How many are coming? Hundreds? Thousands? Millions? A prediction on a number is a wild guess at best. No matter the number of these accidental invaders, most people don't like to see even one in their homes. The invader I'm talking about is the uninvited "multi-colored Asian lady beetle".
The multicolored Asian lady beetle has become permanently established throughout the U.S. and Iowa. I try to remember that they serve a useful purpose as a beneficial biological control eating aphids and other insect pests in trees during the summer, and in fields and gardens during the fall. However, most people are exposed to them only when they become a household nuisance during late fall and winter as they accumulate on the sides of light-colored buildings and walls and wander indoors.
I understand they are only following their instinctive behavior of flying to sunny, exposed surfaces as they prepare to hibernate through the winter. The time of beetle flight varies, but is usually from mid-September through October, dependent upon day length and the weather. The first warm day after the first fall frost will find the beetles scurrying to find an overwintering spot.
The Asian lady beetle is like other accidental invaders; that is, "outdoor" insects that create a nuisance by wandering indoors during a limited portion of their life cycle. Accidental invaders do not feed or reproduce indoors. Insects that emerge from overwintering sites inside a home during winter and spring all entered the building the previous fall. They cannot attack the house structure, furniture, or fabric. They cannot sting or carry diseases, however, they infrequently pinch exposed skin. Lady beetles may leave a slimy smear and they have a distinct odor when squashed.
As with other accidental invaders, the most effective management option is to prevent invasion by sealing cracks, gaps and openings around windows, doors, eaves, roofs, siding and other points of access before the beetles appear. Experience tells us, however, that comprehensive pest proofing is time-consuming, often impractical and usually not 100% effective. Spraying pyrethroid insecticides such as permethrin or esfenvalerate to the outside of buildings when the beetles appear may help prevent pest entry. Treatment must be applied before the beetles begin to enter buildings to be effective. Homeowner insecticides other than permethrin usually do not provide satisfactory prevention.
Indoor sprays are of very limited benefit and should be used sparingly. The practical solution for homeowners in fall and winter is to vacuum and discard invader lady beetles as they appear. Long-term relief may come from planting trees that will grow up to shade the south and west sides of the house.
The time to insect-proof your house is BEFORE the hundreds, thousands, or millions of invaders make their way from the gardens, fields, and trees. The time and effort put forth may save you the frustration of seeing them openly wander around your home throughout the winter.