Adorable but Devastating: Vanquishing the
Vexatious Voles
Everyone
seems to know about the “mouse problem” on our farm. The signs of rodent damage
can be seen everywhere, and it’s not uncommon to see a small blur of brown
streaking from one bed to another. But what exactly do we know about our furry
neighbors? I say not enough. In my time on the farm, I’ve seen perhaps just one
true mouse. The real culprit is actually a species of vole (most likely
Microtus californicus) a rodent which is somewhat similar to a mouse yet has
some key differences in behavior. To minimize damage caused by them, we must learn
how to identify our pests and better understand how they fill their niche in
the ecosystem.
Voles have a different body
shape from mice to suit their lifestyle. While mice are small, skinny and adept
at climbing, voles are relatively large, fat, and have short limbs, tails and
ears. Below I’ve constructed a diagram which should help with some key
identifying characteristics. Adult female voles are smaller than adult males.
The most telltale sign of
vole presence is the extensive system of ‘runways’ they produce. Voles spend
much of their time in shallow burrows, but also construct worn networks near or
above the surface, often through vegetation or mulch, that they utilize to get
from place to place. The burrows and runways both pose serious threat to crop
yield as the voles disturb the soil and remove vegetation to create them.
Burrows can expose crops’ roots to air and cause dehydration; they can also cause
the entire plant to collapse and die from lost soil structure. Voles line
the insides of their burrows with vegetation that they gnaw down, so even
calorie-poor crops like young rye and vetch aren’t safe from damage. I
personally had most of my group's rye patch destroyed by voles; first they displaced
the plants by constructing a runway through the middle of it, then they gnawed
all the stalks down to line their burrows, and finally they dug out a burrow
underneath it and caused half of the rootstock to collapse and die.
In
addition to these secondary effects, voles directly consume almost any plant
matter. They will relish root crops like beets and potatoes, but will also
consume many other plants like cabbage, broccoli, artichoke, lettuce and
brussel sprouts. They will also gnaw on the bark around the trunk of young fruit trees and can cause permanent damage or death.
The fluctuating nature and
fertility of wild vole populations makes them a top priority pest. Females
become sexually mature at just a month of old and can give birth to about a
dozen litters of 3-6 offspring in its year-long lifetime (Hatfield, 1935). In
the wild, vole populations commonly fluctuate and can explode exponentially
under ideal conditions until the number of individuals can reach plague
proportions. Our farm does provide optimal habitat for them, so the risk of
this happening shouldn’t be ignored. Although the population would decline on
its own, the damage caused to our crops from the ravenous voles would be
extensive to say the least. We must continue to keep a close eye on the number
of voles implement population control methods when needed.
Voles are active at all times
of day and night, though they their activity has a correlation to the weather.
During the heat of summer, they’re more active during the night, and in the
middle of winter they come out during the day. They are also more prone to
venture into the open during the evening, night and dawn. They don’t hibernate
but tend to have fewer pups in the wintertime. Because they’re always active,
we can attempt to catch them year-round during all times of day.
Voles love to live in and
around very dense plant and weed growth. Unfortunately for us, most traditional
vole control strategies involve reducing or removing this cover. Since this
isn’t an option we must use other methods. One of our most effective ways of
controlling vole populations are the use of live traps. Since I began using
these traps during winter break, I’ve relocated about twenty individuals from
our farm to surrounding areas. Traps are best set up in close proximity to a
burrow entrance or runway and near some cover, like a tall plant or some logs.
This will help the voles to feel more secure and encourage them to go into the
trap. The best bait I’ve found is some white bread with peanut butter smeared
on it. I smush the bread into a dense ball so it stays together, then tie it to
the trigger mechanism with some cotton string or a blade of grass. Most food
items will work, but smelly foods which won’t fall apart are best.
Maintaining these traps is a
big commitment since you must check the traps once or twice every single day,
first thing in the morning. If they’re not checked, a mouse or vole will
quickly die. We’ve also had reports of small birds (dark-eyed juncos) becoming
trapped inside. It seems that my own as well as other students’ first instinct
to re-set a trap that appears to have been sprung with nothing inside of it. To
avoid miscommunication and potential incidence of death, we should agree to not
set a trap unless you plan on checking it the following day, and to store traps
not in use far away from the beds. Catching voles by hand can also work, but it
is very limited in its ability to make a real dent in the population since
there are only a few structures under which you can unreliably find rodents.
Vole caught in a live trap
In addition to direct
intervention, efforts can be taken to increase natural predation on rodents.
Local red-shouldered hawks and white-tailed kites both relish voles, as do
night-hunting owl species. Building an owl box and a raptor perch nearby the
farm may help keep the population in check. Gopher snakes, garter snakes and
western racers are also natural predators, and all have been sighted in the
surrounding areas before. Perhaps the racer from last semester will return this
spring to lend us a hand.
workers constructing a raptor perch
Despite
all the destruction they caused, voles are actually a positive indicator
species. This doesn’t mean that we want them on the farm, but their presence
does indicate that we produce enough food on the farm to support their
population. Several years ago when our site was just a field of grass, no vole
could have survived long term due to lack of food and habitat.
-Jeremy Morrell
