Wondering about the weevil

Superintendents have been dealing with the annual bluegrass weevil (ABW) for more than three quarters of a century. As spring approaches, turf pros ready for the pest’s annual emergence, which is especially problematic in the Northeastern US and along the Eastern Seaboard.

Rutgers University entomologist Dr. Albrecht Koppenhofer, considered one of the world’s foremost ABW authorities, has spent considerable time studying the pest. In fact, Koppenhofer theorizes there is a distinct strain of the insect that proliferates on golf courses.

“My theory – but it’s a pure theory – is that the insect at some point adapted to the golf course environment, and so there is the ‘golf course strain’ if you want to call it that,” he says. “But, again, that’s just a theory.”

ABW was first identified as a golf course pest in Connecticut circa 1931. But records indicate it was prevalent in other areas apart from the golf course environment prior to that, particularly in wet areas such as riverbeds. As a golf course pest, however, the ABW is concentrated in portions of southern Canada, New England, the Northeast and along the Eastern Seaboard, south into Virginia. It has also migrated westward and southwestward into West Virginia and Ohio, and certain mountainous inland areas of North Carolina. How and why this migration has occurred is a matter of conjecture.

Dr. Ben McGraw, a Penn State University plant entomologist, says ABW migration could be the result of natural movement, as well as unintended movement via equipment, sod or even golfers.

McGraw, however, shares Koppenhofer’s theory. “We might be witnessing something of a speciation event,” he says, “or maybe not even going that far, two different races of the same insect, one that might feed on plants that aren’t important to (the golf industry) and another that has evolved to feed on golf course turfgrasses.”

ABW thrives on closely mown annual bluegrass (Poa annua) and creeping bentgrass, both of which are prevalent in the golf course environment in the Northeast and in the Transition Zone farther south. But it has less impact on grasses with a higher height of cut such as Kentucky bluegrass, perennial ryegrass or tall fescue.

It spends the cold-weather months essentially in a state of suspended animation, in areas shielded from the worst of the winter weather, such as tree lines, in or around leaf litter or under shrubbery. Research has shown that the adult insect will situate itself in a protected area up to one-inch deep in the soil.

It emerges in early spring, and climate and topography dictate exactly when. But the University of Rhode Island’s Dr. Steven Alm notes other natural clues as to when the insect will appear, and when superintendents need to begin treating.

“In Rhode Island, peak emergence of adults is normally between forsythia and flowering dogwood full bloom – approximately April 23 to May 7,” he says. “However, the first adults can be seen as early as a month before forsythia full bloom depending upon the temperature in any given year … The adults overwintering in the leaf litter will be ready to emerge as soon as temperatures are favorable for development.”

The pest migrates by walking; its wing muscles weaken during the winter, so when it emerges from suspended animation it cannot fly. Instead, it treks across the turf, be it a tee box, a fairway or green, or even a green collar.

A few weeks after they emerge, the adults begin laying eggs, which hatch into larvae. Depending on location, there will likely be two generations per season and perhaps three or four. The adults fly back to their winter home before the cold weather returns.

While superintendents strive to control the adult population (to prevent them from laying eggs), the larvae are the greatest concern. They thrive within and adjacent to the plant stem and can wreak havoc.

While the ABW larvae also feeds on creeping bentgrass, particularly in areas where annual bluegrass isn’t as common, Koppenhofer notes Poa is far more susceptible to damage.

“Annual bluegrass basically gets wiped out by this insect,” he says. “In the center of the plant, if the crown is killed, that plant dies and then the larva moves on to the next one. Whereas with creeping bentgrass you might kill that one plant, but then you have the rhizomes and stolons connecting all the various plants and so the plant is much more able to draw resources from other areas … So not only can it tolerate more before it shows damage, but it can also recover much better.”

Opinions vary considerably on if and how the ABW should be eradicated or simply managed. Some superintendents rely on repeated applications of pyrethroid-based insecticides. For some time, these products proved effective against ABW and a broad range of other pests. They are also inexpensive, and thus very popular among superintendents concerned about their bottom line.

Over time, however, ABW has built up an increased resistance to pyrethroids, rendering them less effective. “Some populations are quite resistant to pyrethroids now,” Alm says. “Dr. Darryl Ramoutar, working in my lab, determined there was resistance to two of the synthetic pyrethoids. Resistance is often a problem with insects that have a high reproductive potential and multiple generations per year, which the ABW has.”

Koppenhofer contends resistance problems are often created by superintendents, some of whom overspray at the first sign of ABW. “They just start spraying and go nuts, and in the process that gets the insect more and more resistant,” he says. “There are populations where there are only a few (products) that still work.”

There are various strategies and philosophies for managing ABW and each golf facility is always a unique environment. Alm advises superintendents to be on the lookout for the pest and to get outside help when it’s needed, particularly if they are confronting ABW for the first time. “First, be on the lookout for any yellowing Poa annua,” he says. “Take cup changer cores and either submerge the cores in a saturated salt solution (four cups of salt in one gallon of warm water) and look for floating adults and larvae; or take damaged plugs to your local turfgrass entomologist for evaluation.”

Rick Woelfel is a Philadelphia-based writer, and is a frequent GCI contributor.