Diamondback moth

Diamondback moth larvae in canola have already been detected and appear to be increasing, but for July, crop growth might outgrow the impact of this pest

 

Where have they been reported?

Early instars of diamondback moth (DBM) (Plutella xylostella) caterpillars have been recorded causing minor damage to canola crops south of Robinvale in the Victorian Mallee. Numbers of larvae sampled in sweep nets have increased since early July. DBM have also been observed in low numbers near Elmore in the Victorian Northern Country. In both cases, caterpillar feeding was restricted to minor foliar damage. DBM moth activity has been widespread in establishing canola across South Australia and western Victoria in recent weeks according to SARDI researcher Kym Perry.

Diamondback moth habits and behaviour

DBM are distributed across Australia, but are most common in southern states. They are most abundant during spring and summer, although the milder conditions of this autumn and winter have resulted in greater activity than we normally experience. However, the current rate of development of caterpillars will be relatively slow due to the cool weather conditions, and mortality from rain dislodgement. Zoopthora fungal infection (observed this week in SA by SARDI entomologist Greg Baker) will also limit their build-up while wet weather conditions persists. As temperatures warm, DBM development increases considerably and they can have significant effects on canola yields.

Our advice

We recommend checking canola crops sown in the vicinity of volunteer canola and weed brassicas for DBM larvae and damage. Spray decisions should be based on whether larval populations pose an immediate threat of yield loss to the crop; during this mid-winter phase, most crops should out-grow the impact of DBM. Growers should not be concerned with potential spring populations at this stage. Canola can tolerate considerable foliar feeding damage without impacting yield. The economic threshold for DBM in unstressed Pre-flowering canola crops is 50 larvae per 10 sweeps.

Infested crops should be regularly monitored, particularly if conditions become relatively dry or warm. Pay particular attention to damage targeting the tip of young inflorescences. As temperatures warm up in late-winter and early spring, growers should regularly monitor canola crops. Search for the presence of larvae on leaves, buds and flowers, especially during flowering and podding. Sample crops by sweep netting.

Control options

Sampling crops at several locations is important to determine whether numbers are increasing or decreasing before any spray decision is made. Click here for further information on monitoring and threshold guidelines. If control is needed, keep in mind that moderate to high levels of resistance is widespread in Australian populations. Avoid pyrethroids and organophosphates, either alone or in mixtures, because they are ineffective for DBM control and will destroy beneficial insects, potentially leading to other problems (e.g. DBM or aphid resurgence).

Bacillus thuringiensis products and two new synthetic insecticides, Affirm® and Success Neo®, are registered for DBM control in canola, and are less disruptive to beneficial insects. No single insecticide application will completely eliminate the DBM population. In years when populations are large, a two-spray strategy is recommended.

Description

DBM caterpillars are pale yellowish green and tapered at each end of their body, which grows to about 12 mm long. They often wriggle rapidly when disturbed. The moths are about 10 mm long and grey-brown in colour. They have a characteristic whitish strip of uneven width down the back, which resembles diamond patterns. Diseased caterpillars are white, brittle, flat and covered with fungus and attached to the plant leaves.

 

* Sources of field reports of diamondback moths

Greg Baker – Researcher, SARDI (South Australia)

Chris Dunn – Agronomist, Landmark (Victorian Northern Country)

Andrew McMahen – Agronomist, Landmark (Victorian Mallee)

Kym Perry – Researcher, SARDI (South Australia)