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- New UA research has discovered that seed beetles from the desert Southwest shelter their broods from attacking parasitic wasps under a stack of dummy eggs.
"I actually took the eggs and weighed them. I weighed individual eggs and bottom eggs from a stack and then top eggs from a stack." Deas found that the individual eggs that had not been stacked weighed the same as the bottom eggs in a stack. But the top eggs only weighed half as much."
"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."
Deas believes that the top eggs simply don't have enough nutrients to fully develop a beetle – or a wasp.
"If a wasp attacks it, the wasp larvae have reduced survivorship," said Deas. "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," said Deas.
"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."
In addition, Deas found that beetles in an environment with more parasitoids stack their eggs much more frequently than beetles in an environment where the parasitoids are few or none.
Then Deas switched the treatments, removing the wasps from the first group and adding wasps to the group that had been parasitoid-free. "I wanted to see how plastic the beetles were. Could they adjust according to what risk there was?" said Deas. "And sure enough they could."
The beetles can adjust how often they stack their eggs depending on the risk of parasitism, but some beetles seem to have a preferred strategy regardless of the risk.
"It looked like there are those beetles that lay tons of stacks no matter what level of parasitism they are exposed to, or they only lay a few stacks. Part of what I'm going to try to do next is figure out whether or not there are different strategies," said Deas.
"It's such a common organism that does a really curious thing," said Deas. "You walk by on campus getting your coffee, and you just don't realize that there's this intriguing thing happening. You know a lot about the larger, more charismatic organisms, but a lot of these smaller species have cool traits that are just aching to be discovered."
Deas's research is funded by the National Science Foundation's Doctoral Dissertation Improvement Grant, the Center for Insect Science and by an MGE@MSA grant from Arizona State University.