SMALL MAMMAL POPULATIONS IN MARYLAND MEADOWS DURING 4 YEARS OF HERBICIDE (BROMINAL(R)) APPLICATIONS

The herbicide Brominal(R) was applied at the recommended rate to one p lot in each of three paired 0.6-ha plots; the other three plots were u sed as controls. Plots were sprayed once in the fall of 1988 and 1989 and twice in the spring of 1990 and 1991. Small mammals were trapped t hree times during each activity season (April-October) to obtain popul ation estimates before and after spraying and in the spring preceding fall spraying or the fall following spring spraying. Population estima tes immediately after spraying gave no evidence of direct mortality. B y 1991, dicot vegetation on treated plots was suppressed and mean numb ers of meadow voles (Microtus pennsylvanicus) were less than on contro l plots. Because meadow voles favor dicots over monocots in their diet , reduced availability of dicots may have been related to the smaller vole population estimates. Species diversity of small mammals was nega tively correlated with size of vole populations, but was not different between treated and control plots. Brominal apparently induced opaque corneas in nine voles. The condition was found in two voles too small to have been conceived at the time of the last previous spray nearly 8 months earlier, suggesting exposure to residue alone.