The Evolution of Communities 



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NO TREATMENT 



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I \ European red mite 



Two-spotted mite 



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EARLY EUROPEAN RED MITE CONTROL 



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Fig. 92. Seasonal population trends of the European red mite and the two- 

 spotted spider mite. Above, on untreated apple trees when both species were 

 present; below, on trees treated by pre-blossom sprays in which the European 

 red mite was virtually eliminated. (After Lienk and Chapman.) 



numerical density is related to the fact that the active forms (larva 

 and adult) eat the passive forms (egg and pupa) whenever they 

 encounter them. This goes on indiscriminately when two species of 

 Triboliiim coexist in the same sample, so that each species is partly 

 a predator on the other. With only a slight change in relationship, 

 one species could become a habitual predator, the other species 

 its habitual prey. 



The predatory habit has evolved between closely related species 

 many times. In certain bees (Apoidea) the adult of one species 

 lays its eggs on the food gathered by a provisioning species, a type 

 of predatism known as social parasitism. Social parasitism has 

 evolved independently in many different subfamilies and tribes of 

 bees. Frequently the parasite bee lives only at the expense of one 



