Abundance and Diversity of Grasshoppers and their Ectoparasitic Mites in South Dakota

In South Dakota, grasshoppers (Orthoptera: Acrididae) are sporadic pests that can cause economic injury to rangeland and crops during outbreaks. It is important to know which grasshopper species are present as not all have the same potential to cause damage. USDA-APHIS conducts annual grasshopper surveys in western South Dakota rangelands, but the last published survey was in 1925. Of the potential biological control agents existing, grasshopper mites feed on grasshopper eggs and the larvae are ectoparasites of nymph and adult grasshoppers. Previous studies suggest that mite larvae reduce grasshopper fecundity and mobility, making them useful for integrated pest management of grasshopper populations. Yet, a study evaluating grasshopper mites in South Dakota has not been conducted since 1944. The purpose of the first study was to determine the abundance and species diversity of grasshoppers and the second study was to determine the density and distribution of grasshopper mites in South Dakota. Data for both studies was obtained by sampling grasshoppers in both 2017 and 2018 using sweep nets with 40 pendulum sweeps. Samples from western South Dakota were collected in rangeland and donated by USDA-APHIS. For eastern South Dakota, 400 sites were sampled once with two samples collected simultaneously from ditches alongside crop and rangeland. The first study determined that the most abundant species were Melanoplus femurrubrum and Phoetaliotes nebrascensis. For both years, a majority of grasshopper populations did not exceed the recommended thresholds; however, there were localized “hot spots” that greatly exceeded the thresholds. Results of the second study indicated that the most abundant mite (i.e., over 90%) was Eutrombidium spp., which was most commonly found on M. femurrubrum nymphs. For both years, the majority of mite populations were relatively low or absent. However, there were localized, increased populations that were related to increased grasshopper populations. The results from the first study suggest that annual grasshopper surveys are necessary to detect potential outbreaks and forecast “hot spots” in the future, while the results from the second study suggest that annual grasshopper mite surveys could improve the overall understanding of the importance and impact that grasshopper mites could serve for integrated pest management purposes