Search

The survival and persistence of Nosema locustae Canning in field soils and resident grasshopper populations in Saskatchewan were assessed in a 3-year study. Low levels of N. locustae spores persist in soils but not on vegetation from treated fields. Large numbers of spores were sometimes detected in soil depending on the extent of infection in the resident grasshopper populations. Indigenous soil microorganisms appear to prey upon N. locustae spores or utilize nutrients during spore decomposition. Nosema locustae infection in resident grasshoppers varied, depending on the viability of the spores applied, and its level was higher following repeated (3-year) applications. Low numbers of N. locustae spores persist in soil but large populations probably will not accumulate because of leaching and because of their interaction with soil microorganisms. Spores appeared to leach through soil as a function of water movement.

Nosema acridophagus Henry was shown to be more virulent than N. cuneatum Henry to 3rd-instar nymphs of Melanoplus sanguinipes (Fab.) following peroral inoculation with 1.0 × 106, 105, 104, or 103 spores/individual. The lethal time was dose dependent and significantly shorter and the lethal dose (LD50) was significantly lower in grasshoppers inoculated with N. acridophagus than for those inoculated with N. cuneatum. Grasshoppers inoculated with N. acridophagus died earlier and a smaller proportion survived to the adult stage compared with those inoculated with N. cuneatum. Nosema acridophagus was more infectious at the lower doses than was N. cuneatum; however, N. cuneatum reached an intensity of infection in host tissues which was 2–10 times higher. Both microsporidians appeared to have similar pathogenicity for both individually and group-reared M. sanguinipes.

Second- and 3rd-instar nymphs of Melanoplus sanguinipes (Fab.) were more susceptible to Nosema cuneatum Henry infection than were 5th-instar nymphs. Lethal times were significantly lower for 2nd- and 3rd-instar nymphs inoculated with 1.0 × 106 and 104N. cuneatum spores/individual than for similarly treated 5th-instar nymphs and a much higher percentage of the latter were uninfected at the end of the experimental period (40–60 days post-inoculation). Fifth-instar nymphs inoculated with 1.0 × 106 spores/individual showed reduced adult longevity. The number of eggs laid per female was not significantly different for inoculated or untreated 5th-instar nymphs. However, the dry weights of eggs laid by inoculated grasshoppers were significantly lower and fewer eggs hatched than for those laid by untreated grasshoppers. Twenty to 30% of the progeny from inoculated 5th-instar nymphs had detectable levels of N. cuneatum sporoblasts and spores.

The dosage of carbofuran as a bait required to reduce third-instar grasshopper populations to non-economic levels (5–10 grasshoppers per square metre in short mixed grass hay pasture) was considerably lower than the recommended rate for spray application. The results from 2 years’ field trials showed that 35.0 g active ingredient per hectare as a bait would reduce grasshopper populations by about 70%, 3 days after application.

Bran treated with four different insecticides was field-tested as a crop protection measure against mainly third instar grasshopper populations composed mostly of Melanoplus sanguinipes (Fab.). The order of toxicity of these four insecticides was dimethoate 〉 fenvalerate 〉 propoxur 〉 pyridaphenthion. Five dosages of dimethoate were tested against mainly adult grasshopper populations. Rates of mortality due to both aqueous sprays (previously published results) and bran baits were compared. Degradation of both dimethoate and fenvalerate mixed with bran was small when stored in the laboratory at either 1°C or 20°C.

Cypermethrin was tested at nine dosages from 2 to 72 g AI/ha and at five temperatures from 15 °to 35 °C against second-instar nymphs of a non-diapause strain of the migratory grasshopper, Melanoplus sanguinipes (Fab.). At lower dosages, mortality increased with increasing temperatures from 15 °to 30 °C and then leveled off. At dosages of 12 g AI/ha or higher, mortality increased slightly with increasing temperature from 15 °to 20 °C, then declined at higher temperatures. Cypermethrin was about 2.2 times less toxic (LD50) at 35 °C than at 15 °or 20 °C and 70% of this reduction in toxicity occurred above 25 °C. Thus, the toxicity of cypermethrin was negatively correlated with temperature above 20 °C for nymphs of the migratory grasshopper.

Cypermethrin and carbaryl, as both aqueous sprays and bran baits, were tested in the field as crop protection measures against third-instar grasshopper populations infesting bromegrass and alfalfa–bromegrass pastures. Application of 15 g cypermethrin/ha as a spray reduced the grasshopper populations by 97% within 120 h; 700 g carbaryl/ha as spray reduced the populations by 90% during the same period of time after application. Neither insecticide applied on bran bait sufficiently reduced populations to be useful as crop protection measures against grasshoppers.

The life history and ultrastructure of the protozoan Malameba locustae (King and Taylor) were studied in the migratory grasshopper Melanoplus sanguinipes (F.) using feeding and injection studies. Insects fed cysts developed infection in the Malpighian tubules 5–6 days later; no trophozoites were observed in haemolymph samples taken 2–20 days post-feeding. After excystment, a few trophozoites entered the midgut epithelium and many were located near the basement membrane of the epithelial cells, where they appeared to degenerate. Trophozoites were not seen to divide in the midgut epithelium, and apparently did not damage this tissue. Trophozoites injected directly into the haemocoel could not be recovered even 4 h after injection, and the Malpighian tubules did not become infected. It was concluded that trophozoites did not penetrate the midgut to enter the haemocoel or move through the haemocoel to infect the Malpighian tubules, but instead probably entered the tubules directly from the gut. Trophozoite ultrastructure in midgut and Malpighian tubules, and cyst wall deposition were described.

The synthetic ability of type-A neurosecretory cells (NSC) and corpus allatum (CA) was investigated autoradiographically in both normal virgin females and those from which the frontal ganglion had been removed. Incorporation of3H-cystine and3H-uridine into the NSC was significantly lower in frontal ganglionectomized insects than in controls. There appeared to be no relationship between the level of isotope uptake and the amount of stainable material in the NSC. Ability of the CA to incorporate3H-uridine seemed unaffected by the operation.

The potential usefulness of Nosema locustae Canning as a microbial agent for grasshopper control was first demonstrated by Henry (1971) and Henry et al. (1973). These authors found that a 50–60% reduction in grasshopper density (as compared with untreated populations) could be achieved, and they speculated that 35–50% of the survivors would be sufficiently infected so that their fecundity would be reduced. The value of incorporating such an agent into a grasshopper pest-management system is obvious: By suppressing high insect densities to manageable levels, the need for conventional chemical insecticides could be reduced and perhaps even restricted only to “hot spot” areas. The objectives of the study we report here were to: (1) Determine if N. locustae was infective to the predominant species of grasshoppers in Saskatchewan; (2) if so, monitor the development of the disease within the populations; and (3) determine if the pathogen could survive over the winter and infect next year's populations. Because this was a preliminary experiment, we did not attempt to determine any effect the pathogen might have on population reduction.

Since its description by Canning (1953) from cultures of the African migratory locust, Locusta migratoria migratorioides (R. & F.), in England, Nosema locustae has been found in grasshoppers in southern Ontario, Canada (Smith 1965), and in most of the western United States (Henry 1969; Henry and Oma 1981). It probably was the unnamed organism noted earlier by Goodwin (1949) in locusts from Africa (Canning 1962). Thus, the known geographic range is England, central Canada, the western United States, and probably Africa. However, as Henry and Oma (1981) speculated, its known range undoubtedly will be extended as grasshoppers are examined from other locations. This report extends the geographic range of N. locustae to several locations in southern and central Saskatchewan.