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Cover Story
The 411 on Annual Bluegrass Weevils
By Gram Grant and Terri Billeisen, Ph.D., Turfgrass Entomology Lab, North Carolina State University
Introduction
The annual bluegrass weevil (ABW; Listronotus maculicollis) is an insect pest of cool-season turfgrasses, primarily annual bluegrass (Poa annua) and creeping bentgrass (Agrostis stolonifera). Native to the United States, ABW was first detected causing damage on golf courses in Connecticut in 1931. Since then, ABW has caused damage in managed turfgrass in the Midwest and southeastern US, as well as five provinces in Canada (Fig. 1).
ABW adults are small (⅛ in length), black or dark-brown beetles with a long, curved proboscis (Fig. 2), similar to that of other weevils. Adults are often confused with hunting billbugs as the elongate body shape and proboscis look similar among weevil species. However, ABWs are less than half of the length of an adult hunting billbug which measures 0.3–0.4 inches (Fig. 3). Additionally, the snout of ABWs is shorter and thicker than that of hunting billbugs. ABW larvae are small and legless with a cream-colored body and a brown head.
Distribution and Damage
ABW damage is most common in short-mown turf, typically fairways, collars, approaches, and tees on golf courses. Damage to greens is possible but uncommon as fewer larvae are present (Czyzewski and McGraw, 2017). Adults overwinter in fairways and, depending on location, emerge from deep in the soil profile from early to mid-February in North Carolina. Although capable of flying very short distances, ABW moves primarily by walking the turfgrass surface at night, approximately one hour after sunset. In addition to walking, ABW also can move around the course on mowing equipment.
Although ABWs are known primarily for damage to annual bluegrass, they also can cause wilting and yellowing in creeping bentgrass, which can be slower to show damage symptoms. Preliminary trials have indicated that ABW can survive on warm-season turfgrasses, including bermudagrass ( Cynodon dactylon ) and zoysiagrass ( Zoysia spp.), but damage potential in these turf species has not yet been evaluated. During this period, adults will feed on leaf tissue and females will chew a small hole in the stem where eggs are laid. After hatching, larvae develop through five larval instars before pupating. Early-instar larvae tunnel throughout the stem of the turf plant to feed. As they grow larger, larvae exit the stem and feed on the crown and roots of the turf. Damage from this feeding may begin as yellow spots on the turf ( Fig. 4 ) that expands as the season continues ( Fig. 5 ). ABW adults and larvae feed on turfgrass from February to October although damage in North Carolina is most common and severe mid- to late May. ABW completes three generations in North Carolina, when adults move deep in the soil to overwinter in November-December.
Monitoring
While recommendations for monitoring and application timing for ABWs often rely on plant phenological indicators such as bloom periods for Forsythia spp. and flowering dogwood (Cornus florida) in the northeastern US, inconstant and fluctuating spring temperatures in North Carolina render this strategy ineffective. Weekly monitoring for ABW is essential for managing infestations. Traditional monitoring efforts focus on both adult and larval collection, applying control products when either life stage reaches a threshold for treatment. In North Carolina, larvae are active deep in the soil profile (5+ inches) compared to northern sites and are recovered from closer to the soil surface only in areas of moderate-severe damage. NC ABW monitoring programs, therefore, focus on tracking weekly adult activity and using peaks to determine application timing.
Although ABW can be monitored via vacuum sampling, pitfall traps or hand collection, the easiest method for quantifying ABW adults in a fixed area is using the soap flush technique. Soap flushing forces mobile insects to the soil surface by flooding the top few inches of the soil with a soapy liquid that is irritating to the insect exoskeleton. Conduct a soap flush by mixing a dyed and heavily-scented, such as a yellow “lemon” or green “laundry” scent, dish detergent with water and apply to turf via bucket or watering can. Thoroughly soak the area in both damaged and adjacent non-damaged areas and wait 2–3 minutes. If present, adult ABWs will appear on the surface, climbing up and clinging to the tips of leaf blades.
Treatment
Pyrethroids (bifenthrin, lambda-cyhalothrin) are most effective against ABW adults while anthranilic diamides (chlorantraniliprole, cyantraniliprole) and an insect growth regulator (novaluron) are more effective against the larvae. Each of these insecticides may be used in a control program to target different life stages. Pyrethroid applications targeting adults should be timed when adult activity peaks.
Similar to other multivoltine insects in turfgrass, such as chinch bugs, repeated applications throughout the year can result in resistance issues if managers do not rotate insecticide classes. Resistance in ABW resulting in field control failures can build in as short as five years when the same mode of action is used exclusively and repeatedly. Once resistance has been established, the use of the pesticide that led to resistance is ineffective and can no longer be used. Furthermore, pyrethroid resistance can also lead to reduced efficacy in other insecticide classes by as much as 57% (Koppenhöfer et al. 2012) For control of ABW, pyrethroid resistance is the greatest management concern as they provide the highest and most consistent level of adult control.
Biological options for ABW control are very limited. Previous research has indicated entomopathogenic nematodes can reduce ABW larval populations by as much as 50% (McGraw and Koppenhofer, 2009), but fail to provide a consistent level of control. Products containing Bt can also be efficacious against ABW but, similar to nematodes, results will vary. Biological organisms used for pest control are highly sensitive and efficacy of these products are reduced when applied in direct sunlight, in dry conditions and high temperatures.
Using cultural control techniques, such as selecting less susceptible turfgrass species and altering mowing height can help mitigate ABW damage. ABW damage is more prevalent in annual bluegrass stands compared to creeping bentgrass.
This appears to be less about host feeding preference; rather, creeping bentgrass can tolerate more feeding before showing symptoms of damage. Mowing height can also affect the incidence and severity of ABW damage. Damage to greens is rarely reported as low mowing height is not optimal for feeding and oviposition. As mowing height increases from green height to fairway height, reports of damage become more common.
As we move into spring and begin to see warmer temperatures, it is important to start monitoring for ABW infestations. Applying soap flushes and keeping tracking of adult counts each week provides an effective way to monitor adult activity and apply adulticides when peak adult activity is reached. Remember, May is when we observe some of the most severe ABW damage in North Carolina, so stay vigilant in the coming weeks. For more information, including a template for recording adult counts from soap flushing, be sure to visit NC Turf Bugs.
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ABW FAQs
1. How quickly after an application can I expect to see a reduction in adults?
A. It depends on which active ingredient you are using. After applying a product that controls adults (think: pyrethroids), you should see a decrease in the number of adults you find in your next soap flush. If it is a product that is better for larval control, you may or may not see a decrease in adult numbers, depending on where the insect is in its life cycle.
2. I am just now seeing ABW in my fairways and haven’t managed them before. How often should I expect to spray for them?
A. The number of applications will fluctuate from yearto-year. In NC, courses with established ABW populations spray anywhere from two to seven times per year. In some years, more control is necessary and other years are quieter. The key is to keep up with the weekly tracking so you can anticipate problem areas as early as possible.
3. Do ABW tend to pop up in the same areas from year to year?
A. Yes and no. Yes, they will cause damage in the same areas so make sure to focus your monitoring efforts in areas of past damage. However, they can also spread to new areas within a course either by walking or short flight. Keep an eye out, especially in the spring and early summer, for areas where the turf looks stressed and put out a few soap flushes. It’s easy to rule out the possibility of ABW damage with a quick soap flush!
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References
McGraw. B.A., Koppenhofer, A.M. 2009. Population dynamics and interactions between endemic entomopathogenic nematodes and annual bluegrass weevil populations in golf course turfgrass. Applied soil ecology 41 (1), 77-89.
Koppenhöfer, A. M., Alm S. R., Cowles R. S., McGraw B. A., Swier S., and Vittum P. J. 2012. Controlling annual bluegrass weevil: optimal timing and rates. Golf Course Management, March 2012, 98–104.
Albrecht M. Koppenhöfer, Kostromytska, O. S., & Wu, S. 2018. Pyrethroid-resistance level affects performance of larvicides and adulticides from different insecticide classes in populations of Listronotus maculicollis (Coleoptera: Curculionidae). Journal of Economic Entomology, 111(4), 1851-1859.
Czyzewski, B. D., & McGraw, B. A. 2017. Mowing height influences Listronotus maculicollis (Coleoptera: Curculionidae) oviposition behavior and mechanical removal from golf course putting greens, but not larval development. Journal of Economic Entomology, 110(5), 2165-2171.
