The annual bluegrass weevil (ABW, Listronotus maculicollis Kirby), one of the “snout beetles” in the family Curculionidae, is a destructive pest of short-mown turfgrass in golf courses and sports fields. Adults measure less than 1/5 inch (< 5 millimeters) in length, with antennae near the end of their snout (Figure 1). Newly emerged adults appear mottled brown, while older adults are shiny black. The ABW primarily infests short-cut annual bluegrass (Poa annua) and occasionally infests creeping bentgrass (Agrostis stolonifera). Adults overwinter in grass near tree lines, in leaf litter, moss, high-cut grass, and pine litter. In spring, they emerge and migrate to the nearest short-mown turf to feed, mate, and lay eggs. Eggs are deposited under the leaf sheaths of grass stems. The eggs are oblong and pale yellow to white, transitioning to a smoky black color within days of being laid. After hatching, the larvae develop through five instars before pupation. Larvae are legless and cream-colored with an orange-brown head capsule that increases in size when the larvae molt to the next instar. Early instar larvae are generally hidden within the plant as stem borers. Late instars become more visible as they exit the stem to externally feed on the plant’s crown. The last (fifth) instar larvae pupate near the soil surface and then emerge as adults, giving rise to additional generations (totaling three generations per year in Ohio).
The distribution range of ABW stretches from the northeastern United States into the Midwest and eastern Canada. Initially detected in the 1950s on Long Island golf courses, this destructive pest has spread radially in recent decades (Figure 2). Isolated populations have been detected as far west as Nebraska and as far south as Georgia. In Ohio, the ABW was first reported in 2007 and has spread to regions across the state. It is suspected that the movement of sods may have facilitated the spread of this pest.
ABW adult feeding causes superficial damage to grass blades, visible as notches in the blades. Early instar larvae tunnel into stems, which may cause moderate damage in the form of yellowing tillers. The most substantial damage comes from the late instar larvae that feed on the crown of turfgrass and the base of stems. During ABW development, each larva can kill a dozen of grass stems. As adults emerge in late March to late April from their overwintering sites, damage is typically first spotted in collars and along the edges of fairways, progressing inward and spreading more widely as the ABW population increases through the year. The first generation typically causes the most severe damage in annual bluegrass; however, late-season damage may also occur (Figure 3). Damage to creeping bentgrass usually occurs in southern and western ranges of ABW distribution, where annual bluegrass is less commonly grown.
Plant phenology may be used to indicate ABW developmental stages. For example, ABW adults usually migrate from overwintering sites in increasing numbers when Forsythia spp. are in full bloom. Peak adult populations often occur when forsythia reaches “half gold (flower) and half green (leaf)”, or when eastern redbud is in early bloom. Egg-laying begins during the full bloom of eastern redbud or flowering dogwood. Larvae start to appear in the soil during the full bloom of Catawba rhododendron hybrid. However, the phenology method should be based on multiple plant stands on or near sites because of the variability of individual plants. Also, it is recommended to use plant phenology together with degree-day models and pest scouting for optimal monitoring of the pest.
Adults may be monitored using a disclosing solution (“soap flushing”), vacuuming, and examining mowing clippings. Among these methods, soap flushing by pouring 1 pint of 0.8% solution (1 fluid ounce of lemon-scented dish detergent per gallon of water) per square foot and then observing the site for 5-20 minutes, is most efficient, followed by vacuuming. Monitoring clippings is most effective for detecting adults on putting greens. When adults are active, grass clippings should be destroyed or disposed of far away from at-risk playing surfaces, as most adults could survive the mowing and may return to the site or spread to new areas. Besides, linear pitfall traps may be used to monitor ABW adult activity.
Larvae may be extracted from soil cores using saline solution. Cores taken with a turf plugger (2 ¼-inch diameter) are torn into three or four pieces and submerged in tepid to warm water saturated with salt. The salt irritates the small larvae out of the stems and floats all stages to the surface (stir and observe every 15 minutes for one hour). Alternatively, larvae may be extracted from soil cores with heat using Berlese funnels. To use this method, put the core upside down above a funnel that is over a receptacle filled with soapy water (to receive larvae) and expose it to a high temperature of 104 degrees Fahrenheit (40 Celsius) for two to three days. However, this method is only suitable for assessing larvae, not pupae.
The control of ABW has primarily relied on synthetic insecticides, such as organophosphates and pyrethroids, targeting the adult stage, especially overwintered adults in spring. However, this pest has developed resistance to pyrethroids, which is associated with a general resistance mechanism of enhanced enzymatic detoxification. Also, because of the short residual efficacy of contact insecticides (such as pyrethroids) used for adult control, usually these insecticides need to be reapplied for adult emergence over a long time period from the overwintered sites. This encourages the development of insecticide resistance, or building-up of resistance levels. For populations with resistance to adulticides, larvicides may be more viable options, although their efficacy might be compromised in highly resistant populations. Control options for adulticides and larvicides are listed in Table 1.
Varying with target stages and residual activities, insecticides may be applied either in preventive (before damage occurs) or curative approaches. For targeting overwintered adults and first-generation larvae, timing of applications should be based on scouting of ABW stages and densities, growing degree days, and plant phenology (adulticides: forsythia half gold/ half green; larvicides for early instars inside of stem: dogwood full bloom to just past bloom; larvicides for late instars outside of stem: rhododendron full bloom). Larvicides need to be watered in with approximately 1/10 inch (2.5 millimeters) of water post application. For the second and third generations of ABW, applications should be based on pest scouting. Control options with different modes of action need to be rotated to reduce the potential development of insecticide resistance.
Biological control agents, e.g., beneficial nematodes Steinernema carpocapsae and Heterorhabditis bacteriophora, may serve as effective alternatives to insecticides, especially for control of insecticide-resistant larvae, although their efficacy may be reduced against highly resistant populations. Other biorationals, such as insecticidal fungi (Beauveria and Metarhizium spp., causing white and green muscardine diseases of insects, respectively), may be used but provide only moderate control levels. These biopesticides are compatible with many synthetic pesticides and may be used in combination with chemical insecticides to achieve higher control levels while reducing chemical use.
Disclaimer: Pesticide recommendations are subject to change with time and are provided only as a guide. It is the pesticide applicator’s responsibility, by law, to read and follow current label instructions for the specific pesticide being used. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.