WSU Whatcom County Extension

Integrated Pest Management for Raspberries

Voles

Vertebrates

 

Vole

 

Symptoms

Voles are active year round, day and night, and are normally found in areas with dense vegetation. They feed on plant roots and foliage near the ground. Their gnawing and chewing can girdle roots, crowns, canes, and may lead to cane loss. Subterranean feeding activity also creates air pockets along the root zone. The presence of voles is indicated by chewing marks on canes and roots. Girdling of canes extending to just above the soil line and well-used tunnels through the soil and/or grass are signs of an infestation. Rabbits usually damage trunks and twigs higher up, leaving larger tooth marks at a 45° angle, and beavers generally clip branches, leaving 2-inch stubs. Although voles do spend considerable time aboveground and may occasionally be seen scurrying about, most of their time is spent below ground in their burrow system. Voles often leave 1 ½ to 2 inch open holes in areas of heavy activity, while moles always seal up their holes. Fresh clippings of green grass and greenish-colored droppings about 3/16 inch long in the runways and near the burrows are further evidence of voles. With age, the droppings lose the green coloring and turn brown or gray.

In badly infested fields, dead plants can be located during the dormant period by giving them a tug. If they move very much, the entire root system is probably gone. In the growing season, damaged plants are leggy and thinly leaved with a reddish tinge to the foliage.

Identification

Voles are commonly called meadow mice or field mice, although they are distinguished from true mice by their short tails (1 inch), stocky build and small eyes. Common voles found in Pacific Northwest small fruit fields are Townsend’s vole, Microtus towsenii, and the Oregon vole (creeping vole), M. oregoni. The Townsend’s vole is dark brown sprinkled with black along the back; grayish or grayish brown along the belly. The tail is blackish, and indistinctly bi-colored. It has large ears that project well above the harsh fur. It is about 6 ½ to 9 ½ inches long. The Oregon vole is 5 ½ to 6 ½ inches long. Its fur is gray to brown or yellowish-brown with a darkish belly. The tail is indistinctly bi-colored.

Life History

Voles may breed any time of year, but spring is the peak breeding period. Voles are extremely prolific with females maturing in 35 to 40 days and having five to ten litters per year. Litter size ranges from three to six, although the Townsend’s vole may have up to nine young in one litter. Several adults and young may occupy a burrow system. Vole numbers fluctuate from year to year. Under favorable conditions their populations can increase rapidly. In some areas their numbers are cyclical, reaching peak numbers (up to several thousand per acre) every 3 to 6 years before dropping back to low levels. Voles seldom live longer than 12 months.

 

Vole Tunnel

Runways can be recognized by inch-wide, matted trails in grass and groundcover.

 

Vole Habitat

Voles prefer open field habitats with grass groundcover. Feeding ranges are limited to small areas.

Monitoring

Assessing vole populations in the fall, winter, and spring are important for management decision-making. Monitor vole populations in early fall to determine if any management decisions should be made. Two to three week post-treatment monitoring should be used to evaluate management decisions in the fall. Populations monitored in spring can assess winter mortality and new population potentials. Look for tunnel entrance holes about one inch in diameter, surface runways through grass, droppings and/or chewing marks on canes and roots. In the growing season, look for plants that are leggy and thinly leaved with a reddish tinge to the foliage.

 

Vole Trap

Go to the Ontario Woodlot Association for information on constructing and using a PVC bait station.

 

Monitoring stations can be constructed using a protected shelter to cover a runway or tunnel entrance. Shelters can be constructed using roofing shingles or PVC piping. Place an apple wedge as bait underneath the shelter. Check the apple bait after 24 hours for evidence of feeding. Inspect the apple wedge for feeding damage. Monitor under favorable conditions, such as no/little rainfall and above freezing temperatures. If weather is marginal, leave the bait stations active with apple wedges for a few days to be sure that an optimal foraging night was sampled. Four to eight bait stations per acre can provide an accurate assessment of vole activity.

 

Vole damage to apple

 

Vole monitoring can be assessed using two methods. The first method, the apple sign test, can be used to only detect feeding activity in the field. Apple wedge feeding can be scored as present (+) or not (-) for each station. The second method, feeding index developed by WSU’s Dr. L. R. Askham, can assess feeding activity and relative feeding pressure (or population size). Apple wedges can be scored to relative amount of feeding. See the following table for feeding indexes.

 


Category


% Apple Consumed


Population Rank


Feeding Index Ranking

0
0%
--
0
1
<25%
Low
<1.0
2
25-50%
Moderate
1.0-1.9
3
50-75%
High
2.0-2.9
4
>75%
Severe
>3.0

 

Score each apple at each bait station. Multiply the total number of apples that were scored the same for each category by that categories value. Then add all the values together and divide by the total number of bait stations. This is the feeding index ranking for the whole field. See the following example of eight baiting stations with four showing no damage, three with less than 25% of the apple eaten and one with over 75% of the apple eaten:

 


Category Value

 
Multiply
# Apples/Category
Feeding Index
0
X
4
0
1 (Low)
X
3
3
2 (Moderate)
X
0
0
3 (High)
X
0
0
4 (Severe)
X
1
4
8
7
FI/ # of Apples
7/8
<1 (Low population)

 

 

Thresholds and Management

For the first method, apple sign test, management is needed when 40% of the bait stations show positive feeding damage (+) after 24 hours. When using the feeding index method, management is needed when the total feeding index is greater than two. This second method gives a more accurate assessment of vole population size along with identifying pest problem areas of the field.

One way to effectively manage this pest is to make the habitat less suitable. Weeds, heavy mulch, and dense vegetation cover encourage voles by providing protection from predators. Therefore, keeping weeds under control and regular cultivation of aisle ways can help keep populations under control. If your field is within close proximity to a high vegetation area, consider keeping a 15 foot weed-free buffer to deter voles from entering.

 

Resources

Ontario Woodlot Association, Rodent Problems – Constructing a Bait Station,
http://www.ont-woodlot-assoc.org/sw_rodentbait.html

University of Nebraska-Lincoln, Prevention and Control of Wildlife Damage – Voles, http://www.ces.ncsu.edu/nreos/wild/pdf/wildlife/VOLES.PDF

University of California, IPM Online: Pests in Gardens and Landscapes – Voles, http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7439.html

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WSU Whatcom County Extension 1000 N. Forest St., Suite 201, Bellingham, WA 98225 (360) 778-5800 whatcom@wsu.edu