What does a Warm, Dry Winter Mean for Insect Populations?

What does a Warm, Dry Winter Mean for Insect Populations?

Utah has experienced two mild winters in a row. What are the likely impacts of mild winters on pest insect populations? There is not a precise science to predicting insect populations following a warm winter, but we can make inferences from what is known about mechanisms of insect dormancy, survival, and population growth. Many factors come into play, including effects of repeated thawing and freezing, amount of snow cover, spring frosts, and survival of beneficial insects that will feed on pest insects in spring and summer. In general, it is probable that the long stretches of the warm temperatures in winter 2015 have provided some species the chance to survive in larger numbers.

In temperate regions, such as Utah, many insects enter a form of arrested development to avoid adverse winter conditions, called ‘diapause’. Diapause is much like hibernation in vertebrate animals; it is a slow-down in development of a specific life stage. For example, diapause occurs in eggs of grasshoppers and aphids, in last-instar larvae of codling moth and peach twig borer, in pupae of tomato hornworm and corn earworm, and in adults of alfalfa weevil and squash bug. Diapause is primarily triggered by shortening day length in late summer and fall. In preparation for diapause, the insect sequesters fat reserves and moves to a protected site. Upon entering diapause, metabolism is slowed and food and water intake usually ceases. To protect against water loss during the winter, insect cuticles may accumulate a thicker wax layer or absorb water vapor. Diapausing adults may absorb energy from degenerating flight muscles and cluster together for protection. Insect body fluids and tissues withstand freezing temperatures in a number of ways; the accumulation of antifreeze compounds such as glycerol, sorbitol, or some proteins is a common mechanism.

A mild winter combined with early spring conditions can create a ‘double whammy’. Not only will more insects survive the winter, but an early start to the season can translate into greater numbers, especially for insects with a high reproductive rate, short generation time, and multiple generations per year.

Insects that overwinter as adults above ground

alfalfa weevil clover mites grape leafhoppers minute pirate bug stink bugs
asparagus beetles cucumber beetles honey bee onion thrips two-spotted spider mites
boxelder bug European paper wasp lacewings spider mites western flower thrips
bumble bee flea beetles lady beetles squash bugs western predatory mite

Boxelder bugs overwinter as adults. Adults will become active on warm winter days, such as in 2015, but cold nighttime temperatures and desiccation can kill them.

Insects that overwinter as adults seek protected sites, such as tunneling into leaf litter, specialized nests, or in cracks and crevices of trees and structures. This group of insects is particularly susceptible to cold winter temperatures and loss of moisture. In general, some individuals are killed by cold or even during average winters, thus regulating the population. But warm winter temperatures will increase survival.

In contrast, fluctuating winter temperatures can take their toll on insects. Warm winter days cause some insects to become active (e.g., boxelder bugs, paper wasps) when they normally would be dormant, and subsequent freezing and thawing temperatures may kill them. In addition, these insects use up stored fats they depend on to survive until the spring. Without access to food, these active insects could starve to death before food becomes available.

Insects that Overwinter as larvae or pupae aboveground

bark beetles cabbage worms peach twig borer
borers codling moth raspberry horntail

Insects in this group seek protected shelter, such as bark crevices or plant debris, or create shelters such as silken chambers in or on plant structures.  These protected sites help them avoid severe and fluctuating winter conditions.  In general, overwinter survival of this group following a mild winter will be high, and early spring conditions will stimulate earlier than normal activity.

Codling moth overwinters as late-instar larvae, and then pupate in the spring. These shown at right were located under tree wrap on the trunk of an apple tree.

Insects that Overwinter as adults, larvae, or pupae below ground

ants cutworms flea beetles syrphid fly western cherry fruit fly
apple maggot earwig sawflies turf grubs walnut husk fly

This group of insects is generally less influenced by winter conditions because soil temperatures remain somewhat constant.  A detriment to these insects in Utah’s winter of 2015, however, is the lack of snow cover, which would normally keep ground temperatures more constant.  There could be more survivors than normal if the frost layer of the soil is shallow.  Dry soils in the spring may prompt early development and activity; however, insect survival could be harmed by abnormally dry soils.

Insects that Overwinter as eggs above or below ground

aphids grasshoppers honeylocust plant bug native bees tent caterpillar
cankerworm leafhoppers (some) mosquitoes spiders

With a few exceptions, arthropods that overwinter as eggs typically are not affected by abnormal winter conditions. They are well-designed to withstand external extremes.
For example, some eggs, such as those of the green peach aphid, can tolerate temperatures as low as -50°F.

Populations are more likely to be affected after the eggs hatch in spring, especially if the emergence is earlier than average due to warm temperatures. Insects that hatch when plant buds open, such as aphids or certain leafhoppers, will be susceptible to spring frosts, and are more likely to succumb to the cold at that time. When spiders hatch, if there is an early availability of food such as flies, beetles, and other insects, there will be a greater chance for the spiderlings to survive, leading to a large population later in the summer.

Apple maggots and western cherry fruit fly spend the winter as pupae a few inches below the soil surface.

Aphid eggs, which are laid in near leaf buds, are usually not affected by winter conditions, except they will hatch earlier.

-Marion Murray, IPM Project Leader, and Diane Alston, Entomologist