Attract Beneficial Insects to Your Garden

An insect on a leaf is not automatically the reason a plant looks rough. Aphids may curl tender growth and leave sticky honeydew. A hoverfly larva feeding among them is part of the response, not another thing eating the plant. Look at several plants before you decide what needs attention. A few aphids on a sturdy kale plant can be very different from a colony building on a newly set pepper.

Pay attention to life stage. An adult hoverfly visits flowers for nectar and pollen, while its larva hunts aphids. Look for a small, tapered, legless larva tucked into an aphid colony. It does not have the obvious head capsule or legs of a leaf-chewing caterpillar. If it is moving among aphids instead of chewing foliage, leave it there and check the colony again in a few days. Learning one adult photo is useful, but it will not tell you what every small larva or cocoon in the bed is doing.

Hoverfly larvae are useful when aphids are present. They move slowly through a colony and feed on the insects rather than chewing the leaf. Adult hoverflies are common around flowers and can hover in place, which is why they are often mistaken for small bees or wasps.

Lady beetle larvae belong in the same category. They have six legs and a long, blue-black body with orange or pale markings. You will usually find them where aphids are thickest. They do not chew foliage, so leave them alone. A larva can eat roughly 300 to 400 aphids before it pupates.

Lacewing larvae also take small, soft-bodied prey. They are soft and alligator-shaped, usually cream or brown, with large forward-facing jaws. Those jaws make them look alarming, but they are used on aphids and other prey. A lacewing larva can eat up to 200 aphids in a week.

A spined soldier bug is another predator gardeners may mistake for a plant-feeding stink bug. It is shield-shaped, with a sharp spine at each shoulder. Its stout beak is held beneath its body and is much thicker than the antennae. A plant-feeding stink bug has a thin beak instead. Leave a spined soldier bug on the crop, where it can feed on caterpillars and beetle larvae.

Tiny parasitoid wasps work differently: their young develop in or on a particular host insect, so a wasp that helps with aphids may not help with the caterpillar on a cabbage leaf. Look for aphid mummies as well. A parasitized aphid becomes dry and swollen, with color that can range from gold to black. An intact mummy may still hold a developing wasp. A clean round exit hole shows that an adult wasp has already emerged. Keep both kinds of mummies in place, and avoid spraying a leaf covered with intact ones.

On a tomato plant, a hornworm carrying rows of white, rice-like cocoons is already hosting braconid wasps. By this stage, the wasp larvae have already emerged from the hornworm to spin their cocoons. The hornworm has stopped feeding and will die during the wasps’ pupation. Leave it in place so the adult wasps can emerge.

Natural enemies can slow a colony, but they do not guarantee that every pest will disappear before the crop is damaged. Their presence is a reason to avoid a reflexive whole-bed spray, not to ignore a young plant that is losing ground.

The adult stage of many useful insects needs nectar or pollen even when the immature stage feeds on pests. Small, open flowers are a better fit than blossoms with nectar buried deep inside a tube. The carrot family, Apiaceae, is a useful one to look for: cilantro and dill make flat flower clusters with shallow nectar.

Sweet alyssum, a member of the mustard family, works well as a low strip at the edge of many beds. Do not put a dense alyssum border beside brassicas, though. It can attract flea beetles and other pests that feed on cabbage-family crops. Yarrow or coreopsis are Asteraceae choices for that spot.

Place the flowers beside the vegetables that need help, not across a wide stretch of lawn. Keep one flower source open while the crop is vulnerable. Buckwheat can fill a short vacant patch, but cut it before it reseeds. Flowers make the garden more usable for beneficial insects; they do not make a pest outbreak harmless.

Ants are drawn to honeydew, the sticky material left by aphids and other sap-feeding pests. They often defend those pests from predators and parasitoids. A plant can have plenty of hoverflies nearby and still hold an aphid colony when ants are working the stems.

Follow the ant trail before you blame the helpful insects for being ineffective. On a woody trunk, a band of sticky material applied over a tree wrap or tape collar can block the ant route. Do not put sticky material directly on bark or tender green stems. On annual vegetables, remove easy bridges such as weeds or stems leaning from the soil onto a support.

A contact spray along an ant trail gives only a short-term knockdown. For an identified sugar-feeding ant, a low-concentration liquid borate bait in an enclosed station can give workers time to carry bait back to the nest. Ants share food from foragers to the rest of the colony through trophallaxis. UC IPM recommends 0.5 to 1 percent borate for this use because stronger mixtures can repel ants. Choose a product labeled for the site and keep it where children and animals cannot reach it.

Aphids confined to a few growing tips can often be washed off or removed with the affected tip. On a sturdy plant, a strong stream of water will knock many aphids loose without covering the whole bed in a treatment. Support delicate growth with one hand and do not blast a tender seedling hard enough to tear it. On a disease-prone crop such as a tomato, use the hose early on a dry day so the foliage can dry.

A chewing caterpillar calls for a different tool. For leaf-eating caterpillars, look for Bacillus thuringiensis kurstaki, usually marked Btk, on the label. It works only after a caterpillar eats treated foliage and will not control aphids. Sunlight breaks Btk down quickly, so apply it late in the day or on a cloudy day while small caterpillars are actively feeding. A Bti product is for fly larvae such as mosquitoes and fungus gnats, not caterpillars on a vegetable leaf. If a label names another Bt subspecies, read its pest list before using it.

Organic does not automatically mean safe for beneficial insects. Spinosad can harm syrphid larvae and certain parasitoid wasps. It is highly toxic to bees while wet, though that toxicity falls after the foliage dries. Drying can take from a few hours to a day, depending on the product. If the label permits its use on the crop, apply it in the evening when bees are not active and give it time to dry. Pyrethrins can kill helpful insects that are on the plant when it is sprayed. When a product is necessary, choose one labeled for the crop and pest you have confirmed. Apply only where it is needed and follow the pollinator directions. Oils and soaps have little residual activity once dry, but they can still kill a lacewing larva or hoverfly on the plant at the time of application. On a flowering plant, use them at dawn or dusk when pollinators are not active. Do not spray a drought-stressed plant or work in heat beyond the product label’s stated limit. Broad, persistent treatments can remove the insects that would otherwise catch the next aphid colony. Check the new growth again before deciding the problem needs another response.

You do not need a formal insectary garden to get a benefit. A small flower edge that keeps attracting hoverflies is worth leaving in place. If a bolted herb is feeding useful insects without crowding the crop, let it stay until you need the bed.

Make an outer margin into a useful winter refuge instead of leaving a loose debris pile against the beds. Leave pithy or hollow perennial stems eight to 10 inches high through winter. Some native bees and other insects use them for shelter. A small, undisturbed patch of bunchgrass or dry leaf litter also gives ground beetles cover. Keep diseased foliage out of that margin.

Use the result to make one small adjustment next season. You might move the flower strip closer to the cucumbers, or keep the ant problem from reaching the peppers. That is more useful than buying a collection of insects and hoping they will settle into a garden that offers no food or shelter.