Kirk A. Smith
biosys
Palo Alto, California U.S.A., 1994-10-01

Abstracts in Other Languages: 中文, 日本語, 한국어

I. Control of Weevils with Entomopathogenic Nematodes

Introduction

Gaugler and Kaya (1990) recently published a complete review of current knowledge of entomopathogenic nematodes. Klein (1990) gave a condensed introduction regarding nematode efficacy to the order Insecta. The intent of this paper is to provide a brief overview of entomopathogenic nematode efficacy against weevils, and the recent technological developments which have made possible the commercial success of nematode-based products.

Entomopathogenic nematodes in the genus Steinernema and Heterorhabditis and their associated bacteria (Xenorhabdus spp.) have been successfully commercialized as biological control agents for a variety of curculionid species. They can kill hosts rapidly, are easy to apply, and are exempted from federal and local registration requirements in most countries because of their safety to mammals and plants (Georgis et al. 1991). Difficulties in production, storage, formulation, quality control, and application technology limited their commercial success in the past. Recent public pressure to limit environmental contamination associated with chemical insecticide use has resulted in a dramatic increase in research conducted by scientists in government, universities and industry to overcome some of these technical difficulties (Smith et al. 1992). There are now three major biotechnology companies that have been successful in introducing nematode-based products into some commercial and consumer markets.

Production

Since their discovery as biological control agents, nematodes have been produced in vivo, with an insect host serving as the media for nematode-bacterial growth and production. This method has limitations, because it requires a constant source of healthy insects. It is also sensitive to biological variation, and costs of production are high in terms of equipment and man-hours. More efficient methods of production using in vitro methods have recently been developed. Steinernema spp. are now commercially produced in monoxenic liquid culture systems that use fermentation tank technology. This approach is the most economical of all known methods. Nematode production is taking place in tanks of up to 80,000 liters in volume, which has lowered costs considerably, allowing successful introductions into markets requiring large numbers of nematodes or markets of low cash crop value.

Formulation

The successful market introduction of an entomopathogenic nematode based-product requires a reliable and stable formulation. This has been a difficult task, because most larger markets are demanding a product with a minimum shelf-life of six months when stored at room temperatures (20°-25°C). Nematode products contain living animals that have certain temperature, oxygen and moisture requirements necessary for their survival and effectiveness as control agents. While no nematode formulation has been completely successful in reaching these goals, some have come very close (Georgis 1992).

Application Technology

Strategies must also be developed which insure the successful delivery of the nematode to the target site and target insect, thereby increasing the probability of nematode-insect interaction. This has not been a simple task. Many parameters must be investigated to improve performance. Determining target insect life-stage susceptibility is critical, since different life stages of different species are not equally susceptible. Research by the biosys company has shown that pest population levels and behavior have a great influence on nematode performance and must be considered carefully (unpublished data). Often, larval stages of insects such as borers are not accessible to nematodes. Selecting the most appropriate nematode species and/or strain is important for efficacy and commercial development. Abiotic factors such as soil type, soil temperature and moisture, and biotic factors, including pathogens and predators, can greatly influence the nematodes' ability to effectively kill the target pest.

Application strategies, including field dosage, volume, irrigation and appropriate application methods, are very important, especially if nematodes are to be integrated with other control strategies. Compatibility with a wide range of agrichemicals has been demonstrated (biosys, unpublished data). This has benefited the successful introduction with existing Integrated Pest Management programs. Crop morphology and phenology must be considered in predicting whether nematodes are viable control candidates. Additional research has shown the potential for entomopathogenic nematodes to be used in other habitats (e.g. aquatic, foliar, and cryptic), and in manure.

Efficacy

Steinernematids and heterorhabditids differ in host seeking behavior, tolerance to environmental parameters, behavior in the soil and pathogenicity to various insect species (Gaugler 1988). The success of entomo-pathogenic nematodes is largely due to the extensive amount of scientific research conducted, both in the laboratory and in the field.

Probably the most studied group of weevil pest species are the so-called root weevils such as black vine weevil, Otiorhynchus sulcatus, and strawberry root weevil, O. ovatus. These insects are serious pests on several crops throughout western Europe, and North America. Table 1 summarizes several field studies of black vine weevil in containerized ornamental plants supervised by the biosys company. Our experience with this insect has shown that rates of 7.5x10⁹ nematodes per hectare are necessary for consistent control. Both S. carpocapsae and H. bacteriophora under appropriate environmental conditions give control equal to, or better than, registered insecticides when targeted against the immature life stages. In North America, Helix and BioSafe-N (S. carpocapsae) are sold commercially for use on cranberry. BioVector is sold for other berry crops and also mint, while Exhibit is sold for use on ornamental plants. Several thousand hectares have been treated to date.

Table 2 shows data on the pathogenicity of various Steinernema spp. against neonate larvae of black vine weevil (Shanks, unpublished data). Other studies have shown that all larval and pupal stages are susceptible.

Billbugs, Sphenophorus spp., have become significant pests of turf in North America and Japan. All immature stages are susceptible to entomopathogenic nematodes. Table 3 summarizes field trial results of Exhibit (S. carpocapsae) against bluegrass billbug, S. purvulus, in the United States. Although it was less effective on billbug populations than the insecticide isazophos, the control effect is within acceptable limits. In Japan (Table 4), field trials conducted by SDS Biotech in cooperation with biosys has displayed good levels of control against hunting billbug, S. venatus, with BioSafe (S. carpocapsae). Adults of this species are also susceptible. These results have made it possible to register BioSafe for commercial sale in Japan.

Another commercial success story in the United States has been the introduction of BioVector (S. carpocapsae) for control of the apopka weevil, Diaprepes abbreviatus, and the citrus root weevil, Pachnaeus litus. These two insects cause extensive root damage to citrus in the state of Florida. Thousands of hectares of citrus orchards and nurseries have been treated with BioVector. Table 5 and Table 6 show the results of a seven-month study examining adult insect emergence after a single, early-season application (Bullock, unpublished data).

The sweetpotato weevil, Cylas formicarius, is a serious pest of sweet potato all over the world. Research efforts have shown that there is a good potential for controlling this pest with entomopathogenic nematodes. Table 7 shows results from one recent study (Jansson et al. 1990).

Table 8 lists various "weevil" pests that are reported in the literature to be susceptible to entomopathogenic nematodes. As previously mentioned, some of these pests are now being controlled effectively with commercial entomopathogenic nematode-based products.

References

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• Mannion, C.M., and R.K. Jansson. 1992. Movement and postinfection emergence of entomopathogenic nematodes from sweetpotato weevil, Cylas formicarius (F.) (Coleoptera: Apionidae). Biological Control 2: 297-305.
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Discussion

In the Discussion, after the first session on entomopathogenic nematodes, one participant was interested in the fact that some nematodes are effective against Lepidoptera, while other nematode species are not. He wondered whether this might be related to some internal defence mechanism and/or external morphology which might determine whether the nematode can penetrate the membrane of the insect. Dr. Kaya pointed out that the dense hairs on the body of some species of larvae protect them against nematodes. Furthermore, Steinernema carpocapsae is one of the few nematode species which goes through nictating behavior standing on its tail, and can actually jump onto an insect host. Dr. Smith was asked about the cost of nematode applications to control e.g. turf billbugs. He replied that the cost varies according to the species of nematode, but at application rates of half a billion to three billion nematode nematodes per acre (1 to 7 billion per hectare), the cost would range from US$20 to US$200 per acre (approximately US50 to 500 per hectare).

Participants were also interested in formulation techniques. Dr. Smith explained that an infective juvenile has no mouth and cannot absorb food directly from a nutrient source. One formulation had extended the shelf-life to a commercially viable period by reducing the nematodes' metabolic rate. He pointed out that farmers do not like radically new products, and prefer those which resemble the conventional formulations they are used to. For this reason, commercial firms are now exploring the possibility of granule applications, and also of aerial spraying.

The important question was raised whether entomopathogenic nematodes have an effect on beneficial insects such as natural enemies of pests. Dr. Kaya replied that in general, if the natural enemies of an insect pest live above the ground, they are not affected by nematodes. If they are living in the soil there may be a possibility of infection, although in practice this did not seem to happen. He suggested that more work needs to be carried out on natural enemies which live in the soil, particularly predators. He also pointed out that insects such as scarab beetles are exposed to infection for only a short period of time during the larval stage, since the adults have a hard carapace which is resistant to nematode attack. One participant was interested in what would happen if a scarab beetle were to eat a nematode. Dr. Kaya answered that eating the nematode would probably not lead to infection of the beetle unless infective juveniles were very abundant. However, if a natural enemy were a parasitoid developing inside a larva, it would be exposed to entomopathogenic nematodes and would be killed by them. Dr. Smith pointed out that if a cockroach eats a nematode, the structure of the cockroach's stomach is such that the nematode is torn apart and there is no infection. There is considerable concern in Europe on the effect of nematodes on non-target species, and many studies are under way, but as yet no effect has been found. He pointed out that many different crops in the United States receive nematode applications every year. There is no information on whether there has been any change in the microbial populations of the soil as a result, but suggested that this would be an interesting topic of study. Dr. Smith felt that soil microorganisms seem to have little influence on nematode effectiveness, compared to abiotic factors such as soil moisture. He was asked whether any nematodes are known to eat insect eggs, and replied that in general, insect eggs do not seem to be susceptible. However the nematode Deladenus siricidicola in Australia invades the ovary of Sirex noctilio, which then lays eggs which contain nematodes. The eggs of bark beetles can also be infected with nematodes.

It was pointed out that not all infective juveniles are infectious, and that at most only 10-20% are infectious at any one time. An increase in this rate to e.g. 90% would greatly increase the effectiveness of nematode applications. Dr. Smith was asked whether the efficiency of infective juveniles in terms of their behavior had been considered. Dr. Smith replied that nictating behavior is dominant in young juvenile nematodes. As they age, their lipid content falls and they change from ambushing to hunting behavior. When S. carpocapsae was applied to insects such as weevils which are difficult to kill, there was a population reduction of 20%. Four weeks later, there was a population reduction of 60%. He was interested in the reason for this change in mortality rate, which might reflect a change in nematode behavior, and pointed out that it may not be advantageous for all nematodes to behave in the same way. Dr. Kaya supported this view, and pointed out that nematodes are part of an evolutionary process adapted to survival of the species. From the point of view of efficient use of available resources, it would be disadvantageous for all nematodes to become infective at the same time, however desirable this might be from the commercial point of view.

Table 7. Continued

Index of Images

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  Table 1 Field Efficacy of Entomopathogenic Nematodes against Black Vine Weevil, <I>Otiorhynchus Sulcatus</I>

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  Table 2 Susceptibility of Neonate Black Vine Weevil Larvae, <I>Otiorhynchus Sulcatus </I>to Various Steinernematid Species

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  Table 3 Field Efficacy of Entomopathogenic Nematodes against Bluegrass Billbug, <I>Sphenophorus Purvulus</I>, in the U.S.a.

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  Table 4 Field Efficacy against Hunting Billbug, <I>Sphenophorus Venatus</I> <I>Venatus,</I>

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  Table 5 Field Efficacy of Entomopathogenic Nematodes against Apopka Weevil, <I>Diaprepes Abbreviatus</I>

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  Table 6 Field Efficacy of Entomopathogenic Nematodes against Citrus Root Weevil, <I>Pachnaeus Litus</I>

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  Table 7 Field Efficacy of Entomopathogenic Nematodes against Sweetpotato Weevil, <I>Cylas Formicarius Elegantulus</I>

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  Table 8 Weevils That Are Susceptible to Entomopathogenic Nematodes

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