Novel Strategies for Biopesticide

In some cases, preexisting application technology may not be well suited to the requirements of a biological agent. One approach that takes advantage of the biological nature of entomopathogenic fungi is the "lure and infect" approach, best demonstrated by research on Z. radicans for control of diamondback moth. Furlong et al. (1995) have shown that using pheromone lures to attract moths to traps containing sporulating Z. radicans can result in contamination and spread of the fungus through the target population. Such an approach has been investigated for use with scarab beetles in the Azores (Klein and Lacey 1999). Autodissemination of entomopatho-genic fungi for control of Popillia japonica in the Azores used a trapping system of commercially available attractants with M. anisopliae. The viability of conidia in traps after 6 days was found to be about 35%, but the basic process was successful for introducing fungi into pest populations. Another approach has been bait stations, such as those used with termites (Rath 2000). The entomopathogenic fungus is placed in a trap together with a food-based bait, and the insect becomes contaminated when it enters the trap. The general approach is similar for lure-and-infect and bait stations: attract the insect to an inoculum source, rather than broadcast application to secure contact between pest and disease.

Use of attractants is not restricted to luring to a single trap. Smith et al. (1999) investigated the use of vegetable fat pellets formulated with pheromone and B. bassiana to control the larger grain borer, Prostephanus truncates. Significantly higher numbers of beetles were attracted to pellets containing pheromone than those without pheromone incorporated. The pellets containing pheromone and fungus could be stored for several weeks, indicating this may be a useful strategy to increase the utility of entomopathogenic fungi.

Development of biopesticides for social insects has been problematic because the method by which social insects defend against disease is mainly behaviorally-based rather than biologically-based. For example, hymenopteran wasps such as Vespula spp. have well-developed hygienic behaviour which includes removing all suspected material from a nest before contamination of nestmates occurs. Vespula do not reuse nests and, therefore, disease in one season does not result in disease in another season. Behavioral defense against disease requires novel application and formulation methods for any chance of success for entomopathogenic fungi. Similarly, termites are highly susceptible to entomopatho-genic fungi, including M. anisopliae and B. bassiana but many factors such as avoidance of conidia, the removal and burial of fungus-killed termites, together with defensive secretions and inhibitory components in termite frass (Rath 2000), and grooming to remove spores (Milner and Glare, unpublished observations) reduce field efficacy. Boucias et al. (1996) used a low sublethal dose of a neurotoxin, imidacloropid to disrupt the grooming behaviour of termites, which then became highly susceptible to the fungus B. bassiana.

One proposal is to use more than one pathogen to increase the utility of entomopathogenic fungi. It is often common in the field to find more than one pathogen exerting influence on a pest, such as both a nucleopolyhedrovirus and the fungus Entomophaga maimaiga infecting gypsy moth (Malakar et al.

1999). The possibility of combining multiple species or strains in a single biopesticide to overcome limitations inherent in the single strain approach is intriguing. For example, Inglis et al. (1997) have investigated the use of both M. anisopliae and B. bassiana for control of locusts and grasshoppers, to overcome the temperature limitations of both species. There are potentially many methods whereby the efficacy of biopesticides could be enhanced by combinations, such as those described earlier, but the economics of producing multiple pathogens for a single product are usually too limiting.

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