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: :,~ ~ T A 1- E FOR EST S 0 F NEW SOU T H W ALE S I

FORESTPROTECTION FOREST RESEARCH AND DEVELOPMENT DIVISION

S E R I E S NUM B E R E10 ISSN 0158-3832 ISBN 0731091302

BENEFICIAL INSECTS Deborah Kent

INTRODUCTION , TAT I

Most native insects have a wide range of natural ~R~S~ enemies which help control their numbers, They include parasitic and predatory invertebrates such as insects, mites, spiders and nematodes and predatory vertebrates such as birds, frogs, reptiles and mammals. Vertebrates are obvious and important predators of insects, However, even more important, but much less visible than these animals, are the parasitic and predatory invertebrates and many other organisms such as fungi, bacteria and viruses.

The importance of natural enemies is best illustrated when an insect native to one country is accidentally introduced into another without these enemies. An example is the introduction of sirex wood wasp into Australia (see Leaflet No. P1).

The importance of native predators, parasites and pathogens in the control of potential forest pests and the successful production of native forest crops such as eucalypts cannot be overemphasised. They are so important that most potential forest pests never cause serious economic damage and/or reach epidemic population levels because of their effect.

This leaflet describes some common insect predators and parasites in order to alert tree growers and forestry officers to their presence, to explain how vital they are in maintaining healthy plantations, and to suggest ways to encourage them.

parasitic wasp (ichneumon id)

2.5x

Figllre 3, Robber fly (2x),

PREDATORY INSECTS

A wide range of insects hunt and kill other insects for

food. They normally eat many insects to reach maturity

and are free-living throughout their life cycle. Most are

predatory in both the immature and adult stages, but

some are predatory only as larvae. Their importance is

difficult to determine as many prey indiscriminately on

both beneficial and pest insects.

Beetles (Coleoptera)

Many beetles are predatory but among the most effective

are ladybirds (Coccinellidae). Adult ladybirds are small

shiny round beetles (often spotted black on a red-orange

background); the larvae are highly mobile with long legs

and spiny projections. Ladybirds and their larvae feed

on the eggs and young larvae of leaf beetles (see Leaflet

No. E6) and other pests such as aphids and psyllids.

They can be quite voracious; field observations in

Tasmania have recorded up to 80% predation of leaf

beetle eggs. Unfortunately, the adults can be confused

with leaf beetles.

Lacewings (N europtera)

Adult lacewings are slender bodied insects with long

thin antennae. They have two pairs of delicate

multi veined wings that are held tentwise above the body

when at rest (Fig. 2). They are attracted to lights and are

normally seen at night. The adults lay white, elongate

oval eggs on stalks (Fig. 6) on twigs and foliage as well as

on the walls of buildings. The larvae are active, voracious

predators. A commonly seen species is the green

lacewing. The larvae of this species adorn their bodies

with the dried out remains of previous victims (Fig. 6).

Both adult and larvallacewings feed on psyllids, scales,

aphids, mites and small caterpillars and tend to be more

active in summer and autumn.

Bugs (Hemiptera)

Shield bugs (Pentatomidae e.g. Oechnlia schellebergii) and

mirid bugs (Miridae) feed on caterpillars, sawfly larvae,

aphids, scales, leafhoppers and other small insects by

sucking body fluids.

Assassin bugs (Reduviidae) have a powerful curved

beak to impale their prey and suck them dry (Fig. 7).

They are usually solitary and are often seen near the tips

of shoots. They can be mistaken for sap-sucking bugs but are never found in groups. These predators seem to

prefer soft-bodied prey such as cup moth caterpillars.

Figure 4. Longlegged fly (5x).

Figure 5. Malltid.

Figllre 6. Lacewing eggs and la rva.

Figure 7. Assassin bug (2.8 x).

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Flies (Diptera)

Longlegged flies (Dolichopodidae) (Fig. 4) feed on aphids and free-living psyllids as adults. They have been seen in quite large numbers resting on the leaves of plantation eucalypts on the south coast.

Robber flies (Asilidae) (Fig. 3) capture prey on the wing, puncture them with stout beaks and suck the bodies dry. The larvae live in soil and decaying wood.

Hoverflies (Syrphidae) are only predaceous as larvae; the adults are nectar feeders. Hoverfly larvae are small slug-like, green or yellow maggots that feed on aphids, scale insects and psyllids.

Other Orders

Mantids (Mantodea) (Fig. 5) are well known predators.

Dragonflies and damselflies (Odonata) capture prey on the wing or on foliage. The nymphs are aquatic predators; the adults are strong fliers and may be seen a long distance from water.

Scorpion flies (Mecoptera) are predaceous on a variety of insects and will also feed on nectar.

Predatory caterpillars (Lepidoptera) of several species of moth are specialist feeders on scale insects (e.g. Stathmopoda melanochra on gumtree scale).

PARASITOIDS/PARASITES

True parasites live in a symbiotic relationship with their hosts and rarely kill them. The 'parasites' discussed here are not parasitic in the true sense and are more correctly called parasitoids. They ultimately kill their hosts, while true parasites do not. The larvae may feed externally or, more usually, inside the body of their host and are free­living as adults. They only feed on one individual of their host to complete their life cycle.

Parasitoids are extremely important in controlling populations of pest species as their victims always die before maturity, thus exerting a measure of control on their populations. They are extremely well adapted to their hosts and are good at finding them even when the host is relatively rare.

The wasps and flies contain many important parasitoids. They tend to be highly host-specific and will live and feed only on one host species or a few closely related species.

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Figure 8. Tachinid life cycle: eggs on host -leafbeetle larvae; cocoon alld adlllt.

Parasitoid wasps which attack larvae lay their eggs in or on their hosts during any stage of host larval development. The parasitoid larva lies dormant inside its host, which continues to develop until the prepupal stage. The parasitoid larva then undergoes rapid development, consuming the host tissue and eventually killing it. The fully developed parasite larva then pupates in or near the remains of the host. The adult wasps are free living and are often seen feeding on pollen, nectar, honeydew, and sometimes on the body fluids of their hosts.

Parasitoid wasps which lay their eggs into the eggs of their hosts can have a dramatic effect on the pest population. Parasitised eggs do not hatch in the normal incubation time, but turn black before the parasitoid wasp emerges, its larva having fed on the contents of the

egg.

The small white eggs of parasitoid flies are often observed on the host (Fig. 8), but eggs are sometimes laid on foliage and eaten by the host larvae (some wasps also use this method). Sometimes live maggots are even laid directly on to the host. Pupation may take place inside the empty husk of the host, but more usually the final stage larva pushes its way to the outside through some weak spot in the host skin (Fig. 8).

Wasps (Hymenoptera)

The most common larval parasitoids are the ichneumonids and braconids, thin-bodied and reddish­brown or black wasps, often with yellow markings. Ichneumonid wasps (Fig. 1) are long and slender and may have a conspicuous sting-like ovipositor (egg-laying mechanism). Braconid wasps are sinular to ichneumonids

but are usually smaller, more stout-bodied and not as brightly coloured. Most braconids pupate in silken cocoons on the outside of the host whereas ichneumonids normally pupate in cocoons within the host's body. Their hosts include the larvae of moths, sawflies, beetles, true bugs and flies.

Egg parasitoid wasps are small to very small spt'cies belonging mainly to the Chalcidoidea and related groups. They are sometimes metallic green or blue. Chalcidoids are very important parasites of the egg and larval stages of many insects, especially forest tree pests such as moths and psyllids. The eggs of chrysomelid leaf beetles and eucalyptus weevils are also known to be attacked by these minute wasps.

Flies (Diptera)

Tachinid flies (Tachinidae) are important larval parasitoids of defoliators such as caterpillars and sawflies. They are small black or grey flies not unlike common house flies, but usually have large bristles near the end of the abdomen (Fig. 8). The adults are nectar feeders and are often seen feeding at flowers or sap flows.

CONSERVATION AND ENHANCEMENT

Conservation of beneficial insects and other natural enemies by the use of management techniques that red uce harmful effects on natural enemies can be effected in several ways: reduce the number of chemical insecticide treatments used in a specific area; reduce the concentration of the chemical treatment to reduce its effect on beneficials; and modify the timing of any chemical insecticide treatment to reduce its effect on beneficials.

An important disadvantage of using pesticides is their impact on non-target, often beneficial, organisms. Parasitoids are more vulnerable to insecticides than predators, because they spend part of tl1eir life cycle developing inside their prey. They are also less visible then predators and their presence is usually underestimated. However, in the laboratory it is

sometimes difficult to rear pest species through to adults because of the high level of parasitism.

Beneficial insects can also be enhanced in a eucalypt plantation by tl1e use of management techniques that increase the longevity and reproduction of these natural enemies or increase the attractiveness of a plantation for natural enemies. These can involve: providing certain types of food for adult parasites and predators, e.g. nectar producing flowers, particularly native species, to

increase their reproductive potential and longevity; and by providing a diversity of plants and animals to supply alternative hosts for these natural enemies.

In general, the diversity of rlants and animals is much greater in forests than in agricultural or uniform plantation systems. Mixed forest stands and stand edges around eucalypt plantations ha ve the potential to increase the availability of alternative hosts for pests and adult food supplies for beneficial insects.

FURTHER INFORMATION

State Forests of NSW (1989). Sirex woodwasp. Forest Protection Series No. 1. 5 pp.

State Forests of NSW (1995). Leafbeetles. Forest Protectioll Series No. E6. 4 pp.

This publiration may be reproduced in full provided acknowledgement is made to State Forests afNew South Wales. Extracts may not be published withollt prior reference to

State Forests of New SOllth Wales

Copyright ©: State Forests of New South Wales 1997

Additional copies may be obtained by contacting:

Publications Officer Forest Research and Development Division State Forests of NSW PO Box 100 BEECROFT NSW 2119

Phone: (02) 9872 0111 Fax: (02) 9871 6941