Lady beetles (ladybugs) are widely recognized as beneficial insects in gardens and agriculture because they consume numerous plant-feeding pests, offering natural pest control. However, not every beetle in the Coccinellidae family is helpful. Certain introduced species, particularly those that have become invasive, pose significant problems for plants and native insect populations. Understanding the distinction between helpful native species and harmful foreign ones is essential for effective garden management.
Identifying the Problematic Species
The primary problematic species is the Multicolored Asian Lady Beetle (Harmonia axyridis), a non-native insect originally imported to North America to control aphids and scale insects. This beetle is characterized by significant variation in appearance, making identification challenging. Its color ranges from pale yellow or orange to deep reddish-orange, and it may have zero to 19 black spots on its wing covers.
A definitive marker for this species is the black “M” or “W” shape located on the white shield-like area just behind its head, known as the pronotum. The beetle is also slightly larger than many native species, measuring about one-quarter inch long with a distinctly convex, dome-like body. Unlike many native lady beetles, the Multicolored Asian Lady Beetle tends to aggregate in large clusters, particularly in the autumn as it seeks sheltered places to overwinter. This clustering behavior often leads to household invasions, where they become a severe nuisance.
The Role of Native Lady Beetles
Native lady beetles provide a valuable ecological service by maintaining a natural balance of garden pests. The Convergent Lady Beetle (Hippodamia convergens), for example, is easily identified by the two white lines that meet at the top of its black pronotum. These native species tend to have a more consistent appearance than the invasive species.
The Nine-Spotted Lady Beetle (Coccinella novemnotata), another native species, is recognized by its pale orange to red wing covers, which typically feature nine black spots and a distinct black line running down the center where the wings meet. These native beetles are highly specialized predators whose diet consists primarily of soft-bodied insects, including aphids, mealybugs, and mites. A single lady beetle can consume thousands of aphids during its lifetime, helping to protect plants throughout the growing season.
The presence of the invasive species poses a direct threat to these beneficial insects, often displacing them through aggressive competition for food resources. The Multicolored Asian Lady Beetle is also known to engage in intraguild predation, actively consuming the eggs and larvae of native lady beetle species. This ecological pressure has contributed to the dramatic decline of several native populations, including the once-common Nine-Spotted Lady Beetle.
Plant Damage and Control Measures
The Multicolored Asian Lady Beetle is detrimental to plants because of its feeding habits when preferred prey, like aphids, becomes scarce. Late in the season, the beetle switches its diet to soft, ripe fruits (frugivory). This feeding causes direct damage and cosmetic issues for crops such as grapes, raspberries, and apples, especially where the fruit skin is already bruised or broken.
In vineyards, the beetles can be inadvertently harvested with the grapes. When crushed, they release a defensive chemical that taints the resulting wine with an unpleasant flavor. Beyond crop damage, the beetles can become more aggressive toward humans during this time, occasionally biting when they land on skin.
Management of this invasive species focuses on non-toxic methods that protect beneficial native populations. Since the beetles often cluster in large numbers, particularly on structures in the fall, manual removal is an effective strategy. Gardeners can use a handheld vacuum cleaner with a nylon stocking secured over the nozzle to collect the beetles without harming them, allowing for later disposal away from the garden.
Exclusion techniques are also highly recommended to protect vulnerable fruit crops. Covering ripening plants with fine mesh netting can physically prevent the beetles from accessing the soft fruit late in the season. Broad-spectrum chemical pesticides are discouraged, as these chemicals indiscriminately eliminate native, beneficial lady beetles and other natural predators, which can ultimately worsen pest problems.