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Number 26
Revised June 1994
By Charlma Phillips, Principal Forest Health Scientist

Robberflies and hoverflies are predatory insects. They are very beneficial in controlling many pest insect species. Both are true flies - that is they only have one pair of wings. Both are quite common on eucalypts and may be seen throughout the year, though they are more common in summer and autumn.

Robberflies

Both adult and larval (immature) stages of robberflies are predatory.

Adults (Pictured above): Adult robberflies are very aggressive insects and large individuals look particularly daunting, however, they are harmless to humans. They range in size from 5 to 30 mm in length. They are dark brown in colour with a stout, well developed, humped thorax to which the wings are attached. The abdomen is long and narrow and covered by the folded wings when the insect is at rest. They have long, very strong legs, large eyes and a hairy face. They have a short, sharp beak which they use to suck out the body contents of their prey. The large eyes and well developed thorax mean this insect is particularly well adapted for catching prey on the wing. Prey are captured in flight or dropped onto from above in the case of non-flying insects. The captured insect is held by the powerful legs whilst the insides are sucked dry.

Robberflies tend to patrol regular beats in search of prey and often catch insects much larger than themselves. Unfortunately they do not discriminate between beneficial and pest insects and frequently capture and devour other beneficial insects. One species preys exclusively on honey bees.

Eggs and Larvae: Some species lay eggs in the soil, others attach their eggs to foliage or bark. The larvae either live in the soil, beneath bark or in rotten wood.

Note: Robberflies are very beneficial insects

Hoverflies

Hoverflies are also known as flower flies or syrphids. They are very common. The larval stages are predatory on other insects.
 

Adults (Pictured above): Adult hoverflies are black with a black and yellow striped abdomen. They resemble bees but are not as heavily built and they do not sting. They can be distinguished from bees quite easily since hoverflies have only one pair of wings while bees have two pairs of wings. Hoverflies are approximately 1cm long and have large, well developed eyes. They have a swift, darting flight and can often be seen hovering motionless in the air, their wings beating so fast they cannot be seen by the human eye - hence the name "hoverfly". Adults feed on nectar, pollen and honeydew. They are common throughout summer and autumn.

Eggs: Eggs are white and oval or spindle shaped. They are laid on leaves or stems, among the prey insects. There are several generations per year.

Larvae (Pictured above): Larvae are slug-like. They are green, yellow or mottled in colour, legless and eyeless. They wander about feeding on small, soft bodied insects. They feed by sucking out the body contents of their prey. They have voracious appetites, consuming enormous numbers of insects and are very important in the control of outbreaks of pest insects such as aphids and scale insects.

Note: Hoverflies are very beneficial insects

Summary

When to look:  Look mainly in spring, summer and autumn

Where to look:  Look on leaves/stems for hoverfly larvae.
Look on leaves or in the air for flying hoverfly or robberfly adults.

What to look for:  Look for slug-like larvae (not to be confused with weevil larvae – see Forest Health Fact Sheet Number 7) among aphids, scale insects or other soft bodies insects.
Look for bee-like flies hovering in the air.
Look for large flies with humped thorax and a long, thin abdomen - may be flying in the air or resting on leaves.

For further information contact PIRSA Forestry

Disclaimer: While this publication may be of assistance to you, the Government of South Australia and its officers do not guarantee that it is without flaw of any kind or is wholly appropriate for your particular purpose. The Government therefore disclaims all liability for any error, loss or other consequence that may arise from you relying on any information in this publication.