PLANT PROTECTION 1 – Pests, Diseases and Weeds

Insect secretions
PLANT PROTECTION 1 – Pests, Diseases and Weeds
WAX GLANDS
SILK, WEBBING
Leaf curling spider
curls a dead leaf with silk
to form a hiding place.
ODOURS,
TASTE
Spider web
ATTRACTANTS
POISON GLANDS
Stingers are used
to lay eggs,
for self-defence
and stinging
THE SECRETION OF WAX BY THE EPIDERMAL GLANDS is a normal
process of cuticle formation in all insects. However, in some insects profuse
discharges of wax occur.
Beeswax is the natural secretion of the worker honeybee that is poured out
in thin scales or flakes from glands that open on the underside of the
abdomen. Its production directly follows the consumption and digestion of
a quantity of honey, a kilogram of wax resulting from the consumption of
from 2-10 kg of honey in about 24 hours!
Several insects belonging to the Order Hemiptera also secrete wax
profusely, including the tiny lac scale insect which is native to India and
Burma and produces the substance from which shellac (a varnish) is made.
Woolly aphids and mealybugs secrete large quantities of waxy materials.
SILK AND WEBBING are produced by many insects, mites and spiders.
Silk is used by caterpillars of moths and butterflies for many
purposes, including cocoon construction to protect the pupa, case making
for sheltering caterpillars, binding leaves together and for lowering
themselves for dispersal, eg leafrolling caterpillars. The silk is produced in
special glands and comes out via the mouth. The Chinese untangled the silk
from the pupa of the silkworm cocoons to give the world silk.
Webbing is also produced by ‘spider’ mites, eg twospotted mite, from
glands which open into the mouth. In heavy infestations, the fine silk
threads form a web over the entire plant. The mites crawl over the webbing
and fasten their eggs to it.
All spiders use silk to cover their eggs. The use of silk to form various
types of webs to capture food is widespread. Young spiders (spiderlings)
use silk for dispersal (ballooning).
DERMAL GLANDS, which secrete unpleasant odours which have a protective
function, are common in bugs in the Order Hemiptera, eg bronze orange bug
and the crusader bug.
Stink bugs give out a particularly disagreeable smell when disturbed,
hence their common name.
Birds learn to avoid brightly colored insects that taste unpleasant.
Social insects, eg ants, bees and wasps, use odours to communicate with
each other.
PHEROMONES are substances produced by external glands on insects which
produce specific reactions in other individuals of the same species.
The best known pheromones are the sex attractants which are
commonly found in moths and flies and used in pest control.
MANY ANTS, BEES, WASPS AND SOME ANTS secrete venom from glands
and inject it through a modified ovipositor or a stinger as a mechanism of selfdefence.
Honey bees have a large barbed stinger.
Barbed stinger which the
honey bee uses for self-defence.
Worker bee
SPIDERS BITE and some simultaneously inject venom via their fangs into
their victim.
Insects and allied pests - Biology 19