Summary of Best Management Practices for Annual Bluegrass Weevil Control

1. Regular scouting is essential for determining population structure, which in turn has major implications for product selection and timing. Adult monitoring is less labor intensive than scouting for larvae, though the latter is more useful for determining the potential for turf damage.
   
   
2. Phenological indicators, like the blooming of certain plants, help to define periods when more active sampling measures should take place in spring and can be effective determinants of when to apply both adulticides and larvicides. Unfortunately, no such aids are available to target later generations.
   
   
3. Soap flushing, vacuuming, pitfall traps and monitoring clippings can be effective practices to assess adult activity. Soap flushing is the most efficient method on fairway heights of cut, and is unaffected by temperature or time of day, making it optimal for monitoring adults in springtime. Vacuuming is most effective on greens and provides a means to rapidly estimate population density. Larval monitoring requires core sampling and is best performed in combination with a saline salt extraction.
   
   
4. Adulticide applications should be limited to the end of overwintering adult migration, prior to egg laying. The “half green, half gold” forsythia development point has been a reliable indicator of overwintering adult peak populations. Applications prior to peak have little value as few females are mated, most males are reproductively immature, and adult feeding causes only superficial damage.
   
   
5. Larvicides are most effective during the period when larvae move from the plant to the soil, and prior to crown feeding. This application requires precise timing to prevent significant damage. While phenological indicators aid in timing the first-generation treatments, accurately estimating what stages are present with larval populations in later generations requires soil sampling.
   
   
6. Biological controls, such as the use of insect-parasitic nematodes, offer a promising yet underutilized alternative to insecticides, often demonstrating potential for comparable or superior efficacy in reducing larvae. Challenges such as the variability of biological agents, specific storage and soil condition requirements, and the relative costs and availability hinder their widespread adoption in turf.
   
   
7. Cultural practices alone are unlikely to deliver acceptable population reductions, though they may assist in improving chemical control. Mowing putting surfaces may reduce adult populations by 26% to 38% each time, though most adults are removed alive and therefore attention must be given to where clippings are discarded.
   
   
    

Population Development and Monitoring

Successful ABW management centers around understanding pest biology, population structure and development. In spring, adults move from overwintering sites – e.g., leaf litter, tall grasses or wooded areas – to surrounding short-mown playing surfaces where they will mate and lay eggs (Diaz et al., 2007; McGraw et al., 2020). Eggs are placed by female adults between the leaf sheath and the stem of the plant. After eggs hatch, larvae bore into the stem where they will feed, largely protected from the outside environment and most insecticides. It has been observed that small larvae are motile and will damage up to a dozen stems during development (Cameron & Johnson, 1971a). Fourth- and fifth-instar larvae are generally too large to fit within the plant and will feed externally on the crown. As such, dense populations can cause extensive damage to the turf. The pupa, a dormant non-feeding stage, can be found in the soil-thatch zone. This stage appears to be naturally protected from most chemical controls as well. The time to develop from egg to adult, as well as the number of generations per year, is dependent on temperature. ABW possess one generation per year in northern populations and up to three in New York and New Jersey (Simard et al., 2007; Diaz et al., 2008; McGraw & Koppenhöfer, 2009). No detailed population studies have been conducted in southern or western regions of its distribution, but it stands to reason that more than three generations are possible given greater duration of optimal temperatures for development.

Complicating both the determination of the number of generations and control is that after the first generation in spring, populations become unsynchronized – i.e., stages from one generation overlap with the following. Additionally, adults are capable of depositing their eggs over several weeks if left uncontrolled in spring, leading to even greater asynchrony in stages and difficulty in assessing how many or which generations are actually present in summer (McGraw et al., 2020). Successful control programs are generally those that effectively reduce overwintering adults and/or their offspring, which are the first-generation larvae.

Adult Monitoring and Management in Spring

Traditionally, turfgrass managers have sought to prevent larval feeding by controlling adults following their migration onto short-mown areas (Cameron & Johnson, 1971b). This approach has several advantages since adults can be easily detected and adulticides are relatively inexpensive. However, adulticides are generally broadly toxic and lack residual activity. Several studies have documented overuse of pyrethroids leading to resistance in many populations (Ramoutar et al., 2009). The mechanism of resistance appears to be nonspecific and may reduce the effectiveness of non-pyrethroid adulticides, as well as larvicides (Kostromytska et al., 2018; Koppenhöfer et al., 2019). Therefore, it is important to limit pyrethroid applications in time and space as much as possible. This is most effectively achieved by developing a monitoring program to identify the end of the adult migration from overwintering sites. This signals “adult peak” on short-mown surfaces, and annual adulticide applications should be limited to this period.

Phenological indicators, such as the blooming of ornamental plants, are observable events that occur in a predictable sequence throughout the year, often in response to environmental changes like temperature and day length. Since insect development is connected to temperature as well, these indicators can be used to monitor for insects by providing a biological calendar to predict developmental events. Full bloom of the forsythia (Forsythia spp.) coincides with the initial migration of adults, marking the time to begin monitoring. Once the forsythia loses half of its yellow flowers (half green, half gold phase) or when eastern redbud is in full bloom, all adults will have emerged signaling the peak of the adult population when adulticides are most effective.

Plant phenology should not be the sole means of monitoring adults, but rather serve as a guide for determining when to initiate or intensify monitoring with more active means. Disclosing solutions like “soap flushing”, vacuuming the turf surface, monitoring clippings and linear pitfall traps are all effective means to determine the density of adults – as long as the technique is performed consistently. In a study comparing the first three sampling methods listed above, soap flushing was the most-efficient method and was unaffected by temperature or time of day, making it optimal for monitoring adults in spring (de Sousa et al., 2023). Vacuuming proved more effective on greens than fairways and is considered a viable alternative for scouting on greens. The efficiency of collecting adults in mower clippings varied with mowing height and temperature, with higher recovery rates on greens and during warmer temperatures.

Adults spend the winter in a state of dormancy by significantly reducing their metabolic rate. In doing so, fat stores and wing muscles are depleted, presumably to assist in survival (Vittum, 1980). Therefore, adults must walk from overwintering sites to playing surfaces in spring. Populations do not emerge on fairways or tee boxes simultaneously, but rather exhibit a steady increase in density over time. Spatial analyses of populations demonstrate that adults will walk through the edges of fairways and progress into the middle. Therefore, sampling the edges bordering overwintering sites in spring will give the best chance to detect an infestation (McGraw & Koppenhöfer, 2010).

Spring adulticide applications need to be made prior to oviposition (egg laying) but not too early in the migration that they miss adult peak, given that products have limited residual activity. Examinations of the reproductive and digestive systems of adults demonstrate three key factors affecting adult control. The first is that adults do not feed prior to the time when peak density occurs. Secondly, females emerge with well-developed reproductive systems, but few are inseminated prior to winter. Finally, male reproductive systems are undeveloped and do not reach maturity until adult peak has occurred (McGraw et al., 2020). Combined, these findings suggest little value in adulticide applications prior to adult peak and further emphasize the need to align applications with peak population density on short-turf areas.

Larval Management

Following adult peak, the focus for the rest of the season should be on larval monitoring and control. First-generation larvae and their resulting damage are most often observed on short-mown turf on the edges bordering overwintering sites (McGraw & Koppenhöfer, 2010). Behavioral and electrophysiological studies have suggested that weevils are hardwired to orient toward the volatiles emitted by annual bluegrass and creeping bentgrass, though damage may appear solely to annual bluegrass when the stand is mixed with bentgrasses (McGraw et al., 2011). This phenomenon has led to several studies examining host preference and tolerance. Spatial analyses suggest ABW oviposition behavior is not solely influenced by the presence of host plants, though damage is more likely to occur on annual bluegrass at lower densities (McGraw & Koppenhöfer, 2015). The findings that females may oviposit in areas not dominated by annual bluegrass suggest that ABW have flexibility in host plant selection, and that both annual bluegrass and creeping bentgrass can support larvae. In laboratory assays, females laid significantly fewer eggs in bentgrass than in annual bluegrass, and bentgrasses were less suitable for larval development (Kostromytska & Koppenhöfer, 2014). In follow-up studies, bentgrass species tolerated higher densities of ABW adults and larvae than annual bluegrass before showing any significant decrease in turf quality (Kostromytska & Koppenhöfer, 2016).

As with adults, larval activity can be predicted with phenological indicators in the spring and confirmed by more active measures as the season progresses. Most larvicides work best when larvae are exposed or outside the plant (Koppenhöfer et al., 2019). These are also the stages that inflict the most damage on turf, therefore it is important to precisely time applications. Rhododendron catawbiense in full bloom has been shown to be a reliable indicator of when populations are transitioning from third- to fourth-instar larvae. However, as with adult phenological indicators, this will not help with subsequent generations and more-active measures are required. Soil sampling with either a cup cutter, tubular plugger or knife can help locate larvae in the soil, though given their relatively small size they can often be overlooked. Combining soil sampling with an extraction method, such as placing cores into a saturated saline solution, can assist in irritating small larvae to force them from plants or soil. A hand lens or microscope may be needed to detect small larvae as they float to the surface. Commonly accepted damage thresholds in spring are 40 larvae per square foot, though pure annual bluegrass stands may require much lower thresholds to avoid damage (McGraw & Koppenhöfer, 2009).  

A broader range of insecticide classes are effective against larvae compared to the number of effective adulticides. However, within each class only a few active ingredients are truly efficacious. Koppenhöfer et al. (2019) found that cyantraniliprole (Ference), spinosad (Conserve), indoxacarb (Provaunt), and trichlorfon (Dylox) show variable effectiveness depending on the average larval instar at the time of application. Notably, earlier applications (at instar averages of 2.5 and 3.2) were generally more effective than later applications (at an instar average of 4.0). This indicates that timing – and therefore monitoring – is crucial for effective management.

Another key factor in control is the presence of populations that are resistant to certain classes of ABW insecticides, particularly pyrethroids like bifenthrin, deltamethrin and lambda-cyhalothrin (Table 1). Perhaps equally important is that the pyrethroid-resistance status of a population was shown to reduce efficacy in all tested larvicides except spinosad, demonstrating the need to minimize adulticide applications to reduce the potential for developing resistance.

Targeted Insecticide Applications

Several studies have examined the spatial distribution of adult and larval populations to better understand where damage is likely to occur. Overwintering adults can be observed migrating through rough and short-mown turf borders. Adults progressively move throughout large areas like fairways over the course of several weeks, though their offspring are commonly observed to be strongly aggregated along the short-mown edges (McGraw & Koppenhofer, 2010). This spatial pattern suggests that more-targeted control measures could be employed, potentially reducing the use of broad-spectrum insecticides and their associated costs and potential environmental impacts. Although targeted application techniques like “banding” or “looping” fairways have not been rigorously tested, their potential to reduce insecticide load compared to broadcast treatments is obvious. However, the adoption of these strategies is hindered by the low tolerance for damage and the unpredictable nature of pest outbreaks.

Alternatives to Insecticides

The development of pyrethroid resistance and secondary impacts to non-pyrethroid insecticides – like cross and multiple resistance – necessitates integrating multiple control tactics such as cultural, biological and biorational approaches to minimize turf damage. Biological control, or the use of natural enemies such as predators, parasites and pathogens, is viewed as a nontoxic alternative to managing insect pests, potentially with lasting effects. To date, no specific natural enemies have been observed impacting ABW populations, hindering the development of long-term control programs. However, generalist entomopathogenic or insect-parasitic nematodes (EPN) and fungi (EPF) have been observed impacting natural populations (Grant & Rossi, 2004; McGraw & Koppenhöfer, 2009). EPN and EPF have been shown to reduce larvae in laboratory and/or field studies, often comparable to or greater than levels observed with chemical insecticides (McGraw & Koppenhöfer, 2008; McGraw et al., 2010; Clavet et al., 2014). Both may be applied through standard spray equipment and often in combination with other chemicals or fertilizer. However, biological control agents are living organisms and do not have the same tolerance to storage conditions as insecticides, and also require certain soil conditions to be most effective. Other limitations to the adoption of biological controls include the inherent variability of living organisms in biological systems, the availability of a reliable source and the overall cost relative to insecticides.

Cultural control refers to altering common turf management practices to negatively affect pest populations and represents a potentially sustainable approach to managing ABW. Of the primary cultural practices like mowing, irrigation or nutrition, manipulating ABW populations through mowing has been given the most attention. It is common to observe damage in collars while adjacent putting surfaces composed of the same turfgrasses remain undamaged. These observations led to studies examining the influence of mowing on ABW oviposition behavior and survival. Greenhouse reel mower studies suggest between 26% and 38% of adults are removed at low mowing heights (0.100 inch), and the effect diminishes with increasing heights of cut (Czyzewski & McGraw, 2017). Surprisingly, most adults survive the act of mowing. Therefore, it is important to discard clippings far from high-value turf areas, as adults are capable of walking significant distances, often greater than 100 yards. Adults that are allowed to oviposit into putting greens that are infrequently mowed can produce larvae that become late instars and cause damage. Fairway- and collar-height mowings remove negligible amounts of adult ABW. These studies suggest that while greens mowing can be a part of ABW management, it should be integrated with other control measures.

Conclusion

ABW presents a complex and evolving challenge for turfgrass management on golf courses across eastern North America. Effective management of ABW populations hinges on a comprehensive understanding of their life cycle, behavior and the environmental conditions that influence their development and spread. Increased research devoted to the problem has significantly improved our ability to predict and mitigate ABW damage. However, the increasing instances of insecticide resistance among ABW populations underline the urgent need for non-chemical approaches. While targeted insecticide applications remain a necessary component of ABW management, there is a clear imperative to reduce reliance on chemical controls and to explore and expand the use of alternative methods. Future research should continue to focus on understanding the nuances of ABW biology and ecology, particularly in relation to its rapid spread and evolving insecticide resistance.

References

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Cameron, R.S., & Johnson, N.E. (1971b). Chemical control of the “annual bluegrass weevil,” Hyperodes sp. Nr. Anthracinus. Journal of Economic Entomology, 64(3), 689-693.

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de Sousa A.L., Kostromytska, O.S., Wu, S., & Koppenhöfer, A.M. (2007). Optimizing sampling technique parameters for increased precision and practicality in annual bluegrass weevil population monitoring. Insects, 14(6), 509.

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Diaz, M.D., Seto, M., & Peck, D.C. (2008). Patterns of variation in the seasonal dynamics of Listronotus maculicollis (Coleoptera: Curculionidae) populations on golf course turf. Environmental Entomology, 37(6),1438-1450.

Grant, J.A. & Rossi, F.S. (2004). Evaluation of reduced chemical management systems for putting green turf. USGA Turfgrass and Environmental Research Online, 3(4), 1-13.  

Koppenhöfer, A.M., McGraw, B.A., Kostromytska, O.S., & Wu, S. (2019). Variable effect of larval stage on the efficacy of insecticides against Listronotus maculicollis (Coleoptera: Curculionidae) populations with different levels of pyrethroid resistance. Crop Protection, 125: https://doi.org/10.1016/j.cropro.2019.104888

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Kostromytska, O.S., & Koppenhöfer, A.M. (2016). Responses of Poa annua and three bentgrass species (Agrostis spp.) to adult and larval feeding of annual bluegrass weevil, Listronotus maculicollis (Coleoptera: Curculionidae). Bulletin of Entomological Research, 106(6), 729-739.

Kostromytska, O.S., Wu, S., & Koppenhöfer, A.M. (2018). Cross-resistance patterns to insecticides of several chemical classes among Listronotus maculicollis (Coleoptera: Curculionidae) populations with different levels of resistance to pyrethroids. Journal of Economic Entomology, 111(1), 391-398

McGraw, B.A., & Koppenhöfer, A.M. (2008). Evaluation of two endemic and five commercial entomopathogenic nematode species (Rhabditida: Heterorhabditidae and Steinernematidae) against annual bluegrass weevil (Coleoptera: Curculionidae) larvae and adults. Biological Control, 46(3), 467-475.

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McGraw, B.A., Rodriguez-Saona, C., Holdcraft, R., Szendrei, Z., & Koppenhöfer, A.M. (2011). Behavioral and electrophysiological responses of Listronotus maculicollis (Coleoptera: Curculionidae) to volatiles from intact and mechanically damaged annual bluegrass. Environmental Entomology, 40(2), 412-419.

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