Aphids – to spray or not to spray

Spraying to manage aphid infestations in the northern region does not guarantee increased yields

Image of aphids on a plant

Aphid infestations at late pod set in canola have been shown to have no impact on crop yield.

PHOTOS: Dr Melina Miles, Queensland DAF

Recent outbreaks and regular infestations of cowpea aphids in canola and faba bean crops in the northern grains region have led to questions about whether spraying to control infestations can actually deliver a benefit. Research is beginning to show that controlling aphids may not necessarily lead to higher yields or prevent the spread of viruses.

Entomologist Dr Melina Miles, from the Queensland Department of Agriculture and Fisheries (DAF), says spraying for aphids late in the season, when the crop is mostly set, is not likely to increase yields in infested canola crops.

“Growers have been spending money to control late aphid infestations that looked severe and had built up rapidly,” Dr Miles says. “But such infestations were not going to impact on yields.”

Dr Miles develops management strategies for insect pests in broadacre crops in the northern grains region. She says aphids are regularly occurring pests in many northern crops, such as canola, and are a concern for growers due to the lack of research looking at infestation thresholds for the northern region.

Dr Miles and her team have confirmed, through field trials over the past two years, that spraying for aphids does not increase canola yields unless infestations are extreme and occur again during early flowering.

The research showed that one-off damage to plants, with removal of up to 90 per cent of early flowers and young pods, did not negatively affect yields.

New data for early infestation suggests that plants simply compensate by developing more lateral shoots to replace those damaged by aphids.

However, prolonged or repeated infestations at early growth stages would be expected to result in reduced levels of compensation and likely yield loss. This type of infestation will be evaluated in future research.

Seed dressings

Large outbreaks of cowpea aphids in faba bean crops occurred in the northern grains region during autumn 2014.

Image of an aphid damaged plant

An early and prolonged aphid infestation in this canola plant has stunted the primary raceme, resulting in additional racemes being produced.

While aphids can transmit damaging viruses to faba beans, trying to control virus transmission by spraying for aphids is likely to be of little use in preventing the spread of non-persistently transmitted viruses, such as bean yellow mosaic virus (BYMV). Spraying may actually increase the movement of aphids and consequently increase the spread of the virus in the crop.

Dr Miles, together with Queensland DAF senior plant pathologist Murray Sharman and plant pathologist Joop van Leur from the New South Wales Department of Primary Industries, found that low virus transmission had taken place in Queensland crops despite widespread and high-density aphid infestations. However, all crops surveyed on the Liverpool Plains in NSW showed BYMV infection.

“In the event of similar aphid outbreaks, we may see a different outcome due to the complex interactions between the aphids and different viruses,” Dr Miles says. “In areas where there were previous virus outbreaks, faba bean crops will be at greater risk.

“Using seed dressings that help protect the crop from early aphid establishment and virus transmission is a reasonable way to approach virus management at this point.

“Seed dressings are beneficial in terms of limiting the spread of both persistent viruses, such as bean leaf roll virus, and non-persistent viruses in faba beans.

”More work is still needed, particularly in the areas of predicting aphid outbreaks and virus transmission.

“Cowpea aphids migrated some distance into cropping areas this year, so predicting those outbreaks was extremely difficult,” she says.

Faba beans are considered a research priority because there is limited information on pest management for growers and agronomists.

They are a relatively new crop in the northern grains region, with new varieties making them a viable option for longer-season regions or in seasons with early planting opportunities.

In addition to aphids, helicoverpa species are major pests in faba bean crops.

Sampling effectively for helicoverpa larvae is challenging. Early research results suggest a combination of beat sheet or sweep net sampling and visual inspections is needed.

Without visual inspection, the presence of small larvae may be overlooked, making it difficult to assess the degree of infestation and correctly time controls.

Researchers hope to gain a better understanding of Helicoverpa species. and how other pests impact faba beans in the next two years.

More information:

Dr Melina Miles,
07 4688 1369,
melina.miles@daf.qld.gov.au

Video of Dr Melina Miles discussing the costs and benefits of spraying canola for aphids

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