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Predicting next year’s insect pests this harvest

Small pointed snail in head of wheat
Small pointed snails in head of wheat. Photo courtesy of: DPIRD.

Identifying the species of invertebrates harvested with grain can be an early warning that pest numbers are increasing in paddocks or indicate which pests may cause in-season damage to susceptible crops like canola next year. One of the first signs that these pests are building up to potentially damaging levels is their presence in harvested grain.

Pests such as snails, European earwigs, weevils and vegetable beetle are known as resident pests because they either do not fly or are weak fliers so do not move readily between paddocks. Finding resident pests in grain does not mean you will need to control them but does provide warning for future monitoring. Even though a hot dry summer can decrease pests such as snails, knowing what is in the paddock prior to seeding enables you to have more control options available.

If you find European earwigs and/or snails in harvested grain it is likely that these pests were introduced into paddocks on vehicles and machinery. Cleaning machinery before moving from one paddock to the next can minimise the risk of introducing these pests into new areas but cleaning machinery is time consuming and wont remove all snails so consider harvesting paddocks with snails last.

If crops are swathed, trials have shown that grain from swathed crops can have more insect contaminants than grain from direct harvested crops. Invertebrates such as European earwigs and bronzed field beetle tend to congregate under crop swaths rather than in standing crops.

The less delay there is between swathing and harvest, the fewer vagrant invertebrates were harvested with the grain.

As direct harvesting canola is not always feasible the incidence of vagrant invertebrates (other than snails) in the grain can be reduced by:

  • Swathing crops at the right height. For example cereals need to be swathed at height of 125mm (cool drink can height) and canola should be swathed above the first fork in the plant stem if yield is expected to be one tonne per hectare or more. This allows the swath to be supported by the stubble. If the swath is close to the ground it is more likely that vagrant pests will be harvested with the grain. Also harvest swaths as soon as the grain is ready as the longer swaths are left unharvested the more vagrant insects use them as a refuge which increases the risk of insects in the sample.
  • Harvesting during the heat of the day. DPIRD trials show that grain harvested at night had up to 25% more invertebrate contamination than grain harvested during the day. At night European earwig and other beetle pests were found to move up into swaths and standing crops. During the hottest part of the day all invertebrates are found under the swath or near the ground in standing crop. Trials looking at snail movement have shown snails are more active at night and are more likely to be harvested at this time.

If you have snails in the paddock using a stripper front at harvest can be an effective way of reducing snail contamination as these fronts vibrate snails off standing cereals and take in less material. According to GRDC’s Snail Management fact sheet, stripper fronts reduced snail contamination in grain by 50 per cent compared to a standard open front.

For more information contact Svetlana Micic, Research Officer, Albany on +61 (0)8 9892 8591.

Article author: Svetlana Micic (DPIRD Albany).