WSU Whatcom County Extension

Integrated Pest Management for Blueberries

Voles

Vertebrates

 

Vole

 

Pest Status

Common voles found in Pacific Northwest small fruit fields are Townsend’s vole, Microtus towsenii, and the creeping vole, M. oregoni. Voles (also known as field mice) feed on plant roots and foliage ground covers, such as grasses. In small fruits, voles can feed on the fine root structures and girdle root crowns and trunks of canes and bushes. Subterranean feeding activity also creates air pockets along the root zone. Vole feeding can also damage or disrupt irrigation materials.

 

Life History

Voles resemble typical house mice. Voles are generally gray-brown with gray undersides. Adult voles are about 4 to 5 inches long with reduced tails (1/2 to 1 inch long) and reduced legs. Voles can be very prolific. Voles have litters of 3-7 young. Young reach reproductive maturity in as few as 21 days. Given good conditions, voles can breed throughout the year with peak breeding occurring in spring.

During daylight, voles create subterranean tunnels to feed on plant roots, rear young and store food. Tunnel entrances are approximately one inch wide. Voles also travel and feed above ground, nocturnally, along designated runways.

Tunnels created by Voles

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

Habitat of Voles

Voles prefer open field habitats with grass groundcover. Feeding ranges are limited to small areas, generally 0.05 – 0.2 acres depending on species of vole and food availability.

Vole populations are cyclical with peak populations occurring every 2-5 years. Vole populations are regulated by food availability and climatic conditions. Populations can be greatly reduced during cold winters. There are multiple overlapping generations per year.

Vole Damage

 

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 should be monitored in spring to assess winter mortality and new population potentials.

Monitoring Voles

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 conducive 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 Trap

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. Generally management is needed when 40% of the bait stations show positive feeding damage (+) after 24 hours.

Vole damage of an apple

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 now 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
Total
8
7
FI/# of apple slices
7/8
<1 (Low population)

 

Using the second, 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.

 

Management

Habitat reduction is key to reducing vole pest problems. Regularly mow and manage vegetation along field borders. Keep sod or groundcover between the rows regularly mowed.

Pelletized baits with rodenticides are commonly used to treat vole problems. Broadcast applications of baits can be used but degrade quickly and increase the phenomenon of bait-shyness through sub-lethal doses. Rodenticide bait stations can be more effective at reducing vole populations. Activate bait stations in fall and spring, if populations remain high throughout winter.

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