From the March 2017 issue of GCM magazine:
A survey of annual bluegrass weevil management
Annual bluegrass weevil is spreading to new regions at the same time more populations are becoming resistant to currently available insecticides.
A fifth-instar larva (top) and an adult annual bluegrass weevil.
Photos by B.A. McGraw
Benjamin A. McGraw, Ph.D., and Albrecht M. Koppenhöfer, Ph.D.
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The annual bluegrass weevil (Listronotus maculicollis Kirby) is the most difficult to control insect pest of short-mowed golf course turf in the northeastern United States and eastern Canada. Annual bluegrass weevil was first isolated from damaged turf on Long Island in 1957, and until about 1990, was concentrated around the metropolitan area of New York, including northeastern New Jersey and southwestern Connecticut (7). The pest has consistently expanded its range of impact over the past decades, with infestations now reported from the southern parts of Quebec and Ontario in Canada south through western North Carolina and west to eastern Ohio.
Annual bluegrass weevil larvae can cause severe turf damage on tees, fairways, greens and collars that have high percentages of annual bluegrass (Poa annua L.), as this grass is particularly attractive to egg-laying females and has low tolerance to larval feeding (5,6). Females place eggs between the leaf sheaths or inside the stem of the turfgrass plant. The first through third larval instars feed within the grass stem, causing the central leaf blades to yellow and die. The third instars eventually exit the stem, and the fourth and fifth instars feed externally at the soil/thatch surface. The late-instar larval feeding damage is most severe, because it damages the apical meristem of the turfgrass plant.
Presently, chemical control is the only effective strategy for annual bluegrass weevil management (11). Consequently, turfgrass managers often overuse broad-spectrum insecticides, primarily pyrethroids used to prevent adults from laying eggs. Not surprisingly, pyrethroid resistance has been reported (9) and seems to be on the rise (McGraw and Koppenhöfer, unpublished data). Effective alternatives to pyrethroids include the larvicides chlorantraniliprole and cyantraniliprole (class: anthranilic diamides), spinosad (class: spinosyns), indoxacarb (class: oxadiazines), and trichlorfon (class: organophosphates) (4). However, pyrethroid resistance in annual bluegrass weevil seems to be, at least in part, due to enhanced enzymatic detoxification (10), a rather nonspecific mechanism that breaks down most active ingredients before they can reach their targets in the organism. As a result, most of the insecticides presently available seem to be less effective against resistant annual bluegrass weevil populations (4).
Our ultimate goal is to develop optimal management recommendations for annual bluegrass weevil populations with different levels of insecticide resistance. To this end, we are conducting laboratory and field studies to better understand the scope of resistance in annual bluegrass weevil (degree of resistance to different insecticide classes, stages affected). It is also important, however, to gather information from turfgrass managers to better understand the geographic spread and severity of the pest, incidence of resistance, and current monitoring and management practices.
Figure 1. Distribution of the Listronotus maculicollis (annual bluegrass weevil) management survey responses.
Materials and methods
A Google document survey was created in fall 2014 to capture regional trends in annual bluegrass weevil management, and to help us understand the severity and extent of damage throughout the area where the pest is currently causing problems on golf courses. A link to the survey was sent to golf course superintendents through a variety of avenues, including social media (for example, Twitter), hard copies distributed at educational conferences, and emails from local GCSAA chapters. The survey was made available from Nov. 14, 2014 through the end of January 2015.
The survey consisted of 26 questions, which could be grouped into three general categories:
- Local and regional damage. Turfgrass areas damaged (for example, greens, tees, fairways), number of damaged areas and seasonal occurrence of damage
- Annual bluegrass weevil management. Number of insecticide applications, total insecticide budget, pyrethroid use, and suspected (or confirmed) development of insecticide-resistant populations
- Integrated pest management (IPM) practices. Scouting techniques, spot-treatment frequency, information acquisition, and influence of university recommendations on management decisions
The majority of surveys (79.5%) were completed by superintendents of 18-hole facilities. Therefore, budget values were transformed into 18-hole equivalents for reporting several economic statistics. The proportion of insecticide budgets within the annual maintenance budgets was calculated only from the responses of the 18-hole golf courses, because turf managers were asked for a range of maintenance budgets rather than an exact dollar amount. The responses for the number of damaged turf areas (for example, greens, tees, fairways) were also converted to 18-hole equivalents before using descriptive statistics. Differences between pyrethroid-susceptible and “resistant” courses in number of areas (green, tee, fairway), insecticide budget, applications to control annual bluegrass weevil, and pyrethroid use were analyzed by non-parametric rank-sum tests. All data were transformed into 18-hole equivalents before analysis.
Results and discussion
Table 1. Regions and subregions used for categorizing survey data for Listronotus maculicollis geographically, number of completed surveys by region, and total golf facilities in the region (percent surveyed).
Responses were collected from 293 golf courses in 14 states and two Canadian provinces during the two months the survey remained open (Figure 1). The survey was completed by an estimated 5.6% of the 5,197 facilities in the surveyed area (2; GCSAA, personal communication). The survey required specifying a location at the state or province level (minimum), although more detail was encouraged. Most respondents provided city and state/province data, which allowed us to develop detailed survey maps and divide the responses into regions in most instances (Table 1). The majority of responses (86%) came from courses located in Pennsylvania (25%), New York (19%), New Jersey (11%), Massachusetts (11%), Virginia (11%) and Connecticut (9%). Data were grouped by regions where n > 20, which tended to be by state, or were further subdivided where there were enough responses and it made geographical sense (that is, eastern vs. western Pennsylvania, and Long Island vs. upstate New York). Delaware and Maryland had very few responses and were combined with Virginia. The western (Ohio, West Virginia) and northern peripheral states and provinces (Maine, New Hampshire, Vermont, Ontario, Quebec) were the only regions included in the analyses that had fewer than 20 responses. In total, eight distinct regions and four subregions are represented in the descriptive statistics.
Local and regional annual bluegrass weevil damage
Only surveys that indicated annual bluegrass weevil was a pest (90%) were used in reporting damage incidence to select turf areas. To better understand the average extent of damage across a site and to best gauge the total amount of turf that requires insecticide treatment, we asked superintendents to determine the average number of turf areas (for example, number of tees) that experienced damage in a year. The average course experienced damage to 6.6 fairways, 5.7 tee boxes and 6.4 greens/collars (Table 2). A survey of GCSAA members (3) estimated that an 18-hole golf course consists of 30 acres (12.1 hectares) of fairways, 3 acres (1.2 hectares) of tees, and 1.06 acres (0.43 hectare) of greens/collars. Using these figures as a base, we estimated that, on average, 10.8 acres (4.4 hectares) of fairway, 0.95 acre (0.38 hectare) of tees, and 1.06 acres of greens/collars, for a total of ~12.8 acres (~5.2 hectares) require protection on an 18-hole golf course.
Damage by turf area
Annual bluegrass weevil damage was reported to occur in all turf areas, including areas where turfgrass managers rarely report damage to university personnel (for example, greens and roughs). Damage was most common on fairways (69%) and collars/aprons (58%) (Table 2). Only 10% of respondents reported damage to roughs.
†G = greens, T = tees, C/A = collar/approaches, FW = fairways, R = roughs.
‡G/C = Greens/collars, T = Tee, FW = Fairway. Transformed to an 18-hole equivalent; no responses removed from statistics.
§Percentage of respondents who did not report damage to any area
//Average putting green height-of-cut in inches (mm)
Table 2. Listronotus maculicollis (annual bluegrass weevil) damage to turfgrass by location on golf course and by season.
The incidence of damage to putting greens (26%) was unexpectedly high given the low mowing heights reported across the region and the traditionally high intensity of insecticide use in these areas. Average putting green mowing height in the region varied little, ranging from 0.109 inch (2.77 mm) in western Pennsylvania to 0.126 inch (3.20 mm) in the northern peripheral states and provinces. Two thirds (66%) of the courses that reported annual bluegrass weevil damage to greens had heights of cut greater than the average, which was 0.119 inch (3.02 mm) across all survey responses (Figure 2).
Regions with the highest incidence of damage to putting greens included Connecticut (40%), northern peripheral states and provinces (38%), and New Jersey (31%) (Table 2). The average height of cut on putting greens in these areas (0.120 to 0.126 inch [3.05 to 3.20 mm]) was greater than the survey average. The lowest incidence of annual bluegrass weevil damage to greens was reported on Long Island (11%) and in western Pennsylvania (16%), which were also the two regions with the lowest average height of cut for greens (0.110 and 0.109 inch [2.79 and 2.77 mm], respectively).
Management of annual bluegrass weevil populations
Figure 2. The percentage of surveyed golf courses categorized by putting green mowing height (A), and those that experienced Listronotus maculicollis (annual bluegrass weevil) damage to putting surfaces (B) by mowing height treatment.
The annual maintenance budgets for all surveyed golf courses in the region demonstrated peaks between $1 million and $1.5 million and between $350,000 and $550,000 per year. Annual insecticide budgets averaged $9,270 and ranged between $50 and $75,000 per year. The average annual insecticide budgets, when adjusted for an 18-hole equivalent, ranged between approximately $4,000 and approximately $17,000 in subregions that submitted enough responses to summarize (Table 3). Connecticut ($14,627), New Jersey ($13,770), New York ($10,000) and Massachusetts ($10,843) had the highest average annual insecticide budgets. Upstate New York and the northern peripheral states and provinces ranked the lowest, with median insecticide budgets of $4,072 and $4,781, respectively. Interestingly, despite having the most regional restrictions in terms of insecticide registration and use, Long Island was the highest in median insecticide spending ($17,047).
The percentage of the total annual budget used on insecticides was calculated by using a weighted average of the responses on an 18-hole equivalent and dividing by the median value of the range of the annual maintenance budgets for 18-hole golf courses. Using these values, we determined that insecticides account for 1.23% of the total maintenance budget.
Each superintendent surveyed identified at least one product that was used in managing the weevil, even though 10% of respondents reported no damage to any areas. The pyrethroids and chlorpyrifos (for example, Dursban) used by 79% and 65% of respondents, respectively, were the most popular means of controlling annual bluegrass weevil adults, despite development of pyrethroid resistance and indications that chlorpyrifos efficacy may also be reduced (1). Acelepryn (chlorantraniliprole, Syngenta), an anthranilic diamide, was the most widely used larvicide (51%), which, in part, may be due to its broad spectrum of activity and role in preventive white grub management. Ference (cyantraniliprole, Syngenta), another anthranilic diamide, had not yet been registered for turfgrass at the time of our survey. (A list of insecticides available for control of annual bluegrass weevil at the time the survey was taken is shown in Table 4.)
Courses made an average of 3.9 insecticide applications per year to manage annual bluegrass weevil (Table 3). States/regions where courses had above-average application frequency were located around the epicenter of the annual bluegrass weevil distribution (New Jersey, 4.4; Massachusetts, 4.35; Connecticut, 4.2; Pennsylvania, 4.2; New York, 4.1). Superintendents on Long Island, where the relatively long-residual anthranilic diamides are not registered, made the most annual bluegrass weevil applications per year (5.5), with the highest use of pyrethroids (85%). In this area, 30% of superintendents reported making six or more annual annual bluegrass weevil applications (survey average = 18%), and 20% made 10 or more applications (survey average = 6%). The fewest annual applications occurred in the northern (2.2) and western peripheral states and regions (2.7), and upstate New York (2.3).
Development of pyrethroid-resistant populations
On average, superintendents reported making 2.7 pyrethroid applications per year (including all targeted insect pests) (Table 3). The regions with highest number of pyrethroid applications included Massachusetts (3.9) and the northern peripheral states and provinces (3.6).
One in five courses (20.1% of the courses with damaging annual bluegrass weevil populations) reported having a pyrethroid-resistant population either suspected or confirmed by bioassay. [Editor’s note: Populations are described as “resistant” (in quotation marks) to indicate that resistance has been confirmed through scientific means in some populations, but is only suspected in others.] “Resistant” populations were located across the region, though higher-than-average incidence was reported from areas with long histories of managing annual bluegrass weevil, including Long Island (55% surveyed suspected resistance), Connecticut (48%) and New Jersey (28%) (Table 3). Interestingly, no superintendents in regions with the highest pyrethroid use (Massachusetts and northern peripheral states and provinces), as well as in western peripheral states, suspected they had a pyrethroid-resistant population, possibly because annual bluegrass weevil has become problematic in these areas more recently and pyrethroids have not been used for as long.
On the whole, “resistant” annual bluegrass weevil populations caused more damage (93% of courses) than susceptible populations (74% of courses) when grouped by seasonal damage incidence. Courses with “resistant” populations had a significantly greater number of turf areas damaged (n > 256; U > 3,021.5; P < 0.001), including 84% of collars/approaches and 85% of fairways damaged, compared with 48% and 52% for courses with susceptible populations. Courses reporting resistance also had higher numbers of damaged sites (21.2 combined turf areas vs. 9.8 for courses with susceptible populations). Interestingly, a significantly higher percentage of “resistant” courses (36%) than susceptible courses (21%) reported damage to putting greens (n = 278; U = 3451.5; P < 0.001) despite the “resistant” courses having a significantly lower average height of cut for greens (2.84 mm) than the susceptible courses (3.05 mm) (n = 280; U = 4394.5; P = 0.001).
†ABW, annual bluegrass weevil
Table 3. Average insecticide budgets and trends in chemical management of Listronotus maculicollis populations.
Two-thirds of “resistant” courses but only 42% of susceptible courses had annual maintenance budgets greater than $750,000 (18-hole equivalent). It is not surprising that, given the higher maintenance budgets at “resistant” courses, the average annual insecticide budget of “resistant” courses ($19,241 annually) was more than double that of the susceptible courses ($8,361 annually). This statistic is understandable given that the total area damaged by annual bluegrass weevil on “resistant” courses is more than double that of susceptible populations for most site categories, including fairways (8.3 vs. 3.4 damaged). Greater insecticide budgets on “resistant” courses may also be explained by the significantly greater frequency of annual bluegrass weevil applications (5.8 vs. 3.5 per year) (n = 261; U = 2666; P < 0.001). More than 40% of “resistant” courses but only 10% of susceptible courses reported making more than five applications per year. Additionally, 18% of “resistant” courses reported making 10 or more applications to control annual bluegrass weevil.
Chemical selection differed greatly between the two types of annual bluegrass weevil populations (Figure 3). Larvicides, especially chlorantraniliprole, indoxacarb (Provaunt, Syngenta), spinosad (Conserve, Dow AgroSciences) and trichlorfon (Bayer, Dylox) tended to be used more on courses with “resistant” annual bluegrass weevil than on courses with susceptible populations. Most superintendents of courses with “resistant” populations (93%) used chlorpyrifos (Dow AgroSciences, Dursban) for adult management. However, a relatively high percentage of respondents (64%) still used pyrethroids. The number of annual pyrethroid applications on golf courses with “resistant” annual bluegrass weevil populations was not significantly different from the number with susceptible populations (n = 256; U = 4668.5; P = 0.30). Courses with “resistant” populations were also much more likely than courses with susceptible populations to select older broad-spectrum insecticides such as trichlorfon (Bayer, Dylox) (67% vs. 34% for susceptible courses) or carbaryl (Bayer, Sevin) (21% vs. 11%).
Integrated pest management
Pest control decisions for golf courses are based on aesthetics, playability and avoidance of damage, hence strong emphasis is placed on preventive chemical control of insects, diseases and weeds. Turfgrass managers nevertheless can incorporate many aspects of integrated pest management (IPM) into their management of annual bluegrass weevil. Most respondents (73%) indicated that they “always” or “sometimes” employ spot-treating as a means of controlling annual bluegrass weevil. Only 16% indicated that they never spot-treat. A greater than average number of responses from the Delaware-Maryland-Virginia region (40%) indicated that those superintendents “never” employ spot treatments. This is surprising given the relatively short history of damaging populations in the region and the lower amounts of P. annua present on those golf courses.
Poa annua removal
Annual bluegrass (Poa annua) is believed to be the preferred host plant of annual bluegrass weevil (5), but the pest can also develop in and damage creeping bentgrass (Agrostis stolonifera), even in mixed stands where P. annua is not limiting. Because creeping bentgrass is more tolerant and requires greater larval densities before damage becomes visible (8), promoting creeping bentgrass in mixed stands should help reduce damage and the need for insecticide applications. However, only 54% of respondents indicated they had tried to reduce the amount of P. annua on the course in order to lessen annual bluegrass weevil damage. Many of the courses, especially older courses in the New York City metropolitan area, have large percentages of P. annua in most playing surfaces. Although regrassing the course to reduce weevil damage may not be a viable strategy for such courses, promoting creeping bentgrass over P. annua through selective cultural methods and plant growth regulators may be a practical option.
Table 4. Insecticides available for annual bluegrass weevil control at the time of the survey.
Superintendents regularly employ scouting techniques to estimate annual bluegrass weevil population densities or determine presence. Most respondents (90%) indicated they regularly use two or more monitoring techniques, and 80% used three or more. Only 5.1% indicated that they do not regularly monitor.
Sampling practices can be broken down into two categories: passive (scouting practices that require little manual effort) and active (monitoring practices that estimate pest density). A low percentage of superintendents reported using only one type of technique. Roughly 9% indicated they use only active means to determine population density, and a similar number (8%) indicated they use only passive monitoring techniques (plant phenology, growing degree days). The remaining 79% of responses indicated the use of both types of monitoring techniques.
Passive techniques were favored, even by respondents using multiple means of assessing populations. Syngenta’s proprietary monitoring system, WeevilTrak, gauges population activity and development across the region from growing degree days and soil and vacuum samples on courses within a region (the work is carried out by university personnel and consultants). WeevilTrak was used by 72% of respondents and was the most popular means of monitoring annual bluegrass weevil populations. Smaller percentages of respondents used other passive techniques, such as monitoring plant phenology (65%) and calculating growing degree days (45%). More-active means of population assessment included checking mower baskets for adults (61%) and taking soil cores (56%) to observe stages. Other active measurements that were less popular were the use of soap flushes (40%), traps (for example, linear pitfall traps) (28%), salt flushes (11%) and vacuum sampling (6%).
Information sources and influences
Most respondents indicated that they rely on multiple sources including colleagues (82%), salespeople/distributors (80%) and university personnel (78%) for information on annual bluegrass weevil management. Specific “other” responses (9%) included Syngenta’s WeevilTrak, consultants and the United States Golf Association’s Turf Advisory Service.
When asked which source has the greatest influence on their decision-making, respondents indicated university personnel (43%), followed by colleagues (31%). Colleagues had the greatest influence on management decisions in eastern Pennsylvania (50%), Virginia (46%) and the Delaware-Maryland-Virginia region (44%). Salespeople/distributors were the main source of information for 21% of the surveyed superintendents. Their influence was greatest in western peripheral states and regions (56%) and Pennsylvania (24%). A small percentage of responses indicated “consultant” (3%) and “self” (3%) as having the greatest influence on management decisions. Consultants were most influential in the decision-making process in New York (10%) and in the northern peripheral states and provinces (8%). These low values likely reflect the relatively few turfgrass consulting and scouting services available across the region.
Figure 3. Chemical insecticides used in Listronotus maculicollis (annual bluegrass weevil) management, and comparison between pyrethroid-susceptible and “resistant” courses. Combination products include insecticides with two active ingredients (for example, bifenthrin + imidacloprid).
Changes to management programs
More than half (54%) the surveyed superintendents indicated they had made changes based on presentations or recommendations made by university researchers and staff. Two-thirds of all responses from New Jersey and New York (including 75% from Long Island) indicated changes were made based on information from university personnel. The lowest frequency of impact from university personnel was observed in responses collected from Delaware-Maryland-Virginia (32%) and western Pennsylvania (24%).
Most turf managers (94%) described their management changes in the fill-in-the-blank section of the survey. Timing of controls (43%), chemical selection (39%) and chemical rotations (23%) were the predominant responses given. Some respondents (17%) noted they had moved away from pyrethroids or chemical insecticides (in general) based on university research or recommendations. Less common answers dealt with general IPM recommendations, such as learning about scouting techniques (8%), reducing sprayed areas (2%), alternative controls (2%), and reducing P. annua populations (1%).
This survey provides a broad framework for understanding the importance and spread of annual bluegrass weevil as a golf course pest, the spread and severity of insecticide resistance, and the need for expanded extension efforts to communicate best management practices for the pest. With 90% of respondents indicating damaging densities on their courses and, on average, about one-third of fairways, tees, and greens and collars affected, annual bluegrass weevil clearly is a tremendous problem in the region. Resistance is already widespread, with 20% of respondents throughout the region (regionally up to 55%) suspecting or having confirmed resistance, though it is likely many more courses have unknowingly developed or will develop insecticide-resistant populations in the near future.
Despite reports of pyrethroid resistance, pyrethroids are still the most widely used insecticide class for the weevil's management, followed by another adulticide, chlorpyrifos. While “resistant” courses have made significant changes in insecticide use, particularly greater adoption of larvicides and switching to chlorpyrifos as the primary adulticide, 64% of those courses still used pyrethroids. Greater changes in types of insecticides used are likely being held back by a combination of risk averseness, dominance of preventive approaches by superintendents, and the much lower cost of adulticides compared with the more effective larvicides. However, few classes of insecticides are effective in controlling adult weevils, and the ability for those classes to control pyrethroid-resistant annual bluegrass weevil populations is questionable, as highly pyrethroid-resistant populations already show increased tolerance (4). Our findings highlight the need for novel approaches for controlling adults. Registration of a new, highly effective larvicide (cyantraniliprole, Ference) in 2015 (after this survey was conducted) is likely to increase larvicide use, at least against “resistant” populations. Unfortunately, larvicides are generally much more expensive than adulticides and are therefore cost-prohibitive for many facilities.
Nearly all superintendents who deal with annual bluegrass weevil monitor weevil populations, but the most widely used methods only help with timing of management as opposed to estimating population densities. However, 56% of respondents used soil cores to scout for larval stages, which is the most direct and likely the most precise method of assessing the need for treatments. If more courses move away from primary reliance on adulticides, monitoring of larvae will become more important, which could, in turn, reduce total insecticide use. Because highly resistant weevil populations are also more tolerant of — if not resistant to — most of the currently available larvicides, superintendents will also have to start relying more on biorational insecticides and cultural means to manage weevil populations.
The authors would like to thank the numerous golf course superintendent associations who helped to distribute the survey through chapter emails and social media. Benjamin A. McGraw would like to thank GCSAA for providing golf course facility data by state and province.
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Benjamin A. McGraw is an associate professor in the Department of Plant Science at Penn State University, University Park, Pa. Albrecht M. Koppenhöfer is a professor in the Department of Entomology at Rutgers University, New Brunswick, N.J.