CODLING MOTH—PORTER $50 
codling moth populations to get out of hand in certain localities. 
Closely related is the effect of other present-day orchard practices on 
the parasite population. It may well be that the intensive clean-cul- 
ture or cover-crop systems followed in many modern orchards may 
have eliminated many of the other hosts of the common parasites of the 
codling moth, thus causing the balance to swing in favor of the codling 
moth, 
ADAPTATION ON THE PART OF THE CODLING MOTH 
The factors just outlined, however, are not sufficient to explain the 
marked increases that have developed in the ability of the codling 
moth to thrive in the presence of heavy deposits of lead arsenate. 
Some change seems to be taking place in the insect itself that is modi- 
fying its ability to enter fruit in spite of the presence of a poison. 
The most extensive study that has thrown light on this problem has 
been carried on by W. S. Hough, of the Winchester field laboratory 
of the Virginia Agricultural Experiment Station. His earliest work 
(Hough, 1929 and 1934) included a comparison of codling moth stocks 
from near Grand Junction, Colo., where the insect had become notori- 
ously difficult to control, with stocks from Virginia, where control was 
much easier. Dr. Hough showed that newly hatched codling moth 
larvae from Colorado stock were able successfully to enter fruit heavily 
sprayed with lead arsenate to the extent of 15 to 40 percent or more, 
whereas the proportion of native Virginia larvae entering similarly 
sprayed apples was usually less than 5 percent. Further, this differ- 
ence persisted through 14 or more generations reared in the insectary 
under Virginia conditions. Crosses gave intermediate results. Hough 
later (1943) found that Virginia larvae from stocks from orchards 
having a history of intensive spray programs were able to enter sprayed 
fruit in much greater proportion than those from unsprayed or poorly 
sprayed orchards. Strains from various Virginia orchards fed through 
successive generations in the insectary on sprayed fruit became differ- 
entiated from the parent strains, and showed increased ability to enter 
sprayed fruit. Steiner and associates (1944) have shown similar 
wide differences in codling moth stocks from different orchards in 
the Ohio Valley with respect to their ability to enter sprayed fruit. 
Both of these investigators have found that this condition is not re- 
stricted to lead arsenate, but that differences, although not always so 
wide, exist with respect to other insecticides, including nicotine ben- 
tonite and cryolite. Hough has reached the conclusion that these 
differences are due to differences in general vigor, but Steiner’s obser- 
vations have suggested that they may result from differences in habits. 
Both of these workers believe that the different strains have been 
segregated by the elimination of those larvae that have the least 
ability to enter sprayed fruit, rather than that individuals have be- 
