hand, resistant versus susceptible plants have 
not had sufficient use on the study of insect 
feeding behavior and physiology. Resistance 
must be found in each case before it can be 
studied, 
Much has been learned in recent years of 
the food relations of insects to plants and 
especially of differences between plant species 
in the relationships (Beck, 1965). But there re- 
mains only one example, that of resistance 
factor A (6-methoxybenzooxazolinone) and its 
biochemical precursor (RFC) in corn, in which 
a biochemical difference between varieties of a 
crop plant has been experimentally and satis- 
factorily associated with resistance to an 
insect, 
nubilalis (Hubner), (Beck, 1960). 
A knowledge of the several bases of resist- 
ance will eventually be of value in other ways 
besides contributing to the understanding of 
resistance, Identification of chemical or 
physical factors concerned in preference, non- 
preference, or repellence may lead to other 
uses in control of insects through the identified 
chemicals or related synthesized ones that im- 
prove on nature, Likewise the identification of 
bases of antibiosis could lead to new and 
presently unknown kinds of insecticides, less 
toxic to mammals and perhaps highly specific 
to certain insects. The search for causes of 
tolerance may lead to/ ways of improving the 
ability of plants to withstand related insects, 
perhaps through manipulation of plant 
hormones, enzymes, or similar substances, 
The use of a knowledge of bases of resistance 
in the search for resistance through chemical 
tests of plants instead of using insects is un- 
likely to be practical for several reasons, An 
exception may be with such plants as trees. 
With insects such as the spotted alfalfa aphid, 
for example, thousands of plants can be 
screened in a few weeks, the susceptible 
plants killed by the insect, and only presumed 
resistant ones left. Few chemical tests would 
permit such "'screening'' so quickly or so 
cheaply and still leave only the few resistant 
plants. Furthermore, using insects permits one 
to find other resistance factors with other 
bases that would be missed by chemical tests. 
Such additional findings, as indicated above, 
would be of major importance, 
In contrast to lack of value in a search for 
resistance, the availability of chemical or 

the European corn borer, Ostrinia 
141 
physical tests associated with specific genes 
for resistance could speed up the combining 
of two genes for resistance in a single variety 
when the two can otherwise be identified only 
genetically. 
The search for biochemical substances as- 
sociated with the finding of host plants by 
insects is highly important in insect biology, 
and information obtained should find increasing 
value in insect control and survey. But such a 
study must not be confused with a search for 
the bases of resistance, 
The value of a few examples of resistance 
emphasizes some principles concerned with 
using resistant varieties for insect control. 
The spotted alfalfa aphid, Therioaphis 
maculata (Buckton), was first observed in the 
United States early in 1954 damaging alfalfa 
in the Southwestern States, reaching Kansas 
late the same year, By 1956, ithad spread into 
30 States. Susceptible alfalfa plants of any age . 
or size were rapidly killed. Cooperative studies 
between agronomists and entomologists of the 
Kansas Agricultural Experiment Station, the 
Fort Hays Branch Experiment Station, and the 
U.S. Department of Agriculture were begun in 
1955 in a "'crash'' program to supply farmers 
with an adapted, spotted aphid-resistant alfalfa 
variety (Harvey et al., 1960). 
It was quickly evident that a few individual 
plants resistant to this insect occurred in 
alfalfa varieties. Using a great number of 
plants, the resistant ones were selected from 
the adapted variety Buffalo in a series of 
entomological tests, and the 22 best plants 
were selected on the basis also of agronomic 
tests. The plant progeny from those 22 clones 
constituted the variety Cody. In the green- 
houses (fig. 1) under aphid attack, the dif- 
ference between Cody and Buffalo, from which it 
was selected, is clearly evident (fig. 1). The 
same has been true in the field, where Cody 
was compared with other varieties. The young 
alfalfa plants in August and September were 
especially susceptible to aphid attack, yet in 
fields where plants of susceptible varieties 
were severely injured or destroyed, Cody 
survived with little loss of stand. The original 
records, on which the decision to release Cody 
was made in 1959 (table 1), show thatits resist- 
ance is due to the two components, tolerance and 
antibiosis. Nonpreference may also be involved, 
but information on that point is not yet clear. 
