Unnecessary spraying not good for your crop, your industry, or your bank balance

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Recent reports have surfaced that best practice pest management guidelines are not always being followed in winter and spring-planted mungbean and soybean crops. There are reports of some crops in the Burdekin being sprayed unnecessarily for well-below threshold pest populations, and/or being sprayed with no regard for the GRDC’s widely publicised Insecticide Resistance Management Strategy (IRMS) for Helicoverpa armigera. Similar reports have been received previously from other regions. Given the increasing resistance levels in H. armigera, now is not the time to lapse into pest management strategies that can favour this major pest, and also potentially flare secondary pests such as silverleaf whitefly and mites.

Not only are these practices unsustainable, they threaten the marketability of the very crops that growers and consultants are trying to protect. Underpinning these actions is the (mistaken) belief that a zero tolerance of pests in a crop is the best way to maximise returns to growers. Spraying below-threshold pest populations loses growers money, because by definition, the cost of control is greater than the value of the damage inflicted. Unnecessary spraying also hastens the development of insecticide resistance in H. armigera.

IRMS non-compliance

Of particular concern are reports of multiple applications per crop of Group 22 and 28 insecticides, namely indoxacarb (e.g. Steward®) and chlorantraniliprole (Altacor®) respectively. Note that the national IRMS for Helicoverpa armigera specifically states only one (1) application of these products to be applied per crop, within the IRMS prescribed spray windows. Losing these two insecticides would be a major backwards step for helicoverpa management and is a very real threat, given the low to moderate levels of resistance recorded to date in North and Central Queensland, with resistance to Altacor® and Steward® being 2.3% and 13.6% respectively. Previous experience with H. armigera in the 1990s showed insecticide resistance can increase very rapidly from such levels, and unrestricted applications of Altacor® and Steward® are guaranteed to lead to their demise.

Best practice product windows and restrictions from H. armigera IRMS (Central and Northern Queensland)

Another risk of multiple sprays with the same insecticide is that maximum residue limits in harvested seed may be exceeded. If this occurs, key markets can be lost, and the integrity of the whole industry is jeopardised. When selecting insecticides, we need to be cognisant of this, and also of the timing of the IRMS spray windows for Altacor® and Steward®.

For North and Central Queensland, the first Altacor® spray window has just finished (30 Sept) with a 4.5 month gap until the second window opens in mid-February and ends 30 April (see Table 1 above). However, more than half of this gap is covered by two Steward spray windows; mid-September to the end of October and 1 January to mid-February.

At any time during or outside the Altacor® and Steward® windows, newly registered emamectin benzoate (Affirm® – Group 6) can be applied with a restriction of 2 sprays per crop. Note that helicoverpa larvae sprayed with Affirm rapidly stop feeding but remain in a moribund state for 3-4 days before dying. During this period they are slow moving but otherwise (to the untrained eye) look quite healthy. If in doubt, keep larvae in a container overnight with some leaves, flowers or pods – larvae impacted by the spray will do little if any feeding. For more information, visit the Beatsheet’s research article on the impact of newer insecticides on helicoverpa larvae.

Biopesticide options for helicoverpa

During the soybean crop’s vegetative stage, the preferred option for helicoverpa control is none of the above chemical insecticides, but rather the commercial formulations of Nuclear Polyhedrosis Virus (NPV), VivusMax® and Gemstar®. Soybean’s tolerance of leaf damage during the vegetative stage is reflected in the high helicoverpa threshold of 5-6 larvae per square metre. The trick is to detect larvae before they are 10-12 mm long (1st and 2nd instars), and to apply the virus in a high water volume (at least 100L /ha via a ground rig, and preferably in 200-300L of water/ha).

These viral biopesticides can also be applied legally via overhead irrigation (chemigation). This practice is NOT legal with other insecticides and would be extremely dangerous if a highly-toxic product such as the carbamate methomyl (e.g. Lannate® and Electra® – Group 1A) was used. In contrast, VivusMax® and Gemstar® are highly selective, killing only the target pest (helicoverpa), and having no impact on other life forms, including beneficial insects (predators and parasitoids), and aquatic and terrestrial animals, including humans.

The other key recommendations of the IRMS are to use the softest options available against other pests, and to only spray pests when the crop is susceptible to damage inflicted by that pest. Using soft (selective) products for helicoverpa early in the crop’s life (i.e. ‘going soft early’) preserves natural enemies. A build-up of these beneficial insects buffers the crop against further attack from aphids, mites, whitefly, and helicoverpa, during the crop’s more vulnerable budding to podding stages.

Podsucking bug control

Unfortunately there are currently no effective but selective (soft) insecticide options for podsucking bugs (PSB). The pyrethroid deltamethrin (e.g. Ballistic® – Group 3A) is very effective against green vegetable bug (GVB) (Nezara viridula) but is totally ineffective against redbanded shield bug (Piezodorus oceanicus), however the addition of a 0.5% w:v salt adjuvant lifts control of the latter from 0% to a mediocre 60%. To achieve acceptable control (at least 80%), a permit (PER 86221 – expires 31 August 2021) has been obtained for the neonicotinoid clothianidin (Shield® – Group 4A) against these two bug species in mungbeans, navy beans and soybeans. For best results, add a 0.5% w:v salt adjuvant and a 0.2% MAXX Organosilicone Surfactant™.

As neither of these insecticides are selective, the only IPM strategy is to delay their application until PSB are at threshold and the crop has reached the early podfill stage, as stated in the permit. Note that soybeans and mungbeans are not susceptible to PSB damage during the vegetative, flowering and podset stages, and that seed quality is only affected during the podfill and podripening stages. As well, PSB nymphs can only survive in crops with pods that are filling. Early spraying for PSB pre-podfill is therefore a waste of time and money and is totally at odds with the IRMS for H. armigera.

Take home messages:

  • Go soft early
  • Apply only one Altacor or Steward spray per crop during their respective IRMS spray windows
  • Observe official data-based pest thresholds
  • Delay pod-sucking bug control until early podfill.

The Helicoverpa armigera IRMS and the science behind it are available at https://ipmguidelinesforgrains.com.au/ipm-information/resistance-management-strategies/