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Hoverflies

Also known as Hover flies, Flower flies, Syrphid flies, Syrphidae

Beneficial or harmlessBeneficial
Adult marmalade hoverfly (Episyrphus balteatus): one pair of wings, banded abdomen Photo: Hans Hillewaert · CC BY-SA 4.0

In short

Hoverflies are harmless flies that mimic bees and wasps; the adults pollinate flowers while the larvae of many species eat aphids. The slug-like, legless larvae are frequently mistaken for pests on aphid-covered shoots. They rank as the second most important group of pollinators after wild bees.

How it looks

Adults are 3–25 mm long, often black with yellow, orange or white bands, though some are brown or metallic. Unlike bees and wasps they have only one pair of wings, a broad head with large eyes, and they hover in one spot before darting away; they do not sting. Eggs are white to grey, oblong, about 1 mm, laid singly next to aphid colonies. Larvae are legless maggots that taper towards the head, 5–20 mm when grown, and brown, greenish, pinkish or whitish, sometimes semi-transparent. They are often found lying flat among aphid colonies.

Signs

Small white eggs laid singly among aphids; slug-like larvae on aphid-infested tips; shrivelled, emptied aphids; flies hovering at flowers.

Life cycle

Egg, larva, pupa and adult. During the growing season egg to adult commonly takes 2–4 weeks, and some species have 5–7 generations a year.

What favours it

Adults need nectar and pollen, favouring open white and yellow flowers, and females lay where aphids are present. Not all hoverflies are predators: some larvae live in decaying matter or stagnant water, and a few (bulb flies) feed inside bulbs.

Prevention

  • Grow a few flowering plants that offer nectar and pollen near your collection, keep ants off infested plants, and avoid broad-spectrum, persistent insecticides.
  • Do not wipe off legless, slug-like larvae from aphid colonies before checking what they are.
  • Plant small-flowered, open blooms (for example daisies or umbellifers) near vegetables and ornamentals.

Treatment

Start with the first step and move down only if the problem persists.

  1. No control needed; let larvae work through aphid colonies.
  2. If aphids must be sprayed, spot-treat with insecticidal soap or horticultural oil, avoid flowers where adults feed, and follow the product label.

Pros and cons

Pros

  • A single aphid-eating larva can consume 100–400 aphids before pupating.
  • Larvae also take mealybugs, psyllids, scale insects and thrips.
  • Adults are important pollinators and cannot sting.

Cons

  • Their bee-like colours can cause needless alarm.
  • Only some species are aphid predators; others are decomposers, and a few bulb flies are pests of bulbs.
  • Larvae arrive after aphids do, so they rarely stop a sudden outbreak on their own.

Look-alikes

Bees
Bees have two pairs of wings and hairy bodies, and many carry pollen on their legs; hoverflies have one pair of wings and hover in place.
Wasps
Wasps have two pairs of wings and are carnivorous; hoverflies have a broad, flat abdomen and huge eyes.
Slugs and snails (young)
Hoverfly larvae are much smaller, sit among aphids and have segmented bodies; slugs leave slime trails and chew holes in leaves.
Caterpillars
Caterpillars have a distinct head and true legs and chew leaves; hoverfly larvae are legless and taper to a point at the front.

Where you find it

Aphid-prone plants outdoors or on balconies, including vegetables, herbs, roses and flowering ornamentals.

Sources

  1. Syrphids (flower flies, or hover flies) — UC IPM Natural Enemies Gallery
  2. Hoverfly — Wikipedia
  3. Beneficial Predators — UC IPM Home and Landscape

Written in our own words from these sources. Check a product label before using any treatment.