The potential of Paranosema (Nosema) locustae (Microsporidia: Nosematidae) and its combination with Metarhizium anisopliae var. acridum (Deuteromycotina: Hyphomycetes) for the control of locusts and grasshoppers in West Africa

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Locusts and grasshoppers are the most treating agricultural pests that represent with drought the first reason of famine in Sahelian regions of Africa. Concern about environmental and toxicological issues resulting from wide spread application of synthetic pesticide has stimulated studies on the development of microbiological insecticides based on Microsporidian Paranosema locustae Canning and the entomopathogenic fungus Metarhizium anisopliae var. acridum Driver and Milner. The main objectives of this study were to: (i) investigate the potential of P. locustae to control locusts and grasshoppers (Chapters 2 and 3) and (ii) the possibility of its combination with M. anisopliae to increase the efficacy of the two pathogens in regulation of locust and grasshopper populations in West Africa (chapters 4 and 5).
The evaluation of the relative susceptibility of developmental stages of Oedaleus senegalensis and Schistocerca gregaria to different dosages of P. locustae indicated both lethal and sublethal effects of the pathogen against the hosts. Considering the lethal effects all tested nymphal stages suffered from significantly higher mortality compared to the control with always the younger nymphal instars being more susceptible compared to the older. Sublethal effects of P. locustae in the tested hosts included among other, delayed and abnormal development of the infected hosts, reduced host weight and vertical transmission of the pathogen from infected adults to the offspring. Laboratory investigation of the interaction of P. locustae and M. anisopliae indicated that additional inoculation with M. anisopliae following infection with P. locustae cause additive and synergistic mortality response of the two pathogens in fifth instar S. gregaria, resulting in both a significant reduction of Median Survival Time (MSTs) and a higher mortality in mixed infections compared to the single infection. Significantly fewer P. locustae spores were yielded in mixed infections compared to nymphs infected with P. locustae alone. However, M. anisopliae production was not affected by the mixed infections. Last but not least, based on the laboratory bioassay testing the relative susceptibility of grasshopper and locust developmental stages to P. locustae, a field trial was designed to target the younger instar of grasshopper. The results showed that within season grasshopper population reduction might be feasible when the pathogen is applied earlier after hatching. Moreover combined application of P. locustae and M. anisopliae under field condition confirmed that mixed application of the two pathogens could help to increase host population reduction in short time periods.
In the light of the results from the laboratory bioassays testing different concentrations of P. locustae spores against different nymphal instars of O. senegalensis and S. gregaria, the effects of P. locustae on the two species can be classified as direct and indirect effects. The direct host mortality effect of P. locustae is greatly pathogen concentration dependent with younger instars being more susceptible than older instars. The longer Median Survival Time (MST) recorded in older nymphal developmental stage compared to the younger, confirms the high susceptibility of the latter. Almost 100% mortality can be reached in first and second instars in a maximum of 10 days while most fourth and fifth instar nymphs can survive P. locustae infection up to adulthood. Mortalities recorded when targeting the host at older stages remain low irrespective of the pathogen dosages. Although some of the results show a low susceptibility of the tested host to the pathogen (e.g., low concentrations against fifth instars, the indirect sublethal effect of the pathogen can be considered as an additional important factor that can contribute to overcome its low virulence. Younger instars infected at low P. locustae spore concentrations that did not die from the pathogen infection in most cases did not reach adulthood and remained small in their size with low weight. Moreover common malformations of wings and hind legs observed in infected nymphs that developed to adults could affect the population dynamics of grasshoppers and locusts since morphologically affected hosts are unlikely to move and fly as normal healthy ones. Only infected adults that have survived the pathogen infection were able to mate and produce eggs with very high potential to transmit P. locustae to the hatching nymphs. However the slow development of P. locustae infection remains one important limitation in the use of this pathogen since for the grower the first and probably the only objective is to kill the pest fast and with high efficacy to avoid resurgence of the pest and need for repeated treatments.
Our laboratory study testing the host mortality response to the combination of P. locustae and M. anisopliae indicated that a combination of the two pathogens increased significantly host acute mortality with consistent reduction in host survival time. This is further supported by the fact that after inoculation of Paranosema-infested fifth instar nymphs, total mortality rates were recorded within 4 to 7 days in host infected at the highest concentrations of the two pathogens. However, although the two pathogens showed ability to develop in the same host, the production of P. locustae spores was significantly lowered in dual infection. A successful integration of the two control agents, i.e. P. locustae and M. anisopliae var. acridum, depends on the possibility to minimize direct and indirect detrimental effects on the relatively less virulent P. locustae.
Application of P. locustae alone or in mixture with sugar or M. anisopliae resulted in significant grasshopper density reduction in three weeks, with up to 40% P. locustae infection in surviving grasshopper in plots that have received P. locustae spores, suggesting the possible persistence of the pathogens in the following generations. The general ability of M. anisopliae var. acridum and P. locustae to infect locusts and grasshoppers (Brooks, 1988; Henry, 1990; Langewald et al., 1999), the high relative humidity (leading to a >90%) and temperature (24-31 °C) at the time of pathogen applications, which correspond with the optimum growth conditions for both pathogens (Ouedraogo et al., 1997; Hallsworth and Magan, 1999; Lowman et al., 2000) might have contributed to the high infection and density reduction recorded in the present experiment. Moreover, the relatively high predominance of younger instars of the Senegalese grasshopper, O. senegalensis (>76% were represented by first, second and third instar nymphs) expands the previous laboratory observations on the high virulence of P. locustae against young grasshopper and locust instars. The field trial confirmed the efficacy of combination of P. locustae and M. anisopliae for control of locust and grasshopper populations.
Results obtained from this study suggest that P. locustae is of a value as long-term biological control agent of S. gregaria and O. senegalensis in West Africa. Although P. locustae does not cause conspicuous mortality as do the chemicals currently used, its increased developmental time, the decreased survival of the infected host, and the fact that infected nymphs are apparently unable to reach adulthood, would have an impact on the intrinsic rate of population increase, leading to lower population density levels in the long term. In our study we have observed that both O. senegalensis and S. gregaria readily consumed wheat bran the way in which P. locustae is delivered (Henry and Oma, 1981). In large sector of the locust and grasshopper outbreak area, ground cover is relatively poor, meaning that nymphs would easily locate baits.
Pathogens used in the present study were derived from commercial products (M. anisopliae var. acridum from Green Muscle® IITA-Benin and P. locustae from M and R Durango, Inc. P. O. Box 886 Bayfield, CO 81122 USA). This suggests that microbial control of locusts and grasshoppers combining the two pathogens in bait could be readily implemented. Innovative products such as P. locustae and M. anisopliae formulations, however, are unlikely to be accepted unless they are easy to handle. The oil-based M. anisopliae formulation has proved to be easy to handle, involving a minimal amount of preparation before spraying, and thus helped to achieve an acceptably high work rate (Douro-Kpindou et al., 1995; Bateman et al., 1997; Johnson, 1997; Langewald et al., 1999; Lomer et al., 1999; Lomer et al., 2001). Interestingly, manual application of wheat bran formulation of P. locustae and M. anisopliae in the present study has proven to induce consistent pathogen infections in grasshopper populations in Sahel. Such method could be more appropriated for the Sahelian growers given the small handled quantity of bran (2kg) applied per hectare. Compared to the use of toxic pesticide that requires or need more protection or risk avoiding technique for application, the hand spreading method required little work and less experiment from the operator.