Annual Bluegrass Weevil
Some turf-damaging insects just look like bad news. Case in point is the long-hated annual bluegrass weevil (ABW), a highly destructive pest that has been plaguing professional turf managers since the 1930s. While initially only devasting turf in the northeastern United States, over time they have now migrated into the Midwest and southern parts of the country…making them a serious threat to most turf situations.
Originally identified as the Hyperodes weevil, the ABW, Listronotus maculicollis, is a native beetle that is most prevalent and injurious in low-cut, high maintenance turf such as golf course greens, tees and fairways. As its name suggests, ABW larvae and adults feed primarily on annual bluegrass, but also feeds on creeping bentgrass and perennial ryegrass. It’s an especially aggressive invader of newly stands of creeping bentgrass. In areas where annual bluegrass is prevalent, high populations of weevils will cause substantial areas of dead turf that affect both the visual and functional quality of golf course turf.
Biology and Identification
The ABW has a complete life cycle with 1-3 generations per year possible, depending on location. The adults are small, compact and differ in color from black to gray. Adults are about 1/8 inch long with a characteristic weevil snout. Their body is covered with fine hairs and scales, which are easily observed under magnification. It is difficult to distinguish between male and females. When the adult emerges from the pupal stage they appear reddish, however, their outer shell will harden and the hairs and scales wear off and the weevil appears charcoal-gray in color. The eggs of the ABW are small and oblong, which can be found in leaf sheaths of the grass plant. Initially, the eggs are white in color, but they soon turn gray. Once the larvae emerge, they are legless, with a white body and dark brown head. The older larvae appear slightly curved, but not nearly as C-shaped as other white grubs. Pupae look similar to an adult ABW, but are smaller in size and have a reddish- brown color that gradually darkens.
Damage and Symptoms
Damage often is first noticed in the perimeter of greens and fairways that support a high population of Poa annua. Most of the damage is caused by the larval stage and can go unnoticed for some time. Damage to plants begins when the adult female chews into the outer sheaths of the grass blade and lays her eggs between sheaths. This process may just weaken the plant and cause yellowing, but rarely will it kill a plant. When the eggs hatch, the larvae feed on the stem and later instars move into the crown tissue. When crown feeding occurs, the turf can be easily pulled from the soil. The hollowed grass stem is the key diagnostic feature. As more eggs hatch, more severe damage can occur and the damage associated with the fifth instar often is the most destructive. At this point, the turf will appear to be under severe drought stress due to damaged stems. Turf will appear purple before it turns brown and dies-out. Significant damage generally becomes obvious in late May or early June, and often is mistaken for other problems.
Monitoring Techniques and Thresholds
There are a couple of techniques that are recommended to help identify the level of infestation of the ABW. The threshold for damage early in the season is 30 to 80 larvae per square foot. As the season continues and turf becomes stressed, this threshold reduces as the number of ABW larvae needed to damage the turf becomes significantly less.
- Run a reverse air flow leaf blower with mesh netting attached to the front of the nozzle across the turf surface. Note that this technique does not appear to work in areas with lower ABW populations.
- Cut a wedge of turf with a knife and inspect the area where the turf and thatch layer connect.
- Flush an area of turf with a soapy water mixture (1 oz. dishwashing soap to 2 gallons of water). This will bring the adults to the surface.
- Cut out a 3” x 3” x 3” section of turf and submerge it into a saltwater solution (3/4 cup salt to 1 quart of water). This will also bring the adults to the surface.
- Black light traps can also be use to attract adults and assess the population.
Cultural control options include proper nutrition and irrigation, which often mask the symptoms. One of the most effective cultural control methods is to convert from a susceptible turfgrass species to one that is tolerant or resistant to infestation. The ABW over-winters in adjacent tree litter and leaves and removing this litter may help to reduce populations.
Biological control options are somewhat limited and varied effectiveness. Some research has shown that the beneficial nematode Steinernema carpocapsae can reduce annual bluegrass weevil populations by almost 50%. Bacillus thuringiensis, a microbe naturally found in the soil, makes a protein that is toxic to ABW larvae and has been used to reduce larval populations by 50-65%. Spinosad, a natural substance made by a soil bacterium that can be toxic to insects, can also be very effective (80% control) against larvae but only when used at a high rate. As with all control options, accurate timing is essential to successfully reducing populations.
Chemical controls options are the most reliable and widely used. There are multiple professional insecticide products available that are labeled for ABW suppression and control. Choose a relatively insoluble insecticide that stays in the thatch where adults are active. Synthetic pyrethroids (Bifenthrin, Cyfluthrin, lambda-Cyhalothrin, Deltamethrin) are the best options. Water in the application lightly to move the material off the leaves. Widespread applications are usually not necessary. It should be sufficient to limit applications to periphery sprays along historically susceptible turf areas.