Why Some Bugs Are Attracted to the Wrong Species

By Elizabeth Preston | May 20, 2015 9:18 am

andresii-tristis pair2

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.

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  • Bob Naylor

    They like big bugs…

  • Jen H

    …and they cannot lie

    • Treehopper

      Actually (Jen Hamel of the article here), I didn’t make this comment. :-)

  • link

    The question “So what are these bugs of different species doing together?” begs another question: Could this not be strategic in order to establish numerical dominance of one species over the other? Since:
    “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.

    There are numerous examples in nature where actions to gain dominance are the rule, even in plants. Well, in every realm of nature that I’m aware of.
    Thank you for the great observation Jennifer Hamel.
    You and Bob Naylor both earn comment gold by the way.
    Kudos.

    • Treehopper

      Actually (Jen Hamel of the article here), I didn’t make the other comment. :-)

      • link

        Oh..oh well..Say..what about those other questions I pose? Any thoughts or perspectives on that?

        • Treehopper

          I’d have to disagree that this is a strategy for species dominance…

          Our framework for understanding why animals behave the way they do is centered around individual reproductive success – how many kids one bug produces, relative to others in the same population. The bugs that produce the most offspring also pass on their traits more than do other individuals. Individuals who produce fewer or no kids don’t pass their traits.

          In its historic range (west of the Mississippi and south into Mexico), the preference of male A. andresii for larger females is probably adaptive – meaning that males who prefer big females likely sire more offspring, and they therefore pass that trait on to lots of offspring… This is because female body size is correlated with how many eggs a female can produce.

          However, in North Central Florida, male A. andresii are confronted with very large females of a different species in addition to females of their own species. Now, male A. andresii who mate with the largest females are mating with the wrong species (A. tristis), and they do not sire any offspring from those matings. In other words, in this new context, the behavior is not beneficial for the males of species A OR for the females for species B. Make sense?

          (FYI… from specimen collections, we know A. andresii was introduced to north central Florida by the late 70s. It is almost certain that A. andresii was introduced accidentally, by humans – this is something that happens very, very frequently. When we move plants or plant materials from state to state or country to country, we move insects too – sometimes in the form of eggs, larvae, or pupae. This creates interesting opportunities to study evolution in action, but also causes problems… for example, when we move firewood, we also unwittingly transport pest insects to new places.)

          • link

            Thank you for your terrific reply and I was delighted with the disagreement as well. I’m cool with it. Maybe I’m missing a wrinkle there but I’ll stay open to it. I’m just looking at it as nature doing what nature does. Sometimes behaviors appear unusual at first until we understand more and I’ll be the first to state that I’m far more ignorant than knowledgeable on anything. Articles on biological interference and altruism come to mind as far as the sum of it.

            So if “matings are not beneficial for the males of species A OR for the females for species B because no offsprings are produced” could not that nullification of offspring not play into one populations favor specifically the species belonging to species A?
            I see it as a chess game. Strategic? Altruistic? Is it that neither profits or that one species does not from the cost if you like but the investment.
            You’ve got to pay if you’re gonna play they say.
            Life in whatever form it take will forever find me endlessly fascinated to learn more.
            Just a mere observer I am.
            Thank you Jen for the time you devoted to all of this.
            Kindest regards,
            Link

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Inkfish

Like the wily and many-armed cephalopod, Inkfish reaches into the far corners of science news and brings you back surprises (and the occasional sea creature). The ink is virtual but the research is real.

About Elizabeth Preston

Elizabeth Preston is a science writer whose articles have appeared in publications including Slate, Nautilus, and National Geographic. She's also the former editor of the children's science magazine Muse, where she still writes in the voice of a know-it-all bovine. She lives in Massachusetts. Read more and see her other writing here.

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