mber of
injurious insect species. However, some species do at times cause a
heavy loss of nuts and may also damage the vegetation growth of the
trees. Injury by insects will vary from year to year, due to various
causes, and insects frequently show varietal host preferences. Timely
use of insecticides is the most effective means of combating many
harmful species.

Until the beginning of World War II a rather limited number of
insecticides was available, such as lead arsenate, cryolite, nicotine,
mineral oil emulsions, and rotenone. Some injurious insects were
satisfactorily controlled through the timely application of one or the
other of these materials, or combinations of them; others survived in
damaging numbers in spite of all attempts to suppress them.

During and since World War II, both in the United States and abroad,
work on insecticides has been stepped up markedly. As a result, many new
insecticides have been developed and are available for general use.

The first of the new insecticides about which we heard was DDT.
Actually, the compound itself was not a new one, since it was prepared
by a German student chemist in 1874. However, no use was found for it
until 1939, when a Swiss chemist found it promising as an insecticide
against the Colorado potato beetle. It was first tested in the United
States a few years later.

Since the successful introduction of DDT, promising new insecticides
have become available more frequently and in greater numbers than ever
before. Among these materials are certain chlorinated hydrocarbons
related to DDT. These include methoxychlor and TDE, neither of which is,
on the whole, as useful as DDT but both of which are of value and have
an important advantage over DDT in that they are reported to be less
toxic to warm-blooded animals. Other new chlorinated hydrocarbons
include benzene hexachloride, synthesized in 1828 and first tested
against insects in France in 1941 and discovered about the same time in
England; chlordane, developed in the United States a few years ago; and
toxaphene. Several organic phosphorus compounds, including hexaethyl
tetraphosphate, tetraethyl pyrophosphate, and parathion, have also been
developed.

Technical benzene hexachloride is a mixture of several isomers, the
gamma isomer being the most toxic to insects. The practically pure
isomer is known as lindane. A handicap to the general use of benzene
hexachloride on fruit is its tendency to cause off-f