A memory of mosquitoes 1/10/02. By Giles Newton How mosquitoes learn to associate odours with their human hosts to bite or with their preferred egg-laying places to lay their eggs is almost unknown at present. Philip McCall hopes to find out.

With every breath, we exhale carbon dioxide and our bodies emit a rich bouquet of odours – sweet perfume to a mosquito searching for a human to feed upon. Within the mosquito's antennae are sensors tuned to these chemicals enabling the mosquito to find its host to bloodfeed. But the mosquito might also learn about its environment by associating such cues with experiences.

Some people or species are often bitten by mosquitoes more than others, and some places are preferred by mosquitoes as sites to lay their eggs. Much of this selection occurs through the mosquito's response to visual or odour cues, but little is known of how much mosquitoes might learn about these cues. Does experience improve their ability to find and choose a host to feed on, or a place to lay their eggs? Dr Philip McCall at the Liverpool School of Tropical Medicine is investigating.

Background: The life cycle of Anopheles mosquitoes

"We're looking at the behaviour of individual mosquitoes," says Dr McCall. "While they have predetermined mechanisms of behaviour – genetic memory or instinct – there is a degree of flexibility or plasticity within this. Like many animals, the mosquito can be moulded by its environment. If we can find out how, this would lead to a greater understanding of how they find resources and to the design of new ways to prevent or divert their biting behaviour."

The ability of insects to learn has been studied particularly closely in bees and wasps, ideal model systems for understanding insect memory.

"Bees are known to have very sophisticated learning mechanisms, as they can remember a source of food and then communicate the location to the rest of the hive," says Dr McCall. "They have complex short- and long-term memory, and can learn to associate visual and odour cues with the food resource."

Studies on the fruit fly Drosophila have also provided many insights into the genes that underlie memory, with learning mutants, deficient in specific aspects of memory, being used to understand many of the individual steps in the processes of learning and retaining memory.

While bees and Drosophila can provide clues as to how insects learn, little is known about the capabilities of mosquitoes and other vectors (black flies, the vectors of onchocerciasis or river blindness, or sand flies, the vectors of kala azar or leishmaniasis).

"These vectors of disease have not been investigated to the same extent," says Dr McCall. "We don't know enough about the odours or visual cues they respond to, or the impact such learning can have on those responses."

Although memory may help mosquitoes and other vectors finding suitable resources, they are also likely to be great opportunists. While they may have learned to prefer something particular, they may also accept the first suitable alternative they come across. Thus, under certain circumstances learned changes in behaviour could be profound, while elsewhere they may not alter behaviour very much at all.

The egg-laying behaviour of mosquitoes is known to be influenced by odours, and many species are attracted to compounds such as phenols and skatole (the latter smelling like an subtle blend of faeces and halitosis) emitted by a suitable breeding site.

"If we rear mosquitoes in a specific environment with a certain odour, one which is normally repellent, they appear to associate that odour with 'good breeding sites'. Then given a choice, they preferentially lay their eggs in such breeding sites over a week later. Perhaps because they successfully developed in it, they have come to 'classify it as suitable for their offspring'."

If we knew more about the odours that attract mosquitoes to feed on human blood, similar studies on host selection might be possible. "We don't really know precisely what human odours are attractive to most blood-feeding insects," says Dr McCall. "Specific compounds in exhaled breath or foot odour might attract some malaria mosquitoes, but their identity remains elusive."

Knowing how a mosquito responded to these odours would allow us to understand what she experienced at the time of feeding. "If she finishes a blood meal, she may associate certain odours with this positive experience, and an interrupted meal with a negative experience," says Dr McCall.

"Some malaria mosquitoes – such as Anopheles arabiensis – bite both humans and animals, so if there are more cattle near a community, completing the bloodmeal is more likely to occur. This would be a positive experience, and she may come to prefer cattle next time she comes back to feed. If we knew how this occurred, we might be able to manipulate the system to reduce mosquito human contact and, ultimately, reduce malaria transmission."

Dr Philip McCall is in the Vector Research Group, Liverpool School of Tropical Medicine, University of Liverpool.