Insects and their world 



that plant-feeding insects normally have a very high rate of mortality : 

 they lay a great many eggs, but few of the offspring survive, nearly all 

 of them being killed by unfavourable weather, or unsuitable food, or 

 eaten by predators. 



Just now and then, however, the insect has a run of luck, and all the 

 factors happen for a while to go in its favour. The balance between a 

 herbivorous insect and its enemies is a complex one, and Thompson 

 (1956) maintains that there is no simple sequence of cause and effect. 

 On the contrary, with so many different factors involved the final result 

 follows the laws of pure chance. 'The usual variation in the numerical 

 value of field populations from year to year corresponds to curves based 

 on numbers selected at random.' 



Be that as it may, the population of any insect varies greatly from 

 one year to another. Collectors of insects can quote examples of insects 

 that are thought of as rare for many years, and which suddenly re- 

 appear. For several years they flourish, then quietly fade away, and again 

 become a rare curiosity. An interesting case is that of the seaweed flies, 

 which may normally be found in small numbers on the shores of Great 

 Britain, but which suddenly came into the headlines of the newspapers 

 in the autumn of 1953 because of their immense numbers, and their 

 appearance inland. Perhaps the weather of that year had something 

 to do with it, but quite possibly the species was already due for a rise 

 in numbers as part of its long-term fluctuation, and the warm autumn 

 merely exaggerated this. 



One cause of sudden increases of numbers of insects is our habit of 

 planting big areas covered with one crop, because this provides the 

 insects that feed on that plant with an unlimited supply of food, and a 

 chance to increase more rapidly than their enemies. In time the enemies 

 also increase, and restore a balance, but in the meantime the planter is 

 likely to suffer heavy losses, and for financial reasons might be forced 

 to give up trying to grow the crop. The planter's remedy is either to 

 attack the insect with chemical insecticides, which gives quick results, 

 but is expensive and will have to be repeated; or to try to introduce 

 other insects that will attack the pest, and perhaps control it perman- 

 ently, by what is called biological control. 



An illustration of this is the aphid Therioaphis maculata, which attacks 

 clovers, alfalfa and related leguminous plants in the United States, 

 Middle East and India. According to Dickson, et al. (1955), when these 

 are planted as a fodder crop for feeding animals, 'The aphid reduces 

 the hay-crop by feeding on the leaves, causing them to dry up and drop, 

 by retarding the growth of the . . . new shoots after the field is cut, 

 and by killing some of the plants, particularly seedlings, and so thinning 

 the stand. The copious honeydew secreted (see p. 107) compHcates 

 harvesting operations.' 



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