Diamondback moth (DBM) in canola

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.

Insects and damage

Image: SARDI

Image: SARDI

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

  • Check the crop at 5-7 day intervals using a sweep net.  Recheck 5–7 days after a spray treatment.
  • Take a minimum of 10 sweeps at each of 5 locations in the crop. Start at the edge and sample towards the centre of the crop (infestations may be higher at the edge).  Increased sample numbers give greater confidence levels as distribution can be patchy.
  • Record the number and size of larvae in each set of sweeps (recording as small, med, large to help in decision-making and optimise the timing of Bt and other insecticide sprays). Calculate the average field density = Total number larvae found/ number of sets of 10 sweeps
  • Also take note of the crop stage, number of beneficial insects and evidence of disease, and any visible damage symptoms.

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:

  • 113 days at a daily mean temperature of 12ºC
  • 47 days at a daily mean temperature of 15ºC
  • 17 days at a daily mean temperature of 25ºC

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

  • Diadegma sp  – attack all larval stages
  • Apanteles ippeus – attacks larvae
  • Diadromus collaris – attack pre-pupae and pupae
  • Trichogramma sp – attack eggs

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.

Cultural control
  • DBM does not diapause, so needs a year-round source of hosts to survive.
  • Control summer Brassica weeds and canola volunteers
  • Control weed hosts in adjacent paddocks which allow build up of DBM.
  • Self sown canola pre-cropping will contribute to DBM population build up.
  • Monitor forage brassicas in proximity to canola, they may be a source of DBM.
Thresholds Thresholds are speculative guides only:

  • an average of  >30 larvae per 10 sweeps for pre-flowering, moisture stressed crops.
  • an average of  >50 larvae per 10 sweeps for pre-flowering, unstressed crops.
  • 100-200 larvae per 10 sweeps for unstressed crops with the majority of plants in flower
  • If crop is drying off and only 10-12 days from swathing – no action needed

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.

  • Good coverage is essential for effective control. Sprays must be able to target the significant portion of the population in the lower canopy.
  • Target small larvae (<5 mm in length) for maximum efficacy of Bt and insecticides.
  • Be aware of withholding periods of any products used for DBM control.
  • A 2-spray strategy can be more successful than a single application. This strategy aims to kill survivors, or newly hatched larvae in the second spray. If using this tactic, it is essential that insecticides are rotated.
  • Consider spot spraying for outbreaks at edges of field or where DBM are moving from areas of weed hosts. This approach may suppress these populations and prevent further spread across the field.
  • Late season control is less economic as the impact on yield is reduced once crops have formed most pods.
  • Refer to the impact of pesticides on beneficials table

Using Bt (Bacillus thuringiensis)

  • Consider using Bt in the first instance – it may not kill all larvae but can keep populations below threshold and preserve beneficial populations
  • Spray at dusk for greater effectiveness; Bt breaks down quickly with exposure to UV.

Insecticide Resistance

Multi-pest considerations
  • Other pests that occur at the same time as DBM include; aphids, helicoverpa and Rutherglen bugs.
  • Where possible – always consider the use of biopesticides first, as these do not impact on other pests/beneficials.
  • Bt can provide effective control of DBM and small helicoverpa.

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.

Further information

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.

Useful links

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