They lead modest lives among the palo verde, mesquite and acacia trees throughout the Southwestern U.S., laying their eggs on seed pods and defending the survival of their offspring against the parasitic wasp species that attacks their eggs before their young can develop.

They are the seed beetles Mimosestes amicus, living all around us in the trees of Tucson, and yet remaining all but invisible to our eyes – or nearly so.

Now, doctoral candidate Joseph Deas in the University of Arizona's Graduate Interdisciplinary Program in Entomology and Insect Science, along with his faculty advisor Martha Hunter in the department of entomology, is peering into their world through his microscope and has discovered something novel: The beetles, whose eggs frequently are parasitized by the wasp Uscana semifumipennis, have a strategy to protect their offspring that goes beyond a helpful habit.

"They're stacking their eggs in order to protect them from these parasitic wasps," said Deas, whose research appears in the Proceedings of the Royal Society B on Sept. 14.

The eggs have eyes

The wasps, called parasitoids because they kill their host rather than just taking advantage of its resources, deposit their own eggs inside the beetles'. The wasp larva gets a head start in life and develops before the beetle larva, hijacking the beetle egg yolk for its own nourishment.

"You can tell when an egg has been parasitized because the egg will start to darken and blacken," said Deas. "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."

As often happens in science, Deas came upon the discovery of M. amicus' strategy through the course of a different investigation.

"I wanted to see how these wasps were performing in different-sized eggs of another beetle species," said Deas. "My idea was the wasps preferred to attack the largest eggs, because the largest eggs lead to larger offspring and larger offspring have a lot of advantages. For example, they can be faster foragers and lay more eggs if they're female."

"Then I saw that they were attacking these other eggs from M. amicus that were much larger."

Wondering how the wasp larvae would do in the apparently larger eggs, Deas investigated and was surprised to find that the larvae were not doing well at all.

"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."