Of mice and men: Managing mice
in apple orchards
Voles
Voles (or mice as they are commonly referred to) can cause significant
economic injury in Ontario apple orchards. They are most commonly a problem
in orchards when there is a lack of food available. Voles are brown or
gray rodents with stocky bodies short legs and a short tail. They are
active day and night, year round .
Voles most commonly breed in the spring. Females are capable of reproducing
at the age of 3 weeks, and litters of two to nine young are produced monthly.
Populations can increase dramatically, followed by a crash. Population
levels generally peak every two to five years, however these cycles are
not predictable. Voles often feed on grass seeds, herbs and bulbs during
the spring and summer and then switch to feeding on bark in the fall and
winter. Indications of the presence of voles in orchards appear at harvest
when tunnels, droppings and chewed apples begin to appear in fruit. Leaving
dropped apples from the trees after harvest can attract rodents into the
orchard.
The two species of voles in Ontario are the meadow vole (or meadow mouse)
and the pine vole. The most common mouse in orchards is the meadow vole
which is about 150 mm to 195 mm in length, dark brown in colour with a
gray belly. Its tail is twice as long as its hind foot, the ears are furred
and projecting slightly above the fur on its head, and it has prominent
eyes. Meadow mice thrive in extensive grassy or weedy areas such as old
fields and areas with succulent green plants (orchards, pastures, hay
fields and fence rows). They create shallow tunnels in the soil and surface
runways in the grass. The pine vole is smaller than the meadow vole, light
brown fur and a tail about as long as its hind foot. It has sunken eyes
and its ears are buried in fur. The pine vole is present south of the
line between Goderich and Ottawa. Pine voles dig deep tunnels but make
few surface runways. They require a set amount of organic matter and clay
content in the soil so their tunnels can hold up; they are rarely found
in sandy locations.
Both types of voles girdle tree trunks in the fall and winter especially
in years with heavy and prolonged snow cover. Vole damage can result tree
stress or death.
Management Options
Biological
A variety of wild animals such as hawks, owls, crows, ravens, weasels,
foxes, coyotes, raccoons, skunks, cats and snakes feed on voles. Predation
may not manage high vole populations, but it may help to manage populations
in normal years to prevent some damage.
Habitat modification
Reducing or eliminating long grass and cover is a good method of reducing
vole populations. Grass should be mowed between 7.5 and 15 cm to limit
food and cover and expose the voles to predators. Waiting long intervals
between mowing or mowing with a sickle-bar mower may produce a thatch
layer that provides cover for voles. Maintaining a wide herbicide strip
in the tree row or under individual trees coupled with regular close mowing
of orchard ground cover helps to discourage mice from taking up residence
in orchards. Avoid leaving mulch, prunings or decaying vegetation around
the base of trees or in tree rows.
Exclusion
Mouse guards can be placed around trees to prevent voles from damaging
the bark. Guards should be used on young trees, where small amounts of
vole damage can severely damage or kill the tree. Guards should be approximately
45 cm high and should be buried in the soil 5 cm deep. Mouse guards should
be checked each year to make sure they are working and to ensure that
they are not interfering with trunk expansion. Avoid the use of guards
which do not allow air circulation. Always use light coloured tree guards
since dark materials can increase the temperature of the bark on sunny
days in winter and low temperatures at night can cause injury to bark
tissue.
Trapping
While trapping is not effective for controlling high vole populations,
it can be used to monitor or control small populations. Mouse traps baited
with peanut butter, oatmeal or apples slices can be placed near runways
or tunnels.
Repellents
Repellents such as thiram are registered for voles. However there is
some uncertainty about the long term efficacy of these products.
Rodenticides
Poison baits can be used to help manage mouse populations. Baits are
more effective if other food sources (dropped apples) are picked up and
removed from the orchard. Baits should be applied before the ground is
covered with snow and the grass falls over. Baits should be applied on
bright days with no rain expected for several days. Baits are poisonous
to many non-target organisms (cats, dogs, turkeys, pheasants, raccoons
and skunks.)
Bait stations are often a better way of managing mice because they provide
long-term control of pest populations. They are also considered to be
more environmentally friendly because they minimize the risk of other
animals, and or children coming in contact with the poison. Slabs of wood,
pieces of board or tin cans with one end removed can be used for bait
stations. Bait is placed under the board or in the can with the can lying
on its side. The inverted T bait station is made of 3.8 cm ABS pipe and
is probably the most effective type of bait station available. In this
station several table spoons of bait are placed in the neck which is capped
protecting the bait from the elements. Use 25 stations per ha (10 per
acre). Recently a reduced-risk biopesticide Rode-trol Rodent Control Bait
was registered for rats/mice control for use in buildings, warehouses,
barns and empty feed storage areas.
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