Agriculture Reference
In-Depth Information
CHAPTER 1
INTRODUCTION
Pesticides are chemicals or biological substances used to kill or control
pests. They fall into three major classes: insecticides, fungicides, and herbi-
cides (or weed killers). There are also rodenticides (for control of vertebrate
pests), nematicides (to kill eelworms, etc.), molluscicides (to kill slugs and
snails), and acaricides (to kill mites). These chemicals are typically manmade
synthetic organic compounds, but there are exceptions which occur naturally
that are plant derivatives or naturally occurring inorganic minerals.
Pesticides may also be divided into two main types contact or nons-
ystemic pesticides and systemic pesticides. Contact or surface coating pesticides
do not appreciably penetrate plant tissue and are consequently not transported,
or translocated, within the plant vascular system. The earlier pesticides were
of this type; their disadvantages were that they are susceptible to the effects of
the weather and new plant growth was not protected.
In contrast, most of recently developed pesticides are systemically active
and therefore they penetrate the plant cuticle and move through the plant vas-
cular system. Examples of systemic fungicides are benomyl and hexacona-
zole. These systemic agents can not only protect a plant from attack but also
inhibit or cure established infections. They are not affected by weathering and
also confer immunity to all new plant growth.
The use of pesticides has been traced by historians to before 1000 B.C.
Homer mentioned the use of sulfur as a fumigant to avert disease and control
insects. Theophrastus, in 300 B.C., described many plant diseases known
today such as scorch, rot, scab, and rust. There are also several references in
the Old Testament to the plagues of Egypt for which the locust was chiefly
responsible, and even today locusts cause vast food losses in the Near East and
Africa. Pliny in 79 A.D. advocated the use of arsenic as an insecticide and by
900 A.D., the Chinese were using arsenic and other inorganic chemicals in
their gardens to kill insects.
In the seventeenth century the first naturally occurring insecticide, nico-
tine from extracts of tobacco leaves, was used to control the plum curculio and
the lace bug. Hamberg (1705) proposed mercuric chloride as a wood preserva-
tive and a hundred years later Prevost described the inhibition of smut spores
by copper sulfate.
