Clues in the Clay: Decoy Insect Larvae Reveal Predator Presence in Turfgrass
By Ed Ricciuti
It’s a jungle out there in the lawn or, perhaps more aptly, a mini-Serengeti. Hidden amid the blades of turfgrass, sharp-jawed beetles, ants, and others predatory insects kill black cutworms, Japanese beetles, and other herbivores with all the ferocity of lions preying upon wildebeest and zebras on the African savanna.
Lions leave remains of a kill for the vultures, but scientists studying predator-prey relationships in the turf find scant, if any, pickings. This absence of remains makes it difficult to develop natural pest control measures for turfgrass farm operators and even lawn-infatuated homeowners. But, perhaps no longer. Researchers at the University of Georgia have figured they can find evidence of predator arthropods by tricking them into attacking clay models—a technique pioneered three decades ago with snakes—and leaving behind signature impressions of their jaws, claws, and other weapons. Their findings were published in October in the open-access Journal of Insect Science.
The research, done in the field, builds upon an earlier laboratory study by the Georgia team showing that the technique can work as successfully with insects as it has with amphibians and birds. It could significantly improve non-chemical, less-expensive biological pest control for commercial sod farming, no small development since turfgrass is the largest agricultural crop in the nation, covering 20.2-million hectares and adding $58 billion annually to the economy. Homeowners looking for eco-friendly ways to maintain their lawns could also benefit. The method could provide homeowners with a neat window into the hidden and complex goings-on in the world amidst the grass blades, says entomology doctoral candidate Fawad Khan, who led the study under Shimat Joseph, Ph.D., assistant professor in the Department of Entomology at the University of Georgia.
As models go, those used in the study were not detailed representations of prey species but gross replicas. One look at a typical fishing lure testifies that an exact replica is not necessary to trigger a predatory response to prey organisms. Many game fish strike for the eyes of prey, which is why so many lures have googly eye spots. In other words, a shape or particular feature or a movement can be enough of a cue to set off an attack. Making models, preferably in the shape of a caterpillar, is a do-it-yourself job with commercially available clay, says Khan.
The results of the study show that predators prowl the grass most actively at night and that the color of the models made little difference in the dark, whereas blue and green models were attacked more than those colored white or yellow during the day. The bigger the model, moreover, the more impressions were left upon it. Perhaps understandably, models shaped like soft, squishy caterpillars bore more paired marks, scratches, cuts, and pricks than those fashioned after adult beetles. Other impressions were deep distortion, dents, scoops, and U-shaped marks. Some models were so savaged that parts were detached.
“Predators leave unique impressions when they interact with models,” says Khan. “The type of impression predators create can indicate what type of predators were active.”
Like on the savanna, the young are particularly targeted by predators. “Although it is unclear why predators preferred one shape over the other,” the researchers write, “it is possible that arthropod predators evolved on preying on larval stages of insects, and they are selected for traits that can recognize less mobile immature stages of arthropods.”
Tracking predator activity could help create better integrated pest management (IPM) methods against insects that damage turf. The clay model system will reveal what predatory insects prowl the hidden world of turfgrass, making it easier to manipulate as a control method for the pests.
Khan says the ecosystem-based approach of IPM considers beneficial insects including predators, parasites, and pollinators; the environment; costs; economic loss; and other factors surrounding any crop. Says Joseph, “In IPM, we tend to use multiple tactics. Pesticide is an important tactic, but there are others, like biological control. My viewpoint is more conservation. Here we are looking at biological control so we can … develop a population of predators and beneficial insects. When the fall armyworm attacks, (predators) can provide the first layer of control. If the population is overwhelming, we have to come up with a remedy. Chemical management comes into play if the numbers go beyond a certain threshold.”
Journal of Insect Science
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.