By Leslie Mertz
Turfgrass covers three times more land area than any other irrigated crop in the United States, and brings in tens of billions of dollars in annual revenues. Yet one of the major pests of this crop still remains a mystery in many parts of the country, according to Madeleine Dupuy, lead author of a new Journal of Integrated Pest Management article on this damaging insect. The insects are billbugs, a type of weevil that is found from southern Canada to Mexico, as well as parts of the Caribbean.
“The information on billbugs is very regionally limited, so if you’re living in a region where there hasn’t been a lot of research done on billbugs, you may find that the fact sheets available to you are relying on information from another region that is not necessarily applicable,” said Dupuy, a doctoral student at Utah State University. She explained that at least 10 species of billbugs damage turfgrass in the United States.
The article, which she co-authored with her advisor Ricardo Ramirez, coalesces the information about billbugs. “For instance, this paper includes a table that lists the known host plants of common species of billbugs and the known ranges of billbugs in the United States. Previously, this has always been pretty disjointed information: incomplete and in several different sources,” she said. “I think we finally brought that all together, and hopefully, this will make it easier for turfgrass managers to get answers about what species might be causing them problems.”
Insecticides and Beyond
Billbugs not only feed on turfgrass, but also live within the plants for part of their lives. Female billbugs deposit one to three eggs in grass stems and sometimes in shoots, and the eggs hatch into larvae that feed on tissue inside the stem until they finally emerge and drop into the soil. Once in the soil, the larvae consume the roots and crowns, which can kill the grass plant. The larvae eventually metamorphose into pupae and then adults. Feeding by larvae can leave behind unsightly patches of brown grass, which can be a major problem for managers of golf courses, sports fields, and sod farms.
Telltale signs of a billbug infestation include patchy, yellowing spots that can progress to large sections of brown and dying grass that pulls up easily; and noticeable collections of sawdust-like frass resulting from the larvae chewing their way out of the stems.
The most common control measure for billbugs is the use of preventive chemical insecticides, such as neonicotinoids (e.g., clothianidin and imidacloprid) or, more recently, anthranilic diamides (e.g., chlorantraniliprole and cyantraniliprole), but they are not the only solution.
“There are other methods of cultural control and biological control that I think deserve a closer look,” Dupuy said.
One good alternative to chemical insecticides is planting a turfgrass variety that does better against billbugs.
“If you have a problem with billbugs in one part of your golf course every year, you might consider overseeding that area with a resistant variety of turfgrass, such as a resistant Kentucky bluegrass cultivar, or an endophyte-enhanced rye grass or fescue,” she said.
Endophytes are fungi that produce billbug-fighting toxins. If light to moderate billbug infestation still occurs, strategic irrigation and fertilization can encourage robust plant growth and effectively mask the billbug damage, as damage is most evident in stressed turf.
Another option for battling billbugs may be to encourage the growth of existing populations of natural billbug predators, such as ground beetles and wolf spiders.
“There is a diverse and well-documented predatory arthropod community inhabiting managed turfgrass, and some studies have shown these existing populations to have beneficial impacts on pest populations,” Dupuy said. “In our studies using linear pitfall traps, we have seen adult billbugs wrapped up in spider’s silk with their insides sucked out. We’re not sure if this is opportunistic feeding — just because the spiders and billbugs are trapped together — or if the spiders are also feeding on billbug adults in the wild, but wolf spiders are definitely out there and they are abundant, so this may deserve more research.”
To keep predator numbers high, she suggests planting strips of bunch grass and flowering plants, which provide habitat for predators.
“Flowering plants are also an excellent attraction for parasitoid hymenopterans, some of which are known to parasitize billbugs,” Dupuy said.
Making predictions, filling gaps
Dupuy’s research group is also developing a model to predict the timing of billbug development in her part of the country, the Intermountain West, which is the geographic area surrounded by the Rocky Mountain, Cascade, and Sierra Nevada ranges. Such timing of billbug development is critical for insecticide application because the insecticides commonly used against billbugs are most effective against younger larvae that are still in the stems. This model, which is based on a temperature-tracking method called degree-days, can predict that specific point in time so managers can plan their insecticide application for maximum effectiveness.
“Temperature determines insect development, so if we have a cold spring, the best day to spray may be on June 1, but if we have a warm spring, the best day could be on April 1,” Dupuy said.
Dupuy’s group is also working to fill a large gap in knowledge of this insect.
“We are the only ones looking at billbug biology and management in the Intermountain West, and we’re also looking at entomopathogenic nematodes (insect-attacking roundworms) and the conditions under which they’re most effective against billbugs,” she said. “I think this work will go a long way toward both determining whether entomopathogenic nematodes are a viable control option for billbugs, and widening our regional knowledge of billbug biology.”
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Leslie Mertz, PhD, teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.