Modern insect control may be approached from four main standpoints and these will be briefly reviewed in their application to the insect enemies of agricultural crops. There is, firstly, the direct application of chemicals, otherwise insecticides, to kill or deter the insect; secondly, there are what may be termed ‘cultural methods’, where the agriculturist by variation in his farm practice endeavours to put the insect continually at a disadvantage. The third method, known as ‘biological’ or ‘natural’ control, invokes the aid of parasites and predators which are artificially introduced into the neighbourhood of the pest in question. Lastly, the importation of foreign insects and the movement of resident pests within a country are restricted by means of legislation.

CHEMICALS. The use of insecticides in the agricultural practice of the British Isles is very limited and is governed largely by the question of price, the profit on the average farm crop, especially at the present time, allowing small margin for expenditure on insecticides. The use of chemicals and sprays, however, has been found practical and efficacious in the case of certain insects and some of these instances are briefly indicated. The most recent development is the control of leatherjackets (Tipula spp.) and cutworms (Euxoa, Feltia, etc.), by the use of Paris green (acetoarsenite of copper) as a poison bait. (See pp. 74, 161.) This has proved a cheap and practical method of control for these two serious agricultural pests. The use of calcium cyanide as a soil insecticide is practicable under certain conditions and though too expensive for application on a large scale, it may be used for poisoning wireworms which have previously been assembled in a small area by the use of bait crops. (See p. 108.) Crude naphthalene has also proved its worth as a cheap soil insecticide in certain cases. As regards vegetables some success has been achieved with the use of tar oils as deterrents and of mercury compounds as poisons against Dipterous pests. (See p. 221.) Certain chemicals have been proved efficacious for killing the larvae of the warble fly in the backs of cattle; among these may be mentioned derris-iodoform and vaseline and a mixture of nicotine sulphate, calcium hydrate and water.