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Sniffing out behaviour in caterpillars 

By Dawn Wiseman

You see a group of caterpillars traipsing around in circles; how do you know which one is leading? Melanie McClure can tell you. Her PhD work in Emma Despland’s biology lab has her studying the behaviour of the forest tent caterpillar.

“The leader is the hungriest,” she explained with a smile.

Forest tent caterpillars emerge from a single mass of between 30 to 150 eggs and spend most of their larval stage as a seething, foraging throng. Unlike their relatives that build silky nests as bases to venture out from and return to, forest tent caterpillars are nomadic and move from site to site in search of food. Their preference is the foliage of broad-leafed trees.

When food gets scarce in one area, it takes a while for the animals to collectively move on.

As McClure explained, “Group movement seems to be governed by physical contact and a trail pheromone.” After marching in circles for a while, “the hungriest member of the group will cautiously initiate a new direction and in doing so lay down a pheromone trail — the others will follow.”

McClure’s PhD work is building on what she learned during her Master’s at the Université de Montréal. There she studied a tiny wasp, Aphidius ervi (A. ervi), which preys on pea aphids.

Like the forest tent caterpillar, aphids are considered pests because of the damage they do while foraging. The pea aphid, which dines on field peas, alfalfa and clovers, was inadvertently introduced to North America when these products were imported. A. ervi, a parasite of the aphid, was subsequently deliberately introduced in the 1960s as a natural means of controlling it.

A. ervi uses pea aphids as hosts for their larvae. Once deposited in an aphid, a larva ingests the bug from the inside out in about five to six days. All that’s left of the aphid by the time an adult wasp emerges is a dried husk called (appropriately enough) a mummie.

McClure’s studies focused on courtship and mating behaviour in relation to the female sex pheromones in A. ervi.

While the wasps can reproduce asexually, the resulting offspring are exclusively male. Female offspring can only be produced sexually, and A. ervi adults emerge from aphid mummies raring to go.

“Their lifespan as adults is about eight to 10 days, so the females tend to be very goal-oriented,” McClure said.

To attract mates, the females appear to emit two distinct pheromones. One works over long distances, the other at close range. McClure and two colleagues from Ontario, Jay Whistlecraft and Jeremy McNeil, first established the conditions under which mating was most successful.

“We tested for a number of variables including the age of females and males, previous matings, and so on,” said McClure. Their findings indicate that as the females age they become less attractive to males; a significant drop in pheromone production over time is suspected. Because the vast majority of females mate only once, the researchers also postulate that pheromone production is significantly reduced after one coupling.

“It’s also possible that the males leave some pheromone of their own on mated females which discourages subsequent suitors,” she added.

Once they established the optimal biological conditions for mating, they began looking at the impact of environmental conditions.

“The pheromones the female releases are volatile,” explained McClure. “Wind is therefore essential for carrying the smell to distant males, but it is also a limiting factor on mating success.”

A. ervi is so small (about the size of fruit fly), even low wind speeds can delay or preclude males flying towards females. But that doesn’t stop them from trying.

“They will wait for a lull and fly to the female in steps,” said McClure. “If the wind is too strong, they will even walk if they can. But the wind is deadly in a lot of cases. It’s impressive that they eventually get there at all.”