CHAPTER 21
Insect
Behavior
At times it may appear that insects and
their relatives are capable of thought. However, close examination reveals that
they do not think but are merely reacting blindly to their environment. Insects
can be compared to a complex computer, which has been programmed through the
processes of nature. As a result, they are equipped with a large number of
behaviors, such as walking, running, tasting, eating, flying, mating, avoiding
light and many, many, more. In order for a behavior to occur, the proper button
on this computer (insect) has to be pushed. This is accomplished through the
sensory system of the insect (e.g. compound eyes, ocelli, tactile hairs,
antennae, maxillary palps, etc.). Once an insect perceives a stimulus (e.g.
tastes food with its maxillary palps), then the preprogrammed behavior unfolds.
In this case the insect eats food.
It can be demonstrated easily that there
are no thought processes going on by placing the abdomen of a dragonfly in its
own mouth, which it will readily consume. This occurs as a thoughtless
behavioral reaction to the stimulus of protein being placed in its mouth. Of
course, in nature it would indeed be rare for a dragonfly to find its own
abdomen in its mouth, so this behavior would be of no disadvantage to the
species.
Many insects share some characteristics
with higher animals including humans. Two signs of intelligence are the ability
to think and communicate those thoughts to others. As we have already
discussed, insects cannot think but they do have limited abilities of
communication.
INSECT COMMUNICATION
Communication in insects is most commonly
found in, but not limited to, social insects. There are documented cases of
nonsocial insects communicating with individuals of the same species, cases of
insect communicating with insects of a different species and cases of insects
communicating with higher animals.
Although the last situation is extremely
rare, a few species of moths communicate with bats. Most if not all
insect-eating bats use echolocation to find their prey. A feeding bat sends out
a high frequency beep that bounces off its prey (moths), with the returning
beep being detected by the bats large ears. This system works similarly to a
destroyer's sonar in detecting a submarine. Some species of moths are capable
of detecting the beep of an echolocating bat. If the detected beep is weak,
indicating the bat is far away, the moth will fly away as fast as it can in an
attempt to avoid the bat. When the beep is strong, indicating that the bat is
close, the moth will fly erratically or merely drop to the ground. Although
amazing, none of the above is a good example of communication. However, in at
least one species of tiger moth, the beep of an echolocating bat will cause the
moth to return a squeak of its own, which is detected by the bat. This tiger
moth is foul tasting and any bat that has previously fed on another squeaking
tiger moth of the same species will associate that sound with the foul taste
and break off the hunt.
Unfortunately, a few questionable
entrepreneurs have exploited the public by selling pest control devices based
on the principle of avoidance displayed by insects and other pests upon hearing
certain types of sound. One such device is a small electrical box that is
supposed to produce a high frequency sound that drives cockroaches and other
insect pests from the home or disrupts their mating biology. Another is a probe
that, when placed in the ground, produces a beep that drives gophers
from the ground. Unfortunately these devices are totally worthless. I once
walked into the backyard or a homeowner who had placed 8 of these probes in the
ground in attempts to eliminate one gopher. Of course the gopher was still
there after 4 weeks of beeping.
Interspecies Insect Communication.
Although rare, there have been a few
documented cases of one species of insect communicating with another species of
insect. There is a species of caterpillar in
The caterpillar has a ridged area on the
front of the prothorax which, when rubbed by the back margin of the head,
produces a squeaking sound. If a predator attacks the caterpillar, it produces
this sound, which alerts any nearby ants to the danger so they can quickly rush
to its aid.
Interspecies Insect Communication.
Insect communication within the species is
accomplished with the use of all possible media including smell, sight and
sound.
A. Chemical Communication. Insect pheromones, which utilize the sense of
smell, are a component of insect behavior that has been studied in great
detail. A pheromone is a hormone-like chemical released by one individual of a
species that elicits a behavioral response in another individual of the same
species. There are many different types of these chemicals including sex,
aggregation, trail-following, alarm and oviposition
pheromones.
Sex pheromones-Sex
pheromones serve to stimulate the opposite sex to mate and are found throughout
the insect world. They have been studied most thoroughly in the Lepidoptera
(butterflies and moths) and the Coleoptera (beetles). In the former order,
moths rely considerably more on sex pheromones for mating communication than do
butterflies. Butterflies are diurnal and rely much more on sight to recognize
the opposite sex. However, with nocturnal insects such as moths, vision is of
little value, and consequently, they utilize chemical communication.
When a female moth is ready to mate, she
exposes a gland on the tip of her abdomen, which releases a sex pheromone
continuously into the air. Biologically speaking, the female of the species is
the logical sex to release such a chemical, as typically she is more
heavy-bodied (full of eggs) and consequently a less agile flyer.
Once released, the sex pheromone floats
downwind from the female, forming a continuous odor plume. The distance this
plume travels while remaining intact may be several hundred yards, depending on
the species of moth involved and the speed and turbulence of the prevailing
wind. Because the odor plume will remain intact longer and therefore be more
effective when winds are calm and slow moving, most moths mate very late at
night or in the early morning hours (when these conditions occur). Any sexually
receptive male moth of the same species exposed to the female sex odor plume
will immediately begin flying in the direction of the releasing female. The
main environmental clue as to the direction at the female is not the pheromone
itself, but the direction of the wind. Since the pheromone travels downwind,
the responding male orients or flies upwind, thus traveling toward the female.
This phenomenon of upwind orientation is referred to as positive anemotaxsis.
Once near the female, the male does not
recognize her; he relies entirely on a concentration of her sex pheromone that
is more or less uniform at close distances in all females of the same species.
At that point, he releases a sex pheromone of his own--thus alerting her to his
presence.
The author has conducted considerable
research with insect sex pheromones. On more than one occasion I have spilled
small amounts of these chemicals on myself and then gone out at night only to
have large numbers of male moths flock to, and attempt to mate with--me! One
warm summer evening our daughter drove a car containing a vial of synthetic sex
pheromone (used in research) in the glove box to pick up her brother from his
job. As she waited under the lights with the windows down, approximately a
hundred moths flew into the car and circled her frantically. Others waiting in
their cars were understandably astounded.
The sex pheromone of many of the more
important agricultural pests has been chemically identified. This is usually
accomplished by first establishing a large laboratory colony of a species. The
sex pheromone glands of several hundred thousand females is then clipped off
the abdomen in an attempt to chemically extract enough pheromone in a pure form
so its chemical structure can be identified. Once identified, the chemical can
be synthetically produced in the laboratory.
One of the more effective uses of
synthetically produced sex pheromone has been in the control of the pink
bollworm, a small moth that is major cotton pest from U.S. to Southern America.
This moth is a weak flier and only feeds and mates in cotton fields. The
control technique consists of saturating entire cotton fields with the smell of
artificially produced sex pheromone over the entire growing season. The purpose
of this procedure is to prevent the male moths from finding females and
consequently preventing mating and reproduction. If this can be accomplished,
the moths will not reach sufficient numbers to become pests.
Two theories as to why this technique
works have been called the male confusion technique and the male inhibition
technique. The male confusion theory is based on the premise that is if the air
of a field is saturated with artificially produced sex pheromone, when a female
releases her pheromone in that field, there will be no discernible odor plume
for him to follow and find her.
The male inhibition theory is based on a
biological phenomenon that is common to many animals including humans and
moths. For example, when someone is continuously exposed to a strong odor (e.g.
perfume or cologne), after a short period of time, one can no longer smell it.
This is because the olfactory sense cells in the nose adapt to the presence of
this odor. The same phenomenon would occur in a male moth sitting in a field
saturated with the smell of artificial pheromone. If a female moth released her
pheromone in that field, a male would not be able to detect it and therefore
could not find her to mate.
Trail Following Pheromones-Trail
following pheromones are found in a number of insects but can most often be
seen in the ants. These insects forage from their nest randomly in search of
food. Once successful they carry some of the food back to the nest, utilizing
visual cues in their environment and time compensated sun orientation. This
term refers to the ability to use the sun as a compass and adjusting
orientation to the sun as it appears to move across the sky (see later
discussion in this chapter).
As an ant returns to the nest, it drops
small amounts of a trail-following pheromone from the tip of its abdomen onto
the ground or other substrate on which it is traveling. Upon reaching the nest,
it shares the food with other workers, which excites them to leave the nest and
search for food. Upon leaving, the new foragers follow the partially
established trail, reinforcing it with pheromone of their own. Trail-following
pheromones are quite volatile and if not constantly reinforced will soon
disappear, a characteristic that is advantageous once the food source runs out.
This phenomenon can easily be demonstrated by rubbing one's hand through a
trail of foraging ants and thus erase the chemical trail. Once other foraging
ants reach this point in the trail they will appear confused and head off in
any of a number of directions
Another insect that utilizes trail
following pheromones is the honeybee. When a beehive becomes crowded, a new
queen is raised and she will frequently leaves the hive with approximately
one-half the worker bees (up to 40,000) in the form of a swarm. The newly
emerged swarm will not typically fly far from the hive and will quickly settle
down on a nearby fence post, branch or other structure. Because such locations
are not ideal for setting up a new home a number of the worker bees in the
swarm will search out a more suitable location. Unfortunately, a newly found
location often ends up being the hollow wall of an apartment building or house.
Once a worker bee finds this new location, she will return to the swarm and
lead it to that location by releasing a trail following pheromone as she flies.
Aggregation pheromones-Aggregation
pheromones differ from sex pheromones in that they can be released by either
sex of a species and attract both sexes for mating and feeding. They are best
developed in the bark beetles. Bark beetles are one of the major insect pests
of timber, especially when trees are in a weakened condition. Healthy trees
generally possess a natural defensive mechanism against beetle attack in the
form of sap flow, which will drown most boring pests. If a tree is weakened due
to disease, drought, fire, or any of a number of other reasons, sap flow decreases
and bark beetles are attracted in large numbers. These insects bore beneath the
bark forming extensive tunnels, which can eventually girdle and kill the tree.
Aggregation pheromones provide an
efficient mechanism for bark beetles to find susceptible trees. If a bark
beetle flies to and bores into a healthy tree, sap flies will kill it quickly.
However, if it bores beneath the bark of a weakened tree with reduced sap flow
and begins to feed, it releases an aggregation pheromone that in turn attracts
huge numbers of beetles to the tree. This mechanism greatly increases the
chances of beetle survival insuring both mates and a copious source of food.
Aggregation pheromones have been
identified for a number of species of bark beetles. Again, humans have used these
chemicals in the attempt to control these pests. One technique, which has
resulted in marginal successes, is to paint aggregation pheromone on healthy
trees, letting the natural sap flow kill those beetles that are attracted.
Aggregation pheromones also have been used as 'bait' in large traps that are
painted with a sticky material that ensnares beetles. When these traps are
placed in forested areas, each literally can trap buckets of beetles in a few
nights.
Alarm pheromones-Alarm
pheromones are found in many species of insects but are best developed in
social insects, such as bees. When a honeybee stings, it releases an alarm
pheromone from a gland near the stinger. In this case the pheromone draws other
bees to the site of and stimulates them also to sting. Because honeybees are
rarely found together except in the vicinity of a beehive, this is a mechanism
for protecting the colony. Many a beginning beekeeper has been stung by a bee
inside the veil and made the mistake of attempting to remove it only to find
many more angry bees on the attack. Because a honeybee can only sting once, and
dies soon after, a bee proves to be no threat once it as stung.
This reaction was recently demonstrated in
a beekeeping class at Cal Poly. There was an individual in the class who had
gained the reputation as being cool. He was what we call a professional
student, having been at the university for nearly 15 years and having completed
4 majors and 9 minors. The first day of laboratory he got a bee under his veil
which stung him on the ear. Forgetting what we had warned him about, off came
the veil and on came the hasty retreat. He quickly outdistanced the bee until
he ran into a patch of prickly pear cactus. After a few days he healed except
for his bruised dignity.
Oviposition Pheromones-Oviposition
pheromones are associated with the process of egg laying. They are best
developed in the parasitoids, or those insects that parasitize other insects. A
well-known parasitoid is the tarantula hawk, a wasp that deposits an egg inside
a tarantula. Once the egg hatches, the larval wasp feeds and completes its
development inside the tarantula and eventually kills it.
Many parasitoids will place a dot of
oviposition pheromone on their host immediately after they have deposited an
egg in or on it. This phenomenon is referred to as the spore effect. In these
cases the pheromone serves to alert other parasitoids of the same species that
this host is already parasitized, of course this prevents over-parasitization
and insures an adequate food supply for the developing immature parasitoids.
B. Visual Communication
As previously mentioned, butterflies
utilize sight and moth use odor to attract the opposite sex for mating.
Correspondingly, butterflies are typically brightly colored and moths tend to
be dully colored. This fact was recently well documented on a recent insect
collecting trip to Costa Rica. Several of the students had spent most of the
morning attempting to collect the large brilliantly colored Morpho butterflies. They had limited
success as these butterflies tend to be quite fast flying and elusive. Upon
observing their frustration, I pulled out a blue piece of blue paper and waved
it in the air attracting several excited males, which were easily captured.
Another insect that uses visual clues to
communicate mating readiness to the opposite sex is the firefly. The fireflies
are soft-bodied beetles in which the head is not visible when viewed from a
dorsal angle. During the spring and summer months these insects are quite
conspicuous due to their blinking yellow lights. There are small members of
this group in California, but these are not capable of producing light. The
light-producing species are rather common in many areas of the world and in the
southern and eastern United States.
The light emitted by these insects is
unique in that 100% of the energy produced is in the form of light. In a light
bulb only about 10% of the energy produced is light while the rest is heat. The
light is emitted from a gland located on the underside of the firefly's abdomen
and is produced by oxidation of a material called luciferin in the presence of
an enzyme called luciferase. The gland is richly supplied with tracheal
breathing tubes and the beetle has the ability to supply oxygen to the gland when
it is needed to oxidize the luciferin to produce light.
Each species has its own flashing pattern,
with variations occurring in the flash length and intervals between flashes.
The blinking is a form of sexual communication within a species. At twilight the
males of most species fly low over the ground and begin to flash. While sitting
on the ground or vegetation, receptive females of the same species begin to
flash back, thus drawing in the males for mating.
In a few species, large numbers of males
will gather in one bush and flash in unison. This draws both sexes for mating.
This phenomenon has also been observed is some of the "eyed" click
beetles and is similar to some the behavior of some of the long-horned
grasshoppers or cicadas that sing in unison to draw mates. This cooperative
behavior intensifies the signal, which then is carried over longer distances
than can that of individuals.
There is one species of predatory firefly
that mimics the blinking pattern of a smaller species. In this case the males
of the smaller species that respond are consumed rather than finding a mate.
Surprise!
C. Sound in Insect Communication
Katydids, crickets and cicadas are the
most common insects that use sound in mating communication. Male crickets
produce their familiar chirping by rapidly raising and lowering their wings. In
doing so, a hardened area on the forewing (scraper) is rubbed over a ridge area
(file) on the hindwing. Each cricket has its own characteristic chirp--thus
avoiding mating attempts between different species. Mating communication in
cicadas is the loudest sound in the insect world. This sound is produced by a
pair of flap like structures (tymbals) located on the underside of the abdomen.
In the jungles of Malaysia the mating calls of the largest species of cicada in
the world (Pompone imperata) can be
nearly deafening.