THE POTENTIAL OF BACTERIA FOR THE MICROBIAL CONTROL OF GRASSHOPPERS AND LOCUSTS

Bacteria have been implicated in disease epizootics observed in field populations and laboratory-reared locusts and grasshoppers. Two species [Serratia marcescens Bizio and Pseudomonas aeruginosa (Schroeter) Migula] consistently infect locusts when ingested with food and can spread in laboratory populations. However, research on developing these organisms for microbial control of locusts and grasshoppers begun in the 1950s has not been continued. In recent years strains of Bacillus thuringiensis Berliner have been studied for activity against locusts and grasshoppers. Results of additional trials by the authors are reported. Among 393 B. thuringiensis isolates and 93 preparations of other sporeforming bacteria fed to nymphs of Locusta migratoria (L.) and/or Schistocerca gregaria Forsk., none has shown any pathogenicity to the insects. The recent discovery of novel B. thuringiensis strains active against various diverse pests and the many properties of a sporeforming bacterium that satisfy the requirements for a microbial control agent, and the development of Serratia entomophila as a promising agent for control of grass grubs, provide incentive to continue the search for an orthopteran-active sporeforming bacterium and to re-investigate the potential of non-sporeforming bacterial pathogens as microbial control agents of grasshoppers and locusts.

CONTROL OF GRASSHOPPERS, PARTICULARLY HIEROGLYPHUS DAGANENSIS, IN NORTHERN BENIN USING METARHIZIUM FLAVOVIRIDE

Trials on the use of Metarhizium flavoviride Gams and Roszypal conidia in oil-based formulation for the control of grasshoppers, particularly Hieroglyphus daganensis Krauss, in Malanville, north Benin, are described. Preliminary work examined sprayer types, application rate, and time of application. In a trial on 4-ha plots with three replicates, M. flavoviride mycoinsecticide application to H. daganensis nymphs resulted in field population reductions of 70% after 14 days. In samples incubated in cages, mortality was higher in the samples taken 3 or 7 days after application than in the sample taken immediately after application, indicating the possibility of residual pick-up compared with direct spray impact in this environment. Significant mortality was still being observed in samples collected 37 days after application; to investigate this further, a method for bioassaying the spore load in the field was developed and used to monitor the spore load in the field. The possibility that the results indicate the occurrence of secondary infections resulting from horizontal transmission of M. flavoviride is discussed.

SAFETY AND REGISTRATION OF MICROBIAL AGENTS FOR CONTROL OF GRASSHOPPERS AND LOCUSTS

Microbial control agents offer a method of pest control using organisms that are a natural component of the environment and are usually much more selective than chemical pesticides. Furthermore, they can usually be integrated with other methods of control, and may provide prolonged control by establishment within the host population. However, microbial control agents also possess properties that can pose human and environmental risks depending on the nature of the pathogen and its pattern of use. We present an overview of issues concerning the safety and registration of microbial control agents with emphasis on pathogens of locusts and grasshoppers. The potential safety issues and other consequences of concern from the deployment of microorganisms for pest control are: (1) pathogenicity to non-target organisms, (2) toxigenicity to non-target organisms, (3) competitive displacement of microorganisms, and (4) allergenicity. Inundative control methods pose unique risks because the pathogens must be produced in large quantities, stored, transported, and applied, usually in concentrations much higher than would normally ever occur naturally. The overriding concern in introducing an exotic agent is the risk to non-target beneficial organisms, because once the agent becomes established, it will in most situations be impossible to eradicate. However, if indigenous organisms are used, there is relatively little risk of irreversible, long-term detrimental effects. A synopsis of safety testing results of some of the more promising microbial control agents for grasshoppers and locusts and an evaluation of their potential hazards are presented. Safety to vertebrates is evaluated by a tiered series of laboratory test requirements. Assessments on hazards to non-target invertebrates are based principally on results of laboratory bioassays. Safety tests should be chosen with regard to the biological characteristics of the agent and should not impose standards that are more stringent than those imposed on other forms of pest control. Regulatory oversight should assure the integrity of the environment and safety of the public, while at the same time not unduly hampering the development, registration, and use of more sustainable pest control methods.

FORMULATION OF ENTOMOPATHOGENS FOR THE CONTROL OF GRASSHOPPERS AND LOCUSTS

Successful development of a biological pesticide requires attention not only to the biological agent, but also to formulation, application, and the biology of the pest–pathogen interaction in the field. Emphasis in our review is given to fungi, Metarhizium spp. and Beauveria bassiana (Balsamo) Vuillemin, as the most suitable agents, and oil-based ULV formulations or baits as the most promising application techniques for use with locusts and grasshoppers. The efficacy of the pathogen isolate must be maximized; selection is aimed at those that are suitably virulent, specific, and well adapted to the relevant environmental conditions. Opportunities exist for manipulation of the characteristics of the isolate by genetic means and by developments in culturing techniques. Formulation requirements are stability during storage and the ability to carry the active ingredient successfully to the target insect at application. Likely storage methods for fungi would be as dry conidia, perhaps with clay diluents, or in oils; the characteristics of both are briefly discussed. At application, efficacy of dose transfer and protection of the biological agent against environmental constraints such as UV radiation are needed. Baits have advantages in terms of dose transfer but logistical problems associated with the bulkiness of the carrier remain. Technological advances, including those that offer the prospect of carrier production in situ from dense precursors, and better knowledge of feeding behaviour have improved the prospects for baits. Multi-disciplinary research reducing dependency on the biological agent and exploiting formulation chemistry and application technology is required in developing biological pesticides.

METARHIZIUM FLAVOVIRIDE (FI985) AS A PROMISING MYCOINSECTICIDE FOR AUSTRALIAN ACRIDIDS

Only one isolate of Metarhizium flavoviride Gams and Roszypal group 3 has been isolated from a field-infected acridid in Australia. This is isolate FI985 (ARSEF 324) obtained from a spur-throated locust, Austracris guttulosa (Walker), near Rockhampton, Queensland, in 1979. In terms of conidial size and shape as well as phialide morphology, FI985 is intermediate between Metarhizium anisopliae (Metschnikoff) Sorokin and M. flavoviride. It has been compared with other group 3 isolates using RAPDs and sequence analysis of the ITS region and found to be very similar. However the analysis shows that these group 3 isolates are genetically closer to M. anisopliae than to M. flavoviride sensu stricto. Laboratory bioassays have shown that FI985 is virulent for five species of acridid pests in Australia. Comparative bioassays with other isolates of Metarhizium, including other group 3 isolates from Africa and Asia, have not yet revealed any isolate more virulent than FI985. This isolate is amenable to mass-production on rice and has been formulated in oil as a mycoinsecticide. The results from six field tests, mostly against wingless grasshopper, Phaulacridium vittatum (SjÖstedt), using doses of 2–7 × 1012 conidia per hectare and plot sizes up to 50 ha are summarized. These trials (with the exception of the first against the Australian plague locust) have given high levels of disease-related mortality in caged samples of the target collected within 3 days of spraying. In the four trials with wingless grasshopper, population reductions were detected 10–30 days after application; however these reductions were much less than suggested by cage samples as a result of movement of the target acridids. In contrast, positive control plots sprayed with fenitrothion gave a very high initial kill (>90% in 1 day) but were then more rapidly reinvaded. Consequently, 3–4 weeks after spraying the density in the plots treated with chemical insecticide and those treated with mycoinsecticide were similar. Further field trials are needed especially against the Australian plague locust and evaluating lower doses. The results obtained to date show that a mycoinsecticide based on FI985 is likely to be effective over a wide range of target acridids and weather conditions.

METHODS OF APPLICATION OF MICROBIAL PESTICIDE FORMULATIONS FOR THE CONTROL OF GRASSHOPPERS AND LOCUSTS

The use of chemical insecticides, especially as ultra low volume (ULV) formulations, against locusts and grasshoppers will continue for the foreseeable future; therefore application techniques for microbial agents should be as compatible as possible with existing practice. Low volume and ULV spraying of deuteromycete conidia in oil-based formulations have produced very promising acridid control results in the field, although baiting, dusting, and hydraulic application techniques have also been tested for a wide range of pathogens.The key problems for further research and development appear to be the logistics and supply of consistently reliable formulations for application on a large scale, and the determination of mechanisms for effective dose transfer in the field. The application of suspended particulate matter can present special problems with rotary and other atomizers.

LABORATORY AND FIELD EVALUATIONS OF BEAUVERIA BASSIANA (BALSAMO) VUILLEMIN AGAINST GRASSHOPPERS AND LOCUSTS IN AFRICA

Two isolates of the fungus Beauveria bassiana (Balsamo) Vuillemin, GHA and BF, were evaluated in Cape Verde in 1991 and 1992 for infectivity to the Senegalese grasshopper, Oedaleus senegalensis (Krauss), and the migratory locust, Locusta migratoria migratorioides (Reiche and Fairmaire). Evaluations included laboratory bioassays and small-scale field trials. Laboratory bioassays evaluated five different formulations. Four of the formulations tested showed strong dose–response patterns and significantly higher mortality than the untreated control or carriers minus spores. All four formulations achieved high mortality levels when applied at economically feasible dose rates. The GHA and BF isolates, formulated in an oil carrier with an emulsifier, were equally infectious to migratory locust nymphs. Six different formulations of GHA were evaluated in field trials. Field trials evaluated both direct effects (treatment of field plots infested with O. senegalensis) and indirect effects (treatment of plots without grasshoppers, after which grasshoppers were introduced). In both cases, all six formulations showed good biocontrol potential. Grasshoppers exposed to treated plots up to 72 h after application exhibited comparatively high mortality levels, indicating that large numbers of spores remained viable in the field for at least 3 days. This was confirmed by analysis of the viability of conidia from vegetation samples obtained in the field following treatment. In open-plot, small-scale field trials, two different formulations (oil and clay-based) of GHA resulted in high rates of infection and approximately 45% reductions in grasshopper densities in the treated plots 7 days after application, even though density-reduction results were "diluted" by grasshopper migration into and out of the test plots. Results of the Cape Verde evaluations demonstrate that biopesticides developed from B. bassiana represent a promising alternative to chemical pesticides for grasshopper and locust control.

DEVELOPMENT OF BEAUVERIA BASSIANA FOR CONTROL OF GRASSHOPPERS AND LOCUSTS

Recognition of the potential of Beauveria bassiana (Balsamo) Vuillemin as a control agent of grasshoppers and locusts occurred as early as 1936, in South Africa. Field testing of B. bassiana as an inundative control agent of grasshoppers and locusts has been facilitated by development of a solid substrate method for mass-production of the fungus and has resulted in the registration of a strain against grasshoppers in the United States. In some, but not all field trials, application has resulted in substantial reductions in grasshopper populations. Numerous environmental constraints, including temperature and ultraviolet (UV) radiation, may limit field efficacy of the fungus. Laboratory studies suggest that low humidity does not limit the ability of the fungus to initiate disease. Sunlight is the major cause of mortality of conidia on leaf surfaces. The incorporation of UVB protectants in formulations can increase conidial survival; however, these have not yet been evaluated for their effects on field efficacy of B. bassiana against insects. Thermoregulation by grasshoppers has been implicated in resistance to mycosis. Results of laboratory studies indicate that grasshoppers infected with B. bassiana preferentially seek temperatures between 40 and 42 °C and these temperatures are inhibitory to disease development. In field-cage trials, a higher prevalence and more rapid development of disease were observed in grasshoppers placed in shaded cages than in grasshoppers placed in cages exposed to full sunlight. In laboratory experiments simulating grasshopper thermoregulation during daylight periods, application of both Metarhizium flavoviride Gams and Rozsypal and B. bassiana simultaneously resulted in a final prevalence of disease that was greater than M. flavoviride alone in the hot temperature environment, and equal to B. bassiana alone in the cool temperature environment. Incorporation of sublethal levels of Dimilin with conidia of B. bassiana increased efficacy of the fungus against grasshoppers in laboratory and field trials. Once environmental constraints are better quantified, it may be possible to overcome them through improved formulation, strain selection, genetic or phenotypic manipulation, and inoculum targeting. Ultimately, success of B. bassiana as a microbial control agent will depend on our ability to overcome environmental and other constraints and/or to predict its efficacy under various environmental conditions.

STRATEGIES FOR THE USE OF ENTOMOPATHOGENS IN THE CONTROL OF THE DESERT LOCUST AND OTHER ACRIDOID PESTS

Large-scale applications of non-persistent but broad-spectrum chemical insecticides in Africa during the 1980s for control of acridoid pests, particularly the desert locust (Schistocerca gregaria Forskål) and Sahelian pest grasshoppers, raised concern about environmental damage and human safety. Similar concerns have been expressed in Australia, the United States, and Canada and have led to a search for alternative strategies. To lessen dependence on chemicals, an integrated pest management (IPM) approach for grasshopper control has been encouraged in the United States with emphasis on biological control as an important component and this is also desirable elsewhere, but additional biocontrol components are needed. Current strategies for most pest acridoids rely on short-term destruction of outbreak populations. Nymphs are the preferred target wherever possible and inundative augmentation of entomopathogenic deuteromycete fungi formulated as biopesticides could replace chemical spraying in some cases, especially where the major threat is to crops remote from the pest breeding areas. Entomopathogens are slower acting than chemicals and thus best suited for use where the pest is not immediately threatening to crops. Schistocerca gregaria and Oedaleus senegalensis Krauss pose particularly difficult problems because of the very large area and inaccessibility of their potential breeding grounds, their very sudden upsurges, and their great mobility as adult swarms. Fast-acting chemicals are likely to be needed when rapid intervention is required to control these pests, but an IPM strategy could incorporate biopesticide application in the early stages of upsurges and also be used for swarm control in some cases. However, improved prediction and monitoring are needed to facilitate the use of biopesticides and other IPM techniques against these pests.