EXTOMOLOGY AND I'LANT QUAKANTIXE 
39 
planting is an important factor in deterniining amount of infestation and that 
degree of resistance varies with the age of the phmts. 
Infestation by the corn earworm was relatively low in the snmmor of 1936, 
owing both to the severe drought and to the very severe weather conditions that 
prevailed in the previous winter. The results of hibernation studies during the 
winter indicate that this insect hibernated successfully considerably farther 
north in the winter of 1936-37 than in the previous winter, and as a conse- 
quence it appeared much earlier and in greater abundance in 1937. In studies 
conducted cooperatively by the Bureau of Plant Industry and the Illinois Agri- 
cultural Experiment Station, a large series of double crosses, single crosses, 
inbred lines, open-pollinated varieties, and top crosses were studied to determine 
their relative susceptibility to corn earworm attack. Although the data secured 
are only preliminary, 3-1 double-cross hybrids, 11 single crosses, 8 inbred lines, 
and 6 varieties showed possibilities of having resistance and are being given 
additional more severe tests. A method of artificial infestation has been de- 
veloped and is being utilized to secure a uniform infestation irrespective of 
fluctuations in the normal field population. Investigations on insecticides for 
the control of the corn earworm. conducted at New Haven, Conn., in coopera- 
tion with the Connecticut Agricultural ILxperiment Station, and in Florida, have 
not revealed any outstanding iiisccriciue for field application. The nse of a 
fumigant (hexachloroethane) in paper caps for covering high-value sweet corn 
for protection against the carwoiin has been highly effective and may be feasible 
under special conditions. The method, however, requires further development. 
INSECTS ATTACKING SMALL GRAINS 
The hessian fly was at a low ebb as a result of unfavorable climatic con- 
ditions. It has been possible, however, to maintain fairly high infestations in 
nursery plots, and the general low populations have not seriously interfered 
with the work on hessian fly resistance in wheats that is being conducted in 
cooperation with the Bureau of Plant Industry and the agricultural experiment 
stations of California, Kansas, and Indiana. A number of additional varieties 
of wheat have been discovered to have some resistance to the hessian fly. 
Although these are confined i)rimarily to spring wheat varieties, evidence has 
been obtained that the resistance inherent in .spring varieties may be trans- 
ferred to fall-sown wheats by hybridization. In California a distinction has 
been recognized between those wheats having two factors for resistance and 
others having less stable or lowor degree resistance represented by derived 
one-factor lines. Previous results have indicated a marked difference in the 
reaction of given varieties of wheat to California flies as compared with the 
same varieties of wheat when exposed to Indiana flies under Indiana condi- 
tions. Investigations during the year have indicated that this difference in the 
reaction is in some degree due to biological strains of the flies rather than to 
differences in vegetational growth due to differences in environmental conditions. 
There is a marked difference between the ability of California and Indiana flies 
to infest the same varieties of wheat under similar climatic and soil condi- 
tions. Evidence has been accumulated, however, which indicates that resistance 
or susceptibility to fly attack may be materially influenced in some cases by 
modifications of plant structures as a result of environmental differences. 
Resistant types of wheat in the jointing stage have been shown under some 
conditions to be more susceptible to the hessian fly than when in the fall, or 
rosette, form. Infestation tests of wild grasses show that hessian fly larvae 
may develop on numerous species of wild grasses. Strains of these grasses 
vary greatly in resistance. Susceptibility to infestation appears to be correlated 
with the character of the tissues of the leaf sheaths. Usually, although not 
invariably, the grasses found to be most susceptible have stems that are soft 
and yielding. Preliminary results indicate a previously unrecognized direct 
correlation between the mortality in winter wheat and the extent of hessian 
fly infestation. Detailed studies of the relation of the developii^g larvae to 
the tissues of the fly-resistant and susceptible wheat plants show that the flrst- 
instar larvae on certain resistant plants are apparently generally prevented 
from molting by the pressure of the harsh tissue of these plants. The few 
larvae which molt and develop on these resistant plants are usually distorted 
by pressure of the leaf sheaths, while larvae on susceptible plants survive the 
first molt successfully and sliow no distortion at any stage of their development. 
Records at Manhattan, Kans., indicate that some of the Marquillo hybrids 
most resistant to the hes.sian fly are also resistant to the wlieat jointworm. A 
comparison of hessian fly and jointworm infestation records for 146 of these 
