Diamondback moth Plutella xylostella is a major pest of canola and other brassica crops. DBM is a migratory pest in some seasons, so outbreaks are difficult to predict. Non-crop hosts (brassica weeds) can be abundant in wet summers and maintain local populations of DBM.
- View the DBM insecticide resistance management strategy
- View a pdf of the Diamondback moth life cycle
|Insects and damage||Eggs (oval, cream, <1 mm) are laid on stems, leaf stalks and under-surface of leaves.
The small green larvae wriggle violently when disturbed.
Damage includes clear membranous windows in leaves.
From flowering to maturity, larvae can be present in flowering heads feeding on buds and flowers, causing damage to young pods and surface grazing on maturing pods
|Monitor and record||Because of the sporadic, unpredictable nature of DBM outbreaks, be prepared to monitor canola crops intensively each season.
Monitor at first sign of damage and continue into spring
Rainfall (5-8 mm over 24 hours) can cause major decline in DBM populations. Larvae can be dislodged from plants and drown. Re-check after rain if a spray has been scheduled.
There may be multiple, overlapping generations in a crop. DBM development is highly dependant on temperature; the lifecycle from egg to adult takes:
Note: sweep nets are less effective when sampling dense tall crops.
|Beneficial insects||The beneficial species that have the greatest impact on DBM are small wasp parasitoids
Egg parasitoids are effective in reducing crop damage (larvae don’t emerge from eggs), larval parasitoids will reduce the amount of damaged caused by larvae. Diadromus does not prevent damage to the crop by the larvae it attacks at pre-pupae and pupae, but is important in overall population control.
Predatory beneficial insects include brown lacewing, ladybirds, spiders, damsel bug
Rapidly increasing, large populations of DBM will not be controlled by beneficial insects. However, small populations may be contained by the activity of parasitoids and predators.
Outbreaks of the fungal disease Zoophthera radicans can cause greater than 90% reduction in DBM population density following a period of warm temperatures, rainfall and high humidity. Diseased larvae become yellow, sluggish and swollen before dying. Dead larvae are white, brittle, flat and covered with fungus.
|Thresholds||Thresholds are speculative guides only:
If the DBM numbers are well above or well below threshold, 5 sets of 10 sweeps is sufficient to make a management decision. More sampling is required if average is near threshold, or re-check the crop again at a shorter interval e.g 2-3 days.
Research conducted in the 1980s show that from late flowering to pod-maturity, a 40% loss of foliage was needed to cause a 5% yield loss.
|Chemical control||DBM is a difficult pest to control with insecticides alone. DBM rapidly develops resistance to insecticides, and sustained insecticide use e.g. synthetic pyrethroids, will result in locally high levels of resistance. It is critical that an integrated approach to DBM is considered, rather than total reliance on conventional chemistry.
Using Bt (Bacillus thuringiensis)
Synthetic pyrethroids (SP) may provide control of aphids, helicoverpa and Rutherglen bug, but kill beneficials and are unlikely to control DBM because of resistance. Note that green peach aphid may also have resistance to SPs.
|Communication||An area-wide approach to DBM management (non-crop hosts, insecticide use and resistance management strategy) where outbreaks are severe and insecticide resistance is a growing issue may be useful.
Industry publications provide up to date regional information about pest activity in crops.
Ramachandran, S., D. Buntin, et al. (2000). “Response of canola to simulated diamondback moth (Lepidoptera: Plutellidae) defoliation at different growth stages.” Canadian Journal of Plant Science: 639-646.