Turf Magazine - June, 2009

TURF SCIENCE

Annual Bluegrass Weevil

By Don Dale

It’s a native insect that has taken a liking to annual bluegrass. It’s a pest that started small in the New York City area and has moved outward, and it’s so damaging to its host plants that a turfgrass manager will want to get rid of it as quickly as possible.

So says someone who has been studying the annual bluegrass weevil (commonly referred to as ABW) for 30 years. Pat Vittum, professor of turfgrass entomology at the University of Massachusetts, Amherst, says that the 1/8-inch brown insect with the familiar weevil snout has become a tremendous nuisance. The adult nibbles on Poa annua foliage, but it’s the larval stages that really do a lot of damage.

Photos by Masanori Seto.
Larvae, here emerging from the soil, are the most damaging to turfgrass.
Adult annual bluegrass weevils add to the damage done by larvae.
Damage to Poa annuacan be extensive from the annual bluegrass weevil, but controls are effective.

The adults deposit eggs in Poa annua between the outer leaf sheath and the stem, and the relatively straight, white larvae feed until they ultimately chew their way out. Then they continue feeding on the crown of the plants. They will also feed a bit on bentgrass, but they prefer annual bluegrass, even when it is cut short. The turf manager will see patchy damage that resembles that made by anthracnose, and in New York state, two to four generations can emerge.

“So you can have larvae feeding all summer long,” Vittum says. Once there are overlapping phases of larvae and adults feeding, the damage can be extensive. It is first noticed around the edges of the turfgrass because ABW adults overwinter in surrounding leaf litter and walk in to feed in the spring.

Vittum notes that there are several effective treatments. The primary controls for ABW for many years were the pyrethroid chemicals, but in areas where they have been used extensively, resistance to the insecticides has developed. In some areas there is up to 95 percent resistance in weevil populations. She still lists the pyrethroids in her controls because in the pest’s outer range—as far south as Maryland and north into Canada, with sightings in about 40 states—they can be effective.

The old standbys for her are bifenthrin (Telstar), cyfluthrin (Tempo) and cyhalothrin (Scimitar or Battle). The first application is made to curb-emerging adults, with a handy timing device in New York being the period after the full bloom of local forsythia bushes when the plants are acquiring green leaves. Repeated treatments may be needed, and it is smart to rotate labels to avert resistance as much as possible.

Another methodology is to attack the larvae after hatching, Vittum says, and there are several options here. One is the use of trichlorfon (Dylox), a wettable powder, as soon as you start seeing the wilt on Poa from ABW, Vittum says. She notes that in the New York City area, that will typically be around the first of June. Treatment should be repeated as new larvae emerge and foliage wilts, about three weeks apart.

She has also tested the use of flowable spinosad (Conserve) in place of trichlorfon, and found that it works when used in the same methodology, but she doesn’t recommend it as consistently, even though it may be preferred because it is a synthetic, bio-based chemical.

A wettable powder approved in 2007, indoxacarb (Provaunt), is a new class of material that affects the insect’s nerve function and is highly efficacious, Vittum says. This larvae killer may also have to be applied more than once as subsequent generations emerge during the summer. Again, the university’s recommendation is that managers rotate usage of these larvae controls to avoid long-term development of resistance.

Another new chemical, approved last year, is chlorantraniliprole (Acelepryn). Vittum says this affects muscle control in both adult and larval ABW, but has low toxicity to humans, pets and wildlife. She has tested it for three years, and it has proven especially deadly on the larvae.

“I’m very excited about this one,” she says of chlorantraniliprole, because it is the only treatment that has some carryover effects. A systemic pesticide, one application definitely will kill two generations of the pest and may be good for the entire season. This is the first year it has been widely available.

Vittum says that there is one other chemical approach to ABW control, and that is compounds that are a combination of two products. The first is Aloft, which is a mix of a neonicotinoid and a generic bifenthrin. The other is Allectus, a blend of a neonicotinoid and the bifenthrin Telstar. She says that neonicotinoids by themselves are not efficacious.

She says there are also some cultural methods that can help avert ABW through management. In general, they consist of minimizing either the amount of Poa, or minimizing the amount of stress to the Poa.

Another way to reduce the number of emerging adults in the springtime is to remove their winter shelters. This is usually plant litter where adults overwinter by burrowing under leaves or other detritus and gaining some protection from the cold. The litter under white pines has been found to be a favorite hiding place, so particular attention should be paid to removing it.

Nowadays, the weevil is a primary spring pest in a wide area on the Eastern Seaboard and beyond, and a damaging one, but it can be controlled, particularly with the use of the newest insecticides.

Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.