Pests & Diseases

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Description and Enemies

The diamondback moth is the most damaging pest of brassicaceous crops (e.g, cabbage, broccoli, Brussels sprouts) throughout Australia.

Its eggs are oval, and cream coloured and less than 1mm long (Fig. 1). They and are usually found on the undersurfaces of leaves. The larvae or caterpillars are minute when they hatch. There are four larval stages or instars, which can be distinguished by the width of the head capsule. Mature larvae are spindle-shaped and reach 12 mm in length (Fig. 2). When larvae are disturbed, they wiggle vigorously and may hang from the plant on a silken thread. The mature larvae spin a loose, spindle-shaped cocoon on the plant in which the pupa can be seen (Fig. 3). The duration of development from egg to adult depends on temperature. Developmental times range from 14 days at 28C to 4 months at 12C.

The adult moths are about 9 mm long and greyish with a characteristic cream-coloured row of diamond-shaped markings along the mid-line of the folded wings (Fig. 4). This gives them their other common name - the diamondback moth. The tips of the hind wings are fringed with long grey hairs.

Oval eggs of the diamondback moth

Figure 1: The oval eggs of the diamondback moth are less than 1 mm long.

Mature larva of the diamondback moth

Figure 2: Mature larva of the diamondback moth resting on a fresh feeding window.

 Puoa of the diamondback moth

Figure 3: The pupa of the diamondback moth can be seen inside its loose spindle-shpaed cocoon. Pupae range in colour from green to dark brown. Cocoons are often found on the underside of leaves.

 Adult diamondback moth

Figure 4: Adult diamondback moth.

 

Figure 5. The newly hatched first instar larvae feed inside leaves as leaf miners. In the remaining three instars, they feed externally, often leaving a "window" in the leaf where they have fed.

 

Figure 6. Old damage by larval DBM dries and clearly reveals its extent. Feeding windows often open and mines turn white (left). A magnified view of an old mine caused by a first instar larva (right) shows where it entered and left the leaf. Fresh mines are not easy to see because they retain the colour of an undamaged leaf.

Natural Enemies of the Diamondback Moth

The diamondback moth is attacked by a wide variety of natural enemies. These include parasitic wasps, predators and diseases.

Parasitic wasps often kill a large percentage of larval DBM. These wasps are attracted to damage on leaves where the larvae have been feeding. When they find a larval DBM, they sting it and inject one or more eggs. The egg hatches and the immature wasp feeds internally on the larva, eventually killing it. The wasp larva then becomes a pupa, which transforms into the adult wasp.

Parasitic wasps are also called parasitoids. Most species are known by their Latin names. Some species of parasitic wasps specifically attack only one or a few species of host insects. They are harmless to humans. More than a dozen species of parasitic wasps have been reared from the egg, larval and pupal stages of DBM in Australia.

Diadegma semiclausum is the most abundant of the parasitic wasps that attack DBM in Australia (Fig. 6). This species is often seen flying around Brassica plants that are under attack by DBM. Diadegma lays its eggs in all larval instars of DBM. The wasp larvae mature when the DBM spins it cocoons. The developing wasp spins its silken cocoon inside the cocoon of DBM. In some instances, more than 90% of larval DBM have been found to be parasitised by Diadegma.

 
     
 

Figure 7: Adult (left) and cocoon (right) of Diadegma semiclausum. The cocoon is shaped like a capsule inside the spindle shaped cocoon of DBM.

Apanteles ippeus is another parasitoid that commonly attacks larval DBM (Fig. 8). The developing wasp kills the host larva before it spins a cocoon. The cocoons of Apanteles are bright white.

Figure 8. Adult Apanteles ippeus (left). Adult Apanteles waits to attack a larval DBM that is suspended on a silk thread.

It is worthwhile to consider the level of parasitism when deciding whether or not to spray DBM. The electronic project sampling plan (Victorian Department of Primary Industries, external site) has been developed to assist with decision-making when the level of parasitism is known.

Most insecticides kill natural enemies, with the exception of Bt products. The activities of most parasitoids and predators are negatively affected by insecticide sprays. The presence of parasitic wasps can usually be indicated by spotting their cocoons on leaves. To see if larvae are parasitised, hold a few in a jar with a piece of leaf to see if any cocoons of parasitic wasps appear after a few days. If a DBM larva is dissected by simply pulling each end of the body, the developing larva of a parasitoid wasp can sometimes be found inside it (Fig. 9). Also look for the adult wasps walking and flying in crop fields.

Figure 9: Larva of a parasitic wasp (bottom) that was dissected from a DBM larva (top).

Many predators also feed on DBM (Fig. 10). These include spiders, brown lacewings, damsel bugs, and ladybird beetles. The feeding activities of predators are poorly understood. However, they can be abundant in unsprayed crops, where they must be consuming large numbers of insects and other arthropods.

Figure 10: The damsel bug, Nabis kinbergii, is a predator that is commonly seen in Brassica crops. It is known to feed on eggs and larvae of DBM.

Several diseases attack larval DBM. The most common is a fungal disease called Zoophthora radicans (Fig. 11). This fungus typically appears when wet conditions occur for several days. When the larvae die, the fungus begins to grow outside their bodies. Eventually, the larvae become flattened and covered by a creamy to yellow coating of fungi.

Figure 11: The fungus Zoophthora emerges from a larval DBM (left). The fungus causes the dead larva to flatten as it grows out of it (right).

More Diamondback Moth Information

Crop monitoring
Integrated pest management
Impact of insecticides chart
Insecticide resistance management
Diamondback Moth Newsletters
Publications
Useful websites
Contacts