The Desert Locust

FRES 1010

Fall 2005

Prof. E. Kraemer

Intro

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.

Background

one of about a dozen species of grasshoppers known as locusts

unlike other grasshoppers, are able to change their behavior in response to population density

form swarms that can migrate over large distances

between plagues, typically exist in band across sub-Saharan Africa and into India

Non-swarming behavior

shy, solitary avoid others except to mate fly mainly at night eats its weight in food daily predators include birds, mammals, insect

Swarm behavior

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

control 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 including mammals

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

Studying Locusts

What are scientists learning?

How locusts gain information about their environment

How they use and respond to this info. Involves integration of findings from :

animal behaviour biochemistry neurophysiology computer modelling.

Swarming

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.

Switching states

Switch involves not only a behavioral change, but also, in the case of immature locusts, a change in their appearance at their next molt.

clip

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?

Touch

is the major trigger of swarming contact between crowded individuals makes them

become gregarious. 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.

Experimental Evidence

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.

Detecting touch

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 impulses.

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).

Smell

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 of locusts.

Egg Laying

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 individuals.

A solitarious mother will cause her hatchlings to be gregarious if: she mated with a

gregarious male she was recently

crowdedHow recently mother was crowded affects how hatchlings emerge.

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 solitarious mother  

Washing off the froth from eggs laid by a gregarious mother causes them to develop into solitarious young

Identifying the active ingredient in the froth could lead to new commercial products for controlling locust swarms

 

Food distribution

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 clumps.

Food Distribution

Food Distribution

Similar results have been obtained in field trials in Morocco and Mauritania.

This research is providing information that enhances the ability to predict swarming, and so offers the potential of earlier intervention to control locusts.

Finding the food

Locusts select food by its chemical composition

use hair-like chemoreceptors on the body. At the tip of each is a pore. Inside the shaft are several chemical-sensing cells. (Another cell responds to touch).

Locusts have taste and touch sensors all over their bodies. They help the animal to: feed find a site for egg laying move away from unpleasant chemical stimuli.

Receptors

Identifying food

A chemical's identity is coded in the pattern of activation of several chemical-sensing cells.

The greater the concentration of chemical, the greater the nerve pulse frequency.

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 are replete.

Other responses

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 chemicals

Brain Power Locusts recognise tastes via a range of nerve cell responses

(from "suitable" to "aversive") which resemble those used by humans.

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.

Locust responses

Next …

computer modeling of locust behavior