When most people think of parasite behaviour, horrific tales of behavioural and physiological manipulation are what come to mind. This is not without cause; many parasites are definitely scary to think about. However, many pursuers of the parasitic lifestyle also display behaviour that would be thought of as normal, perhaps even charming in an anthropomorphic kind of way. An example of this is seen in the parasitoid wasp Sclerodermus harmandi, in the form of maternal care.
|Photo of multiple female Sclerodermus harmandi engaging in brood care from Figure 1 of this paper
This stories begins when the female wasp finds a suitable host for her eggs. She injects the host with paralysing venom, and cleans an area of the body to lay eggs on. Once laid, she routinely inspects the eggs with her antennae and mouthparts. If an egg is found to have detached from the host, she would gently reattach it. Maternal behaviour continues when the eggs hatch, when the larvae must be fed. To do so, the mother wasp bites a hole in the host, which is still alive at this point and injected with paralysing venom periodically to prevent it from moving. The hole fills with haemolymph, the insect equivalent of blood, which is consumed by the larvae.
During this stage the mother S. harmandi also moves the larvae around to prevent them from overlapping each other as they grow. If a larva dies, the mother moves the body far from the other larvae to prevent their habitat becoming unsanitary. Even during the cocoon stage the mother continues to rub the offspring, despite them being encased. Eventually, the males of the clutch hatch out as adults. These few males (there is considerable female bias in the ratio of this species) chew holes in the female wasps’ cocoons to assist them in emerging, after which they mate with them. While it has negative affects in many taxa, this kind of inbreeding is less likely to have negative effects in hymenopteran insects, where haploid males act as a purge of deleterious alleles.
So why does S. harmandi provide such comprehensive maternal care? Because it increases the likelihood of offspring surviving. Experiments in which the wasp mothers were removed at varying stages of offspring development showed that not only were offspring that received maternal care more likely to survive to adulthood, but that this was proportional to how much maternal care they received. Experiments also showed that when a mother was taken away and replaced with another female who has previously laid eggs, the ‘stepmother’ will exhibit the same behaviour as the mother would, with the same rise in offspring survival.
Why the stepmother expend her own energy to raise another wasp’s offspring is just as interesting; it is because of the high levels of inbreeding in the population. Since most of the reproduction in this species is done through inbreeding, there isn’t much genetic variation going around. This means that there is a good chance of two wasps being related, so the stepmother may increase the chance of her genes being passed on by raising another wasp’s offspring. Female S. harmandi that haven’t laid eggs yet do not exhibit this behaviour, preferring to leave the offspring alone; this would indicate that the maternal behaviour is initiated by laying eggs.
So for a parasitoid wasp, it turns out that the females of S. harmandi make for very responsible parents or stepparents. That is, if you consider letting your children live on the body of something you paralysed, feeding off its blood until they grow up, and then mate with each other to be “responsible”.
Hu, Z., Zhao, X., Li, Y., Liu, X., & Zhang, Q. (2012). Maternal care in the parasitoid Sclerodermus harmandi (Hymenoptera: Bethylidae). PloS One 7: e51246.
This post was written by Jarrod Mesken