Second Year of Trapping (2023)

A Full Trapping Season

We trapped at three sites in winter 2023: an irrigated dairy pasture, an irrigated forage field, and an unirrigated forage field. These sites were on three different farms, one in Marion County and two in Polk County. Each site was trapped once every two weeks, starting in early January through mid-March. (The end date was determined by the first appearance of juvenile voles in the traps.)

On each farm, two sets of plots were marked. Each plot was a quarter-acre and designated as “human” (a human identified where traps would be set), “dog” (a dog identified where traps would be set), or “control” (untrapped). This set of three plots was replicated at each site. Three dog + handler teams rotated across the “dog” plots on the different farms. Human searcher-trappers handled the “human” plots, set all the traps, and collected the data and, ahem, the bodies the day after trap setting (Figures 1 and 2).

Rumors of a vole population crash were well founded. The number of voles caught only totaled 124 over the season. Details of “the catch” are in Table 1. Like in 2022, all voles caught were contributed to the study on leptospirosis strains in the Willamette Valley being done by our colleagues in the OSU Carlson College of Veterinary Medicine.

Table 1. Number of voles trapped and related efficiencies when a dog identified the locations to set traps vs. human-identified locations.

Forage Assessment (Is the trapping making a difference in reducing crop loss?)

We collected in-field data and took samples twice per farm: once in late March/early April (“spring”,  soon after we stopped trapping) and again in June. In each plot, we ran a transect diagonally from one corner to the opposite corner. Along that transect, we sampled 8, evenly-spaced quadrats (squares) measuring 50 x 50 cm. (See Figures 3 and 4.) We also sampled two of the “worst” spots within the plot but off the transect.

In each quadrat, we estimated the ground cover by type (e.g., grass, legume, duff, etc.) and counted the number of vole holes. (See Figure 5.) We then cut and collected all plants to ground level in half the quadrat (so an area 50 x 25 cm). The collected plant matter was then dried overnight to get the dry matter weights, in order to estimate how much plant material was there.

The results of the forage assessment were inconclusive. The standard deviation values were large. (That is, there was a lot of variability in the dry matter within each plot type.) As can be seen in Table 2, the dog plot appears to have significantly more dry matter (less vole damage?) in the spring sampling. As would be expected, we do see much higher dry matter values in the June measurements vs. the spring.

Table 2.  Mean dry matter (DM) values (in grams per quadrat) from the three plot type in late March/early April (“spring”) and in June, with standard deviation values (SD). DM values also converted to pounds per acre.

In the 2024 trapping season, we will again assess forage quantity in the control and trapped plots. We will be adjusting our sampling methods and timing to produce more robust data. We also hope to employ some drone- and GPS-assisted data collection.