Ladybug

Everyone knows you can tell a ladybug’s age by counting its spots. It is one of the most widely repeated facts about any insect, and it is entirely false.

The number of spots tells you the species. A seven-spot ladybug has seven spots for life, from the day its shell hardens. A two-spot has two. A twenty-two-spot has twenty-two.

An adult ladybug never changes its markings and never grows, because insects do all their growing as larvae. The beetle that lands on your hand is the finished article, and it is as old as it will ever look.

Ladybugs are the family Coccinellidae, with around 6,000 species, so there is no single name. The seven-spot ladybird is Coccinella septempunctata, and the harlequin ladybird is Harmonia axyridis.

• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Coleoptera
• Family: Coccinellidae

A ladybug is a beetle, not a bug. True bugs are a separate order with piercing mouthparts, so entomologists prefer ladybird beetle or lady beetle. Ladybug is the American name, ladybird the British one, and both refer to the Virgin Mary, who was often depicted in a red cloak: the seven spots were read as her seven sorrows.

The affection is ancient and practical. Farmers across medieval Europe noticed that these beetles appeared when aphids were eating the crop, and the name was a thank-you.

Ladybugs are predators, and the numbers are the reason farmers care about them.

A single adult eats around 50 aphids a day and may get through 5,000 in its lifetime. The larvae are worse: a ladybug larva is a fast-moving, spiny, alligator-shaped thing that spends two to three weeks doing almost nothing but eating aphids, and eats more in that period than the adult does in months.

That larva is the stage nobody recognizes. It looks nothing like a ladybug, and a great many gardeners kill them on sight thinking they are pests, which is the single most useful thing to correct about this insect.

Their appetite produced one of the first great successes in biological pest control. In the 1880s, California’s citrus industry was being destroyed by cottony cushion scale, an introduced insect, and the orchards were failing. An entomologist traced the pest to Australia, went there, and brought back the vedalia beetle, a ladybug that eats it. Within about two years the scale was under control across California and the citrus industry was saved.

That success was so complete that it set the pattern for a century of importing predators to deal with pests, which turned out to have a serious catch, as the harlequin story below shows.

Ladybugs are 0.04 to 0.4 inches long and weigh a few hundredths of a gram, and the familiar domed round shape is almost unique among beetles.

The spotted shell is not wings as such. The hard outer covers are elytra, modified front wings that protect the delicate flight wings folded underneath, and the flight wings are roughly twice the length of the body, folded along crease lines like origami. Engineers have studied those folds for deployable structures, since the wing unfolds and locks in a fraction of a second.

Not all ladybugs are red with black spots. There are yellow, orange, brown, black, black with red spots, striped and checkered species, and plenty with no markings at all.

The color is a warning, which is the point of it. Ladybugs are distasteful, and a bird that eats one usually does not repeat the experience.

Ladybugs live on every continent except Antarctica, in gardens, farmland, forest, grassland, wetland and desert.

They follow their food, which means they are found wherever aphids, scale insects or mites are, and that includes almost any garden.

In the fall many species gather to overwinter, sometimes in enormous aggregations. Convergent ladybugs in the western United States assemble in mountain sites in masses of thousands, returning to the same spots year after year, and in Europe and North America others pile into leaf litter, under bark and into the window frames and lofts of houses.

That last habit is why people find clusters of ladybugs indoors in October. They are not breeding or feeding; they are hibernating, and the warm side of a building reads to them like a sunny rock face.

Most ladybugs are predators, and the family as a whole is omnivorous because the adults also take pollen, nectar and honeydew, which they need for the energy to fly and to get through winter.

Aphids are the staple, with scale insects, mealybugs, mites, whitefly and insect eggs taken too.

Some break the pattern entirely. A number of species are fungus eaters, including the orange ladybird which eats mildew, and a handful are plant eaters and genuine agricultural pests, notably the Mexican bean beetle, which is a ladybug that eats beans.

They are also cannibals when food is short, and larvae eat eggs and smaller larvae of their own species, which is partly why females lay some infertile eggs alongside the fertile ones as a first meal for the hatchlings.

The defense is worth describing properly, because the harmless-looking beetle has two unpleasant tricks.

The first is reflex bleeding. Threatened, a ladybug releases a drop of foul yellow fluid from its leg joints, which is its own blood, loaded with alkaloids that taste appalling. The yellow stain people find on their hands after holding one is that fluid.

The second is simply playing dead, tucking in its legs and dropping off the leaf.

They are good fliers, beating their wings around 85 times a second, and tracking has shown ladybugs flying at considerable altitude and traveling many miles, which is how they find new aphid colonies.

They also do something genuinely grim. A parasitic wasp, Dinocampus coccinellae, lays an egg inside a ladybug; the larva feeds inside it, then emerges and spins a cocoon between the beetle’s legs, and the ladybug, still alive and partly paralyzed, stands over the cocoon and twitches at anything that comes near, guarding its own parasite until it dies. A virus carried by the wasp appears to be what alters the beetle’s behavior.

A female lays 200 to 1,000 eggs over her life, in small clusters on the undersides of leaves, deliberately placed next to an aphid colony so the larvae hatch into a food supply.

Eggs hatch in three to ten days. The larva then eats continuously for two to three weeks, molting four times.

It attaches itself to a leaf and pupates for about a week, and the adult emerges pale yellow and soft, with no spots at all. The color deepens and the markings appear over the next day or two as the shell hardens, and that is the only time a ladybug’s appearance changes.

Adults live a few weeks to a year depending on whether they overwinter, and most species manage one to two generations a year.

Birds, spiders, ants, dragonflies, frogs, parasitic wasps and other ladybugs eat them, and ants are a serious problem because ants farm aphids for honeydew and will defend them, attacking ladybugs that approach.

The largest threat to native ladybugs, though, is another ladybug. The harlequin ladybird was introduced to North America and Europe for pest control and has spread explosively, and it is bigger, breeds faster, tolerates a wider diet, and eats the eggs and larvae of native species. Several native ladybugs have declined sharply where it arrived, and the nine-spotted ladybug, once common and the state insect of New York, became so scarce that it was presumed gone from the eastern United States before small populations were rediscovered.

Pesticides, habitat loss and the removal of hedgerows and weedy margins that hold aphid populations also matter.

Most of the 6,000 species have never been assessed, and the honest position is that nobody knows how most ladybugs are doing.

What is documented is the shuffling of species. Several formerly common North American natives have declined dramatically while the introduced harlequin and seven-spot have spread, so the total number of ladybugs in a garden may be unchanged while the mix has been replaced.

Public recording projects have been unusually valuable here. The Lost Ladybug Project in the United States and the UK Ladybird Survey have both relied on people photographing beetles in their gardens, and that is how the surviving nine-spot populations were found.

Helping them is easy: tolerate some aphids, since a garden with no aphids supports no ladybugs, skip the insecticide, leave leaf litter and hollow stems for overwintering, and learn to recognize the larva.

Ladybugs are among the most economically valuable insects on earth after bees. The aphids, scale insects and mites they eat are major agricultural pests, and ladybugs suppress them for free across millions of acres.

They are also pollinators in a small way, carrying pollen between the flowers they feed at, and they are food for a long list of birds and invertebrates.

Their history carries a useful warning. The vedalia beetle saved California’s citrus and made the case for biological control, and the harlequin ladybird, imported on exactly the same reasoning, is now the main threat to native ladybugs across two continents. Both were the same idea, applied with the same confidence, a century apart.

And the spot-counting myth is worth retiring for its own sake. It is a pleasant idea, it is taught to children constantly, and the truth is better: the spots are a species label, fixed within hours of the beetle emerging, and the little red dome on your finger is already fully grown.