Insects and their world 



plants, particularly seedlings, and so thinning the stand. The copious 

 honeydew secreted (see p. 107) complicates harvesting operations.' 



In Israel (Harpaz, 1955) this same pest has about forty generations a 

 year, and it is able to multiply very rapidly when it is presented with 

 plenty of food, under good weather conditions. It is attacked by several 

 insect enemies, lacewings, ladybirds and the larvae of hoverflies, but 

 both in California and in Israel it was found that the increase of parasites 

 lagged behind the increase of the aphid, and insecticides had to be used 

 to keep the pest under control. 



One of the most valuable field crops is cotton, which by its cultivation 

 in big areas is much exposed to attack by inseas. It was recently esti- 

 mated that the annual loss through insects in the United States was well 

 over 200 million dollars. 



In spring the young plant is attacked by thrips (Thysanoptera) and 

 aphids (Homoptera), which cause malformation and loss of foliage. The 

 cotton flea-hopper (Heteroptera) reduces the number of fruiting 

 branches. The boll-weevil (Coleoptera) is the most damaging pest of all, 

 and lays its eggs in the flowers and fruit, where the larvae feed. The boll- 

 worms are caterpillars of moths which destroy the boll (or fruit, with its 

 coating of cotton fibres), whereas the leaf-worms are caterpillars of a 

 tropical moth that flies in each year from the South. Other moths do 

 minor damage. 



A most important group of pests of cotton is the ' cotton-stainers ', 

 plant-feeding bugs (Heteroptera) which attack the bolls, causing the 

 young ones to fall off", and the cotton lint in the older bolls to become 

 discoloured. The leaves of the cotton-plant are damaged by a number of 

 other insects of different Orders, as well as by red spider-mites. 



To combat such a sequence of enemies, the cotton-planter must carry 

 out a systematic programme of spraying through the year, but even then 

 his losses may be substantial. 



Biological Control 

 A classical example of biological control is that of the Coconut Moth, 

 Levuana iridescens, in Fiji in the year 1925-6. The coconut is a long- 

 lived tree, and once it is mature it normally produces a crop of nuts 

 every month. The Levuana caterpillar eats the leaf, which gradually 

 changes colour from a healthy green to brown, then to grey or white, 

 and finally breaks away. After a tree has been completely stripped of 



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