Australasian Science: Australia's authority on science since 1938

Locust Mating a Risky Business

By Stephen Luntz

A wasp targeted copulating locust during the most recent plague.

During the peak of the 2010–11 swarm, Australian locust faced a 10% chance of dying a particularly horrible death if they chose to have sex. Black digger wasps reproduce by paralysing grasshoppers and crickets, dragging them to a burrow and laying eggs inside the body. When the eggs hatch they feed on the still-living Orthoptera.

Dr Darrell Kemp of Macquarie University’s Evolutionary and Behavioural Ecology Research Group says that little is known about the lifecycle of this particular species, but related wasps feed only on nectar after they emerge or “don’t feed at all”. Most, if not all, nutrition comes from the unfortunate individual in which the wasp was laid.

Kemp observed plague locusts around the time of their first mating and found that females had a one-in-200 chance of being stung by a wasp while on their own. The risk for males was even lower, as the wasps would usually let a single male go, possibly because the smaller males lack the nutrients needed to sustain the young wasps to adulthood.

However, during a single mating event lasting 10–20 minutes, the risk of the wasp stinging the female was one-in-ten. “Male locust were never directly paralysed by wasps – either while solo or in couple – but males in captured pairs lost their lives solely because they could not detach from their paralysed female partner and were ultimately buried alive,” Kemp says.

Only once did he see a male break free from a paralysed partner before being buried, and he doubts it would be possible for them to escape thereafter.

Kemp says a typical locust might mate three or four times in its life.

It is not clear if the wasps preferentially target couples because they cannot escape while mating, or if the presence of the male provides a desirable food bonus.

The presence of so many locust during the 2010–11 plague made Kemp’s studies easier, but he says so little is known about the wasps that we cannot be sure whether the population has swelled since. He made the observations while visiting his parents for Christmas, and says he did not see wasps the following year, but they may simply have been more dispersed.

“This study is rare empirical proof that illustrates the cost of reproduction in real terms,” Kemp says. “For locust it can literally be a matter of life or death.”