Ladybug

A predator’s diet

Ladybugs (“ladybird beetles” in British English) are predatory beetles, eating aphids, mites, scale insects, and other soft-bodied invertebrates. A single adult ladybug can eat 5,000 aphids over its lifetime. Both the larva (a tiny dragon-like creature with spikes) and the adult are predators.

This diet is exactly what gardeners want: aphids and scale insects are among the most destructive pests of vegetables, fruit trees, and ornamental plants. Ladybugs have been deliberately introduced to greenhouses and farms for biological pest control for over a century.

The “lady” in the name

The name comes from European agricultural folklore: medieval farmers prayed to the Virgin Mary (“Our Lady”) for help against crop-destroying pests, and the appearance of these beetles was taken as her response. The seven-spotted ladybug (Coccinella septempunctata) was named for the seven spots referencing the Seven Sorrows of the Virgin in Christian iconography.

Aposematic warning colors

Bright red-and-black coloration in nature is almost always an aposematic warning — a signal that the animal is toxic, distasteful, or dangerous. Ladybugs are no exception: when alarmed, they secrete a yellow oily fluid from their leg joints (a process called reflex bleeding) that contains alkaloid toxins. The fluid stains and tastes vile, and most birds quickly learn to leave ladybugs alone.

An invasive cousin

The multicolored Asian lady beetle (Harmonia axyridis) was deliberately introduced to North America and Europe in the 1980s as a biocontrol agent. It thrived — sometimes too well. The species:

• Outcompetes native ladybug species for prey.
• Aggregates indoors in autumn, sometimes by the thousands, creating nuisance infestations.
• Stains walls and curtains with reflex bleeding.
• Bites humans (mildly).

Native North American ladybug species have declined dramatically in regions where the Asian beetle has established. The case is a textbook cautionary tale of biological control gone awry.

Larvae look nothing like adults

Ladybug larvae are dragon-like — long, dark, with spikes and orange or yellow markings. They look entirely unrelated to the dome-shaped spotted adults. Many gardeners mistake them for pests and kill them, ironically eliminating the pest control they’re trying to encourage. Knowing what ladybug larvae look like is one of the most useful pieces of garden knowledge.

Spots that aren’t really useful

Despite folk beliefs, the number of spots on a ladybug does not indicate its age. Different species have different spot counts (zero to over 20). Within a species, spot count is genetically determined and doesn’t change over the beetle’s life.