Cattail Management Symposium

Effects of Rodeo® Herbicide on Aquatic Invertebrates and Fathead Minnows

Catherine J. Henry and Kenneth Higgins
South Dakota Cooperative U.S. Fish and Wildlife Research Unit
Brookings, SD

ABSTRACT

Waterfowl prefer wetlands with about a 50:50 ratio of emergent vegetation to open water. Growth of dense monotypic stands of cattail has contributed to loss of wetland habitat and to the declining trends of some waterfowl populations. Rodeo herbicide (glyphosate) may be an effective chemical tool for altering emergent vegetative cover, but little is known about its effects on nontarget organisms such as aquatic invertebrates that are important as a food source for waterfowl. The objectives of this study were to evaluate the effects of Rodeo on the survival of aquatic invertebrates in wetlands, and to determine in the laboratory the acute toxicity of Rodeo and the associated surfactant and drift retardant used in aerial treatment.

I evaluated the effects of Rodeo on the survival of six species of invertebrates and fathead minnows by placing them in enclosures in eight North Dakota wetlands that were aerially treated with Rodeo at 5.8 L/ha and in four that were untreated. The number of animals alive and dead were then counted up to 21 days post-treatment. Probit analysis was used to figure regression coefficients and median lethal times, and a t-test was used to detect differences. Water samples were collected prior to treatment to test for presence of agricultural chemicals and after treatment to test for the presence of glyphosate. Limnological conditions were also monitored.

In field trials, no significant differences (P>0.05) occurred in mortality rates of invertebrates between treated and reference wetlands. A significant difference did occur in mortality rates of fathead minnows, however deaths of fathead minnows were attributed to factors other than the herbicide treatment.

Laboratory static acute toxicity testing was done on the same species to determine EC50's or LC50's of Rodeo, X-77 Spreader, and Chem-Trol both individually and in mixtures. Rodeo alone and the field application mixture of all three chemicals were rated as practically nontoxic (100-1000 mg/L), X-77 as moderately toxic (1-10 mg/L), and Chem-Trol as an insignificant hazard (>1000 mg/L). The field application mixture was significantly more toxic than Rodeo by itself with X-77 being the most toxic component. I found no evidence of synergistic effects among the three chemicals. The most toxic EC50 for Rodeo of all species tested was 485 times the highest concentration of glyphosate (0.600 mg/L) found in wetland water samples.

I concluded that the benefits to waterfowl by restoring degraded habitat through the use of Rodeo outweigh the limited chance of invertebrate populations being greatly reduced or eliminated as a result of the treatment. Recommendations are given for future research and for managing cattails in wetland complexes with Rodeo herbicide.

LITERATURE CITED

Henry, C. J.  1992.  Effects of Rodeo herbicide on aquatic invertebrtes and 
     fathead minnows.  M.S. Thesis, South Dakota State University, Brookings.  
     63pp.

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