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Asian lady beetles invading homes in Western Montana

There have been a few reports of lady beetles invading homes in the Western part of Montana this spring. These insects are probably Asian lady beetles (Harmonia axyridis) a non-native insect that was first detected in Montana in 2009. They come in a range of colors, from yellow to red, and may have no black spots or many. They can usually be identified by the clear black 'M'-shaped marking behind their head (Figure 1).

Figure 1. Asian lady beetle color and pattern variation.

Asian lady beetles are commonly sold commercially as beneficial predators for soft-bodied pests such as aphids. However, they can be problematic when they invade buildings in the fall/winter, and they may also be pests when they feed on over-ripe or damaged apples and other fruits at the end of summer. They do not infest wood, destroy fabrics, eat food or damage other property, and they cannot sting and do not carry disease. They secrete a strong-smelling liquid from their leg joints, which can stain light colored surfaces. In late fall before freezing temperatures arrive, these beetles may swarm on the southern and western sides of buildings, looking for a place to overwinter; they can enter through cracks and crevices as narrow as 1/8 inch. Once inside, some may move into the living areas of the home where they move towards windows and other sunny locations and eventually die (Figure 2.) Others find suitable sites within cooler areas of the home to spend the winter, such as within wall voids or attics. They remain dormant in these cool areas until outside temperatures warm up in the spring and they wake up and move further into the living areas of the house. Asian lady beetles do not reproduce indoors. All the lady beetles seen inside during winter and spring entered the building the previous fall.

Figure 2. Asian lady beetles invading a home.

Management: Spraying pesticides indoors is not effective. Once lady beetles move into wall voids in the fall there is no way to prevent them from emerging later during winter or spring. Once they move into the living areas of a home, the most effective way to manage them is to vacuum them up. Change the bag frequently because they aren't automatically killed when they are vacuumed. To prevent an infestation from occurring again next fall, seal south and west-facing cracks and crevices on the outside of the building. This includes spaces around doors, windows and fascia and areas where cable TV wires, phone lines, and other utility wires and pipes, outdoor facets, dryer vents and similar objects enter buildings. In the fall before lady beetles begin to aggregate on buildings, pyrethroid insecticide barriers can be applied around the doors, windows and rooflines on the south/west sides of buildings. 

Marni Rolston, MSU Insect Diagnostician
[email protected]

MSU Extension
Montana State University
P.O. Box 172230
Bozeman, MT 59717-223

Tel: (406) 994-1750
Fax: (406) 994-1756
Location: Culberston Hall
[email protected]