Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Photo of Asian lady beetle. © Scott Bauer

The Asian lady beetle is an effective predator of aphids.
© Scott Bauer

August 2011

Beware beetle juice

Asian lady beetles are on the move.

Susan Mahr

Harmonia axyridis is a yellow to orange-colored lady beetle that is often seen in large congregations on buildings around the end of October, hence one of its common names, the Halloween lady beetle. The multicolored Asian lady beetle is the preferred common name for this lady beetle introduced from Japan for control of tree-inhabiting aphids. The adult lady beetle is quite variable in appearance. Individuals can be any color from a pale yellow-orange to a deep orange-red, and have from none to more than 20 black spots. They are very prolific and may live up to three years.

The beetles lay bright yellow eggs in clusters of about 20 on the undersides of leaves. The eggs hatch in three to five days, and the larvae feed up in the trees for 12 to 14 days. They then pupate on the leaves and adults emerge in five to six days. The USDA made several releases throughout the eastern United States in an attempt to introduce this beetle into North America. It took a long time for releases made in Louisiana and Mississippi in 1979 and 1980 to become established and spread, but by 1994 the beetle was found in Alabama, Georgia, South Carolina, and Florida. It can now be found in many locations throughout the Midwest.

The multicolored Asian lady beetle inhabits various trees, including maple, walnut, willow and oak, and feeds on various aphids, certain scales, and a few other insects. It is an effective predator of aphids on pecans, pine trees, ornamental shrubs, roses and other plants. In many areas pecan growers no longer need to spray their trees for pecan aphids because this lady beetle has done such a good job of biological control. Beetle populations tend to explode when there is an abundance of aphids, often eliminating the local aphid population.

Even though this lady beetle is an important biological control agent, it can become a nuisance when it aggregates in large numbers on homes or other buildings. For several days during autumn they typically cluster on sunny, southwest sides of light-colored rock outcroppings or structures where nearby crevices serve as overwintering sites.

Homeowners complain when there are thousands of beetles covering their house, they have to walk across piles of them on the deck, they get in picnic food and drinks, "swarm" like bees and land on people, especially when the beetles get in the house by crawling through openings such as uncaulked window frames. Outdoor aggregations may leave voluntarily after a few days or weeks.

The best way to prevent beetles from becoming uninvited houseguests is to seal cracks around windows, doors, siding, utility pipes or other openings with a good quality caulk.

Replace or repair damaged screens, and install screens over roof vents. Indoor aggregations can be removed with a vacuum cleaner with a crevice tool. They can also be swept up with a broom and dustpan and be deposited outside well away from the house. Killing them with insecticides, squashing them, or handling them may result in orange stains on walls and fabric. When stressed the lady beetles secrete a harmless, but staining, orange substance. This pungent liquid is actually blood that comes out of the joints of the legs – a phenomenon called reflex bleeding (all lady beetles do this when stressed).

Susan Mahr is an outreach specialist and master gardener at the University of Wisconsin – Madison Department of Horticulture.