Join us at Double J Jerseys dairy farm for lunch and farmer-to-farmer conversations to learn about vole ecology and management. We will describe OSU research into canine-assisted vole trapping and discuss how you can work with your own dog. We’ll also look at raptor perches and owl boxes and discuss how to encourage these predators on your farm. This workshop includes outdoor demonstrations on uneven ground, so please dress appropriately.
A red-tailed hawk sits on a recently-installed perch.
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).
Figure 1. Humans identifying vole holes and setting traps.Figure 2. Traps set in runways from active hole.
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.
Dog
Human
Total traps set
2141
1276
Total voles caught
64
60
Voles killed per number traps set
0.029
0.047
Total search time
593 min.
1207 min.
Voles killed per time spent searching and flagging holes to trap
0.108 voles/min.
0.049 voles/min.
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.
Figure 3. Layout of transect and quadrat positions.Figure 4. Cutting forage samples from quadrats (in the rain).
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.
Figure 5. Quadrat about to be sampled. The “bottom” half was cut and bagged.
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.
mean DM spring (SD) g/quadrat
mean DM spring lbs/acre
mean DM June (SD) g/quadrat
mean DM June lbs/acre
control
41.3 (22.6)
2947
64.1 (35.9)
4575
dog
52.8 (35.1)
3768
63.3 (32.5)
4517
human
40.8 (22.9)
2912
62.8 (26.5)
4482
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.
The delays we experienced due to the leptospirosis issue meant we missed the January-February 2022 window of opportunity to focus our removals on would-be breeders before breeding season began. However, we persisted in running one round of on-farm trials in March to gain insights as to whether we should continue our work. We conducted a single-round of trapping (24 hours from trap set to trap check) on five Willamette Valley farms (1 hazelnut grove, 2 dairy pastures, 2 vegetable farms), with each search team getting their own ¼ acre plot in the same fields.
Our primary comparison is between unassisted human searchers and canine-assisted searchers, with each trying to identify holes where a trap should be set. Unassisted human searchers rely on their eyes, looking for trails and holes with fresh scat and perhaps nibbled vegetation in the neighborhood. The dogs also use their powerful noses and ears.
Fresh scat outside a vole burrow entrance.
Promising Results
Despite the many challenges and limitations, the results were encouraging. Canine-assisted teams and unassisted humans were virtually tied in trap success (0.41 voles/trap by canine teams, 0.40 voles/trap by unassisted human), but differences in efficiency were telling. We had the opportunity to compare efficiency among three different canine-handler teams, which revealed a range from 0.29 voles to 0.65 voles killed per minute of search time. When we compared the average efficiency and total search time spent by canine-assisted trappers (0.48 voles killed/minute of searching over a total of 97 search minutes) to that of an unassisted human (0.29 voles killed/minute of searching over a total of 232 search minutes), the value of canine-assistance became very clear. Our much-delayed 2022 season limited our data and the confidence we have in the results, but there were enough promising indications that we pressed on to trap again in winter of 2023.
Daphne (dog) and David (human) searching their plot.
Companion Study on Leptospirosis Infection Rates
Voles killed during our 2022 season are contributing to a parallel investigation being conducted by Drs. Sargent and Beechler of OSU’s Carlson College of Veterinary Medicine. This study seeks to better understand which Leptospira strains and serovars occur in voles in the Willamette Valley, in addition to estimating infection rates. This work is important in understanding whether current vaccinations for pets and livestock are a good “match” to the potential challenges on our region’s landscape.
OSU is hosting a workshop on Voles, Drones, and Dogs to introduce two current vole management projects. Please consider joining us.
When: Thursday, December 1, 2022, 1:00-4:30
Where: Chemeketa Community College Agricultural Hub (Building 60) Room 102, 4000 Lancaster Dr. NE, Salem. (The Ag Hub is on the corner of 45th Ave and Fire Protection Way.)
Register here: Registration is free but space is limited. Pre-registration is required.
Join us to learn about vole ecology and management, how OSU is exploring the use of canine-assisted detection to locate active vole tunnels in pastures and crop fields, and how drones may help assess crop damage in grass seed. The workshop will include time for discussions and, if weather permits, outdoor demonstrations. Presenters and facilitators include (from OSU) Dana Sanchez, Nick Andrews, Christy Tanner, Jenifer Cruickshank, Vanessa Blackstone, Tim Stock, and (from ODA) Matthew Bucy.
To develop our trapping protocols, we did some preliminary field work in the fall of 2021. In a dairy pasture with signs of vole activity (holes, runs, fresh feces in runs) and even some vole sightings, we investigated burrow structure and timed ourselves identifying good candidate holes and deploying our trapping equipment. The general plan was to set traps in the runs around one hole and block nearby entrances to that burrow.
Trap Setting
To get a better sense of how many entrances a vole burrow might have, we used an insecticide fogger (heating vegetable oil). We inserted the smoke-emitting tube into one hole and observed how many other holes the smoke escaped from. The most we saw was about 40 holes in a network spanning about 11 feet. We got a sense of how close holes from one burrow are to each other, so we would know what our “block-the-holes” radius should be.
Identifying vole burrow holes with smoke. We used burlap to block holes.
In this preliminary work, we configured our trap sets. A “trap set” consists of a numbered flag, 2 snap mouse traps in each run (trail) going to/from the flagged hole (usually 2 runs), burlap stuffed in neighboring holes to block them, and milk carton “roofs” to shelter the traps from other animals. Traps are checked ~24 hours later and removed.
A trap set before the milk cartons are placed.A trap set with milk cartons placed over the traps and burlap blocking nearby entrance holes.
Dog Training
To train our collaborating dogs and dog handlers, we set up a system where the dogs could practice detecting live voles. Six live voles were caught earlier and housed at an on-campus small animal facility (see photo of vole in a jar—temporarily; their homes were much bigger than that). We dug some holes and trenches where we could put live voles contained in PVC tubes like those used to keep rats safe in competition barn hunts. Several training sessions with the dogs and live voles were run. However, we discovered that at least some of the voles were positive for leptospirosis, a bacterial disease that can be transmitted to humans and dogs. Biosafety protocols were followed throughout our training sessions. (Health advisory: vaccinate your dog[s] against leptospirosis. It is in our environment.) The voles were subsequently euthanized by veterinary personnel.
A captive vole, temporarily in a glass quart jar.Training dogs to identify vole scent with some tubes containing a live vole. Dog Daphne is lying down to tell her handler that there is a vole in that PVC tube.Training dogs to identify vole scent. Dog Sierra is staring intently at a vole-containing tube just protruding from a hole.The volunteer dog handlers and our dog trainer at the last training session before the winter 2022 field trials began.
Once the dogs had been trained on the vole scent, dogs and handlers tackled some “real” pastures with vole problems. Each dog/handler pair was assigned a search area, where the handler flagged holes that their dog alerted on. At each of those holes a trap set was installed and checked the next day. The more successful dogs (who identified holes where we caught voles) were invited to participate in the data-collection trials that ran in March.
Dog Streya looking up from her task identifying active vole holes.
We’ve started this project because we’re interested in learning how to better manage rodent pests, with a focus on voles. Voles, you say? Yes, voles. Sometimes mis-referred to as field mice, these small rodents resemble mice, but they have a distinctively shorter tail, among other, less obvious differences. The Willamette Valley’s most common vole species is the gray-tailed vole (Microtus canicaudus). Deer mice (Peromyscus maniculatus) are also common in the Valley and are often found in fields occupying the same tunnel networks as voles. They are easy to differentiate from voles because of their protruding ears and much longer tail. Although deer mice can damage crops, they are thought to be less important pests than voles. Shrews (e.g. Sorex bairdi) also inhabit the same networks at much lower levels, but they are insectivores and don’t damage crops. Their long flexible snout makes them very easy to recognize (Figure 1).
Figure1. From left to right: grey tailed vole, adult and juvenile; deer mouse; shrew.
Voles are quite happy to eat a large variety of plants: grass (pasture or seed field, they don’t care), small grains, vegetables, tree bark, and many, many others. Their populations can explode in numbers and then die back, usually in a 3-to-5-year cycle. These cycles are influenced by many factors. The recent vole irruption appears to be declining in 2022. It is possible that climate change could increase the frequency and severity of vole irruptions.
Voles are a serious agricultural pest, and control options are limited, especially for certified organic farmers, who cannot use zinc phosphide (the most common rodenticide used for voles, which has its own challenges). Trapping with old-fashioned mouse traps can make an impact, but it is laborious. In this project, we are looking at whether using trained nose work dogs to assist in placing traps can make the trapping process more efficient. Trapping voles when their populations are high in the summer is usually futile. During the winter they stop reproducing and their population drops, so most control methods are more effective if the number of breeding pairs can be reduced before populations begin to explode again in the spring (i.e., March).
Figure 2. Vole burrow holes in a pasture.
A vole infested field can have thousands of entrance holes per acre (Figures 2 and 3), but during the winter most of those tunnels can be empty. Dogs can search more quickly than humans and if they are well trained, they can tell their handler when there is a vole present by detecting the odor of live voles. Humans can be quite good at this by visually detecting signs of recent vole activity. Our research is essentially a race between dog and handler teams and human teams. Who can catch the most voles in the least amount of time? Our initial results seem to be showing that the dog teams are more efficient – more about that in a later post.
Figure 3. A clear vole trail or “run” in a pasture.
Financial support for this vole trapping project is from the Western Sustainable Agriculture Research and Education (SARE) program, which is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture (https://western.sare.org/).
