Olga S. Kostromytska, Ph.D., and Albrecht M. Koppenhöfer, Ph.D.
The annual bluegrass weevil is a severe and expanding pest of short-mowed golf course turf in the Mid-Atlantic and Northeast regions
of the United States.
Photo by B. McGraw
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The annual bluegrass weevil, Listronotus maculicollis, is a severe and expanding pest of short-mowed golf course turf (fairways, tees, approaches, collars, greens) in the Mid- Atlantic and Northeast regions of the United States and in southeastern Canada. The annual bluegrass weevil goes through two to three generations per year, with the first generation typically causing the greatest damage. Adults emerge from overwintering sites such as leaf litter and tall grasses around the golf course from late March to mid-April to migrate to the short-mowed turf over the course of several weeks. Mated females deposit eggs in the grass stem, either singly or in small batches between the leaf sheaths. The stem-boring young larvae cause limited damage. Older larvae (fourth and fifth instars) cause severe damage by feeding externally on the turf plant crowns.
Chemical control is the predominant strategy for annual bluegrass weevil management, but insecticide overuse has led to high resistance to pyrethroids on many golf courses (3,6). To make matters worse, resistance is not specific, with most of the presently available insecticides being less effective against resistant populations (3). Clearly, relying only on chemical control for annual bluegrass weevil management is not sustainable.
In the areas of eastern North America affected by annual bluegrass weevil, creeping bentgrass (Agrostis stolonifera) is the dominant intended grass species on short-mowed turf areas of golf courses. However, the invasive weed annual bluegrass (Poa annua) often constitutes a high percentage of the turf stand in those areas. In earlier research (4), pure creeping bentgrass stands were found to be more tolerant to annual bluegrass weevil larval feeding than mixed stands of annual bluegrass and creeping bentgrass. No information regarding susceptibility to annual bluegrass weevil is available for colonial bentgrass (A. capillaris) and velvet bentgrass (A. canina), which are being improved for use on golf courses.
Annual bluegrass weevil adults (shown) overwinter in leaf litter and tall grasses around the golf course before emerging in late March to mid-April to migrate to the short-mowed turf.
Photo by R. Cowles
Suppressing annual bluegrass in favor of more tolerant/resistant grasses should be the best way to reduce problems with annual bluegrass weevil. However, annual bluegrass is difficult to suppress, let alone eradicate, on operating golf courses. In addition, reports of annual bluegrass weevil damage to creeping bentgrass have increased recently. Therefore, it will be important to select bentgrasses that are not only more tolerant to annual bluegrass weevil feeding but also poor hosts for annual bluegrass weevil (that is, they are resistant). Otherwise, annual bluegrass weevil populations could build up on the bentgrasses that are tolerant but not resistant and then damage adjacent annual bluegrass patches or even the bentgrasses themselves.
The goal of this study was to clarify annual bluegrass weevil host-plant interactions and to compare resistance to annual bluegrass weevil in select cultivars of three bentgrass species to resistance in annual bluegrass. Antixenosis (non-preference) components of resistance were addressed in no-choice and choice laboratory and field bioassays, and antibiosis (negative effects on pest) and plant-tolerance components were studied in larval development experiments in the greenhouse.
Insects and plant material
†Means within columns followed by the same upper- or lowercase letter did not differ among grass species or cultivars, respectively.
Table 1. Mean (± SE) number of eggs laid per 1.8-inch-diameter plug of annual bluegrass or cultivars of three bentgrasses by two overwintering or spring-generation annual bluegrass weevil females within one week in environmental chambers.
Overwintering adults were collected from overwintering sites on a golf course around early November. They were kept in containers with moist sand for two to six months in an incubator (10 hours light at 43 F [6 C]: 14 hours dark at 39 F [4 C]) until one week before use. Overwintered (collected around late April) and spring-generation (collected around early July) adults were collected from fairways or greens. Adults were kept in containers on moist sand in environmental chambers (14 hours light at 72 F [22 C]: 10 hours dark at 57 F [14 C]) and provided with cutworm diet and organic wheat sprouts as food.
Throughout our experiments, cultivars of three bentgrass species were tested and compared to wild type annual bluegrass: L-93, Penncross, 007 and Declaration creeping bentgrass; Tiger II and Capri colonial bentgrass; and Greenwich and Villa velvet bentgrass. Bentgrasses were grown from seed in the greenhouse for two months before use. Annual bluegrass was grown from plugs taken from uniform fields at Rutgers Hort Farm No. 2. Plug roots were washed free of soil before planting. For field experiments, grasses were grown in a nursery area at Rutgers Hort Farm No. 2. Grasses for all experiments were clipped twice per week at 0.5-inch (1.27-centimeter) height.
Host preference for oviposition and feeding
No-choice laboratory experiments
For each of the nine grasses tested in the no-choice laboratory experiments, there were 12 replications with overwintering adults and nine replications with spring-generation adults. Each turf core of greenhouse-grown grass (1.8 inches in diameter × 3 inches deep [4.6 × 7.6 centimeters]) was fit into a plastic vial and placed in the center of a clear round container (7 inches in diameter × 5 inches high [17.8 × 12.7 centimeters]). Pasteurized sandy loam amended with 10% (v/v) peat moss was added to the level of the core’s soil surface. Two male and two female annual bluegrass weevils were kept for one week in the arena inside an environmental chamber (14 hours light at 72 F: 10 hours dark at 57 F). Each core was then recovered, and the number of eggs and feeding scars determined under a dissecting microscope.
Figure 1. Egg-laying preferences of three phenologically different annual bluegrass weevil (ABW) populations in environmental chambers. Females were given a choice between one plug each of annual bluegrass, Penncross and Declaration creeping bentgrass, and Capri colonial bentgrass (left), or between one plug each of annual bluegrass and L-93 and 007 creeping bentgrass and Greenwich velvet bentgrass (right). Means (± SE) within the same weevil population with the same letter are not statistically different.
The number of eggs per plug was higher in annual bluegrass than in all bentgrass species (data combined within species) for both adult types (Table 1). There were fewer eggs in velvet bentgrass than in the other bentgrasses with overwintering adults, and fewer eggs in colonial bentgrass than in the other bentgrasses with spring-generation adults. There were almost four times more eggs in annual bluegrass than in Penncross, the bentgrass with the highest number of eggs (Table 1). There were also more than 10-fold differences in the number of eggs among bentgrasses. Villa and Greenwich velvet bentgrass and Tiger II colonial bentgrass were among the bentgrasses with the fewest eggs. The number of feeding scars followed a similar trend, but differences among grass species and cultivars were much weaker (range: 55.4-130.0).
Choice laboratory experiments
We used the same methodology as for the no-choice test, except that four cores were placed on the corners of a 4 × 4-inch (10 × 10-centimeter) square in the center of a larger box (14 inches × 11 inches × 6 inches high [36 × 28 × 15 centimeters]). Ten females and 10 males were released centrally. Half of the boxes contained one core each of annual bluegrass, L-93 and 007 creeping bentgrass and Greenwich velvet bentgrass; the other boxes had one core each of annual bluegrass, Penncross and Declaration creeping bentgrass and Capri colonial bentgrass. There were a total of 12 replicates per cultivar combination for each adult phenotype: overwintering, overwintered and spring generation.
Bentgrasses are more tolerant than annual bluegrass of feeding by annual bluegrass weevil larvae.
Photo by R. Cowles
The number of eggs per plug was higher in annual bluegrass than in all bentgrasses tested for both cultivar combinations with all adult phenotypes (Figure 1). Fewer eggs were recovered in Declaration than in Capri and Penncross. No consistent differences were detected among Greenwich, L-93 and 007. The number of feeding scars followed a similar trend, but differences among grass species and cultivars were much weaker (range: 49.6-86.0).
Field experiments were conducted at Rutgers Hort Farm No. 2. Experimental arenas consisted of clear containers with the bottoms cut out that were inserted into the ground in bare soil with the top 2 inches (5 centimeters) above the soil surface. Three days before weevil release, turf cores (4 inches in diameter × 3 inches deep [10 × 7.6 centimeters]) were inserted into the arena ground. The arenas were sealed with a well-ventilated, annual bluegrass weevil-proof screen lid. Plants were clipped twice per week (0.5 inch) and irrigated as necessary. To determine the number of stages present in each core, cores were examined under a dissecting microscope and then submerged in saturated salt solution.
In a no-choice experiment, round arenas (7 inches in diameter × 4 inches high [18 × 10 centimeters]) were arranged in a randomized complete block design with seven replicates. Plugs (4 inches [10 centimeters] in diameter) of annual bluegrass or the bentgrasses (L-93, Penncross, Declaration, 007, Villa and Capri) were placed in the center of each arena. Adult annual bluegrass weevils (five males and five females) were released on April 27, 2012, and cores were extracted on May 27, 2012. At evaluation, more larvae were found in annual bluegrass (6.2 + 0.6) and in Capri (3.8 + 0.6) than in the other cultivars (range: 0.6-1.4).
†Means within columns followed by the same upper- or lowercase letter did not differ among grass species (cultivars).
Table 2. Mean (± SE) number per pot of annual bluegrass weevil immatures recovered from Poa annua or cultivars of three bentgrass species in two greenhouse larval-survival pot experiments (2011, 2012).
For choice experiments, rectangular arenas (14.5 inches × 10 inches × 5.5 inches high [37 × 25.4 × 14 centimeters]) were arranged in a randomized complete block design with eight replicates for each of the following experimental designs: (1) annual bluegrass and L-93, 007 and Villa; and (2) annual bluegrass and Penncross, Declaration and Capri. The four cores were randomly placed on the corners of a 6 × 6-inch (15 × 15-centimeter) square in the center of the arena, and 10 males and 10 females released. The first experiment (April 27−May 27, 2012) used overwintered adults and cores were extracted one month after adult release. The second experiment (July 6-13, 2012) used spring-generation adults and plugs 2 inches (5 centimeters) in diameter, and it was evaluated one week after adult release.
In the choice experiments, annual bluegrass was clearly preferred for oviposition for both cultivar combinations in both experiments. In the spring experiment, stages recovered from the plugs were eggs (76%) and first (14%), second (7%) and third (3%) instars. On average, 107.0 + 7.3 stages were recovered per arena, with 72% of the stages recovered from annual bluegrass and no differences observed among bentgrasses in either cultivar combination (range 8%-11% of eggs). In the July experiment, only eggs were recovered (45.3 + 3.3 per arena), and 66% and 70% were found in annual bluegrass. Fewer eggs were found in Declaration (7%) than in Penncross (12%) and Capri (15%), and fewer eggs were found in L-93 (7%) than in Villa (10%) and 007 (13%).
Larval growth and development
Grass was grown in pots for two months in the greenhouse before overwintering adults (five males and five females per container) were introduced and the containers covered with an annual bluegrass weevil-proof screen. After one week, the adults were removed by submersion in lukewarm water. The containers were kept for four more weeks in the greenhouse.
Figure 2. Damage to annual bluegrass and three bentgrass species (data combined across two to four cultivars) after four weeks of feeding by annual bluegrass weevil (ABW) larvae. Means (± SE) within the same year with the same letter are not statistically different.
Grass quality (1-9 scale, where 1 is dead grass) and percent damage (using a 0.4- × 0.4- inch [1- × 1-centimeter] grid) were assessed weekly. After four weeks, the number of annual bluegrass weevil stages in the pots was determined by visual examination followed by submersion in saturated salt solution. All recovered immatures were weighed and their stages were determined. For each turfgrass cultivar, there were nine pots in experiment 1 (2011) and 12 pots in experiment 2 (2012).
Annual bluegrass had the highest number of immatures followed by velvet and colonial bentgrass, with the lowest numbers recovered in creeping bentgrass (Table 2). Annual bluegrass had significantly more immatures than most bentgrass cultivars except for Villa and Capri in experiment 1. L-93 and Penncross were among the cultivars with the lowest numbers in both experiments.
Third through fifth instars and pupae were recovered from the pots. The average instar reached was less advanced in creeping bentgrass than in the other grasses. The immatures weighed more in annual bluegrass (4.9 + 0.1 milligrams) than in bentgrass species, with the lowest weight in creeping bentgrass (3.1 + 0.2 milligrams). Immatures recovered from all tested cultivars (except Capri and Villa) weighed less than those recovered from annual bluegrass.
Percent damage was highest for annual bluegrass (Figure 2), and velvet bentgrass suffered more damage than creeping bentgrass. Percent damage correlated with the total number of immatures recovered. Quality ratings closely (inversely) followed the damage ratings.
Our study demonstrates that annual bluegrass is not only less tolerant of annual bluegrass weevil larval feeding than bentgrasses, but it is also preferred for egg-laying and better for larval development. Non-preference (anti-xenosis) is at least partially involved in the higher resistance level of bentgrasses relative to annual bluegrass as evidenced by decreased levels of egg-laying observed in bentgrasses. However, lower larval weight and delayed larval development suggest that creeping bentgrass is also an inferior host plant for annual bluegrass weevil compared to annual bluegrass (that is, antibiosis).
Several potential mechanisms might be involved in bentgrass resistance. It is possible that annual bluegrass weevil females use long- and short-distance cues to locate and accept hosts for oviposition (5). Thus, volatiles emitted by annual bluegrass may specifically attract females and thereby be responsible for host recognition and ovipositional preferences. It is also possible that bentgrasses emit volatiles that repel annual bluegrass weevil.
Greater tolerance to larval feeding might be correlated with growth habit. Poa annua is a bunchgrass whereas the three bentgrasses are stoloniferous. Creeping bentgrasses, especially the cultivars Declaration and 007, which were the most tolerant to larval feeding of all the species tested, also have the most aggressive growth habit and show the fastest recovery from environmental stress and pest damage among the bentgrasses (1).
Results of our study have several important implications. Because creeping bentgrasses are the most resistant and tolerant of the species tested, this species is the best candidate for annual bluegrass replacement or overseeding. In addition to their resistance to annual bluegrass weevil, the newer cultivars Declaration and 007 have overall better turf quality, higher density (thus, they outcompete annual bluegrass) and improved wear tolerance and dollar spot resistance than older cultivars (1).
Recently, superintendents have overseeded existing turf stands with bentgrasses to improve quality in turf with or without annual bluegrass weevil infestation and damage history (2). To achieve the highest rate of establishment, overseeding should be done from mid-June to July, which coincides with the period of greatest damage by annual bluegrass weevil larvae. Better results are also achieved if annual bluegrass is weakened to ensure better creeping bentgrass establishment (2). Because annual bluegrass is highly susceptible to annual bluegrass weevil damage and some creeping bentgrasses have demonstrated resistance, overseeding with newer creeping bentgrass cultivars should be beneficial in areas with recurring annual bluegrass weevil infestations. However, because annual bluegrass weevils laid eggs and developed in all grasses tested, risk of annual bluegrass weevil population growth still exists even if resistant grasses are used. Therefore, host-plant resistance should be implemented in combination with other sustainable management strategies, and more studies are needed to investigate the practical implementation of this strategy.
This research was funded by the GCSAA and supporting chapters/associations (GCSA of New Jersey, Hudson Valley GCSAA, Keystone AGCS, Long Island GCSA, Metropolitan GCSA, New Jersey Turfgrass Association, Pocono Turfgrass Association), the United States Golf Association, the O.J. Noer Research Foundation, the Tri-State Turf Research Foundation and the Rutgers Center for Turfgrass Science.
The authors also thank the participating golf superintendents and their clubs for their cooperation.
This article was based on a published paper: Kostromytska, O.S., and A.M. Koppenhofer. 2014. Ovipositional preferences and larval survival of annual bluegrass weevil, Listronotus maculicollis, on Poa annua and selected bentgrasses (Agrostis spp.). Entomologia Experimentalis et Applicata 152:108-119.
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Olga S. Kostromytska is a postdoctoral scientist and Albrecht M. Koppenhöfer is an Extension specialist in the in the department of entomology, Rutgers University, New Brunswick, N.J.