Diamondback Moth

Plutella xylostella

Synonym - Plutella maculipennis

Family: Plutellidae

Order: Lepidoptera

Other names:

Cabbage Moth
Diamond Back Moth.
Riddler.

Description:

Adult
Colour - variable with a diamond pattern on its back. Often grey brown with an uneven white stripe down their back that forms 3 distinctive diamond shapes when the wings are folded back.
Body - Small. Stout with long hair scales. 10 mm long.
Wings - 2 Pairs. Membranous. Hairy. Wing span of 7 mm. Folded back when at rest.
Mouthparts - chewing.
Antennae - 3 segments. Filiform (Finely hair like).
Legs - Broad overlapping scales. Feet (Tarsi) have 5 segments.
Head - Smooth. Broad overlapping scales. Large rounded compound eyes.
Thorax - Broad overlapping scales. 3 segments. Front segment much smaller. Hairy.
Abdomen - Broad overlapping scales. 7-11 segments. Spiracles on segments 1-7. Hairy
Egg - Flat. Oval. Pale yellow.
Habits - Can't fly when cold.
Caterpillar -
Colour - Light green with dark head.
Body - 10-12 mm long, 2 mm wide. Tapered at each end.
Mouthparts - Chewing.
Antennae - Short. 3 segments.
Legs - 5 segments. Single claw on end.
Head - Hard.
Thorax - 3 pair of legs.
Abdomen - 10-11 segments. Spiracles on segments 1 to 8. Prolegs on segments 3, 6 and 10.
Wriggle violently when disturbed and will fall on a string web when disturbed.
Pupae -
Colour - Green to dark brown.
Open mesh form.
Habits - Herbivorous.

Biology:

High rainfall with low temperatures lead to decreased survival.
Thus they are more common in drier winters with higher than usual daily maximum temperatures
Early rains that germinate Brassicaceae weeds and volunteers or low rainfall over winter may lead to epidemics.
Eggs laid on the underside of leaves and stems.
Prefer stressed crops and often infest poor areas of a crop first.

Life Cycle:

Eggs laid on the underside of leaves from October to May. Caterpillars hatch 10-12 days later and burrow into leaves forming a small blister on the leaf. As it grows it breaks out of the blister and feeds on the surface, skeletonising the leaf. When disturbed it retreats with a flicking motion and may drop from the leaf suspended by a silken thread. The caterpillar spins an open mesh silken cocoon on the underside of the leaf for pupation. Adults emerge from the pupae and may be carried long distance on the prevailing winds. There may be 4-7 generations per year. There may be overlapping generation of eggs, caterpillars, pupae and moths in canola crops.

What to look for:

Eggs on underside of leaves and stems.
Small agile larvae that may fall on a thread when disturbed.
Irregular holes or clear membranous windows on leaves.
Grazing damage on pods or stems and damage to young pods.
Larvae in flowering heads.
Check for larvae using a sweep net in canola that is preferably dry. Monitor weekly from early flowering to determine level of infestation and whether beneficial insects or fungi are providing control.

Habitats:

Origin and History:

Distribution:

Widespread.

Significance:

Caterpillars feed on Canola, Cabbage, Brussels Sprouts and other Brassicas. They can damage pods but usually aren't a significant problem of canola. They may skeletonise leaves.
On canola they chew on the stems and pods and make clear membranous windows and small holes in the leaves

Management and Control:

Several parasites and fungi attack the caterpillar.
Spraying in canola is only required in exceptional circumstances. Rainfall often encourages biocontrol and decimates larval numbers.
In vegetables multiple sprays are often required because of the large number of generations and its mobility.
It is resistant to a number of insecticides including the organophosphates (OPs), carbamates and synthetic pyrethroids (SPs). Some populations are also resistant to avermectins.
Ensure good penetration of insecticides into the canopy as larvae may be distributed throughout the canopy.
Spray when larvae are small and <5 mm long.
Two sprays 5-7 days apart are often more effective than a single spray as it controls survivors and newly hatched eggs.
Rotate insecticides. Avoid using OPs and SPs in spring.
Spot spray areas of weeds or edges of paddock that may be a source of infestation.

Thresholds:

Canola
Severe defoliation and pod grazing at flowering and pod fill can reduce yields.
Large numbers may develop on foliage after flowering without causing economic loss, but if the move to the pods they can cause much damage in a short time.
6.5 caterpillars per plant did not cause economic loss in Merredin, WA (Berlandier 1999).
20 larvae that are greater than 3 mm long per 10 Canola plants at flowering (or 50 larvae per 10 plants at podding) are probably worth spraying. Pull up the Canola plant and shake it into a sweep net then count the larvae that are longer than 3 mm. Rainfall events of more than 5 mm per day can quickly reduce larvae numbers.
The South Australian Department of Agriculture use the following criteria as indicators of economic thresholds; Cut 20 canola plants and shake into a sweep net.
Count the larvae that are longer than 3-4 mm.
Spray when the number is more than 1 larva per plant during the vegetative to mid-flowering stage; more than 2 larvae per plant during mid to late flowering stage, or
more 5 larvae per plant during the pod maturation stage.
Economic thresholds can vary widely with 30 to 200 larvae per 10 sweeps being reported in research projects.
A recent summary of economic thresholds in canola was
30 or more larvae per 10 sweeps for pre flowering stressed crops.
50 or more larvae per 10 sweeps for healthy crops
100 or more larvae per 10 sweeps for mid to late flowering healthy crops
200 or more larvae per 10 sweeps at pod maturation.

Biological Control:

A naturally occurring fungus (Zoopthora radicans) can significantly reduce populations in some seasons with warm humid conditions (Vickers, 2003). In most seasons this occurs too late to reduce Cabbage Moth damage to winter crops.
Diadegma semiclausum and Apanteles ippeus wasps parasitise young larvae.

Resistance:

Up to 20% of some populations are resistant to synthetic pyrethroid insecticides (Holloway, 2003).
It is resistant to a number of insecticides including the organophosphates (OPs), carbamates and synthetic pyrethroids (SPs). Some populations are also resistant to avermectins (Severtson pers com).
Worldwide DBM has developed resistance to 82 insecticides.

Related Species:

Cabbage Centre Grub (Crocidolomia pavonana), Cucumber Moth (Diaphania indica), Wax moths (Galleria mellonella and Achroia grisella), Rice moth (Corcyra cephalonica), Meal moth (Pyralis farinalis)

Similar Species:

Hoverfly larvae look similar

References:

CSIRO. The Insects of Australia. Melbourne University Press. (1991) p857.

Chapman, B., Penman, D. and Hicks, P. The Garden Pest Book. Nelson, Melbourne. p14.

WADA. Insects and Allied Pests of Extensive Farming. Department of Agriculture - Western Australia Bulletin No. 4185. p55.

Francoise Berlandier, Agriculture WA (pers. comm.)

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.