Beetles go on egg-stacking offensive against parasitic wasps

If seed beetles aren't careful, wasps will invade their eggs and have their own young kill the beetle larvae for nutrients. But these beetles aren't taking the threat lying down - they've developed an ingenious strategy to fight back.

University of Arizona researcher Joseph Deas has discovered how these beetles, common throughout the southwestern United States, protect their eggs from the invading wasp species Uscana semifumipennis. These wasps are known as parasitoids, rather than just parasites, because they actually kill their hosts as oppose to simply leech off of them. Their strategy is to deposit their eggs inside the beetles'. The wasps hatch first, and devour the beetle egg yolk before the new larva can finish forming.

Deas describes this grim process:

"You can tell when an egg has been parasitized because the egg will start to darken and blacken. The beetle larva by that time will never form because all of the yolk is going inside the wasp larva. And then you can see little red eyes in there; the beetles don't have red eyes. It looks very evil."

At the beginning of his research, Deas simply wanted to see how well the wasps did when they laid their eggs inside particularly large eggs. He assumed that this would give them access to greater supplies of nutrients and allow them to really thrive. But he discovered quite the opposite - the wasps born inside the largest beetle eggs were dying en masse. What could be causing this?

Deas explains his initial hypothesis:

"I didn't understand why. My idea was that there's just too much yolk in there. There are instances where wasps have been developing in size and they sort of drown because there's still yolk around them and they can't pupate in moisture, they can't turn into a cocoon. I thought that was the reason. I was looking at the eggs under the microscope, and I saw when I looked on the side that it seemed like there was another egg underneath it. I took some really small insect pins and started to scrape around the edge of the egg. Eventually I had a layer come off."

This was the big secret - the large eggs weren't single eggs at all - instead, they were stacks of multiple, smaller eggs. The beetles were laying eggs one on top of the other. What's more, the eggs at the top of the stack weren't even regular eggs - they were unusually small and seemingly incapable of nurturing any beetle larva, even if parasites didn't end up attacking:

"I found that a lot of the top eggs didn't result in any beetles, even if they weren't parasitized. So then I started thinking, 'Are these real eggs; what are these? The beetles are somehow able to reduce the size of these eggs before they lay them. They're able to control how big that top egg is so that they can save resources."

This unique behavior was all done to deter the wasps. By placing multiple decoy eggs on top of the real eggs, the beetles were protecting their eggs from parasitic attack and effectively laying a trap for the waps. Deas continues:

"If a wasp attacks it, the wasp larvae have reduced survivorship. At least a quarter of the wasps will die. I've seen the beetles lay from one to three eggs on top of a bottom egg. These are all shield eggs: They all have this flat, sort of shield-type look to them. For eggs with three shields on top, I've never found a bottom egg that was attacked. Those with just one egg on top get attacked much more than those with two eggs on top or three.

"It was clear that the beetles were stacking their eggs in response to parasitoids. I exposed some beetles to parasitoids and some not. I found that those beetles that were not exposed to parasitoids laid very few stacks, and those that were exposed to parasitoids laid anything between 50 and 90 percent stacks."

Via Proceedings of the Royal Society B: Biological Sciences. Image by Tuan Cao.