Abstract

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Selby, G. 2006. Effects of grazing on the Dakota skipper butterfly; prairie butterfly status surveys 2003-2005. Final report submitted to the Minnesota Department of Natural Resources. 123 pp.

Abstract:

A research project was conducted to examine the effects of cattle grazing on the Dakota skipper (Hesperia dacotae [Skinner]) (Lepidoptera: Nymphalidae) from 2003-2005. The study area included public and private properties in and around Glacial Lakes State Park, Pope County, Minnesota. The study was intended to examine the impacts of grazing intensity, duration, and timing on each of the key stages of the Dakota skipper life cycle (e.g. egg, larva, pupa, and adult). Sites representing three grazing regimes (moderate rotational, moderate open seasonlong, and intense open season-long) and various non-grazing regimes (short-term and long-term ungrazed) were selected to test the impacts. For each combination of grazing factors, the objectives of the study included the following: 1) examine grazing impacts on adult usage (e.g. distribution, abundance, and behavior); 2) examine grazing impacts on oviposition site selection and egg survivorship; and 3) examine grazing impacts on larval survivorship during early development (late summer to fall), diapause (winter), and late development (spring to early summer). Unfortunately, the Dakota skipper population in the study area experienced a major population crash prior to or perhaps during the 2003 field season, and did not recover enough by the end of the 3-year study to allow adequate data to be collected. Also, the rotational grazing program that was a central component of the study still had not been implemented by the end of the final field season. Some general trends were apparent from the adult butterfly data. Severe overgrazing and highly degraded prairie have predictably negative impacts on most of the prairie butterfly species, but Dakota skippers appeared to do as well in areas with intermediate grazing as they did in ungrazed areas. It was impossible to draw any conclusions about grazing impacts on the other life stages, but the study did provide an opportunity to refine the methodology that might be used for those components in future studies. The dramatic population declines for the prairie specialist skippers (Arogos and Dakota skipper, and Poweshiek skipperling) in the study area may be widespread in west-central Minnesota, but those same species appear to be doing well at sites in southwestern Minnesota. Dakota skippers also appear to be doing fairly well at Felton Prairie SNA in northern Minnesota.

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