By Leslie Mertz
When a Chinese mantid (Tenodera sinensis) grabs a nice, plump monarch caterpillar (Danaus plexippus), it not only “bites into the fleshy bits” but also eventually gnaws deeply enough to rip through the lining of the gut, the contents of which are propelled out of the still-alive larva as it writhes, describes biologist Jamie Rafter, Ph.D., lead author on a new article in the journal Environmental Entomology. “When the mantid encounters the gut material, you can see its little mandibles working and touching the gut material, but then the mantid will either toss it out of the way or ignore it and feed only on the adjacent tissue,” she says. Rather unnecessarily, she adds, “It’s not very pretty.”
Nonetheless, Rafter says she finds this mealtime behavior fascinating. “The questions are, one, why are mantids gutting the monarch caterpillars when they eat them? And, two, why are they eating them in the first place, since they are supposed to be toxic?” says Rafter, who is an assistant professor at Muskingum University in Concord, Ohio. The caterpillars get their toxicity courtesy of their diet of milkweed, a group of plants that are known for their poisonous substances called cardenolides.
To find out what exactly is going on in this bizarre feeding behavior, Rafter conducted a number of experiments. The first, which she conducted as a graduate student at the University of Rhode Island, compared the toxins as well as the nutritional content of the discarded gut material versus the “fleshy bits” of the caterpillar’s body. “To do that, we had to pry the partially eaten caterpillar biomass from the mantids—not an easy task, let me tell you!” she says. The experiment revealed that while the concentration of cardenolides is about the same in the body and gut, the composition is different. At the same time, she and her colleagues found that the nutritional value was far lower in the gut material, which comprised mainly plants, compared to the body.
Based on those results, however, they still couldn’t be sure if the mantids were gutting the caterpillars because the cardenolides made the gut less appealing than the rest of the body or because the gut contents were simply not as nutritionally valuable.
That led to a more extensive project to try to tease out the details. Rafter and her research group began by raising monarchs from egg through full-size (fifth instar) caterpillar on one of two kinds of milkweed: either common milkweed (Asclepias syriaca), which contains cardenolides; or swamp milkweed (A. incarnata), which has no cardenolides. Then they gave the prey to the mantids. “I was so excited that there might be a difference here, but the little buggers gutted everything regardless of whether the caterpillar was reared on a toxic plant or non-toxic plant, so that experiment suggested that it’s not cardenolides,” Rafter says.
Next, they wanted to determine if the mantids were avoiding plant material found in the gut and finding all other parts of the caterpillar palatable. To test that, the researchers starved a subset of the caterpillars reared on toxic common milkweed and a subset reared on the non-toxic swamp milkweed, so their guts were empty. They fed these starved caterpillars to the mantids, and the mantids ate everything, including the empty gut and gut lining.
With undigested plant material and the cardenolides out of the picture, that left nutrition as a possible reason for the gutting behavior. To determine whether nutritional value is the sole motivation for gutting, they turned to another prey, the caterpillar of a moth called a European corn borer. This caterpillar is non-toxic, but its gut and body have the same nutritional values. Sure enough, the mantids ate the entire corn borer caterpillars, gut and all. Although this seems to show that the gutting behavior is all about nutrition, Rafter points out a problem with that generalization: The nutritional value of corn borer tissues is quite low overall, and actually equal or less than that of the gut material of monarchs that was rejected.
So, what does it all mean? It’s complicated, she says. Based on the findings of the series of experiments, the mantid behavior appears to result from both an assessment of varying nutritional qualities within a prey type as well as an appraisal of gut materials for the presence of cardenolides and other secondary compounds found in some plants, such as swamp milkweed, to fight herbivory.
With this mystery solved, Rafter is turning her attention to other feeding behaviors, but she hopes she can continue to learn more about mantids. “I’m very much intrigued by predator-prey interactions, how predators handle prey, and potential impacts on prey populations,” she says. “If a different organism is better suited for the study question I come up with, I’d of course have to be willing to switch to a different species, but I would like to stick with mantids as my focal predators. They are such extremely interesting insects.”
Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.