Kansas, many acres were planted with Hessian 
fly-resistant varieties in the eastern two- 
thirds of the State, where the insect pre- 
viously had been abundant. Surveys each year 
showed only an occasional wheatfield with a 
low Hessian fly infestation. Over much of the 
State the insect was absent or nearly so (fig. 
3). Hence a conservative estimate of saving 
from planting Hessian fly-resistant wheats in 
Kansas would be 5 million bushels per year. 
During the earlier part of the period the wheat 
was needed to help feed war-torn Europe. 
SUMMARY 
A consideration of plant resistance as a 
means of controlling insects and reducing 
their damage suggests several characteristics 
and principles concerned with such practical 
use of resistant varieties. 
(1) Successful use of host plant resistance 
requires team research by at least an en- 
tomologist and a plant breeder. 
(2) The effect of a resistant variety is 
ordinarily cumulative and persistent in its ef- 
fect on the insect population in contrast to 
insecticides, which are sudden and decreasing 
in effectiveness unless reapplied. Where re- 
sistant varieties are effective, farmers and 
others tend to forget about the insect con- 
trolled and why, until they change to asuscep- 
tible variety and learn again, 
(3) Most young entomologists have little or 
no knowledge of the use of resistant varieties, 
primarily because of misinformation or lack of 
information in many currently used applied 
entomology textbooks--a situation that should 
be corrected, 
(4) The first step in any resistance program 
is to find inherited factors for resistance. The 
chance of finding such resistance is largely 
proportional to the number and diversity of 
plants and varieties studied. Thus far, genetic 
resistance has been found wherever adequate 
search has been made. Some examples include 
resistance to the Hessian fly, the cereal leaf 
beetle, and resistance of corn to insects. 
(5) Evidence of the inheritance of resistance 
factors and of the components of resistance 
involved are necessary early in the studies 
of resistance. It is best to make full biological 
analyses before proceeding with chemical or 
147 
physical analyses. Detailed research on bases 
of resistance have so far been of little value 
in practical use of insect resistance. Since 
such information is of high theoretical interest, 
eventually patterns of importance in practical 
ways of insect control should emerge. 
(6) The same high levels of resistance to 
insects in two plant varieties may be reached 
by different components of resistance or 
different levels of effects of the same com- 
ponents. For example, Cody alfalfa carries a 
high level of tolerance to the spotted alfalfa 
aphid and also an ability to suppress aphid 
reproduction almost completely incomparison 
with susceptible varieties, 
Will winter barley suppresses greenbug 
reproduction by only about 50 percent, but has 
a very high level of tolerance tothe insect. Yet 
field or greenhouse experiments show about the 
same levels of resistance in Cody alfalfa to the 
spotted alfalfa aphid and Will barley to green- 
bugs in comparisons with susceptible varieties 
of the same crops under heavy attack by the 
two aphid species. 
(7) During 32 years, the Kansas Agricultural 
Experiment Station has produced and recom- 
mended four Hessian fly-resistant wheat 
varieties for farmers. They have been grown 
where adapted in Kansas in "Large wheat 
growing areas'' from 1/2 million to 6 2/3 
million acres at time of maximum planting of 
each, 
A continuing change in plant disease condi- 
tions, milling and baking requirements, and 
availability of varieties improved in other 
respects has resultedin replacement of earlier 
fly-resistant varieties by others superior in 
such resistance or in other characteristics and 
appears to be the likely pattern in many cases 
of insect resistance. 
(8) The Hessian fly-resistant wheat varieties 
available in the United States and adapted to 
and recommended in a variety of areas should 
afford low-cost control of the Hessian fly to 
individual farmers in any important wheat- 
growing area where fly infestation normally 
has occurred, 
(9) Research on breeding crop varieties 
resistant to the more important insect pests and 
some minor ones should always be acontinuing 
part of any plant breeding-insect control pro- 
gram, 
