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Section I: Biological Control (2 of 12)
I.2 Nosema locustae
D. A. Streett
Introduction
Unpredictability
of Nosema
Detection
of Nosema locustae
Nosema Transmission
Effect
on Grasshopper Egg Production
Conclusions
References
Introduction
Grasshoppers are the most economically important insect pests on rangeland in the Western United States (Hewitt and Onsager 1982). A conservative estimate for the average value of rangeland forage loss to grasshoppers in the West each year is about $393 million (Hewitt and Onsager 1983). Since the late 1960’s, controlling major infestations of grasshoppers on rangeland has involved the use of chemical insecticides, primarily malathion and carbaryl. However, increasing awareness of the environmental risk associated with the exclusive use of chemical insecticides led to the establishment of the Grasshopper Integrated Pest Management (GHIPM) Project.
Disease-causing micro-organisms have been investigated as potential biological control agents of grasshoppers for many years. Probably the most well-known case has been the parasite Nosema locustae, a pathogen that was selected in the early 1960’s for development as a microbial control agent for use in long-term suppression of grasshoppers (Henry 1978, Onsager 1988). Nosema locustae is the only registered microbial agent that is commercially available for control of rangeland grasshoppers.
Nosema has been studied more than any other microbial control agent for the suppression of grasshopper populations. Applications of Nosema formulated on a wheat bran bait have resulted in numerous successful introductions of the pathogen into field populations. However, while this parasite has proven a potentially effective tool in grasshopper management, several questions have been raised regarding the effectiveness of Nosema in the field.
Unpredictability of Nosema
Vaughn et al. (I.4) attributed the apparent failures of Nosema to low-quality material, equipment failure, poor formulation, inappropriate target species, and unreasonable expectations by users. Onsager (1988) also discussed some of the reasons for this lack of confidence in Nosema for controlling grasshopper populations. He noted that the traditional sampling approach used to estimate grasshopper reductions in field trials with chemical insecticides may not be appropriate to assess the effectiveness of Nosema. Typically Nosema requires much longer to kill a grasshopper than chemicals. Grasshoppers are then able to disperse and conceal differences between treated and control plots.
Reuter et al. (1990) suggested that the standard application rate of Nosema (1 x 109 spores/acre) was too low to induce immediate grasshopper population suppression. In a field evaluation, an untreated control plot was compared to plots receiving either the standard rate (1 x 109 spores/acre) or a higher (100 x) rate (1 x 1011 spores/ acre) of Nosema. Density estimates were taken weekly, and bottomless field cages and small rearing cages were used to estimate mortality. The lack of treatment replication, the small plot size, and the close proximity of plots made it impossible to draw firm conclusions about the grasshopper densities or relative rates of suppression after treatment. However, significant mortality was observed at the higher application rate for Melanoplus sanguinipes in the small rearing cages 7 weeks after application (Reuter et al. 1990). These preliminary mortality results lend support to Henry’s (1981) contention that applying higher dosages of Nosema will not necessarily produce a commensurate gain in density reduction.
A more immediate density reduction has been demonstrated in field studies using wheat bran bait formulations of Nosema and carbaryl in which significant short-term response to carbaryl was followed by a later response to N. locustae (Onsager et al. 1981). Further studies on the response of grasshoppers to higher application rates of Nosema may be warranted.
A review of the literature on the effectiveness of Nosema in the field identifies dispersal as a common problem. Movement between plots was cited as affecting results in six of eight studies that evaluated the effects of Nosema in the field (Henry 1971; Henry and Oma 1974, 1981; Henry and Onsager 1982; Henry et al. 1973, 1978). Only Johnson and Henry (1987) suggested that there was little movement of infected individuals into control plots within 31 days of application.
Detection of Nosema locustae
In the past, visual examinations with phase contrast microscopy for spores have been required to detect Nosema infection in grasshoppers. Generally, Nosema spores are detectable about 21 days after application (Henry and Oma 1974). Most protocols recommend microscopic examinations at 28 days following application (Henry 1978). Thus, it has not been possible to assess some of the earlier events in a Nosema treatment program.
Dispersal and death that occur prior to the detection of Nosema reduce estimates of its presence in the field. Early detection of Nosema infections is therefore necessary to obtain unbiased estimates of initial prevalence. Scientists have developed a sensitive nucleic acid probe for the detection of Nosema in grasshoppers. Data indicate that the probe can reliably detect Nosema in grasshoppers within 7–10 days after infection. Use of a probe to estimate infection rates should eliminate much of the inherent bias associated with visual examination.
Nosema Transmission
A recent laboratory study by Raina et al. (1995) has reported transovarial transmission of N. locustae in Locusta migratoria migratorioides with the incidence of infection ranging from 72 percent to 92 percent among progeny up to the F7 generation. N. locustae spores also were found in all nymphal instars for the F1 and F2 generations.
The mechanisms and rates of Nosema transmission in the field have not been addressed adequately. Spores have been observed in feces (Henry 1969 unpubl.), but the scavenging of Nosema-infected cadavers by healthy grasshoppers may represent the greatest potential for transmission to uninfected grasshoppers of the same generation. Scavenging of cadavers is common in many species of grasshoppers (Lavigne and Pfadt 1964, Lockwood 1988). Henry (1969 unpubl.) observed feeding on Nosema-infected cadavers in the field. Scavenging may offer a very efficient means for transmission of Nosema during the year of treatment and possibly into later generations (O’Neill et al. 1994).
Spores of Nosema have been observed in ovaries from and in eggs produced by infected females (Henry 1969 unpubl.). Although Ewen and Mukerji (1980) were unable to find spores in eggs collected from Nosema-treated plots, they did observe Nosema infection among nymphs raised from field-collected eggs. Henry and Onsager (1982) also reported infection in grasshopper populations during the year after treatment. These observations indicate that transmission to subsequent generations is indeed likely, but the details of Nosema transmission in field populations of grasshoppers have never been fully explained.
Effect on Grasshopper Egg Production
Nosema-infected females produce fewer eggs than healthy females (Henry and Oma 1981). Henry (1969, 1971) reported detecting little ovarial or egg debris in infected grasshoppers that were ground up, which suggests that infected females fail to develop reproductively. Ewen and Mukerji (1980) reported substantially lower rates of egg laying after applications of Nosema in the field. Henry and Oma (1981) suggested the need to measure the effects of Nosema on egg numbers and egg viability. Lockwood and Debrey (1990) also observed some evidence of lower egg production in higher populations (greater than 11.5 grasshoppers/yd2 or 9.6 grasshoppers/ m2 ) of grasshoppers treated with Nosema.
Conclusions
Until the reasons for the inconsistent response of Nosema to grasshoppers are better understood, its effectiveness will probably continue to be disputed (See I.4). The grasshopper species complex, the age of the grasshoppers, and population density can affect the response to a Nosema application. Therefore, a more comprehensive approach is needed to adequately assess Nosema against grasshoppers. This approach must include a better understanding of the major disease processes of Nosema. Vaughn’s team (I.4) recommends that Nosema be used to suppress rangeland grasshoppers in environmentally sensitive areas where cost and acute insecticide control are not primary concerns and proposes the use of higher rates and/or multiple applications when environmental issues outweigh the economic issues.
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References Cited
Ewen, A. B.; Mukerji, M. K. 1980. Evaluation of Nosema locustae (Microsporidia) as a control agent of grasshopper populations in Saskatchewan. Journal of Invertebrate Pathology 35: 295–303.
Henry, J. E. 1971. Experimental application of Nosema locustae for control of grasshoppers. Journal of Invertebrate Pathology 18: 389–394.
Henry, J. E. 1978. Microbial control of grasshoppers with Nosema locustae Canning. In Selected topics of the genus Nosema. Miscellaneous Publications of the Entomological Society of America 11: 85–95.
Henry, J. E. 1981. Natural and applied control of insects by protozoa. Annual Review of Entomology 26: 49–73.
Henry, J. E.; Oma. E. A. 1974. Effects of infections by Nosema locustae Canning, Nosema acridophagus Henry, and Nosema cuneatum Henry (Microsporida: Nosematidae) in Melanoplus bivittatus (Say) (Orthoptera: Acrididae). Acrida 3: 223–231.
Henry, J. E.; Oma, E. A. 1981. Pest control by Nosema locustae, a pathogen of grasshoppers and crickets. In Burges, H. D., ed. Microbial control of pests and plant diseases 1970–1980. New York: Academic Press: 573–586.
Henry, J. E.; Onsager, J. A. 1982. Large-scale field test of control of grasshoppers on rangeland with Nosema locustae. Journal of Economic Entomology 75: 31–35.
Henry, J. E.; Tiahrt, K.; Oma, E. A. 1973. Importance of timing, spore concentrations, and levels of spore carrier in applications of Nosema locustae (Microsporida: Nosematidae) for control of grasshoppers. Journal of Invertebrate Pathology 21: 263–272.
Henry, J. E.; Oma, E. A.; Onsager, J. A. 1978. Relative effectiveness of ULV spray applications of spores of Nosema locustae against grasshoppers. Journal of Economic Entomology 71: 629–632.
Hewitt, G. B.; Onsager, J. A. 1982. Grasshoppers: yesterday, today and forever. Rangelands 4: 207–209.
Hewitt, G. B.; Onsager, J. A. 1983. Control of grasshoppers on rangeland in the United States––A perspective. Journal of Range Management 36: 202–207.
Johnson, D. L.; Henry, J. E. 1987. Low rates of insecticides and Nosema locustae (Microsporidia: Nosematidae) on baits applied to roadsides for grasshopper (Orthoptera: Acrididae) control. Journal of Economic Entomology 80: 685–689.
Lavigne, R. J.; Pfadt, R. E. 1964. The role of rangeland grasshoppers as scavengers. Journal of Kansas Entomological Society 37: 1–4.
Lockwood, J. A. 1988. Cannibalism in rangeland grasshoppers (Orthoptera: Acrididae): attraction to cadavers. Journal of the Kansas Entomological Society 61: 379–387.
Lockwood, J. A.; DeBrey, L. D. 1990. Direct and indirect effects of Nosema locustae (Canning) (Microsporidia: Nosematidae) on rangeland grasshoppers (Orthoptera: Acrididae). Journal of Economic Entomology 83: 377–383.
O’Neill, K. M.; Streett, D.; O’Neill, R. P. 1994. Scavenging behavior of grasshoppers (Orthoptera: Acrididae): feeding and thermal responses to newly available resources. Environmental Entomology 23: 1260–1268.
Onsager, J. A. 1988. Assessing the effectiveness of Nosema locustae for grasshopper control: traditional insecticide-based sampling criteria cannot accurately evaluate efficacy of Nosema. Montana AgResearch 5: 12–16.
Onsager, J. A.; Rees, N. E.; Henry, J. E.; Foster, N. 1981. Integration of bait formulation of Nosema locustae and carbaryl for control of rangeland grasshoppers. Journal of Economic Entomology 74: 183–187.
Raina, S. K.; Dos, S.; Rai, M. M.; Khurad, A. M. 1995. Transovarial transmission of Nosema locustae (Microsporida: Nosematidae) in the migratory locust Locusta migratoria migratorioides. Parasitology Research 81: 38–44.
References Cited—Unpublished
Henry, J. E. 1969. Protozoan and viral pathogens of grasshoppers. Ph.D. dissertation. Bozeman, MT: Montana State University. 153 p.
Reuter, K. C.; Foster, R. N.; Hildreth, M.; Colletto, D.; Cushing, W. J.; Pucelik, M. J.; Kohler, D.; Houston, R.; Scott, A. 1990. Preliminary investigation of the effect of a greatly increased rate of Nosema locustae on rangeland grasshopper populations. In: Cooperative Grasshopper Integrated Pest Management Project, 1990 annual report. Boise, ID: U.S. Department of Agriculture, Animal and Plant Health Inspection Service: 165–174.
