19161 ECONOMIC ZOOLOGY — ENTOMOLOGY. 159 



bugs, belonging to 34 species of the genus Phyllophaga aad collected in 42 

 counties from 1905 to 1911, inclusive, and in 1913. 



" Thirty-four species of May beetles are recognized in Illinois. They vary 

 greatly in abuudance, the above collection containing but two specimens of the 

 rarest species and 43.349 of the commonest. Ninety-one per cent of the speci- 

 mens collected belonged to 10 of the species, the other 9 per cent being dis- 

 tributed among the 24 species remaining. 



"A detailed discussion of the species, taken separately, shows for each its 

 numbers in each year and in each of the three sections of the State, the dates 

 in each year of its first appearance and its greatest abundance, and its com- 

 parative numbers on each of its food plants. By means of the data of num- 

 bers and distribution, the dominant and subdorainant species are distinguished 

 for each year and district, and the intervals between their periods of greatest 

 abundance are considered with reference to the length of the life cycle of the 

 species concerned. 



" From a comparison of the May beetles derived from northern, central, 

 and southern Illinois, respectively, it appears that three species are practically 

 limited to northern Illinois, three to the northern and central parts of the 

 State, two to the central and southern, and 11 to southern Illinois. The actual 

 boundary lines between these areas of distribution are. however, irregular and 

 meandering, especially that between southern and central Illinois, which is in- 

 fluenced by the coui'se of the streams, the southern species following them 

 northward toward their headwaters in a way to bring several such species 

 far into the central division of the State. 



" The seasonal succession of the species — that is, the order in wliich they 

 make their first appearance in spring— is worked out for each section of the 

 State as carefully as the wide distribution and irregular time limits of the 

 collections will permit. 



" Generally speaking, successive periods of extraordinary abundance of a 

 species in any locality or district show little correspondence to any possible 

 life cycle, being too various and irregular for that interpretation. Extensive 

 parasitism of imagos and larvfe by insects, annelids. Protozoa, and fungi pro- 

 duces widespread and destructive epidemic diseases, a knowledge of whose 

 prevalence and status is essential to any safe prediction of periods of de- 

 structive abundance of the white grubs. 



" The May beetle species known as PliyUophaga fnsca and P. futilis were 

 evidently those which produced most of the white grubs which were so abundant 

 in northern Illinois in 1912 as to do heavy damage to farm crops in several 

 counties. Two-thirds of the collections made in that section in 1914 were of 

 these species, the first of the two mentioned being, however, nearly four times 

 as abundant as the second. 



"The facts concerning the food plants of the more abundant species are 

 grouped and classified in a way to distinguish trees and shrubs especially at- 

 tractive to them, and consequently dangerous to adjacent crops by reason of 

 the abundance of white grubs to descend from them." 



The influence of trees and crops on injury by white g-rubs, S. A. Foebes 

 (Illinois Sta. Bui. 187 {1916), pp. 261-265).— The natural supposition that fields 

 nearest to the food plants of May beetles, that is trees, must become most 

 heavily stocked with eggs and consequently worst injured by grubs when these 

 eggs are hatched led the author in 1904 to commence the collection of infor- 

 mation bearing on the subject. 



Collections were made of white grubs in fields that were being plowed in 

 the full or spring by walking behind the plowman and making note of the 



