Does a Hard Winter Mean Fewer Bugs?

19 December 2013, written by Barbara Pleasant

Winter officially begins on the Winter Solstice, the day when the sun stops sagging ever southward, which falls on December 21 this year. Ahem. At my house we have already seen snow twice, and the look and feel of winter has people talking about what this might mean to next year's garden pests. Does a hard winter mean fewer bugs?

According to recent research, the answer depends on what we mean by a hard winter. Insects have had about 400 million years to figure out how to survive cold, and they are very good at it. Starting in fall, overwintering insects accumulate chemicals that limit the ability of ice crystals to damage the organism, whether the insect spends winter as an adult, pupae, or egg. The pupae of cabbageworms, for example, can survive temperatures to -25°C (-13°F); the eggs of bean aphids don't begin to crack until temperatures drop to -34°C (-29°F).

Weather this frigid is often accompanied by snow, which works to insects' advantage by keeping temperatures steady. Insulated from weather changes at the surface, cold-hardy insects enjoy a restful, uneventful winter in years with good snow cover.

The kind of winter that is likely to take a heavy toll on garden pests is much more of an up-and-down affair, with mild periods falling in between hard freezes, and scant snow to protect places where insects like to hide. Overwintering insects that are repeatedly exposed to freezing and thawing are more likely to die, and those than survive typically lay fewer eggs. Up-and-down winters that are brutal one week and balmy the next are bad news for overwintering insects.

Changeable weather is also rough on slugs, which are so traumatized by hard freezes that they lay low for four to five days afterward, even if the cold snap is following by a mild spell. In cool maritime climates where slugs are active year round, the garden benefits most from erratic winters. In very cold climates, however, slug adults and eggs stay deep in the ground until the soil thaws in spring. Then you have slugs like the Spanish slug (Arion vulgaris), a cold-sensitive species from southern Europe that has successfully invaded northern Europe by breeding with native species, thus increasing its cold tolerance. Citizen scientists please note: The John Innes Centre is hosting the SlugWatch project, which hopes to track the Spanish slug (sometimes called the giant killer slug) in the UK.

We gardeners pray for a warm spring so we can get busy planting, but a cool spring is harder on pesky insects. In fact, when spring comes so slowly that plants get a head start before pests emerge, insects tend to be easier to manage. But when mild weather causes overwintering insects to emerge ahead of schedule, their numbers can increase rapidly, so they are ready and waiting to damage tender young plants.

And so, with the Winter Solstice upon us, we must rely on our own observations as to what the effects of winter might be on our gardens and the insects that inhabit them. Why not wish for the best? Borrowing from ancient Winter Solstice rituals, you might start the day in darkness illuminated by a candle that is surrounded by stems of evergreen foliage. Symbolically, the lit candle uses fire to call for returning light, while evergreens represent the continuity of life according to Nature's rhythms. Now make your wish.

By Barbara Pleasant