Adult female prairie voles and meadow voles do not suppress reproduction in their daughters.

Reproductive suppression of young females by conspecific females has been reported from laboratory studies on several species of rodents, including the prairie vole, Microtus ochrogaster, but not meadow voles, M. pennsylvanicus. We exposed female prairie voles and meadow voles to two treatments: a mother and one 23-26-day-old daughter paired with a strange male and a 23-26-day-old daughter paired with a different strange male. We found no differences in the proportion of daughters breeding or the time to sexual maturation for daughters raised in the two treatments for either species. Thus, we have no indication that mothers had any adverse effect on reproductive efforts of their daughters. These results differ from previous studies that concluded young female prairie voles were reproductively suppressed by female relatives. The difference between our and previous studies on reproductive suppression is that we examined breeding in young females rather than proximate measures of growth and reproductive development. We question the evolutionary significance of reproductive suppression among related female microtine rodents, especially in that it has not been documented from field populations.

Response of bobwhite quail and gray-tailed voles to granular and flowable diazinon applications.

We used gray-tailed voles (Microtus canicaudus) and northern bobwhite quail (Colinus virginianus) as experimental model species to field test whether small mammals and birds respond differently to equivalent concentrations of a pesticide applied in granular and flowable formulations. In mid-May 1998, we placed voles into 15, 0.2-ha enclosures planted with a mixture of pasture grasses. In mid-July, we placed quail into the same enclosures with the voles. In late July, we applied the organophosphorus insecticide diazinon in five treatments; a control (all habitats sprayed with water), liquid formulation of diazinon at 0.55 kg/ha, liquid formulation of diazinon at 1.11 kg/ha, broadcast of granular diazinon at 1.11 kg/ha, and broadcast of granular diazinon at 2.22 kg/ha. The diazinon treatment in liquid and granular formulations did not depress population size or growth rate, or survival rate of voles. We found a significant difference in the survival rate of the quail between the controls and treatments; granular diazinon caused a measurable decline of quail survival, whereas the liquid application at an equivalent rate did not significantly affect quail survival. Analysis of our results suggests that ground-feeding birds are more susceptible to granular insecticides than flowable applications, but voles were not susceptible to either formulation at the rate we used.

Effects of acorn production and mouse abundance on abundance and Borrelia burgdorferi infection prevalence of nymphal Ixodes scapularis ticks.

Risk of exposure to Lyme disease is a function of the local abundance of nymphal Ixodes ticks that are infected with the etiological agent, the spirochete Borrelia burgdorferi. We monitored abundance of white-footed mice (the principal B. burgdorferi reservoir in the eastern and central United States) and acorns (a critical food resource for mice), and Ixodes scapularis ticks, as well as ambient temperature (cumulative growing degree days) and growing season precipitation, in a forested landscape of southeastern New York State from 1994 to 2000. We found that acorn production in autumn strongly influenced abundance of white-footed mice the following summer and that abundance of mice in summer, when larval ticks are active, influenced the abundance of infected nymphs the following year. Consequently, the abundance of infected nymphal ticks can be predicted from acorn production 1.75 years earlier. Monitoring of natural fluctuations in acorn production thus supports results of prior acorn addition experiments that were conducted at small spatial scales. Growing degree days and precipitation either had no significant effect on density of nymphs or marginally increased the explanatory power of models that included acorns or mouse density as independent variables. We conclude that, at our study site in New York, the risk of human exposure to Lyme disease is affected by mouse density in the prior year and by acorn production 2 years previously.

A field test of the quotient method for predicting risk to Microtus canicaudus in grasslands.

We used gray-tailed voles, Microtus canicaudus, as an experimental model species to field test a prediction of the Quotient Method (QM) for risk to small mammals of an insecticide in grasslands. In May 1997, we placed voles into 12 0.2-ha enclosures planted with a mixture of pasture grasses. In late July, we applied 1.55 kg/ha of the insecticide Guthion(R) 2S (azinphos-methyl) in three treatments: a control (all habitat sprayed with water), full spray (all of the habitat sprayed with Guthion 2S), and half-spray (one-half of the habitat sprayed with Guthion 2S and one half with water). Five replicates were used for the half-spray and control, and two replicates for the full-spray. The Guthion 2S treatment did not depress population size, growth rate, or survival of voles in half-spray or full-spray enclosures. Our results were inconsistent with the QM prediction for use of Guthion 2S in grass habitats. These results also differed from our previous studies in alfalfa habitats in which measurable responses were detected at the same application rate. The differences probably are due to the different types of vegetation between current and previous studies. The grass habitat in our enclosures was an average of 60 cm high and was very thick from ground level to about 40 cm. Much of the residue likely accumulated in the upper strata of vegetation and did not reach ground level at this application rate. Under the conditions of this study, voles were less affected in grassland habitat than they were in alfalfa habitat.

Chain reactions linking acorns to gypsy moth outbreaks and Lyme disease risk.

In eastern U.S. oak forests, defoliation by gypsy moths and the risk of Lyme disease are determined by interactions among acorns, white-footed mice, moths, deer, and ticks. Experimental removal of mice, which eat moth pupae, demonstrated that moth outbreaks are caused by reductions in mouse density that occur when there are no acorns. Experimental acorn addition increased mouse density. Acorn addition also increased densities of black-legged ticks, evidently by attracting deer, which are key tick hosts. Mice are primarily responsible for infecting ticks with the Lyme disease agent. The results have important implications for predicting and managing forest health and human health.

Comparative toxicity of azinphos-methyl to house mice, laboratory mice, deer mice, and gray-tailed voles.

A laboratory toxicity study on house mice and laboratory mice (Mus musculus), gray-tailed voles (Microtus canicaudus), and deer mice (Peromyscus maniculatus) was conducted as part of a comprehensive laboratory and field study to field validate laboratory-based risk assessment of pesticides. The single dose oral LD50 for the organophosphorus insecticide azinphos-methyl (Guthion) was 10, 11, 32, and 48 mg/kg body weight in wild house mice, laboratory mice, gray-tailed voles, and deer mice, respectively. Ten-day dietary LC50s were 277 ppm for laboratory mice, 297 ppm for gray-tailed voles, and 1,180 ppm for deer mice. All treated animals lost more weight, consumed less food, and had depressed brain cholinesterase (ChE) activity compared to controls. Five-day LC50s were significantly higher than 10-day LC50s for laboratory mice and deer mice. For all three species, animals that died during dietary LC50 tests had mean ChE activity of 50-55% while survivors had 56-70% of controls. The conclusions were that: (1) Laboratory mice were not representative of deer mice or gray-tailed voles with respect to sensitivity to azinphos-methyl, but provided a conservative estimate for risk assessment; (2) 10-day dietary LC50 tests indicate substantially greater estimates of toxicity of azinphos-methyl to rodents than do 5-day tests; and (3) brain ChE depression of 45-50% was lethal in these species.

Parents suppress reproduction and stimulate dispersal in opposite-sex juvenile white-footed mice.

Juvenile dispersal is sex-biased in many mammals and birds: one sex often disperses more often or farther than the other. Two hypotheses are generally presented for sex-biased dispersal. The first holds that juvenile dispersal reduces reproductive and/or resource competition between parents and same-sexed offspring. If so, presence of a parent on the natal home range should both promote dispersal of same-sex offspring and suppress reproduction of those that remain. The second is that juvenile dispersal reduces matings between parents and offspring, thus decreasing the likelihood of inbreeding depression. If so, presence of a parent should favour dispersal and reproductive suppression of offspring of the opposite sex. Here I present evidence that juvenile dispersal in white-footed mice, Peromyscus leucopus, is due to inbreeding avoidance. When population density was high, experimental removal of one parent delayed dispersal of opposite-sexed offspring and only the presence of the parents of opposite sex suppressed juvenile reproduction.