Finding a good nursery for the kids is a dilemma that many new parents can relate to and parasitic wasps are no different. Competition between wasp mums can be keen when the species that will host their offspring can support only one wasp larva at a time. Add to that the sheer effort that a soon-to-be mum must invest to drill through the tough exterior of a fruit or seed to access the host larva within, and parasitic wasp mothers might have to invest significant amounts of energy to ensure that their offspring get the best start in life. Wondering how much importance pregnant wasp females place on these costs when staking a claim to a host larva, Romain Boisseau and Marlène Goubault from the Université François-Rabelais, France, and Art Woods from the University of Montana, USA, measured the amount of energy expended by female Eupelmus vuilleti parasitic wasps as they duel over and then bore through a black-eyed pea in search of weevil (Callosobruchus maculatus) larvae within.
‘Females that start drilling a seed kick and chase intruders until the loser retreats’, explains Boisseau, who recorded the amount of carbon dioxide exhaled by duelling E. vuilleti females as they grappled over a weevil-infested black-eyed pea to find out how costly the exertions were. However, the main technical challenge was finding a way to directly measure the carbon dioxide produced by the duelling wasps while the weevil larva inside the pea continued exhaling carbon dioxide. Fortunately, the team discovered that the wasps were content to lay their eggs on weevils that had just died, so Boisseau provided the sparring females with recently frozen black-eyed peas containing a dead weevil to battle over. Yet, when he compared the amount of carbon dioxide produced by the females during combat with the amount of carbon dioxide that they produced when there was no weevil larva to squabble over, the total daily cost was a measly 0.001%, in contrast to the massive energy costs incurred by other species during combat.
In contrast, when Boisseau recorded the amount of carbon dioxide exhaled by individual females as they bored through the tough seed in search of the weevil larva and calculated the total amount of energy that they expend drilling over the course of a day (16 holes on average), he was amazed that the exertion used as much as 15% of the wasp's energy budget. However, when Boisseau offered the wasps a seed that had already been drilled by a previous visitor (who had already laid her egg within), he was impressed to see that the late arrival saved time (using only 12% of the time it took to drill into a fresh pea). In total, a female could save up to 6% of her daily energy budget, despite the risk that her larva has a lower chance of surviving on a weevil that has already been parasitised – superparasitised – by another wasp mum.
Although superparasitism might not seem like a great idea when it reduces a larva's chance of survival, Boisseau, Goubault and Woods suspect that the energy and time saved using a second-hand hole may partly outweigh the risk that the larva won't make it. They also add that reusing a drilled hole could help to avoid wear and tear at the tip of the wasp's egg-laying microinjector. And the trio suggests that the sheer exertion of egg laying could also account for the wasps’ lacklustre combat performance; ‘There may be little benefit to fighting fiercely to access an unparasitised host’, they suggest.