Pest mole crickets (Scapteriscus spp.) have been the targets of a classical biological control program during the past 25 years (Frank & Parkman 1999). Three species, including the tawny (S. vicinus Scudder), southern (S. borellii Giglio-Tos), and shortwinged (S. abbreviatus Scudder) mole crickets, were inadvertently brought without their natural enemies to the southern U.S. from South America via ship ballast in the early 1900s (Walker 1985). They became established and damaging in pastures and managed turfgrass areas, such as golf courses, athletic fields, and home lawns. Their root-feeding and tunneling in the soil kills patches of turfgrass, which significantly reduces turfgrass quality and aesthetics. The importation, rearing, subsequent release, and dispersal of three natural enemies (Larra bicolor F., Ormia depleta (Wiedemann), Steinernema scapterisci Nguyen and Smart) has helped suppress mole cricket populations, but insecticides are still frequently used to provide control.

The insect parasitic nematode Steinernema scapterisci was discovered in Uruguay in the early 1980s, cultured in the U.S., and released in Florida in 1985 (Parkman et al. 1993b). This nematode infects older mole cricket nymphs and adults and recycles within the soil environment. The nematode, once inside the mole cricket host, releases Xenorhabdus innexi bacterium into the hemolymph (Lengyel et al. 2005). The bacterium reproduces and kills the mole cricket through septicemia, producing a nutrient rich bacterial soup that the nematodes consume. The infective juvenile nematodes then exit the body and infect other mole crickets in the soil (Nguyen & Smart 1991). Steinernema scapterisci (Nematac S®, Becker Underwood, Ames, IA) can be used in inoculative releases for mole cricket control (Parkman et al. 1994). Other insect parasitic nematodes (e.g., Steinernema carpocapsae Weiser; S. feltiae (Filipjev); S. riobrave Cabanillas, Poinar and Raulston; and Heterorhabditis bacteriophora Poinar) used against pest mole crickets have a broader host range and may infect non-target organisms.

Various methods to optimize the survival and establishment of insect parasitic nematodes have been tested. For example, commercial formulations may be sprayed onto turfgrass (Parkman et al. 1993a, 1993b, 1994), chiseled, injected, or buried into the ground (Parkman et al. 1993b; Adjei et al. 2003), or target pests may be trapped into containers, treated with nematodes, and then released (Parkman & Frank 1992). To enhance the establishment of S. scapterisci, adult mole crickets can be attracted to a treated area with synthetic, electronic male mole cricket songs (Parkman & Frank 1992). Application timing is limited to when large nymphs and adults are present and actively tunneling through the soil, primarily late August to late October and March to May in the southern U.S. Usually only one application of S. scapterisci is necessary to successfully establish and recover S. scapterisci populations on a site (Parkman et al. 1993b), but annual applications are recommended (Lombardo et al. 1999).

Because nematode applications may be more expensive and labor-intensive than insecticide applications (Lombardo et al. 1999), spot treatments of nematodes may be more economical. Treating smaller areas of mole cricket damage should reduce costs, and it takes advantage of mole cricket behavior. Adult mole crickets infected with S. scapterisci can fly several kilometers before dying (Walker 1985), thus spreading nematodes to uninfected sites. Steinernema scapterisci can also live in moist soil and survive without a host for at least 10 wk (Nguyen & Smart 1990), which increases its value in areas of low mole cricket density. The goals of this study were to assess the baseline level of mole cricket infection from a previous inoculative application and determine whether subsequent augmentative applications could increase mole cricket infection rates over time.

Materials and Methods

Results and Discussion

Spot treatments of S. scapterisci successfully increased the percentage of mole crickets infected in bermudagrass roughs within 4-8 wk after application at both golf courses, compared to pretreatment levels. The increased percentage of infection lasted for 17 months at IGC and 8 months at GGCC (Fig. 1 and Fig. 2). Likely because of mole cricket movement over time, infection rates in untreated plots (>80 m from treated areas) equaled the percent infection in treated plots after ∼5 months. Turfgrass density ratings (0-9 scale), which indicate the relative amount of damaged turfgrass, did not differ among nematode-treated and control plots in this test at each sampling time (Barbara 2005), and are thus not reliable indicators of nematode establishment.

Nematode infection levels seemed to fluctuate primarily with mole cricket population density, age, and environmental conditions over time (15-40% of mole crickets infected at IGC, 15-75% at GGCC). However, the mean cumulative percentages (± SE) of infection for mole cricket trap collections from 2001 to 2003 from the sites GGCC (22.1 ± 10.5%) and IGC (15.8 ± 4.6%) did not differ (t = 2.00; df = 2, 56; P > 0.05). Fewer mole crickets were collected in year 2 than in year 1 at IGC (t = 2.47; df = 1, 37; P < 0.01). Although S. scapterisci persisted throughout the study period, the percent infection also decreased in year 2 at IGC compared to year 1 (t = 6.63; df = 1, 37; P < 0.01). Mole crickets that were large enough to be vulnerable to S. scapterisci infection were present from late August to April, but cooler winter temperatures likely reduced mole cricket activity and possibly nematode infectivity (Molyneux 1985) from December to February. Similar to Parkman et al. (1994), we found more mole cricket adults infected with S. scapterisci than nymphs. But, contrary to Parkman et al. (1994), more S. vicinus were collected in traps and infected than S. borellii (Table 1). Other factors that may have influenced mole cricket numbers, nematode infectivity, and/or survival include pesticide use (Barbara & Buss 2005), exposure to ultraviolet light (Gaugler & Boush 1978), soil saturation after excessive rain or irrigation (Molyneux & Bedding 1984; Hudson & Nguyen 1989b), infection by or competition with other nematodes (i.e., Heterorhabditis spp., Steinernema spp.), infection by fungi (i.e., Beauvaria bassiana), microbial consumption of the nematodes, parasitism by Larra bicolor F. or Ormia depleta (Wiedemann), or predation. Over 8,400 predaceous arthropods were collected in the pitfall traps with the mole crickets (Table 2), and the most abundant families included Carabidae (>16.6%), Formicidae (>19.9%), and Staphylinidae (>24.8%). However, earwigs were specifically observed attacking mole crickets in the traps. Fluctuations in predator and parasitoid abundance with mole cricket numbers were not examined.

Steinernema scapterisci may become established after one application to turfgrass, but this is the first known study to demonstrate that augmentative spot treatments can also increase the percent of mole crickets infected by a nematode. Given the background infection levels on both golf courses, it is possible that S. scapterisci populations can either persist on intensively managed golf courses for ≥13 years (initial applications had been made in 1988 and 1989) (Parkman et al. 1994), and/or infected mole crickets may reintroduce nematodes periodically during mating flights from other areas. Because infected or septic mole crickets might not be as mobile as healthy mole crickets, and thus not fall into traps and be detected, the reported percentage of mole cricket infection may be an underestimate. It is possible that S. scapterisci can synchronously cycle with mole cricket density over time, since it can reproduce in its host. These nematodes can also kill adult mole crickets before they lay all of their egg clutches (Barbara 2005), which reduces the potential population size of the next generation. Insect parasitic nematodes are viable, environmentally-friendly alternatives to insecticides for long-term mole cricket management.