Controlling summer pulse pests in wet weather

Strong La Nina conditions featuring extended periods of wet weather and flooding generally delays the planting of many summer pulse crops.

However, for crops that have been planted, wet weather can make sampling difficult, and may provide perfect conditions for some pests.

Continuing to sample your crop, when possible, can help you to identify damage and the likely pests that caused it and put effective control measures in place quickly. Identifying insect pests correctly is critical in a wet summer.

Sampling pulse crops in wet weather

If possible, you should continue to sample your crop in wet weather. The most critical time to sample summer pulse crops is from flowering onwards, through pod set and fill and, for some pests, well into pod ripening.

Heavy rain or flooding may make sampling impossible, which may see your crop suffer damage from pests at critical stages. While not ideal, this should not be too great a cause for concern. In many cases, the crop will be able to compensate for early damage, especially as soil moisture is unlikely to be limiting in a wet summer. You must, however, ensure you minimise any further pest activity.

Learn how to use a beat sheet to sample crops for pests and beneficial insects.

Identifying insect pests

Correctly identifying pests is critical in a wet summer. You may not have time to deal with the consequences of a misdiagnosis and the subsequent pesticide application later in the season.

For example, legume webspinner damage is often confused with damage caused by soybean moth larvae. The latter is usually a minor pest but sometimes occurs in devastating plagues. While soybean moth larvae are readily controlled with a pesticide registered in soybeans, this product is far less effective against legume webspinner. Larvae of these species are easily differentiated by their size and colour.

Read our identification guides:

Common wet weather pests

A wetter than average summer will favour some pests more than others.

For example, high helicoverpa and podsucking bug populations are often observed in summer pulse crops in a wet season, and on weed hosts. An abundance of hosts during spring and early summer will boost populations of these pests. However, wetter conditions may eventually act against many caterpillar pests, with major helicoverpa virus outbreaks evident in many sorghum crops, and very likely in peanut crops.

Cooler conditions are also likely to favour soybean aphids, and the red shouldered or Monolpeta beetle often emerges in plague numbers after heavy rain.

In contrast, wet conditions are unfavourable for etiella in peanuts, and damaging populations close to harvest will not threaten crops as they have in recent drought years (when dry soil allowed larvae to reach the underground pods).

Spraying for insect pests

Before spraying for insect pests, consider the size and density of the pests when you detected them and the impact of weather conditions post spray when deciding whether to spray or not in a La Nina summer.

If wet weather is threatening, you may decide to spray immediately in case the weather sets in for an extended period of time. In these cases, consider options that:

  • are less likely to flare pests, such as those that kill their predators and parasites
  • have a long period of activity post spray.

For example, in soybean, a virus-based biopesticide such as VivusMax might be a viable option for helicoverpa (provided larvae are small, i.e. = 7mm), as the wetter conditions will hasten the secondary infection and spread of the virus, and beneficial insects attacking helicoverpa will be preserved. If high helicoverpa pressure is detected, consider a more selective pesticide such as indoxacarb, which has a much longer period of activity than a methomyl-based product, and which is far less likely to flare secondary pests such as aphids and whitefly.

However, for other pests – in particular the problematic bean podborer in mungbeans – prompt action may be needed at flowering as larvae are very difficult to control at a later date once they are inside the pods.

Learn about the chemical control of insects and how to choose the right insecticide.

Reducing your spray costs

Pest management costs can be reduced by avoiding the temptation to spray pests if the at-risk period for a pest is still some time away. This is especially true for podsucking bugs, which are not a threat until pulse crops reach the podfill stage. Unless there is extreme adult pressure, pulse crops can compensate for pre-podfill damage and young nymphs inflict very little damage.

Panic spraying your crop too early with non-selective pesticides can also flare secondary pests and kill beneficial insects, which may further reduce your yields.

You can keep costs down by only spraying the part of the crop infested with the pest. This particularly applies to red shouldered or Monolpeta beetle, which often emerges in plague numbers after heavy rain, but which is often most abundant along the edges of crops.

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