209 



It was impossible to account for the outbreaks of certain insects or to 

 discover effective means of control until a study was made of the 

 reactions of insects to their environment, to each other and to the 

 different biological constituents of their environment. For example. 

 it was not until Forbes worked out the relation of Aphis maidiradicis 

 (the corn root-aphis) to the ant Lasius niger americanus, on which it 

 depends for its well-being, that any success in controUing this serious 

 maize pest could be attained. Such control measures as the breaking 

 up of the ant colonies in the spring, and the destruction of weeds on 

 which the ants establish the wingless Aphids before the growth of the 

 maize, are based solely on a knowledge of behaviour. 



The chief tropisms or reactions of insects to environmental influences 

 are considered in detail. The importance of chemotropism in feeding 

 and reproduction is evinced by the females of Pier is rapae and 

 P. brassicae, which select the leaves of cruciferous plants on which to 

 oviposit, being attracted by the mustard oils present in them. The 

 larvae of the sawfly, Priophoncs padi, which feed on the foliage of 

 rosaceous plants, are probably attracted by the glucoside, amygdaline. 

 The chemotropic reactions on the part of carrion beetles, and to 

 excrement on the part of coprophagous Coleoptera and Diptera are well 

 known. Flies of the genus Sarcoyhaga have been induced to oviposit 

 in a bottle containing scatol, a decomposition product of albuminous 

 substances, and the oviposition response in Stomoxys calcitrans has 

 been stimulated by means of valerianic acid. House-flies have been 

 induced to oviposit apparently in response to an attraction of ammonia 

 in conjunction vnih butyric and valerianic acid. The positive reaction 

 of Drosophiln to fermenting fruit is largely due to amyl, especially 

 ethyl alcohol, acetic and lactic acid and acetic ether. 



In studying the different behaviour of the same species of insect to 

 different plants, it is foimd that a chemotropic reaction is sometimes 

 responsible for the creation of a biologically distinct race of the same 

 species. A form of Rhagoletis pomoneUa infesting Vaccinium (blue- 

 berries) and Gaylussacia haccata (huckleberries) is below the normal 

 size, and appears to be long-established, while the larger apple-bred 

 race refuses to oviposit on blueberry and vice versa. 



A further example of chemotropic reaction, which promises results 

 of great practical value, is the study of the resistance of plants to 

 insect attack, with a view to the production of insect-resisting varieties 

 in crops subject to injury, as a preventive measure against insect pests. 



Other tropisms dealt with are thermotropism, temperature being a 

 far-reaching and universal influence on insect behaviour and a potent 

 factor in determining the range of insect activity. It is inseparably 

 allied with hydrotropism, especially in the effects of climate on insect 

 distribution and migration. Phototropism will undoubtedly place a 

 valuable weapon in the hand of the applied entomologist, and has 

 already been used to advantage, as in the case of the larvae of Mono- 

 chamus {Monohammus), which can be prevented from destroying 

 piled logs when these are thickly shaded. Anemotropism has a large 

 influence in the distribution of insects. The Rocky Mountain locust 

 [Melanoplus spretus] moves with the wind and when the air current is 

 feeble is headed away from the source. The brown-tail moth [Euproctis 

 chrysorrhoea] in New England and Eastern Canada is largely distributed 

 by wind, while the first stage larvae of the gipsy moth [I/yniantria dispar] 



