CONTROL OF INSECT PESTS WITH ENTOMOPATHOGENIC NEMATODES I. Control of Weevils with Entomopathogenic Nematodes Kirk A. Smith biosys Palo Alto, California U.S.A. ABSTRACT Entomopathogenic nematodes in the genus Steinernema and Heterorhabditis have been suc- cessfully commercialized as biological control agents for a variety of curculionid species. Nema- tode-based products have been introduced for: 1) root weevils, Otiorhynchus spp. 2) billbugs, Sphenophorus spp. 3) the apopka weevil, Diaprepes abbreviatus, and 4) the citrus root weevil, Pachnaeus litus. Several other Curculionidae species and other closely related families are potential candidates for future commercial introductions. INTRODUCTION government, universities and industry to overcome some of these technical difficulties (Smith et al. 1992). Gaugler and Kaya (1990) recently published a There are now three major biotechnology companies complete review of current knowledge of that have been successful in introducing nematode- entomopathogenic nematodes. Klein (1990) gave a based products into some commercial and consumer condensed introduction regarding nematode efficacy markets. to the order Insecta. The intent of this paper is to provide a brief overview of entomopathogenic Production nematode efficacy against weevils, and the recent technological developments which have made Since their discovery as biological control possible the commercial success of nematode-based agents, nematodes have been produced in vivo, with products. an insect host serving as the media for nematode- Entomopathogenic nematodes in the genus bacterial growth and production. This method has Steinernema and Heterorhabditis and their limitations, because it requires a constant source of associated bacteria (Xenorhabdus spp.) have been healthy insects. It is also sensitive to biological successfully commercialized as biological control variation, and costs of production are high in terms agents for a variety of curculionid species. They can of equipment and man-hours. More efficient methods kill hosts rapidly, are easy to apply, and are exempted of production using in vitro methods have recently from federal and local registration requirements in been developed. Steinernema spp. are now most countries because of their safety to mammals commercially produced in monoxenic liquid culture and plants (Georgis et al. 1991). Difficulties in systems that use fermentation tank technology. This production, storage, formulation, quality control, approach is the most economical of all known methods. and application technology limited their commercial Nematode production is taking place in tanks of up success in the past. Recent public pressure to limit to 80,000 liters in volume, which has lowered costs environmental contamination associated with considerably, allowing successful introductions into chemical insecticide use has resulted in a dramatic markets requiring large numbers of nematodes or increase in research conducted by scientists in markets of low cash crop value. Keywords: billbugs, biological control, entomopathogenic nematodes, weevils, 1 Formulation Efficacy The successful market introduction of an Steinernematids and heterorhabditids differ in entomopathogenic nematode based-product host seeking behavior, tolerance to environmental requires a reliable and stable formulation. This has parameters, behavior in the soil and pathogenicity to been a difficult task, because most larger markets are various insect species (Gaugler 1988). The success demanding a product with a minimum shelf-life of six of entomo-pathogenic nematodes is largely due to months when stored at room temperatures (20°- the extensive amount of scientific research 25°C). Nematode products contain living animals conducted, both in the laboratory and in the field. that have certain temperature, oxygen and moisture Probably the most studied group of weevil requirements necessary for their survival and pest species are the so-called root weevils such as effectiveness as control agents. While no nematode black vine weevil, Otiorhynchus sulcatus, and formulation has been completely successful in strawberry root weevil, O. ovatus. These insects are reaching these goals, some have come very close serious pests on several crops throughout western (Georgis 1992). Europe, and North America. Table 1 summarizes several field studies of black vine weevil in Application Technology containerized ornamental plants supervised by the biosys company. Our experience with this insect has Strategies must also be developed which insure shown that rates of 7.5x109 nematodes per hectare are the successful delivery of the nematode to the target necessary for consistent control. Both S. site and target insect, thereby increasing the carpocapsae and H. bacteriophora under probability of nematode-insect interaction. This has appropriate environmental conditions give control not been a simple task. Many parameters must be equal to, or better than, registered insecticides when investigated to improve performance. Determining targeted against the immature life stages. In North target insect life-stage susceptibility is critical, since America, Helix and BioSafe-N (S. carpocapsae) are different life stages of different species are not equally sold commercially for use on cranberry. BioVector is susceptible. Research by the biosys company has sold for other berry crops and also mint, while Exhibit shown that pest population levels and behavior have is sold for use on ornamental plants. Several thousand a great influence on nematode performance and must hectares have been treated to date. be considered carefully (unpublished data). Often, Table 2 shows data on the pathogenicity of larval stages of insects such as borers are not various Steinernema spp. against neonate larvae of accessible to nematodes. Selecting the most black vine weevil (Shanks, unpublished data). Other appropriate nematode species and/or strain is studies have shown that all larval and pupal stages important for efficacy and commercial development. are susceptible. Abiotic factors such as soil type, soil temperature Billbugs, Sphenophorus spp., have become and moisture, and biotic factors, including pathogens significant pests of turf in North America and Japan. and predators, can greatly influence the nematodes’ All immature stages are susceptible to ability to effectively kill the target pest. entomopathogenic nematodes. Table 3 summarizes Application strategies, including field dosage, field trial results of Exhibit (S. carpocapsae) against volume, irrigation and appropriate application bluegrass billbug, S. purvulus, in the United States. methods, are very important, especially if nematodes Although it was less effective on billbug populations are to be integrated with other control strategies. than the insecticide isazophos, the control effect is Compatibility with a wide range of agrichemicals has within acceptable limits. In Japan (Table 4), field trials been demonstrated (biosys, unpublished data). This conducted by SDS Biotech in cooperation with biosys has benefited the successful introduction with has displayed good levels of control against hunting existing Integrated Pest Management programs. Crop billbug, S. venatus, with BioSafe (S. carpocapsae). morphology and phenology must be considered in Adults of this species are also susceptible. These predicting whether nematodes are viable control results have made it possible to register BioSafe for candidates. Additional research has shown the commercial sale in Japan. potential for entomopathogenic nematodes to be Another commercial success story in the United used in other habitats (e.g. aquatic, foliar, and States has been the introduction of BioVector (S. cryptic), and in manure. carpocapsae) for control of the apopka weevil, Diaprepes abbreviatus, and the citrus root weevil, Pachnaeus litus. These two insects cause extensive 2 Table 1. Field efficacy of entomopathogenic nematodes against black vine weevil, Otiorhynchus sulcatus Table 2. Susceptibility of neonate black vine weevil larvae, Otiorhynchus sulcatus to various steinernematid species 3 Table 3. Field efficacy of entomopathogenic nematodes against bluegrass billbug, Sphenophorus purvulus, in the U.S.A. Table 4. Field efficacy against hunting billbug, Sphenophorus venatus venatus, in Japan root damage to citrus in the state of Florida. Thousands REFERENCES of hectares of citrus orchards and nurseries have been treated with BioVector. Tables 5a and 5b show Ahmad, R. 1974. Investigations on the white- the results of a seven-month study examining adult fringed weevils Naupactus durius Boh. insect emergence after a single, early-season and Pantomorus auripes Hustache application (Bullock, unpublished data). (Coleoptrea, Curculionidae) and their The sweetpotato weevil, Cylas formicarius, is natural enemies in Argentina. Technical a serious pest of sweet potato all over the world. Bulletin of the Commonwealth Institute of Research efforts have shown that there is a good Biological Control 17: 37-51. potential for controlling this pest with Bedding, R.A., and L.A. Miller. 1981. Use of entomopathogenic nematodes. Table 6 shows results a nematode, Heterorhabditis heliothidis, to from one recent study (Jansson et al. 1990). control black vine weevil, Otiorhynchus Table 7 lists various “weevil” pests that are sulcatus, in potted plants. Annals of reported in the literature to be susceptible to Applied Biology 99: 211-216. entomopathogenic nematodes. As previously Bélair, G.,