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United States Department of Agriculture

Agricultural Research Service

Title: Pathogen Prevalence among Grasshoppers Populations As Affected by Rotational Grazing - An Interim Report

Author
item JARONSKI, STEFAN

Submitted to: Grasshopper Management Board Meeting
Publication Type: Abstract Only
Publication Acceptance Date: January 13, 2002
Publication Date: December 30, 2003
Citation: JARONSKI, S. PATHOGEN PREVALENCE AMONG GRASSHOPPERS POPULATIONS AS AFFECTED BY ROTATIONAL GRAZING - AN INTERIM REPORT. Grasshopper Management Board Meeting. Denver, CO. January 30-31, 2003.

Interpretive Summary: We have been making biweekly collections in paired continuous and twice-over rotational grazing study sites through the grasshopper season, typically late June to mid-September, beginning in 2000. For each sample we determined pathogen prevalence by pathogen, by sample, by host species, and estimated the season-long pathogen prevalence. After the first two years we have seen little correlation between grazing scheme and pathogen prevalence. The insect pathogenic fungus Beauveria bassiana was the most common grasshopper pathogen observed, reaching a prevalence of 14% in a sample. Beauveria attacked a variety of grasshopper species - 17 species in 8 genera ¿ all the predominant species at these sites. Other pathogens were rare. Our studies will be continuing for at least another three years in an effort to build a more comprehensive picture.

Technical Abstract: It has been observed that rangeland subject to rotational grazing had markedly lower grasshopper populations than paired sites subject to continuous grazing. The mechanism or mechanisms for this phenomenon are not clear. One hypothesis is that twice-over rotational grazing reduces bare ground and increases shading, slowing grasshopper growth rates and lowering fecundity. The role that the natural pathogen complex may play is not understood. We have been making biweekly collections in paired study sites through the grasshopper season, typically late June to mid-September, beginning in 2000. For each sample we determine pathogen prevalence by pathogen, by sample, by host species, and estimate the season-long pathogen prevalence. After the first two years we have seen little correlation between grazing scheme and pathogen prevalence. In 2000, overall season-long prevalence of all pathogens considered as a group was 0.5-8% at the three paired sites, and 4-10% in 2001. The insect pathogenic fungus Beauveria bassiana predominated in both years, while microsporida, notably Nosema locustae, had a substantial presence only in 2001. Entomopox virus was extremely rare. Parasites were present in less than 2% of the collected grasshoppers at all sites. The fungus was present at all sites. Beauveria was present from early July until mid-August and in individual samples the prevalence rate reached 14%. Infected grasshoppers generally died 6-8 days post-capture, indicating a well established infection at time of collection. Beauveria attacked a variety of grasshopper species -- 17 species in 8 genera ¿ all the predominant species at these sites. Our studies will be continuing for at least another three years in an effort to build a more comprehensive picture.

Last Modified: 9/10/2014