DDT, for example, is used against the codling moth in apple orchards 
but does not control leafrollers, aphids, mites, and scale insects on the 
same trees. Instead, DDT kills some of the natural enemies of these pests. 
Oddly enough, DDT destroys the predatory mites which prey on the plant- 
eating mites but does little harm to the latter. And when California orange 
groves were sprayed with DDT in 1945 to control the citricola scale, the 
DDT killed vedalia lady beetles, which had kept the cottony-cushion scale 
under control since about 1890. This permitted a resurgence of cottony- 
cushion scale until the use of DDT against the citricola scale was abandoned 
and the lady beetle recolonized. 
In spite of these examples, it should not be inferred that all of the 
friendly insects are being killed by chemicals. Claims of wide scale destruc- 
tion of parasites and predators by insecticides have not been substantial. 
Some of these beneficial insects may be protected at the time of spraying, 
even inside the treated host, and survive to destroy the weakened host. 
Parasites with alternate hosts are even less susceptible since they can 
survive on one host in an untreated area and then spread to destroy indi- 
viduals of other host species still present in the treated area. 
THE BALANCE OF NATURE 
Scientists do not agree on the interpretation of the term, ''balance of 
nature.'' At best, the term has inexact and faulty implications. 
Opponents of chemicals for insect control frequently assume that all 
nature was once in "harmonious equilibrium" before modern man disturbed 
this plant-animal utopia. However, in ancient times when man did little to 
disturb the insect world nature permitted the plagues of the locusts, with 
the fat and the lean years. Food supplies shrank, man became hungry, but 
the locust populations eventually receded. That balancing lefta deep imprint 
on mankind. Animal and plant populations have fluctuated continuously, and 
some species have disappeared. 
A curve drawn to represent the population of an insect species over a 
period of time generally is a wavy line reflecting the oscillations in num- 
bers--up when conditions are favorable, down when adverse. So balance in 
such cases is an average of conditions over the years. 
The insect world has its own balance as between beneficial and destruc- 
tive species, but this balance is not always favorable to man. When the food 
supply of one species is reduced or the food of another is increased, the 
equilibrium of nature is disturbed and adjustments must again be made. In 
a sense, therefore, man has upset the balance of nature by replacing great 
expanses of forests and native grass with crops, cities, and towns. Thus he 
has favored certain pests. 
BIOLOGICAL CONTROL 
Biological control of insects is the suppression of pest species through 
the action of other living organisms--the predaceous and parasitic insects 
and other animals and such insect disease organisms as viruses, bacteria, 
and fungi. 
