Twenty-Year Study Shows How Climate and Habitat Change Impact One Mantid Species
By Paige Embry
Ask someone what they know about praying mantids and chances are they’ll bring up the female biting the male’s head off during mating. It happens, albeit only about 17 percent of the time, but those deaths can be a surprisingly useful tool when studying mantid population changes over time. It’s one of the pieces of information tracked by Lawrence Hurd, Ph.D., a professor of biology at Washington and Lee University, during a 20-year study (1999-2018) of Tenodera aridifolia sinensis, the Chinese praying mantid. The results were published in January in Annals of the Entomological Society of America.
In the last few years, studies finding widespread declines in insect abundance have made headlines. Hurd’s long-term study uses one insect in one northern Virginia field to show how such declines can happen. Although the study only followed one species, Hurd and co-authors note that the findings should apply to other insects and spiders with a similar life cycle.
For this study, Hurd made good use of his resources. He had an insect of unusual size (7-10 centimeters) that beginners (his college ecology lab students) could easily recognize and catch. He also had a nearby field beginning its natural succession, which functioned as a laboratory because the mantids couldn’t easily escape from it. No other suitable fields were close by, and the mantids aren’t very mobile.
Five times between 1999 and 2018, on approximately the same day in September, Hurd sent his students across the field in a “skirmish line” to collect, mark, and note every possible T. a. sinensis. Hurd writes in an email, “I always try to base it [the class] on gathering good, usable data instead of just make-work data collection on a question that has already been answered.”
To assess the reproductive success of the mantids, they went back after the first frost to collect the oothecae (eggs laid in a gooey substance that hardens into a protective case). They brought the oothecae back to the lab, weighed them, and then returned them to the field. For the oothecae found on the stems of herbaceous plants, that meant “tying [them] on with sewing thread run through the dried foam surrounding the eggs.”
Mantids do well in flowery fields with lots of arthropod prey. When succession trends in an area lead to more trees, the population of mantids should shrink. Over the 20 years of this study, two-thirds of the open field area was replaced by trees, and the number of mantids decreased dramatically. However, succession was not the only factor impacting the mantids—climate change was as well.
When a Chinese praying mantid lays her eggs, the sex ratio is even. By the time the mantids reach adulthood, males outnumber females. Once mating begins, the percentage of males starts to fall, prey to the females as well as any other predators in the field. Eventually, the females become more common. Even though Hurd and the students sampled on essentially the same calendar day (September 12, 13, or 14) each year, they found that the proportion of males to the total population declined from more than 60 percent in 1999 to about 25 percent in 2018, showing that the mantids were further along in their life cycle.
It’s no surprise. For the last 40 years the growing season in northern Virginia has gotten longer and the summers hotter, so the mantids both hatch and reach maturity earlier. This means that some eggs may hatch before frost can put them into diapause, leading to death of the young nymphs and potentially adding to the population losses caused by the successional change.
In 2018, Hurd and his students found only three oothecae. In the fall of 2019, he saw no mantids, and found no oothecae after the first frost.
This study demonstrates the potential double whammy of habitat loss—even a naturally occurring one—and climate change. Hurd writes, “People are becoming worried about having to include insects in the mass extinction episode that many (including me) feel is already underway.” He says when he talks about this, people often respond with, “‘We gotta worry about bugs, too?'”
Unfortunately, as this study illustrates, the answer to that question is “yes.”
Annals of the Entomological Society of America
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.