J Insects
Crop Losses due to Insects
Insects are frequently the most serious problem for food and livestock producers in
tropical agriculture. The percentage of crop yield lost on a global scale as a result of
damage by insects has been estimated as follows: Europe 5%, North and Central
America 9%, South America 10%, Africa 13% and Asia 21%. These figures represent
enormous quantities of food, and underline the importance of having some
understanding of insects and how to control them.
Life Cycles of Insects
Most insects have four stages in their life cycle: egg, larva, pupa and adult. The larval
stage (the caterpillar or maggot) is the principal feeding stage of most insects and is
often the most damaging, for plants and animals. Adults are often merely the insects
means of reproduction and/or dispersal.
Other insects such as aphids and slugs have a three-stage life cycle. The eggs hatch
out into nymphs, which are similar to the adult but smaller. The nymphs feed and
develop, and often shed several skins as they grow larger. When they have become fully
fed the nymphs have passed into the adult stage, and the life cycle continues.
Eelworms
This is the term used for several worms of the class Nematoda (phylum Aschelminthes).
They are tiny, from 400 micrometers to 5 mm in length, and are found throughout the
world. Some species occur in both animals and plants.
Eelworms are notorious soil pests, and they have their own form of life cycle. The
eggs hatch out in the soil and the young female larvae then feed on roots of plants. After
fertilisation by the male she produces eggs but keeps them in her body until she
becomes a bag full of eggs, known as a cyst. The eggs are released into the soil when the
cysts break open. Control is normally by crop rotation to break the life cycle.
Many eelworms are specific pests because the damage they cause is specific to
attacks only plants of the Solanaceae family. Eelworms increase in the soil particularly
when a susceptible crop is grown too often in the same field, so they can be controlled
with crop rotation ie not growing that crop in that field for many years.
Eelworms are not all bad, however. Beneficial nematodes are increasingly being
used on a small scale to control a range of insect pests. Details of this form of biocontrol
are widely available online.
Aphids
from plant to plant. The best way to control aphids is with systemic insecticides,
feeding on them but usually cause more damage because they transmit virus diseases
Aphids are also known as green fly or black fly. They do some damage to crops by
although sometimes more harm than good is done due to the phenomenon of resur-
gence—see page 80. Safer control methods are described below:
78 TONY WINCH
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certain crops—for example, the potato cyst eelworm (Heterodera rostochiensis)