Helicoverpa in canola

  • Native budworm (Helicoverpa punctigera) is common when wet winters in breeding areas of central Australia combine with suitable weather conditions for spring migrations.
  • Corn earworm (Helicoverpa armigera) survive over winter as diapausing pupae in the ground. Moths emerge in Oct-Nov and the first generation goes through in spring-flowering crops (e.g. chickpea, canola), or on weed hosts.

In the northern region (Dubbo north) H. armigera and H. punctigera can occur together in the crop. It can be difficult to distinguish small larvae, but larger larvae can be identified to species.  It is important to determine the species in the crop to guide insecticide choices and timing, if control is warranted

Insects and damage

(see Helicoverpa page)

  • Larvae feed on flowers and pods
  • Yield loss occurs when larvae burrow into pods consuming seed
  • Pod damage allows entry of disease that destroy or damage seed.
  • Larval feeding and diseases lead to loss of yield and quality.
Monitor and record Monitor weekly from late flowering until harvest;

  • In small crops it may be possible to use a sweep net to sample for Helicoverpa larvae. In larger, more dense crops, a method of beating flowering and podding racemes will be most effective. Use a large tray and stick to beat a defined length of row (e.g. 0.5 -1.0 m) or shake whole plants into a fertiliser bag or large ‘bucket’.
  • Consistently monitor the same unit (e.g. 10 metre of row, 10 plants) in a field to compare densities between checks. To standardise an estimate based on sampling plants, determine the average number of plants per m2
  • To assess pod burrowing – split open 20-40 pods and look for holes and feeding damage
  • If a spray is applied, assess efficacy within 10 days of application.
  • Continue to monitor the crop until harvest, or when no longer susceptible to damage.

Record

  • Number, size (e.g. small, medium, large) and stage of crop development (to estimate time to harvest).
  • Percentage of pods damaged ((number of pods damaged ÷ number of pods examined)*100)
Conserve beneficial insects A suite of beneficial insects will eat and parasitise Helicoverpa eggs and larvae. In combination they can suppress low to moderate populations. These beneficials include:

  • Egg parasitoids, Trichogamma wasps
  • Larval parasitoids, Microplitis, Heteropelma, Netelia sp..
  • Predatory bugs such as the Big-eyed Bug (Geocoris) and the Damsel bug (Nabis) prey on eggs and small larvae. Glossy shield bug (Cermatulus) and spined predatory shield bug (Oechalia) also attack larger larvae.
  • Ants and spiders attack Helicoverpa eggs and larvae.

NPV is a virus that only infects Helicoverpa species. This occurs naturally in the environment, and natural outbreaks occur in some seasons. A commercial formulation of NPV can also be applied to a crop to control larvae.

Cultural control Helicoverpa punctigera is a migratory species which builds up in on native plants in central Australia before being carried to eastern, southern and western cropping regions by weather systems in spring. Control of local hosts will have no impact on H. punctigera populations. Enhance the activity of beneficial insects and consider native vegetation or farmscaping in cropping regions

Weed control is important for managing H. armigera populations. Larvae can develop on weeds in the crop and then move as large larvae onto the crop as the weeds dry down in early summer. If this happens, damage can occur very rapidly as the larvae infest the crop at a development stage capable of damaging pods. Pupae busting after summer crops (e.g. cotton) can reduce numbers of Helicoverpa armigera larvae emerging in spring

Thresholds
  • Research from WA indicates a yield loss of 6 kg/ha for every H. punctigera larva per 10 sweeps.
    Economic threshold = cost of control / ((6 kg/ha * no. larvae) * grain price $/t)
  • Speculative thresholds from Victoria – 5-10 Helicoverpa larvae per m2  + visible damage to pods

An alternative approach to waiting until pod damage is visible is to control larvae at the threshold density before pod damage appears (but only when it is clear that there is time for pod damage to accumulate if larvae are left uncontrolled). Delaying control as long as possible is a useful strategy, if the influx of moths is prolonged. Delaying a spray until the threshold is exceeded by larvae that will soon be capable of damaging pods (late small–medium larvae) may avoid the need to respray.

Pesticides
  • Aim to control larvae before they are large enough to damage pods
  • Synthetic pyrethroid insectides are very effective on H. punctigera, but their broad spectrum activity negatively impact on beneficial insects present. Refer to the impact of pesticides on beneficials table. Synthetic pyrethroid insectides are generally not effective against H. armigera. If the population is largely H. punctigera, then treating larvae <7 mm in length will give SPs the best chance of controlling the H. armigera proportion in the population.
  • Commercially available NPV and Bt are effective against H. punctigera, but need to target larvae smaller than 7 mm in length.
  • Inspect crops after spraying to assess efficacy of the application.
Other considerations
  • Control of H. punctigera with broad spectrum insecticides (e.g. methomyl, synthetic pyrethroids) can increase the likelihood of an outbreak of aphids if beneficial insects are killed.
  • Where both Helicoverpa species occur (north of Dubbo) be aware that insecticide resistance in H. armigera can affect the efficacy of SPs and OPs in mixed populations
  • Use the Helicoverpa emergence model to determine the likelihood of H. armigera being present in your district.
Communication
  • A national network of pheromone traps for adult Helicoverpa is maintained during migration and emergence periods, and results are reported in PestFacts, PestFax and the Beatsheet.
  • An Area Wide Management approach to H. armigera management may be worth considering in a region with a number of susceptible crops in the rotation
  • Industry publications provide up to date regional information about pest activity in crops.

 

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