Beneficial Insects: Your Garden’s Best Hired Hands

Beneficial insects are your garden’s best hired hands, because ladybirds, hoverflies and wasps control pests without sprays.

Most garden pests are held down by a small group of predators and parasitoids that you can recognise, feed and keep alive. Aphids, caterpillars, mites and slugs all have natural enemies, but those enemies arrive late, need flowers as adults and die easily from sprays. This guide covers who they are, which plants feed them, how to read the signs of their work and how to time any treatment so it spares them.

It belongs to the Pests and Diseases guides. For the pests themselves see Common Garden Pests and Organic Control, and for treatments that do the least damage to allies read 7 Natural Remedies Every Gardener Should Know How to Make.

At a glance

Learn to recognise them at every stage

Larvae look nothing like adults, which is why they are so often squashed. A ladybird larva is a small grey or black alligator-shaped grub with orange or yellow spots, and it can eat hundreds of aphids before it pupates. Adult ladybirds feed on aphids too, but they fly off when food runs short.

Hoverfly larvae are legless, slug-like and green, cream or brown, often tucked into the middle of an aphid colony. The adults are the striped flies that hang motionless over flowers. Lacewing larvae are tan, bristly and shaped like tiny pincered crocodiles, while the eggs hang on thin silk stalks under leaves.

Parasitoids leave signs rather than bodies. Aphid mummies are swollen, tan or bronze shells, each with a neat round hole if the wasp has already emerged. A cabbage white caterpillar surrounded by small yellow cocoons has been parasitised by Cotesia wasps, so leave it where it’s. The caterpillar is already doomed, and the next wasp generation is inside those cocoons.

Plants that keep adult predators in your garden

The adults of hoverflies, lacewings and many parasitoid wasps feed on nectar and pollen, and they need it within easy reach. Hoverflies and tiny wasps have short mouthparts, so deep tubular flowers such as foxgloves are useless to them. Choose shallow, open or clustered flowers.

• Umbellifers: dill, fennel, coriander allowed to flower, wild carrot and Ammi. Their flat flower heads suit tiny wasps and hoverflies.
• Daisy types: cosmos, calendula, yarrow, single marigolds and coneflower.
• Low edging: sweet alyssum and phacelia, which flower fast from seed and sit right beside crops.
• Succession: aim for something in bloom from early spring to first frost, because gaps in flowering are gaps in protection.

Double-flowered cultivars often have their nectar bred out and the reachable parts hidden, so choose singles. Edge vegetable beds with a strip at least 30 to 50 cm (12 to 20 in) wide, and place flowers within a few metres of crops. A broader pollinator planting is covered in Flowers That Attract Pollinators, and plant partnerships that help in the vegetable bed are in Companion Planting.

Habitat: shelter, water and the untidy corner

Allies need somewhere to spend the winter and somewhere to hide by day. Ladybirds and hoverfly larvae overwinter in leaf litter, grass tussocks and hollow stems. Ground beetles live under stones, boards and dense mulch, and hunt after dark. Leave some stems standing until spring, keep a log pile or stone heap in a quiet corner, and mulch beds so ground beetles have cover.

A shallow dish of water with stones to land on helps all of them. Purpose-built insect hotels and lacewing boxes have uneven evidence for improving pest control, so treat them as optional. A varied border does far more.

Ants are the exception. They farm aphids for honeydew and chase off ladybirds and lacewings. If you see ants running up a shoot, deal with the ants first by trapping them at the stem base with a band of sticky barrier on trunks of fruit trees, and the aphids often decline once their guards are gone.

Two more habits help. Leave a small patch of stinging nettle in a sunny corner if you can: it hosts aphid species that don’t attack garden plants, and ladybirds breed on them early in spring, before your crops have any aphids at all. And avoid slug pellets where you can. Ground beetles eat poisoned slugs and can die from them, so reach for a beer trap, a night-time torch patrol or a ferric phosphate product used exactly as labelled instead.

Spraying without wrecking the team

Broad-spectrum insecticides kill predators faster than pests, because predators are fewer and reproduce more slowly. A pest population can rebound within weeks, while an ally population may take a season. Use the following rules if you must act.

1. Identify the pest and the stage before choosing anything. Tools such as Bt (Bacillus thuringiensis kurstaki) only affect caterpillars, while soap and oil sprays affect anything soft-bodied they touch.
2. Spot-treat. Spray the infested shoot tips, not the whole bed, so untouched plants stay a refuge.
3. Spray in the evening once bees and hoverflies have stopped flying, and when temperatures are below about 28°C (82°F) to avoid leaf scorch.
4. Skip open flowers. Remove or avoid spraying any plant in bloom.
5. Read the label for the pre-harvest interval and bee warnings and follow them exactly.
6. Recount after 5 to 7 days. A second spray should be a decision, not a schedule.

Counting what is working

Judge your garden by trend, not by single visits. Once a week through spring, look at the same three or four shoots at the same time of day and note aphid numbers, larvae, mummies and eggs. A healthy pattern is aphids rising for two weeks, then larvae and mummies appearing, then the colony shrinking. If aphids keep growing for a third week and no allies show up, the cause is usually missing flowers nearby or a recent spray, so fix those rather than reaching for another product. Yellow sticky traps catch hoverflies and parasitoid wasps along with pests, so avoid them in open beds.

When buying insects makes sense

Purchased control works best in enclosed spaces. In a greenhouse, the parasitic wasp Encarsia formosa for whitefly and the predatory mite Phytoseiulus persimilis for spider mites do well once temperatures are consistently warm, usually above about 18°C (64°F). Beneficial nematodes for vine weevil grubs and slug nematodes are watered onto moist soil that’s warm enough, with the minimum temperature set by the product, so check the pack.

In open beds, released adult ladybirds and lacewings mostly leave, and wild-collected ladybirds can carry diseases or alter local populations. Improving habitat is a longer-lasting investment.

Diagnosing what you see

How beneficial insects protect your garden as hired hands

Ladybird and lacewing larvae eat aphid and control outbreaks on rose and bean, while hoverfly larvae also feed on aphid and the adults pollinate. Parasitic wasp lays eggs in caterpillar and whitefly, which kills pests without spraying. Ground beetle hides under mulch and eats slug and cutworm.

Dill, fennel, marigold and catmint attract hoverfly and lacewing, because small open flowers supply nectar. Bee needs lavender and borage, and a log pile hosts overwintering beetle. Pesticide kills these allies as well as pests, which means you shouldn’t spray unless a problem gets serious. A shallow dish of water supports them in a heatwave of 30°C. Predators follow pests, so you’ll see aphid first and ladybird a week or two later. A bug hotel with hollow stems of 10 cm hosts solitary bee, and earwig feeds on aphid at night in a pot of straw. Nematode watering controls vine weevil and slug in damp soil at 12°C, and a compost heap hosts frog and hedgehog that eat slug. A strip of clover and borage 1 m wide holds predators near your tomato and cabbage. The RHS and your extension service both advise patience before you reach for a spray, and the organic pest control guide shows what to use when numbers climb.

Common mistakes

• Squashing unfamiliar larvae. Most grubs among aphids are predators. Learn the larval forms first, then act.
• Spraying at the first aphid. Predators need prey to arrive, so a clean plant attracts none. Tolerate a small colony for a week.
• Growing only double flowers. They look generous, but the pollen and nectar are often unreachable. Choose single or open forms.
• Spraying open blooms in daylight. This hits bees, hoverflies and wasps together. Wait until evening and skip flowering plants.
• Clearing every leaf in autumn. Overwintering allies live in the litter. Clear diseased material only, as described in Garden Hygiene.

Next steps

Build a routine around what you have learned with Preventing Garden Problems Before They Start, check the pests you actually face in Common Garden Pests and Organic Control, and pick the gentlest tool from natural remedies. Return to the Pests and Diseases guides.