48 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 16 
call attention to some facts relating to the numerical abundance of some 
other insects of the several types just mentioned. 
It is evident after a moment’s consideration, that destructive agri¬ 
cultural pests, or in other words, numerically abundant species that 
feed upon useful plants, may be either polyphagous, oligophagous, or 
monophagous. Under natural conditions as they existed before agri¬ 
culture changed the face of nature, there can be little question that insect 
species of polyphagous* tastes were represented by larger populations 
than those affecting more restricted diets for it is only under very ex¬ 
ceptional conditions that natural associations of plants are sufficiently 
dominated by a single species of plant to offer ideal conditions for the 
multiplication of monophagous insects. Such notable exceptions are 
illustrated by some of our western coniferous forests and by certain 
types of Savannahs or grass-lands where the dominance of one or of 
several species is almost complete, more nearly so indeed than in the 
average potato patch. To natural associations of plants, common 
migratory locusts are about as destructive as to cultivated areas. The 
non-migratory forms we cannot legitimately compare in this connection 
on account of the destructive action caused by plows and harrows to 
their eggs in cultivated fields. 
Under natural conditions oligophagous species sometimes enjoy the 
presence of their several food-plants in close proximity over more or 
less extensive areas, but much more commonly they do not, for it is by no 
means the rule to find the several food plants of such species occurring 
in the same associations of plants. Rather does it appear that at least ma ny 
oligophagous insect species are composed of several phytophagic races 
as Walsh termed them many years ago. In reference to the species as a 
whole these races appear to bear a relation analagous to the pure and 
impure species of the geneticist and in many cases at least the phyto¬ 
phagic races propagate themselves more or less independently of one 
another. Races of this sort are exemplified by the apple-maggot and 
blueberry-maggot (.Rhagoletis pomonella ), the codling moth of the apple 
and of the walnut (Cydia pomonella), the mangold-fly (. Pegomyia ) and 
others. Such species, so far as numerical abundance is concerned, do 
not appear to profit by their ability to feed on more than one food-plant 
as they do not shift readily from one to another. On the other hand, at 
least one of our very important economic insects has been able to capital¬ 
ize its fondness for several agricultural plants. This is the cotton 
bollworm, an insect which has become numerically abundant under 
the agricultural conditions now prevailing in our southern states. As 
