Abdomen - 10-11 segments. Spiracles on segments 1 to 8. Prolegs on segments 3, 6 and 10.
Habits - Herbivorous.
Biology:
High rainfall with low temperatures lead to decreased survival.
Early rains that germinate Brassicaceae weeds and volunteers or low rainfall over winter may lead to epidemics.
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. There may be 4-7 generations per year.
Habitats:
Origin and History:
Distribution:
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.
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.
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 3mm. 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.
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.
Resistance:
Up to 20% of some populations are resistant to synthetic pyrethroid insecticides (Holloway, 2003).