Asian lady beetles are a common sight. These insects, known scientifically as Harmonia axyridis, are frequently encountered in gardens and around homes. While many people associate them with beneficial roles in gardens, their presence can sometimes lead to questions about their impact on local environments.
Identifying Asian Lady Beetles
Asian lady beetles possess distinct physical characteristics that differentiate them from native ladybug species. They typically measure around 5 to 8 millimeters in length, making them slightly larger than many native ladybugs. Their coloration can vary widely, ranging from yellowish-orange to red, and sometimes even black. The number and size of black spots on their wing covers, known as elytra, are also quite variable; some individuals may have many spots, while others have very few or even none.
A key identifying feature is the prominent black “M” or “W” shaped marking located on their white pronotum, the shield-like structure behind their head. Native ladybugs, in contrast, usually have a black head and thorax with small white dots on the sides, lacking this distinct “M” or “W” shape. Asian lady beetles also tend to have more white on their head, particularly around their “cheeks,” compared to native species. Their legs are often lighter in color.
Why They Are Considered Invasive
Asian lady beetles originated in Asia. They were intentionally introduced to new regions, including the United States, as a biological control agent primarily to manage populations of aphids and other soft-bodied agricultural pests. Early introductions occurred as far back as 1916, with more widespread establishment in North America noted around 1988. Some scientists also suggest accidental transportation via cargo ships contributed to their spread.
These beetles are considered invasive because they outcompete native ladybug species for food resources, such as aphids, and for overwintering sites. Their aggressive nature and higher fecundity allow them to rapidly increase their populations, displacing native species. This competition can reduce the survival and reproductive success of native ladybugs. The Asian lady beetle also consumes the eggs and larvae of native ladybugs, further impacting their populations.
Beyond ecological impacts, Asian lady beetles can become a nuisance to humans, especially when they seek shelter indoors in large numbers during the fall and winter months. They are attracted to buildings, particularly those with light-dark contrasts and warmer, sunnier exposures. When disturbed, these beetles release a yellowish, foul-smelling fluid, known as hemolymph, from their leg joints, which can stain surfaces like walls, furniture, and fabrics. Asian lady beetles can also bite humans, though these bites are generally not serious and do not transmit diseases. Some individuals may experience allergic reactions to the beetles or their secretions.
Managing Their Presence
When Asian lady beetles enter homes, particularly in the fall, several non-toxic methods can help manage their presence. Sealing entry points is a primary preventive measure. This involves caulking cracks around windows, doors, utility wires, pipes, and gaps in siding or foundations. Repairing or replacing damaged window screens also helps prevent their entry.
Once inside, vacuuming is an effective way to remove them without crushing them. Vacuuming helps avoid the release of their defensive chemicals, which can stain surfaces and produce odor. It is advisable to empty the vacuum bag or canister outdoors immediately to prevent escape. Avoiding chemical pesticides indoors is generally recommended, as they are often ineffective against overwintering beetles and can leave unwanted residues. Some natural repellents, like citronella, citrus oil, cloves, or bay leaves, may deter them.