Chemical Communication in Aphids
A Wordless Enigma Only Partially Decoded
Insects use chemical signals that help them find mates, identify their own colonies, locate food sources, warn of danger, stimulate group cohesiveness, hide predators and parasites, or call in insects to protect them. The seemingly insignificant aphid is a connoisseur of chemical communication.
Types of Chemicals
Many communication chemicals are derived from amino acids, fatty acids, and simple sugars. Minor substitutions in the side groups of these molecules produce:
- hormones that regulate growth and the reproductive cycle;
- pheromones that attract mates, warn of danger, and inform other aphids where food is located;
- semiochemicals that communicate with members of other species.
This article describes pheromones and semiochemicals because most hormones are not used for communication.
Pheromones
As aphids reproduce parthenogenetically, sexual attractants are not often involved in aphid life. The very few females that are receptive to mating produce male-attracting pheromones. They stop producing pheromones once mated so males no longer congregate around them.
Other pheromones are extremely important to aphids. Their functions are to attract other aphids to high quality food sources, initiate dispersal movements, and warn of danger.
- Food Attractants
When an aphid flies past a plume of attraction pheromone, she turns into the wind and begins to fly in a zig-zag fashion upwind, moving across the plume of pheromone until she lands on the plant from which the pheromone emanates. Non flying aphids move along the plant until they also accumulate near the source of the pheromone.
- Dispersal Initiators
As plant nutrients become depleted by aphids, the aphids secrete a pheromone that stimulates movement. At first the movement is random, but once a few aphids locate unwilted twigs, buds, or leaves, these individuals secrete the species-specific food attracting pheromone and the others rapidly aggregate on the new food source.
- Alarm Indicators
Insects and birds often consuming aphids in large numbers. When attacked, the aphids release molecules that disperse throughout the colony and stimulate rapid movement on the part of the colony members.
Aphid colony densities are often high and there is a wide array of aphid predators, many of which take only a few individuals before leaving. Responding to each incident would result in constant interruptions of feeding by irrelevant, frenetic attempts to escape. Alarm pheromones evaporate rapidly and only become concentrated enough to cause a response if many aphids produce them. Responding only to concentrated attacks allows the majority of the aphids to continue feeding even while a few are being consumed.
Semiochemicals
Many chemicals are produced in the relationship between ants and aphids, but not necessarily by the ants or aphids. Aphids, ants, plants, and aphid predators produce semiochemicals that regulate the actions of other species. Ants secrete semiochemicals at the food plant that restrict aphid movement and inhibit an aphid's ability to produce winged offspring. Plants often release alarm signals that call in predators of the aphids. Aphid alarm signals cause other aphids to scramble and call in ants to attack predators feeding on the aphids. Many predators of aphids secrete or obtain chemicals that inhibit attack by ants.
- Ant Inhibitory Chemicals
Chemicals released from the feet of ants slow the movements of aphids and restrict the number of aphids that are born with wings. The mechanisms by which these work are discussed here.
- Alarm Signals
While plants produce alarm signals that call in aphid predators and parasites; ants quickly move to the source of aphid alarm signals, search for predators, and attack in numbers that often repel or kill the predator. Ants approach from the sides and rear – biting and stinging legs and soft tissues until the predator leaves or is overwhelmed.
- Chemical Camouflage
Ladybugs, lacewings, parasitic wasps, and others use aphid surface chemicals to effectively make the predator invisible to attacking ants. Some attach the dead bodies or wax-laden hairs of aphids to their skins. Others roll on the bodies of their prey. By altering the odors emanating from their skins, these predators become chemically invisible to ants.
Scientists are beginning to understand the world of chemical communication. There is more to find as the chemicals involved and modes of behavioral initiation and inhibition are yet to be identified. While the mechanisms involved may be quite complex, the chemicals are often simple modifications of the same molecules that form the proteins, carbohydrates and lipids of all organisms.
Insect Semiochemicals
|
What do you think about this article?
Latest Articles