CORVALLIS, ORE. — Christy Tanner crouches on a dirt road next to a grass seed field, with a drone to her right and a calibration control panel in front of her.
Tanner, an Oregon State University Extension Service seed production agronomist, photographs the calibration panel with the drone camera. After a few more preparations, the drone takes off.
"We have a general idea of what damage voles can do. We want to document it and quantify it."
Flying in a grid pattern, it covers a trial area in about 20 minutes. The drone captures one image per second, which will be stitched together into a map of the plot and analyzed for indicators of vole damage.
In Oregon’s Willamette Valley, grass seed fields provide prime habitat for gray-tailed voles, small rodents that can cause significant crop loss. Outbreaks from 2020 to early 2023 raised industry concerns, with growers estimating yield losses between $35 million and $50 million.
Tanner, an associate professor of practice in the Department of Crop and Soil Science in the College of Agricultural Sciences, uses the drone’s two cameras to measure vole damage in grass seed crops.
The multispectral camera calculates a value called the normalized difference vegetation index, or NDVI, which predicts the amount of green, healthy plant tissue. Images reveal vole activity by identifying shorter crop canopies and increased bare soil — both signs of grazing damage.
Tanner’s image classification model distinguishes damaged from undamaged areas with 90% accuracy, providing a scalable alternative to the time-consuming “capture-mark-recapture” methods historically used to estimate vole populations.
“With the regular camera and some software, I can create a 3D map of the crop, which tells me how tall the crop is,” Tanner said. “Voles eat the grass, which makes it shorter and reduces the amount of plant tissue.”
That plant tissue is critical to a grass seed grower’s success.
Addressing gaps in vole control
Tanner is leading a multi-year research effort to develop and test new methods for monitoring and managing vole infestations. Her work uses drone technology, precision imaging and alternative rodenticide delivery systems to improve current control practices.
Vole populations follow a peak-and-crash cycle every four to eight years. Although they live in small tunnels, voles spend time above ground eating grass and other plants. The mouse-sized animals leave numerous burrows and connecting runways through the damaged grass.
“We have a general idea of what damage voles can do. We want to document it and quantify it,” Tanner said.
The aerial imagery tools help quantify damage, monitor treatment effectiveness and target management practices more precisely within a field. This results in more efficient use of rodenticides, reducing unnecessary environmental exposure and improving control where needed most.
Bait boxes and thermal imagery
In addition to monitoring damage, Tanner is testing whether alternative rodenticides can be delivered effectively using tamper-proof bait boxes. The trial began in one field and expanded to three the following season. The bait boxes are placed near active burrows and contain sensors to detect vole activity. All boxes showed some activity, with data confirming voles entered and consumed bait.
Tanner is evaluating two rodenticides not previously registered for bait box use in grass seed fields. The results will support a U.S. Environmental Protection Agency Section 24(c) “special local need” label application under the Federal Insecticide, Fungicide, and Rodenticide Act, which would provide growers access to new, targeted control options.
Sprout Mahoney, an Extension student employee, collects data from sensors inside the bait boxes.
“There aren’t many options, so bait boxes are something new growers might be able to use,” Tanner said.
Through outreach, Tanner has shared her findings with more than 300 attendees in workshops and presentations, helping growers understand vole biology and optimize bait timing based on life cycle patterns.
She has also launched a collaboration with OSU Extension wildlife specialist Dana Sanchez and Laurence Schafer, a wildlife biologist with the U.S. Department of Agriculture, to test the use of thermal imagery for estimating vole population densities — another innovative attempt to modernize monitoring practices.
