Insects and us 



Plants have the advantage over most animals that they can often 

 reproduce themselves vegetatively by splitting, or by sending out 

 runners or rhizomes. Nevertheless, to remain vigorous and healthy they 

 need the help of insects in cross-fertihsation. When an attempt is made 

 to grow crops that are not native to an area it may be necessary to pro- 

 vide the insects to fertihse them : examples are sunflowers in East Africa, 

 and alfalfa (lucerne) in North America. Even in settled countries like 

 England, where a balance has long been reached, there is a danger of 

 upsetting it by using general insecticides and weed-killers. 



Many insects are enemies of man, but even more are enemies of other 

 insects. We have mentioned how parasitic insects can be used in dehb- 

 erate biological control of pests, but these examples are infinitesimal 

 compared with the number that work away to our advantage all the 

 time, without our being aware of it. Not only do insects control each 

 other's numbers continually, but insects also play an essential part in 

 breaking up and disposing of dead animals, dung and refuse of all kinds. 

 It is easy to turn away in disgust from the unsavoury habits of many 

 insects, but they are doing an invaluable operation of clearing up, from 

 which we benefit. 



Fig. 78. Insect pests. Pouring insecticide into a stream in Kenya to kill 



larvae of the fly Simulium. The bites of the adult fly transmit "river 



blindness". Photo by courtesy of the Kenya Information Office. 



131 



