one time (Jones, 1945), with about 6 percent 
considered to be resistant. Over the years, 
two or three times that many have been studied. 
Six genes have been recorded for Hessian fly 
resistance in wheat and several more are known 
to exist. 
For therecently introduced cereal leaf beetle, 
Oulema melanopa (L.), a total of over 30,000 
wheat, oats, and barley varieties have been 
exposed to infestation the first year. About 12 
percent of the 16,095 wheat, 4 percent of the 
5,423 oats, and less than 1 percent of the 
8,634 barley varieties were considered re- 
sistant (unpublished), Since the basis of resis- 
tance is frequently complex, it is important to 
find as many different genetic factors as pos- 
sible for all components of resistance to the 
same insect. Often this can be done by em- 
ploying different measures of resistance: Egg 
counts, estimates of damage, effect of plant on 
the insect, and others. 
"Currently, a cooperative project, being 
sponsored jointly by the International Maize 
and Wheat Center of the Rockefeller Founda- 
tion in Mexico, the Instituto Nacional de In- 
vestigaciones Agricolas of the Government of 
Mexico, and Kansas State University, is con- 
cerned with a search for insect resistance in 
races of corn from the corn germ plasm banks 
of Latin America. These germ plasm banks 
consist of over 18,000 samples of corn still 
being grown near out-of-the-way villages 
throughout Latin America. This seed is kept 
viable by cold storage and periodic replanting. 
The visible range of characters of grain, cob, 
plant, and maturity is far greater than the range 
of such characters in available Corn Belt or 
other inbreds and hybrids. Some races are 
believed to carry various genes from 
Tripsacum,. Both situations suggest strongly 
that these corns should carry genes for insect 
resistance not now available in corn grown in 
the United States. 
In less than a year's study, evidence that this 
is true is beginning to be found. Replicated 
nurseries of 80 samples of these races were 
planted in several places in Kansas and once 
each month at Tepalcingo, Mor., Mexico. Clear 
differences were evident between races indam- 
age by the fall armyworm, Spodoptera frugi- 
perda (J. E. Smith); stalk borers, Zeadiatraea 
spp.; the cornleaf aphid, Rhopalosiphum maidis 
(Fitch); and especially thrips, Frankliniella oc- 
140 
cidentalis (Perg.). Additional research will be 
needed to confirm the results and much more 
to incorporate the genes in useful inbreds or 
new synthetic varieties. The study so far is 
only a small beginning, but it illustrates the 
importance of examining many and diverse 
plants in the search for resistance. 
The next step in a resistance study is to 
obtain evidence that the supposed resistance is 
an inherited character. This requires testing 
of progeny of the plants selected as being 
resistant, and also studying such plants and 
their progeny under various environmental 
conditions. 
If the resistant plants are selections from a 
satisfactory adapted cultivar, they may be com- 
bined or selected to produce an acceptable 
variety for immediate agronomic testing, in- 
crease, and distribution. If, on the other hand, 
the new resistant plants are not adapted in any 
important way, they must becrossed as needed 
with plants of satisfactory varieties and re- 
selected in succeeding generations. Both the 
original and later selection processes must be 
designed (1) to test the maximum number of 
plants with the minimum labor and time and 
(2) to test for as many genetic factors or com- 
ponents of resistance (Painter, 1951, p. 25) as 
possible with the same or succeeding proce- 
dures. 
Resistance as seen in the field or green- 
house is made up of one or any combination of 
these components--nonpreference, antibiosis, 
tolerance. In the field especially it is usually 
impossible to tell which components are in- 
volved in any example of resistance or the 
proportion of each present. It is most important 
to incorporate into any proposed resistant 
variety as many genes as possible for two or 
more components of resistance, since such 
provisions make much more difficult the 
natural selection of biotypes able to infest or 
injure or both the resistant variety. Identifying 
the component or components early in any case 
of resistance is important in designing testing 
procedures. Detailed analyses of the physical 
or biochemical bases of these components have 
been of practical value on only two occasions 
so far. It cannot be overemphasized that a 
detailed knowledge of cause is notneededinthe 
practical use of insect resistance. There have 
been occasions, however, when a futile search 
for cause has delayed such use, On the other 
