Banana scab moth

Scientific name

Nacoleia octasema

Description of adult

The moth is small (25mm wingspan) tan to light brown with small black spots on the wings.

Immature stages

The flattened eggs are laid in clusters ranging from a few to 30 eggs. The eggs resemble shiny overlapping fish scales. The yellow to orange larvae grow to about 25mm before pupating.

Life history

Eggs are laid on or near to an emerging bunch and hatch after about 4 days. The young larvae move under the tightly closed bracts where they feed on the young fruit. Larval development consists of 5 instars and is completed in an average of 14 days. As the bracts and hands progressively lift from the bunch stalk during bunch development, larvae move progressively to the next closed distal hand. Development is usually completed under the bracts which remain attached to the distal male flower bud or 'bell'. The pupal period lasts 8–10 days and is completed on the bunch, in trash or in old leaf axils on the pseudostem.

The life cycle egg to egg is completed in 28 days under hot wet conditions in tropical north Queensland. Adults are short-lived (4–5 days) and have crepuscular habits. Mating and egg laying takes place in the early evening. Adult moths hide in trash and under old leaf axils during the day.

Distribution

Found only in north Queensland, north of Ingham.

Host range

Banana. Pandanus and heliconia are known alternate hosts for scab moth.

Damage

Major and frequent.

Larval feeding causes superficial scarring on young fruit. Damaged areas form a black callous, rendering the fruit unmarketable. Feeding is generally confined to the curve of the fruit adjacent to the bunch stalk and between the fingers. Damage ceases after the hand lifts. Mature larvae can be found under the bracts enclosing the male flower or 'bell'.

This pest is most active during the hot wet summer months but sudden outbreaks can occur throughout the year in some localities. Damage occurs progressively as the hands lift and increases in severity towards the lower hands.

Control

This pest is very damaging and to protect all emerging bunches, treatment must start when activity is first noticed. A decision on action should be made depending on the expected bunching rate and weather conditions, rather than on scab moth populations as these are difficult to predict from monitoring alone.

Cultural

Careful selection of following suckers of equal size will ensure a concentrated bunching cycle that streamlines control.

Biological

A range of spiders and other general predators exert a measure of natural control.

Chemical

To promote biological control of scab moth and other pests, bunch injection is recommended and should be used in preference to aerial or bunch spraying. To be effective, injection must be carried out when the bunch is still upright in the throat of the plant. Dilute insecticide solution is injected approximately a third of the way down from the top of the upright bunch or 'spear'. Injections below this will damage fruit and if done above this area, the dense 'bell' will prevent entry of the chemical.

Chemical registrations and permits

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.