The Spiky Grubs Are The Ladybugs Working.

About a week after a ladybug release, the beetles are mostly gone and something else is on the plant. It is maybe a quarter of an inch long, dark gray or black with orange markings, faintly spiky, and it moves across a leaf like it has somewhere to be. Most people meeting one for the first time assume it is a pest.

It is a ladybug. Specifically it is a ladybug larva, and over the next couple of weeks it will eat more aphids than the beetle that laid it managed during its short stay on your plant.

That is the useful thing to know about a ladybug release, and it is not obvious, because the insect you buy and the insect that does most of the work look nothing like each other.

What you are looking at

A ladybug larva does not look like a smaller ladybug. It looks like a tiny alligator — long, tapered, segmented, dark gray to nearly black with orange or yellow blotches, and covered in short bristly projections. It has six obvious legs and it is faster than you expect.

The resemblance to something harmful is the whole problem. It is the right size and the wrong shape to read as a beneficial, and it turns up on a leaf at exactly the moment somebody is inspecting that leaf closely for pests. If you have released ladybugs in the last fortnight and you find these, do not spray. You are looking at the release working.

It grows through four stages, molting between each, so the ones you find in week two are noticeably larger than the ones from week one. The last two stages are the hungry ones.

Why the larva eats more than the beetle

An adult convergent lady beetle eats about a hundred aphids a day, which sounds decisive until you know how long it stays. Most released adults leave the plant within one or two days — and they do it whether or not there are aphids left to eat. So the adult contributes perhaps two days of very fast feeding.

The larva has no wings. It cannot leave. It hatched on a leaf next to an aphid colony because that is where its mother laid it, and it stays on that plant until it is finished growing.

University of Florida puts the larger larvae at 30 to 50 aphids a day. University of California extension puts the lifetime figure at up to 500 aphids, and gives exactly this reason: the larvae cannot fly away. Fifty a day is half an adult's rate, sustained for ten times as long, on one plant instead of wherever the wind took it.

One finding that looks like it contradicts this, and does not

A 2017 review of ladybird releases in greenhouses reports that released larvae were less effective than adults, measured as aphid reduction per individual released. Read quickly, that says the opposite of this article.

It does not, because it answers a different question. That comparison is between buying larvae and buying adults — a purchased larva has one feeding window and no wings, so it works whatever patch it is placed on and nothing steers it toward the colonies. The claim here is about larvae that hatched on your plant, from eggs a beetle laid deliberately beside prey she picked. Not the same animal in the same situation. We do not sell ladybug larvae in any case.

The same review found the gap closed for flightless ladybird strains, where “larvae were not significantly less effective than adults.” That is the tell: what makes an adult worth more per individual is that it can search, and what makes it worth less is that it can leave.

The clock, and the week where nothing happens

In the laboratory, at 77°F and high humidity, the whole cycle from egg to adult takes about 21 days. University of Florida breaks it down: three and a half days in the egg, then four larval stages of roughly two and a half, two and a half, two, and four days, then about five and a half days as a pupa.

A pupa does not eat. It attaches itself to a leaf or stem, stops moving, and spends the best part of a week rebuilding itself into a beetle. Nothing is being hunted during that time.

Now consider what that looks like from outside. You released a batch of beetles on one evening, so they laid over a few days, so the larvae hatched together, fed together, and pupate together. For roughly a week there are predators visibly stuck to your plant and the aphid numbers stop falling.

This is the week people conclude it failed and reach for a spray. The same trap catches predatory mite releases in week two, for a different reason, and the mistake is the same one: judging a biological control on a snapshot rather than on a trend.

The beetles that emerge from those pupae are then adults, with adult behavior, which includes leaving. Some stay and lay again if there is still prey. Most do not. Indoors, with the windows shut, more of them stay than would outside.

Whether you get larvae at all

None of the above happens unless the beetles lay, and not every release produces eggs. A female lays in clusters of ten to thirty, standing on end, on leaves and green stems close to prey — several hundred over a season, if she has the food to make them.

Two things decide whether that happens on your plant rather than someone else's. The first is prey density: a beetle lays next to a colony worth laying next to, and a plant with a handful of aphids on it is not one. The second is how long she stays, which is where the release-night advice comes from — wetting the foliage, releasing low on the plant, releasing in the cool of late evening. University of California's phrasing is that this “may slow beetle dispersal,” which is a hedge, and it is the best-supported thing you can do.

It also means a release aimed at prevention, onto plants with almost nothing on them, gets you very little. Ladybugs are a response to a colony, not a standing guard. Which of those two jobs you are hiring for is worth settling before you buy anything.

How to read your own release

The trend to watch is the aphids, on one marked shoot, counted the same way each time. Not the beetles.

Common questions

What do ladybug larvae look like?

About a quarter of an inch long, elongated and segmented, dark gray to black with orange or yellow markings, with short spines along the body and six visible legs. The usual description is a tiny alligator. They look nothing like an adult ladybug, which is why they get mistaken for pests.

Are the spiky black and orange bugs on my plant harmful?

If you released ladybugs in the last two or three weeks, almost certainly not — they are the larvae, and they are eating your aphids at 30 to 50 a day each. Do not spray them.

Do ladybug larvae eat more than adult ladybugs?

Over their lives, yes, and the reason is that they cannot fly. An adult eats faster — about a hundred aphids a day — but most released adults leave the plant within one or two days. A larva eats 30 to 50 a day for around two weeks without leaving, which UC extension puts at up to 500 aphids over its development.

My larvae have stopped moving and are stuck to the leaves. Are they dead?

Most likely they are pupating, which is the stage between larva and beetle. It takes about five days at room temperature and the insect does not feed at all during it. A batch released on the same evening tends to pupate at the same time, so there is a flat week where nothing is being eaten. Beetles emerge afterwards.

Will the new beetles stay and start another generation?

Some will if there is still prey worth laying in, and more will indoors than outdoors, because a closed room limits dispersal. But adults disperse as a matter of behavior rather than as a response to conditions, so a self-sustaining population on a houseplant is not the thing to plan around. Plan around waves.

I cannot find any larvae. Did the release fail?

Either the beetles left before laying, or the colony was too thin to lay in, or the eggs are there and you have not found them — they are small, cream to orange, and stood on end in clusters on leaf undersides and stems. Check the aphid trend on one shoot over a week before concluding anything.