parasitoid wasps and their undead babysitters
Raising kids is hard. How much easier would it be if you could strap your baby to a horse and it could eat raw horse steaks until it grows enough to hunt on its own?
That’s more or less the parasitoid wasp playbook. They lay eggs in or on arthropods, like spiders and caterpillars, eventually causing the death of these hosts. But for a period of weeks, the host becomes a living lunchbox.
The venom of some wasps (koinobionts) only causes temporary paralysis, from which the host recovers and goes on to continue to feed, develop, and molt while carrying eggs. Idiobionts paralyze the host permanently; and they are almost always ectoparasitic (eggs attached to the outside the host).
The wasp Glyptapanteles hijacks its host caterpillar’s brain, causing it to become a head-banging zombie. The wasp larvae spin silk cocoons which the caterpillar reinforces. The half-dead caterpillar also fights off attackers that would come eat the larvae, like aphids, by swinging its head around, which successfully deters attacks 60% of the time. This is a parental service it doesn’t even provide to its own offspring.1
I was recently visited on my front porch by one of the largest species of parasitoid wasps, called the long-tailed giant ichneumon wasp Megarhyssa macrurus. Its massive tail is actually three parts: an ovipositor and two shielding parts. It deposits eggs in holes in trees and dead wood occupied by horntail larvae.
Fig wasps Chalcidoidea employ a similar strategy, using a long tail, which is tipped with zinc. But they aren’t considered parasitoids because they feed on galls produced by scarring figs.
Galls are the subject of an even more elaborate interaction. Cynipid wasps scar oak leaves to induce them to produce a gall, where they lay eggs. Galls are a food source for other animals, so it's not a foolproof nursery. But if they're lucky ants will drag a gall into their nest and eat only the fatty acids in the cap, leaving the wasp larvae to mature in the safety of the ant mound.2
All this work is done by females, who are larger and do all the hunting. Males either only spawn to reproduce and die or they work as “helpers” who tend the nest and defend vulnerable larvae.
One nest raider is the blue mud dauber Chalybion californicum, which steals the nests of black and yellow mud daubers. It cleans out old spiders and larvae, catches and sedates a fresh meal, and deposits it along with an egg into its nest.
Wasp larvae have an appetite for protein, and adults have come up with clever ways to stretch out a meal. But adults are vegetarian. They acquire sugar from a secretion their young produce and from flowers, fruit, and your untended lemonade.
Towards the end of summer larvae finish pupating, so adults that rely on sugar secretions get less nutrition and become more aggressive foragers. If you're having a picnic in spring, the nesting season, and wasps show up, set a piece of protein aside to draw them away. If they show up in the fall, set aside something sugary. And if they sting you, be grateful you aren’t being asked to babysit.
Head banging caterpillar https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0002276