Elm Leaf Beetle
The elm leaf beetle is a common insect on elm trees in Iowa. It is also a common indoor, household nuisance in the fall, winter and spring. The adult beetles are accidental invaders; that is, they wander into the house by mistake.
The larvae of the elm leaf beetles are leaf-feeding pests on elm trees, particularly Chinese, Siberian and hybrid elms. There are two generations of larvae each summer, the first during June and the second in late July and August. Larvae of the first generation change to adults that immediately lay eggs on elm tree foliage. The adults of the second generation pass the winter hiding in cracks and crevices and may wander into houses and other buildings to hide. They try to return to the trees in May or whenever they are sufficiently warmed by the heat in the house.
Elm leaf beetle feeding on trees is usually not a serious threat to the tree, even though damage often appears to be extensive. Trees can be sprayed in late May and again in late July to protect trees from defoliation and to reduce the population of potential household invaders. Appropriate insecticides include Sevin and Orthene. Because beetle populations vary greatly from year to year, it is difficult to predict when spraying will be warranted. The best indication is to look for large numbers of yellow and black larvae at the times mentioned.
There is little that can be done about elm leaf beetles have migrated into the house. They do not reproduce or feed indoors during the winter and they are harmless -- they cannot bite or sting and they do not attack the house, its furniture or occupants. Household aerosol insecticides have very little effect on these insects. The most practical solution is to vacuum, sweep or pick up and discard beetles as they appear. Caulking and sealing any foundation or siding cracks and openings through which the beetles enter in the fall can be an effective prevention. Also, if beetles are observed in the autumn as they approach the house, they can be sprayed while still outdoors with an outdoor use insecticide labeled for beetle control. For more information on insecticides please see this article.