At this stage of development natural control agents cannot approach the 
effectiveness of chemical sprays against many fruit and vegetable pests. 
To suppress an insect pest without the use of chemicals, it generally 
takes a combination of natural and biotic factors, that may include adverse 
climate and other detrimental influences in the environment, and the insect 
parasites, predators, and diseases. The failure of any one of these may 
prevent control, and often the biotic agent kills a pest too late to prevent 
serious crop damage. A parasite of the squash bug, for example, destroys 
a very high proportion of its host, but only after the bug has caused heavy 
damage to the crop. 
A succession of parasites and predators that attack the host insect in 
different developmental stages is usually needed to adequately suppress the 
host. Several parasitic insects and a disease pathogen, for example, attack 
the several larval stages of the gypsy moth, and predatory beetles devour 
the eggs, larvae, and pupae. But such a biotic complex is not available for 
many pests. 
Another exacting and often unattainable requirement is that many para- 
sites must have an alternate host on which to live until the primary host is 
again available. For example, Dexilla ventralis, a parasite of several 
species of the genus Popillia in Korea, was introduced into the eastern 
United States in the 1920's to control the Japanese beetle (P. japonica). 
However, this parasite did not become established in this country, mainly 
due to the lack of suitable alternate hosts to bridge the summer period 
when grubs of the Japanese beetle are not available in the field in a suitable 
stage for parasitization. 
Some parasites and predators may be reduced to such low levels during 
light infestations that they cannot increase rapidly enough to be effective in 
a sudden outbreak of the pest. And when the pest population becomes very 
low, the parasites and predators may die out. 
Some parasites and predators attack a pest on one plant quite readily 
but not on another. For example, a parasite that controls the greenhouse 
whitefly effectively on tomatoes is much less effective against the same 
pest on tobacco. The vedalia lady beetle attacks cottony cushion scale on 
citrus but ignores it on Spanish broom, 
Thus, natural biological control agents have important limitations as 
the sole means of insect control. Nevertheless, they are of vital importance 
from an overall standpoint. They keep the numbers of many potentially 
destructive pests to such low levels that they are generally of minor im- 
portance and they prevent population explosions among many of the more 
destructive species, thereby making it possible to deal with them by applying 
other control measures. 
The Appendix (pages 20 to 22) contains a list of insect parasites and 
predators successfully colonized in the Continental United States. 
INTEGRATED CONTROL 
Just as biological control involves the interaction of the whole environ- 
mental complex with the biotic agents and their hosts, so do many of the 
best control programs achieve their highest efficiency whenvarious measures 
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