INSECT BEHAVIOUR

1. DEFENSE IN INSECTS

All species of animals are subject to attack by various enemies. To survive, they must have some means
of defense. Various types of defense are found among insects; some rely on their appearance or location
to avoid attack, some attempt to escape, some attack the predator and some rely on what amounts to
chemical warfare. Defense may take the following forms;

a) Passive means of defense: many insects “play dead” when disturbed. Some beetles fold up legs,
fall to the ground and remain motionless, often resembling a bit of dirt. Many caterpillars freeze
often in a peculiar position
b) Use of shelters: many insects live in situations where enemies have difficulty in attacking them.
Many burrow into plant or animal tissues, under rocks or into soil, and others construct and live
in cases or shelters. Most social insects (termites, ants, wasps and bees) construct fairly
elaborate nests usually in soil. The larvae of many insects that completes metamorphosis, after
completing their feeding and growth, construct a cocoon in which to pupate.
c) Chemical defenses: involves use of body fluids that are distasteful to predators like ladybird
beetles, use of repellant secretions or the injection of poisons into the attacker. A few
caterpillars have stinging hairs or spines that may cause severe skin irritation. Many insects will
bite when handled, and some will bite if given opportunity, whether handles or not. Of all the
chemical defenses used by insects, the use of a sting is probably the most effective (effect is
immediate and often severe)
d) Camoulflage (mimicry): many insects are colored so that they blend perfectly with the
background. Many grass hoppers are colored like the ground, many moths are colored like the
bark of the tree, many beetles, bugs, flies and bees are colored like flowers they visit. Many
insects resemble objects in their environment in both color and shape.

2. COMMUNICATION IN INSECTS (PHEROMONE PRODUCTION)

Semiochemical: a chemical emitted by a plant or animal that evokes a behavioral or physiological

response in another organism. When the semiochemical affects an individual of the same species, it is
called a pheromone. When the semiochemical affects an individual of a different species, it is called an
allelochemical.
Pheromones are chemicals produced by an individual so as to influence another individual of the same
species. Pheromones are produced by specialized glands eg in lepidptera- glands are at the tip of the
abdomen. In hymnoptera, pheromones are produced by the mandibular glands. Pheromone act within
species. They cause behavior and developmental stimuli

Pheromones may be classified according to their biological functions into the following groups:

i) Trail marking pheromones, in a few species of termites and ants pheromones are secreted
by mandibular glands of workers, to mark stationary objects on their flight path. A trail is
made by use of undirected streaks of material from the gut or various epidermal glands.
ii) Sex attracts, female moth releases pheromones from abdominal glands that attract males.
Male insects may also produce sex pheromones that attract females and induce them to
mate.
iii) Olfactory markers and surface pheromones, these attract insects to the secreting insects or
sites that they frequent. These include caste recognition and colony pheromone of social
insects.
iv) Assembly and aggregation pheromone, include those that induce formation of large
temporary or persistent aggregations of species. May cause hibernation. Brings cohesion of
swarms of honey bees.
v) Alarm pheromones, are produced by mandibular and various abdominal glands. These cause
the insect perceiving them to run or fly more actively and show aggressive behavior eg in
bees.
vi) Morphogenic pheromones, include pheromones that affect development eg affect
reproductive system to attain its maturity. Inhibits development of ovary rudiments of
workers in bees. Prevents workers from constructing queen cells and rearing new queens.

Allelochemicals: chemicals substances produced by an individual so as to influence another

individual of a different species. They can be categorized into:

a) Allemone: chemical secreted by an individual of species A (the sender) that has an adverse
effect on an individual of species B (the receiver).
b) Kairomone: is a chemical that has an adverse effect on the sender. For example, some
beetles/predators are attracted by aggregation pheromone of bark beetles, thus locating a
 source of food in the larvae and adults burrowing beneath the bark of the tree. The chemical
may have both negative and positive effects on the sender.
c) Synomone: a chemical secreted by an individual of species A (the sender) and benefit both the
sender and the receivers (species B).

3. INSECT ORIENTATION

This refers to the factors that determine the speed and movement in insects. These factors include;
visual objects, pheromones and environmental factors. Insects are capable animals as far as movement
is concerned. Relative to their size, they tend to move at high speed. This is because they have striated
muscles. An insect’s flight mechanism is very different from those of birds and mammals. There are two
kinds of flight muscles;

Direct flight muscles; they are attached directly to the wing. Found only in primitive groups such as
odonatan

Indirect flight muscles; move wings by deforming the thorax.

The plura and sterna sclerites are attached to form a rigid trough. The tergem is attached via flexible
region. Thorax muscles contract, tergem lowers and wing raises. A second set of muscle then pulls
inwards deforming the box and pushing the tergem upward. These can be synchronous or asynchronous
muscles.

Synchronous muscle – one nerve impulse leads to one wing beat

Asynchronous muscle – one nerve impulse leads to many wing beats. It has two stable positions. Once
the wing clicks into one position thoracic tension is released and it snaps back to the other position. This
allows a single nerve impulse to trigger many wing beats.

Flight in insects is influenced by a number of factors. Some smaller insects fly at a higher rate as a result
of faster wing beat. Smaller insects usually have higher beat frequency eg bees – 180beats/second;
midge – 700-1000beats/second. Larger insects fly at a low rate eg locust flies at a rate of
18beats/second.

In general, smaller insects move relatively faster than the larger ones. The high speed of wings is derived
from efficient striated muscles, but also results from the process of asynchronous contraction/relaxation
of muscles. The system of synchronous consumes a lot of energy, releases a lot of carbon dioxide and
there might be lack of oxygen. This means that insects move very fast for short distances.

NATURE OF INSECT DAMAGE

Insects’ damage may be displayed in a number of ways and these should be born in mind when one is
trying to diagnose the cause of damage. Damage may be direct or indirect effects

Direct effects

1. Biting insects; these damage in a number of ways;

 may eat directly the plant tissue reducing the assimilating tissue leading to hindered
growth
 insects that tunnel inside the stem may interrupt the flow of sap (stem borer and shoot
fly)
 others may cause the ring bark stem eg white coffee borer which bores exteriorly under
the bark and only escape pores are seen
 they may attack flowers and reduce seed production as is the maize tassel beetle
 may injure and destroy seeds eg maize weevil, coffee berry borer
2. piercing and sucking insects; damage plants in the following ways
 remove plant vigour due to excessive removal of plant sap
 damage floral organs and reduce see production eg coffee lygus bugs
 inject toxins into plant causing distortion and necrosis
 opens wounds on plant tissue which then acts as entry point of pathogenic fungi and
bacteria

Indirect effects

1. They make the crop difficult to harvest. This is because infected plants are usually delayed to
mature or crops are not mature at the same time
2. Insects cause contamination and loss of quality or may lower the grade hence losses on
marketability
3. There is transmission of disease organisms. This is done in two ways;
o Mechanical/passive transmission. Takes place during feeding ie the insect injects the
disease causing organism into crops. Sometimes the pathogen is carried on the
proboscis of the insects or on the body
 o Biological transmission. Here, the insect acts as intermediate host. The viruses have a
life cycle phase in the vector which can be completed before it becomes effective to
attack the plant eg tobacco mosaic and cassava mosaic transmission.

FACTORS INVOLVED IN TRANSMISSION OF DISEASE BY ARTHROPODS

In disease control programs, it is important to know factors that affect and sometimes facilitate disease
transmission. Disease transmission by arthropods vectors is affected by many factors.

Extrinsic cycle: this is the cycle of parasite outside the host. In case of malaria, this is the cycle in the
mosquito. The length of the cycle is affected by environmental factors especially temperature. For
malaria parasites, these ranges are limited in a range of 15-30 0C. If the environmental temperatures
goes beyond limits, they won’t survive.

Vector biting rate: the more frequent an organism bites, the more likely it becomes an efficient carrier
of pathogens or disease.

Reproductive rate of vectors: this refers to the index of recruitment. If the rate of recruitment is high,
more vectors appear in a population that will mean increase in the number of bites, hence increased
rate of transmission.

Life expectancy of vectors: this is the same as the survival rate of vectors. When survival rate is high,
individuals produce young ones and vector population remains high. One way to reduce transmission is
to reduce reproductive rate of an organism.

Characteristics of the host:

Host immunity; tissue response aimed at reducing the quantity of parasites in the host body.

Host behavior; how the host displays itself to the vector. Eg being open for mosquitoes, being in forests.