The squash bug mating orgies that biologist Christine Miller began noticing in gardens around Gainesville were nothing unusual. Dozens of insects were crowded together, the petite males along with the bulkier females, to search for partners. The unusual thing was that some males were copulating with females of the wrong species—apparently, they found them irresistible. When Jen Hamel arrived at Miller's University of Florida lab to do her postdoctoral research, she took up the mystery of the swinging squash bugs. Although they're related, the two insect species don't seem to have viable offspring together. And mating with the wrong species can be more than just a waste of time. "There is potentially a big waste of energy for a female," Hamel says, if she lays eggs that aren't fertilized because she's been dallying with the wrong males. The insects could also miss out on opportunities to mate with the right partners, and females might risk damage to their genitalia or reproductive tracts. So what are these bugs doing together? The two species are Anasa tristis, commonly called the squash bug, and Anasa andresii. Both are widespread pests of squash and their relatives. Although the insects share the same host plants, A. andresii is an introduced species that's only been in the area for about 40 years. Both species have small males and large females, but A. andresii individuals are smaller overall. "Males of many insect species prefer larger females," Hamel says. A bigger body often means a female insect is especially fertile. So if a male A. andresii finds large females attractive, then females of the extra-large species A. tristis must look to him like goddesses. To find out whether this was true, Hamel arranged blind dates between A. andresii males and pairs of females. In one setup, the male was introduced to two females of his species. In another, he met two females of the opposite species. A third group of males were presented with one female of each species; the A. tristis female, as usual, was larger than her A. andresii counterpart. Undergrad coauthor Savannah Nease monitored most of the insect dates. She observed each meeting for two hours, although most males had climbed onto a female within the first 20 minutes. A female may reject a male who's mounted her. If she accepts him, they begin copulating—an act which actually takes place with the parties facing away from each other. For every meeting, Nease recorded which female the male mounted first. She also watched to see if females opted to copulate, or if they gave males the boot. Whether the male A. andresii was meeting females of his own species or the opposite, he preferred the larger female. When facing one female of each species, his preference was a little less clear. But overall, the larger females in the study were mounted more than twice as often. This means the males' love of large mates may be what attracts them to the wrong species. Given that many other male insects also prefer large females, Hamel says, her finding wasn't all that surprising. But it was odd that so many females of the opposite species allowed it. Or, as Hamel put it in her paper: "Why are female A. tristis relatively indiscriminate in their choice of mates?" Fifteen percent of A. tristis females that were mounted by a male of the wrong species went on to copulate with him. Hamel says female A. tristis might allow these copulations because it's just not worth their energy to fight the males off. Additionally, the two species have only been sharing their squash plants for about four decades. So it's possible that female A. tristis are still evolving a strategy to avoid these useless matings with A. andresii males. If they do, the male squash bugs will have to give up their fantasy mates and stick to their own species.
Image: A male A. andresii (left) with a female A. tristis (right). Photo taken by Jennifer Hamel at the University of Florida.
Hamel, J., Nease, S., & Miller, C. (2015). Male mate choice and female receptivity lead to reproductive interference Behavioral Ecology and Sociobiology, 69 (6), 951-956 DOI: 10.1007/s00265-015-1907-z
Note: This post has been updated from an earlier version.