Lady beetles (family Coccinellidae) are widely recognized as beneficial insects in agricultural and garden settings. Their predatory nature makes them valuable agents in biological pest management, controlling populations of many plant-damaging organisms. These insects are classified as generalist predators, meaning they consume a variety of small arthropods rather than specializing in one type of prey. This broad diet contributes to their success in various ecosystems and their role in natural pest suppression.
Do Asian Lady Beetles Prey on Spider Mites?
The Asian Lady Beetle (Harmonia axyridis) is a generalist predator that consumes spider mites, though these tiny arachnids are typically a secondary food source. Both adult beetles and their larvae prey upon mites, which are soft-bodied arthropods that weaken plants by feeding on cell contents. Mites are less preferred than larger prey items, meaning a significant population shift is often required for H. axyridis to focus on them.
Beetles shift their diet to mites when their primary food source becomes scarce. If a garden has a severe spider mite infestation but few aphids, the Asian Lady Beetle will readily turn to the mites for sustenance. This opportunistic feeding behavior allows them to adapt to different pest outbreaks, but they do not seek out mites as intensely as specialized mite predators.
Identifying the Asian Lady Beetle
Accurate identification of the Asian Lady Beetle is important, as this non-native species is often confused with native ladybugs. A distinguishing feature on the adult beetle is an “M” or “W” shaped black marking on the white or cream-colored area behind the head, known as the pronotum. This marking can vary in thickness and clarity between individuals.
The coloration of the wing covers (elytra) is highly variable, ranging from pale yellow or orange to deep red, or even entirely black. The number of spots also differs greatly; some individuals have up to 19 spots, while others are completely spotless. Adult Asian Lady Beetles are typically slightly larger than many native species, measuring approximately one-quarter to one-third of an inch in length.
Their larval stage also functions as a predator and has a distinctive, alligator-like appearance. The larvae are elongated and somewhat spiny, with a dark blue or black body often marked with orange or reddish-orange spots or stripes. Both the larval and adult stages are active hunters.
Primary and Secondary Food Sources
The primary food source for the Asian Lady Beetle is soft-bodied insects, specifically aphids (Aphidoidea), which they consume in large numbers. A single adult beetle or larva can consume dozens of aphids per day, making them effective biological control agents. This preference is strong; the presence of large aphid colonies often triggers the beetles to lay eggs nearby, ensuring a ready food supply for the hatching larvae.
Beyond aphids, the broad diet of H. axyridis includes other common plant pests. They prey on scale insects, mealybugs, and whiteflies, as well as the eggs and young larvae of various other insects. When prey is scarce, they supplement their diet with non-prey items such as pollen, nectar, and honeydew produced by sap-sucking insects. This flexibility contributes to their ability to thrive in diverse environments.
Effectiveness as a Garden Control Agent
While Asian Lady Beetles contribute to garden pest control, their effectiveness as a primary control agent for spider mites is limited by their strong preference for aphids. Gardeners should not rely on H. axyridis to eliminate a severe mite infestation if aphids are also present. They are better suited for controlling aphid outbreaks than for consistently managing mite populations.
The use of this beetle also comes with certain disadvantages for homeowners. As temperatures cool in the autumn, the beetles congregate on the sunny sides of buildings, searching for sheltered places to overwinter. They often invade homes through small cracks, becoming a household nuisance. When disturbed, the beetles secrete a foul-smelling, yellowish fluid that can stain surfaces.
Ecologically, the proliferation of the Asian Lady Beetle has raised concerns about their competitive impact on native lady beetle species. Their generalist diet and reproductive success allow them to outcompete native predators for limited food resources. This competition can contribute to the decline of native ladybug populations, creating a complex trade-off between pest control benefits and ecological consequences in the garden ecosystem.