Gut Check: How Stink Bugs’ Insides Illuminate Their Eating Habits
By Paige Embry
No one is likely to be happy when brown marmorated stink bugs (Halyomorpha halys) move into town. Not only does this invasive species release a cocktail of odiferous chemicals onto part of its exoskeleton (aptly named the evapotorium), but it also happily feasts on a wide variety of plants, including a number of economically important crops—ranging from fruit trees to corn and lima beans. Halyomorpha halys is native to Asia and was found in Pennsylvania about 20 years ago. It has since spread throughout much of the country, but it is currently only a significant crop pest in the mid-Atlantic region.
Whenever a new invasive species arrives in a place, scientists start developing management plans. One key piece of information they need is what the pest eats in its new domain. Collecting that information isn’t always easy. Field observations are time-consuming and potentially misleading. For example, you might see the pest on a common and easily sampled plant and assume that plant is a primary host while hidden in the trees above you a multitude of the pest’s siblings gorge on the leaves. A new study published in December in Environmental Entomology looks at another method scientists use for figuring out what an insect eats: Examine its guts and see what genetic evidence lurks there. The scientists found that, for H. halys, certain genes from the plants the bugs had eaten can persist for 14 or more days.
For this study, the scientists fed stink bugs nothing but bean seedlings for seven days and then switched them to carrots. On specific days after the switch (days 0, 1, 3, 7, and 14), they harvested the stink bugs’ guts and used polymerase chain reaction (PCR) technology to make multiple copies of two chloroplast genes, trnF and trnL, that are found in all plants. The genes were then sequenced to see how long the bean genes persisted in the stink bugs’ guts.
Jim Hepler, the lead author on the paper and a Ph.D. candidate at Washington State University, says about harvesting the guts for analysis, “The worst part is actually the scent glands that give stink bugs their name—they are located between a pair of legs on the underside of the thorax. It looks like a tiny, bright orange sack, and it inevitably ruptures during dissection and makes the room smell like angry stink bugs.”
Part of the reason for analyzing only the guts, rather than grinding up the entire stink bug, is to try and ensure that the plant genes found are from plants actually eaten by the stink bug, but this study shows that the process isn’t fool-proof. The DNA analysis showed small quantities of a variety of plants that were not involved in the study. The scientists put the findings down as likely due to “contamination, false matches due to sequence decay, or misbinning.” Misbinning, Hepler says, is “usually small-scale contamination of a sample with reads from another sample being sequenced simultaneously.” The authors note the need for “ground-truthing” when using this method to ensure that what the DNA analysis says is in the stink bug guts actually makes sense.
As for the bean genes, Hepler and the others found that they “declined precipitously” over the first 24 hours but both the trnF and the trnL were still detectable at three days. After that, the trnF dropped out but the trnL persisted to day 14. Hepler and the other researchers were surprised by how long the bean genes endured. “We’d included the 14-day time point more as a formality than out of any expectation of finding anything there,” he says.
The scientists were also surprised by the large difference in persistence between the two genes, but both factors may help scientists build a nuanced dietary history for highly mobile, non-discriminatory eaters like H. halys. For example, the most abundant plant genes found probably belong to the plant eaten most recently, and if only trnL genes are found for a particular plant, the stink bug hadn’t eaten it in the last three days.
All in all, Hepler says, “Host plant DNA sticks around a lot longer than you’d expect in stink bug guts, which opens the door to asking all sorts of interesting ecological questions that will help us manage brown marmorated stink bug more effectively.”
Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.