Winter, Your Best Ally for Managing Insect Pests
By Dr. Ken Fry
Have you ever thought how fortunate we are to live in a climate that has a cold winter season (yes, cool and rainy on the west coast counts as winter)? Beyond the obvious recreational benefits such as hockey, skiing, tobogganing and snowball fights, a cold winter protects us from many insects and insect-borne diseases that thrive in the warmer tropics. Who would have thought cold is good.
The freezing cold of winter prevents a great many insects from surviving year-round in Canada, but not all perish in the cold. Many insects have adapted their life history to allow them to survive during the cold and dark. Have you ever considered where insects go during the winter? The majority spend the winter safely tucked away out of the elements in a resting state called diapause. Shorter days and lengthening nights or increasingly colder temperatures trigger insects into preparing for winter.
Insects overwinter in any of the life stages, egg, immature, pupa or adult, depending on species and preferred habitat. An example of an insect that spends the winter in the egg stage is the aphid. Aphids typically reproduce asexually, the females essentially producing clones during the summer. But come fall, in response to the changing season, males are produced that mate with the females resulting in the production of eggs for the winter. The eggs are laid on the stems of woody perennials. For example, the woolly elm aphid, Eriosoma americanum (Riley), overwinters in the egg stage in cracks and crevices in the bark of white elm trees. Overwintering eggs are destroyed by treating the host tree in early spring with horticultural oil to smother the eggs.
The forest tent caterpillar, Malacosoma disstria Hübner, also overwinters in the egg stage. In the fall the adult moth lays her eggs in a foam-covered ring around small branches of its preferred host, trembling aspen. As a child I was pressed into service every spring by my father to search for egg bands and remove them by hand. This is still the best method of control for this pest.
Some insects choose to overwinter as an immature (i.e. larva or nymph). Wood-boring beetles, including the poplar borer, Saperda calcarata Say, and bronze birch borer, Agrilus anxius Gory, spend the winter under the bark in feeding galleries. The poplar borer can spend up to three years feeding before pupating and emerging as an adult. To combat this stage you have to prune off the infested limb in late fall or early spring before the adult emerges in spring or
Two of three species of birch leaf miner overwinter as a larva as well but not in or on the host plant. Instead, these insects drop out of the leaf and burrow a short distance into the soil and spin a silken cell to spend the winter protected under an insulating blanket of snow. The third species overwinters inside the leaf mine. One option to reduce the number of leaf miners emerging is to turn over the soil at the base of the tree in the fall to expose the larvae to the harsh conditions of winter. To get the third species, you should remove mined leaves
that remain on the tree over winter.
Turning over the soil or putting down a landscape fabric to prevent burrowing can also be used against a common garden pest, root maggots, Delia species, and an ornamental tree pest, the yellow-headed spruce sawfly, Pikonema alaskensis (Rohwer). These insect pests burrow into the soil to change into a pupa, a non-feeding stage where they metamorphose from larva to
adult. Many insects feed on insect pupae in the fall and early spring so encouraging beneficial insects in your yard and garden will help to reduce this stage of pest as well.
There are many other insects that overwinter as adults, including the ash bark beetle, Hylesinus species, Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), and many species of flea beetle. Pruning is the best defense against the ash bark beetle. The CPB burrows very deep so it is likely out of reach and should be dealt with in the spring when it lays its eggs. Flea beetles
commonly burrow into the leaf litter near the garden. They are commonly widespread and therefore difficult to manage effectively by taking action on the overwintering stage.
With a little effort in fall or spring you can reduce the number of pests emerging from their winter slumber. The fewer pests that begin the season, the fewer eggs are laid, the fewer offspring there are to attack your plants and therefore the less impact they will have on your garden and yard.