How Insect Interactions Vary by Height in Turfgrass
By Ed Ricciuti
Scientists who tricked arthropod predators of turfgrass pests into chomping on fake clay caterpillars have revealed how life-and-death struggles hidden in the greenery could result in a huge plus for natural pest control on lawns, golf courses, and sod farms. Experiments using the fakes in bermudagrass (Cynodon dactyl),a key warm-season turfgrass, show that predators are most active close to the ground, according to a paper published this month in the open-access Journal of Insect Science.
Like a forest—but, of course, much shorter—turfgrass is vertically stratified into vegetative zones, the lowest of which is composed of soil and thatch, while the middle and upper zones are stems and leaves. Knowing the levels at which predators forage and feed is key to planning natural pest control, say Fawad Z.A. Khan, Ph.D., and Shimat V. Joseph, Ph.D., of the Department of Entomology at the University of Georgia.
“We observed predator activity at all levels, but predatory interactions were more abundant at the lower level than others, for the most part,” says Joseph.
When natural pest control is an option, predators can be recruited as the first wave of attack on pests, with chemicals following up if necessary. “The behavior of arthropod predators, particularly foraging and feeding behavior within the zones of turfgrass, determines the fate of natural pest management,” the researchers say.
The rub, they claim, is that it has been very hard to figure out exactly where within the turfgrass strata that predators are most active. Unlike the kills of larger predators, remains of prey in the mini-world of turfgrass are few and far between. Ants, for example, crawl through all levels of turfgrass and encounter prey species as they prowl, but they leave little or no evidence of their predation. To remedy the lack, Khan and Joseph stuck clay models to cardboard attached to stakes, driven into the ground and positioned in each of the vertical zones of grass. Predators’ jaws and claws leave behind scratches, cuts, and other signature impressions on the models that can later be examined.
In the new study, these tell-tale signs were most prevalent—and more pronounced—on models placed on or near the thatch surface, say the researchers.
This modeling technique, used with vertebrates, has been adapted for use with arthropods by Kahn and Joseph. The green clay models are gross emulations of caterpillars, lacking detail. As a typical fishing lure proves, an exact replica is not needed to trigger predatory behavior; merely the proper shape or feature will do. Noctuidae moth caterpillars, which include the major turfgrass pests such as the fall armyworm (Spodoptera frugiperda) and black cutworm (Agrotis ipsilon), were the model template. Both of their caterpillars move about the stem, leaves, and thatch of turfgrass.
Turfgrass ranks as one of the largest agricultural crops in the nation. The USDA’s National Agricultural Statistics Survey places the value of the U.S. natural grass sod industry at over $1.1 billion. Turf management is a $40 billion industry, covering about 50 million acres of athletic fields, golf courses, and other carpets of greenery.
Management of turfgrass involves maintaining it at a specific mowing height, which varies with factors such as the aesthetic needs, climatic zone, use, and type of grass. The mowing height can be critical for pest control. Research shows that predatory rove beetles (Coleoptera: Staphylinidae) and spiders (Araneae) increase with the mowing height of cool-season turfgrass. Greater predation of the beetle Ataenius spretulus grubs, which damage roots, has been reported on golf course roughs of annual bluegrass (Poa annua) maintained at 5-centimeter height than on fairways maintained at 1.5 cm. The authors stress, however, that “the vertical distribution of predator activity patterns within the canopy of warm-season turfgrass is still unclear,” a lack their research tries to remedy. They note that “the experiments were conducted on bermudagrass, and it is not certain if predator behavior varies by turfgrass genotype and their growth pattern, leaf texture, and management practices.” It is known, however, that the occurrence and abundance of the beneficial arthropods could vary by turfgrass types.
The authors say that their findings “can be used to refine management strategies, such as mowing height and insecticide use, for effectively managing soil-borne and foliar-feeding arthropod pests and beneficial arthropods.” Deployment of insecticides, for instance, should aim to avoid reducing populations of beneficial predators. The researchers urge more research into factors such as mowing height and cultural practices that could influence predation rates in various types of turfgrass.
“Clay models could be further used to understand the ecology of arthropod predators, such as their distribution within turfgrass landscapes, nontarget effects of pesticides in turfgrass, and to develop strategies to enhance their activity,” Joseph says.
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.