there are few plants they will not attack. Since they hide by day their presence is not always suspected.
They cannot be killed with any of the insecticides mentioned, since they are not insects. They are very fond of bran and so one way to kill them is to mix a slug poison. Such as metaldehyde, with bran and place it near plants that are being attacked. Ready prepared bait is sold by all dealers in horticultural sundries. Another method is to use a liquid slug killer containing metaldehyde, distributing this from a watering-can fitted with a rose and applying it freely to creeping plants under which slugs and snails may be hiding, or to dead leaves. Rubbish and stones. Metaldehyde is most effective in fairly dry conditions. If it is wet, methiocarb is better. It is prepared ready for use to be sprinkled around plants likely to be attacked. But whatever is done, slugs and snails are likely to return and so treatment must be repeated fairly frequently.