186 PROTECTION AGAINST INSECTS. 



Main (1888-89) in the great plague of pine-moths, in the first 

 year, only 8 per cent, of larvae were attacked by ichneumons, 

 in the second year, 30 per cent. 



It was formerly believed that most insects which did not 

 attain full development were killed by insect- parasites, and 

 breeding cages covered with coarse network were maintained 

 in which all larvae infected by ichneumon-wasps or flies were 

 placed and fed. The network allowed the latter when fully 

 developed to escape. 



These cages,* however, have proved useful only in allowing 

 the life-history of the parasites to be studied. 



It is now well known that insects are destroyed in large 

 numbers by bacteria and by fungi, the spores of which find 

 entrance into their bodies either through their skin, or amongst 

 their food. De Bary says : " If one carefully examines the 

 dead leaves and moss of the forest soil in wet seasons, it is 

 astonishing how many fungus-infected insects he will find." 

 The infected caterpillars may be easily recognised by dis- 

 coloured spots on their bodies, and by their reduced activity, 

 and they die when the mycelium of the fungus has spread 

 inside them. Thus, mmcardine is a well-known disease of the 

 silkworm, due to Botrytis bassiana, Bals., and this fungus 

 attacks Noctua piniperda, Panz., Gastropacha pini, L., and other 

 caterpillars. Wet years, being favourable to the fructifica- 

 tion of the fungi, cause these diseases to spread amongst 

 caterpillars.! 



The question whether ichneumons or parasitic plants are of 

 more importance from a forest point of view is still open. It 

 was believed by Eatzeburg that ichneumons attack only 



* In Bengal, where a Tachinid fly attacks silkworms, these are sometimes fed 

 inside a framework covered with gauze to exclude the flies. The most usual plan, 

 however, is to rear the Bengal multivoltine silkworm for silk in alternate months 

 only (it has seven or eight generations in the year) ; during the other months a 

 limited number of worms are carefully kept under gauze to produce eggs for the next 

 brood. The flies, which have also a generation every month, not finding sufficient silk- 

 worms to lay their eggs in, are thus greatly reduced in numbers, whereas, if the silkworms 

 are bred in the open every month, whole broods would be destroyed by the parasites. 



t For an account of fungi attacking insects, ride Cooke's Vegetable Wasps and 

 Plant Worms, London, 1893, also Judeich and Nitche, Mitteleuropaischen Insecten- 

 kunde, Vol. I., pp. 164182 ; De Bary, A., Vergleichende Morphologic und Biologie 

 der Pilze, Mycetozoen, und Bacterien, Leipzig: W. Engelman, 1884. 



