Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
11/11/02
Contacts:
Mark Shour, Extension Entomology, (515) 294-5963, mshour@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for Week beginning Nov. 15
How do Landscape and Garden Pests Deal with the Winter?
By Mark Shour
Extension Entomologist
Iowa State University
It's been a long growing season with the opportunity to spend many hours on your favorite outdoor activities, including gardening. But as the potentially cold, stormy months of winter approach, some Iowans are starting to concentrate on winterizing their homes and making sure they have adequate firewood in storage. Others are preparing to travel to areas with warmer climates. Just like people, insects and other arthropod pests (animals without backbones) that stay in Iowa must prepare for the winter.
Landscape and garden pests in Iowa can survive cold temperatures (overwinter) in many different ways; the way that is used depends on the specific kind of pest. Most find sheltered places outdoors where they are insulated from harsh, drying winter winds and freezing temperatures. Others freeload on humans, spending the cold months in our homes or other structures. Some arthropod pests cannot survive the Iowa winter and must recolonize when warmer temperatures return.
For arthropod pests that spend the winter outdoors, several factors work together to assist them in survival. First, their populations are usually made up of a lot of individuals. Even if many individuals die, their high reproductive potential allows the survivors to build up damaging levels in your garden by midsummer. Second, the small size of most landscape and garden arthropod pests enables them to find shelter in the smallest, deepest recesses on typical urban properties---not to the extent of the journey to the center of the earth---but below the frost line. Third, because the body temperature of arthropods mirrors the temperature of the environment ("cold-blooded" animals), several metabolic changes occur to acclimate them to winter conditions.
At the onset of cooler weather in the fall, overwintering arthropod pests are either in a nonfeeding stage (eggs or pupae) or are active feeders that have stopped taking nourishment. Research shows that some arthropods without food in their gut can withstand lower temperatures than those with food in their gut. Another metabolic change is that overwintering arthropods actively decrease the water content in their body fluids. This decrease is followed by a conversion of glycogen (a sugar built from several glucose molecules) into glycerol, a natural antifreeze. Glycerol content in some overwintering arthropod life stages accounts for 25 percent of the body weight. These adaptations decrease the chance of ice crystal formation in body tissues and thus effectively winterize the animals for below-freezing conditions.
There are many garden and landscape arthropod pests that overwinter outdoors in Iowa. Some pests that pass the winter in the egg stage are spruce spider mite, European pine sawfly, pine needle scale, oystershell scale, white pine aphid, eastern tent caterpillar, true bagworm, iris borer, two-marked treehopper, fourlined plant bug, some grasshopper species, some slug species, and many spider species. Those that overwinter in an immature stage (larvae or nymphs) include the San Jose scale, cottony maple scale, Cooley spruce gall adelgid, Zimmerman pine moth, white grubs (May, June, and Japanese beetles), maple gouty vein gall, wood-boring insects, and some tick species.
Insects overwintering in the pupal stage include blister beetles, tomato hornworm, imported cabbageworm, fall webworm, and mimosa webworm. Examples of arthropod pests surviving the winter as adults are chinch bug, squash bug, flea beetle species, bean leaf beetle, spotted cucumber beetle, striped cucumber beetle, maple bladder gall mite, honeylocust mite, twospotted spider mite, the spotted garden slug, and some spider species.
There are several arthropods that attempt to spend the winter inside man-made structures. Some pests gain access to wall voids by landing on exterior siding and then moving vertically up siding. Others enter through worn or missing door seals, around poorly fitting windows, or through cracks in cement foundations. You can unknowingly bring in overwintering arthropods in firewood or on freshly cut Christmas trees.
Most of these freeloaders are quiescent (inactive) during their hibernation, but very warm conditions (warm days in the winter or occupants recreating summer conditions by turning up the thermostat) cause these pests to move throughout the structure. Fortunately, these pests do not feed, grow, develop, or reproduce during the winter dormant period.
Examples of arthropods that try to spend the winter with us are boxelder bug, multicolored Asian lady beetle, strawberry root weevil, black vine weevil, cluster fly, face fly, wood cockroach, and some tick, spider, millipede, and centipede species.
A few landscape and garden insect pests do not survive Iowa winters. These pests do not have adequate cold hardiness mechanisms and die at the onset of freezing temperatures. Reintroduction of these species into northern states occurs via strong winds in the spring. Although the adult insects have wings, they are not strong enough to make the journey unassisted (for example, aster leafhopper, potato leafhopper, corn earworm, black cutworm, and flower thrips).
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