The Desert Locust
Prof. E. Kraemer
The desert locust is notorious as a swarming
pest with a voracious appetite. In fact, it
spends most of its life as shy, solitary
individual that carefully balances its food
intake to match its body’s needs.
one of about a dozen
species of grasshoppers
known as locusts
unlike other grasshoppers,
are able to change their
behavior in response to
form swarms that can
migrate over large distances
between plagues, typically
exist in band across sub-
Saharan Africa and into India
avoid others except to mate
fly mainly at night
eats its weight in food daily
predators include birds, mammals, insect
fly by day in swarms that can extend over
hundreds of square km, can contain
hundreds of millions of insects per square km
still each eat about their own weight per day
same predators; little impact on swarm
swarms move into neighboring areas of
Africa, Asia, and Europe
recent swarms in Mali, Mauritania,
Kazakhstan, Uzbekistan, Russia, China
Why do scientists study locusts?
to understand the underlying mechanisms of locust-specific
behaviours such as the transition from non-swarming to
swarming forms, and how locusts communicate with each other
to identify targets for improved control of locusts, for example by
preventing or dispersing swarms, and to identify better ways of
predicting swarms so that controls can be introduced earlier and
so be more effective
to use the nervous system of the locust as a model to
understand the basic cellular and physiological processes that
drive behaviour in insects, and other more complex animals
and to use the locust gut microbiota (microorganisms living in
the gut) as a model for the study of animal-microbe interactions
in general, and microbial transformation of plant secondary
compounds in insect guts in particular
What are scientists learning?
How locusts gain information about their
How they use and respond to this info.
Involves integration of findings from :
Unlike its grasshopper relatives, the usually
shy desert locust can switch from a non-
swarming phase to a swarming phase.
Typically happens when individuals crowd
together to compete for dwindling food
supplies after a period of abundance during
which the locust population had risen.
Switch involves not only
a behavioral change,
but also, in the case of
immature locusts, a
change in their
appearance at their next
What’s the trigger?
Until recently nobody knew what triggers the
change from non-swarming ("solitarious") to
swarming ("gregarious") behaviour - the sight,
smell or touch of other locusts? Or a
combination of these or other factors?
is the major trigger of swarming
contact between crowded individuals makes them
Physical contact, not the transfer of chemicals
between individuals, is what is important.
Solitarious individuals can be made to become
gregarious simply by buffeting them with small balls
of papier mâché, or millet seeds.
Sight and smell together can also stimulate
swarming, but each is ineffective on its own.
Effect of 4 hours stimulation by
sight, smell, sight+smell, and touch
on the behavior of locust nymphs.
Experimental stroking of locust
nymphs shows that it is touch on
the hind limb, not elsewhere, that is
significant in triggering swarming.
Tactile stimulation is detected by a nerve cell at the
base of each of thousands of touch-sensitve receptors
on the animal’s body. When the “hair-like” receptor is
deflected it activates the nerve cell with a burst of
The effect of smell
SMELL keeps the swarm together.
Bacteria that live in the locust's hindgut
produce compounds that help to prevent
disease-causing bacteria from establishing
themselves in the gut.
Locusts use two of these volatile compounds
as behavior-controlling signals (pheromones).
These compounds are
given off from the locust’s
feces. They help the
locusts to aggregate.
The mixture of bacteria in the
locust hindgut is dominated by
a single species, Pantoea
agglomerans, which is
implicated in producing guaiacol
and phenol in the fecal pellets
Locusts "taste" their environment and select
egg laying sites on the basis of the chemical
composition of the soil.
The female digs a hole in the ground and lays
up to 100 eggs in a "pod".
She surrounds this with a "froth", secreted by
her accessory reproductive glands, which
stops the eggs from drying out.
Offspring: solitarious or gregarious?
Experiments show that
mated females use their
experience, and that of their
mate, to influence whether
their hatchlings emerge as
solitarious or gregarious
A solitarious mother will
cause her hatchlings to be
she mated with a
she was recently
How recently mother was
crowded affects how
How does she control that?
Chemicals in "froth" (foam)
influence egg development.
Froth from a gregarious
mother will "gregarise"
young from eggs laid by a
Washing off the froth from
eggs laid by a gregarious
mother causes them to
develop into solitarious
Identifying the active
ingredient in the froth could
lead to new commercial
products for controlling
Laboratory studies show that the extent to
which food is dispersed or clumped is
important in triggering swarming.
When food was supplied in a single clump,
solitarious individuals switched to gregarious
behaviour after 4 hours
But they remained solitarious when the same
amount of food was supplied in several
Similar results have
been obtained in field
trials in Morocco and
This research is
that enhances the
ability to predict
swarming, and so
offers the potential of
earlier intervention to
Finding the food
Locusts select food by its chemical
use hair-like chemoreceptors on the body.
At the tip of each is a pore.
Inside the shaft are several chemical-sensing
(Another cell responds to touch).
Locusts have taste and touch sensors all
over their bodies. They help the animal to:
find a site for egg laying
move away from unpleasant chemical stimuli.
A chemical's identity is
coded in the pattern of
activation of several
The greater the
chemical, the greater
the nerve pulse
What are they hungry for??
Locusts also select their
food on the basis of
their current nutritional
status and needs.
Their taste receptors
are very sensitive to
nutrients which they
lack, but unresponsive
to those with which they
Experimental application of salts,
carbohydrates and feeding deterrents such as
nicotine hydrogen tartrate (NHT) to a locust's
leg causes it to move the limb away. The
effect is dose dependent in all cases, but
there are different thresholds for different
Locusts recognise tastes via a range of nerve cell responses
(from "suitable" to "aversive") which resemble those used by
Locusts decide about the acceptability of a chemical mainly on
the basis of information derived from the initial coding by the
sensory cells, and this drives their behaviour.
Sensory cells in the chemoreceptors transmit coded information
about the nature of the chemicals they are experiencing as
nerve impulses to specialist cells, called interneurones, in the
central nervous system. The interneurones process the signals
and pass on their codes to motor neurones that convey
information back to the leg and activate its muscles.
In experiments, all interneurones and motor neurones respond
when stimulated by all test chemicals. The concentration of the
stimulating chemical is critical. The central nervous system can
add "context" to the signal so that avoidance behaviour is
triggered only when appropriate.