54 



INSECTS AND HUMAN WELFARE 



occurring on, and probably original enemies of native Ameri- 

 can grasses, several of which are very closely related to the 

 cultivated wheat plant. A still more interesting case is that 

 of the Colorado potato beetle (Fig. 26), a well-known and 

 very destructive enemy of the potato (Solanum tubewsiim) . 

 We have good evidence that this is a neotropical insect of 

 Mexican or Central American origin which extended its 

 range within historic times into the middle west as far as 



FIG. 26. The Colorado potato beetle (Leptinotarsa decemlineata), originally 

 a wanderer from Mexico, now firmly established in practically every potato 

 patch. The larva is shown at the left. 



Colorado, following the probable northward migration of its 

 original food plant, Solanum rostratum, which is a common 

 roadside weed. In the late sixties of the past century it was 

 first noticed feeding on the foliage of cultivated potatoes and 

 within twenty-five years had encompassed the entire United 

 States east of the Rockies, breeding upon potato plants 

 everywhere, and even invading Europe where its incipient 

 colonies were wiped out. 



This same group of plants, the Solanums, also illustrates 

 nicely the vagaries of numerous insects in relation to food 

 plants. Thus we find that the Colorado potato beetle now 

 feeds upon Solanum rostratum far less commonly than upon 

 potatoes and that it does not occur at all on the tomato or 



