Biological control in an outdoor pepper patch works, but not the way the catalogs imply. Roughly 95 percent of purchased convergent lady beetles leave within 48 hours. Praying mantids eat lacewings as happily as aphids. The beneficials that actually hold a garden are the resident ones, and your job is mostly habitat: flowers, mulch, water, and no broad-spectrum sprays.

Who eats what

BeneficialMain targetsHow to get it
Lady beetles (Coccinellidae)Aphids, thrips, scale, whitefly, mitesDill, fennel, yarrow; buy only in quantity.
Green lacewings (Chrysopidae)Aphids, thrips, small caterpillarsBuy eggs or larvae early in the season.
Parasitic wasps (Braconidae, Aphelinidae)Aphids, whitefly, hornwormsAlyssum, coriander, carrot-family flowers.
Minute pirate bug (Orius insidiosus)Thrips, mites, aphids, insect eggsResident in most gardens; pollen sustains it.
Hoverflies (Syrphidae)Aphids (larvae only)Flat-topped flowers: alyssum, dill, parsley.
Predatory mitesSpider mites, thrips, whitefly larvaeBuy and release before the outbreak.
Ground beetles (Carabidae)Caterpillars, grubs, slugsUndisturbed mulch and bed edges.
Entomopathogenic nematodesFungus gnats, grubs, root weevilsSoil drench into moist, shaded soil.

Do released ladybugs actually stay in the garden?

Mostly no. Commercial lady beetles are wild-collected Hippodamia convergens, and they arrive still carrying the migratory drive that follows overwintering in the mountains. Research puts dispersal at about 95 percent within 48 hours, with the rest gone in four or five days, and they leave before laying eggs, so no second generation stays behind. University of California work found a single release of 175 to 350 beetles per shrub did nothing to aphid numbers, while releases of 1,400 to 1,750 per shrub brought aphids to near zero. Release in the evening onto watered foliage if you do it. Buy far more than the package suggests, or skip it and grow the flowers instead. The Xerces Society also warns that wild-collected beetles can carry pathogens into local populations.

Should I release praying mantises?

No. Mantids catch whatever moves within reach, which includes lady beetles, lacewings, bumblebees and butterflies. University of New Hampshire Extension concludes it is probably not worthwhile to buy and release them. The two species sold as oothecae, the Chinese mantis and the European mantis, are both introduced, and Chinese mantis populations appear to crowd out native mantids. A mantis in the garden is a fine thing to watch and a neutral factor in pest control. Do not pay for one.

The ones worth buying

Three groups return the money reliably, and all three work best as a preventive release rather than a rescue.

Predatory mites. Phytoseiulus persimilis specializes in two-spotted spider mite and starves once it clears them. Amblyseius swirskii takes thrips and whitefly larvae and holds on in warm, humid conditions. Put them out ahead of hot dry weather, when spider mites build, rather than after webbing appears.

Parasitoid wasps. Aphidius colemani, a braconid, lays into aphids and leaves the mummified husk behind as a visible score. Encarsia formosa was the first biological control agent developed for greenhouses and still runs whitefly programs on commercial tomato and cucumber. Both work at low pest density and fail once the population runs away, so start when you find a few per plant.

Entomopathogenic nematodes. Match the species to the depth. Steinernema feltiae cruises shallow and handles fungus gnat larvae, shore flies and thrips pupae. Heterorhabditis bacteriophora hunts deeper and takes scarab grubs and root weevils. Each carries a symbiotic bacterium, Xenorhabdus or Photorhabdus, that kills the host inside 24 to 72 hours. Apply as a drench into moist soil, out of direct sun.

The residents doing most of the work

Minute pirate bugs deserve more credit than any purchased insect. Orius insidiosus eats around a dozen thrips a day and kills more than it eats, takes mites, aphids and eggs besides, and switches to pollen when prey runs short. It needs temperatures over 59 °F and humidity over 60 percent. Give it flowering plants and it stays.

Green lacewing larvae work through aphids, thrips and small caterpillars while the adults feed on nectar and pollen. Hoverfly larvae eat aphids and the adults pollinate. Spiders hunt around the clock in mulch and webs. Ground beetles work at night on caterpillars, grubs and slugs. Braconid wasps handle hornworms: a hornworm carrying white rice-grain cocoons is already finished, so leave it on the plant and let the wasps emerge. The pest guide and the diagnosis chart cover identification.

Practices that build the population

Skip the broad-spectrum insecticides first. They cost you the predators faster than the pests, and pest populations rebound quicker. Grow flowering plants across the season so nectar and pollen never run out: alyssum, dill, coriander, fennel, yarrow. Leave mulch and leaf litter alone, because that is where ground beetles and spiders live. Set out a shallow dish with stones so insects can drink without drowning. Stagger plantings and keep bare soil to a minimum. See the companion planting guide for what to plant, and the soil guide on protecting the soil food web that feeds this from below.

Under cover the calculation changes. A greenhouse or tunnel holds released beneficials in place, which is why commercial biological control developed there first and why most of the research behind these species comes from protected culture. The greenhouse guide covers that setting, and the glossary defines the terms.

Sources & Further Reading