August 9, 2012
Winter weather does a good job of keeping insects such as aphids to a minimum, but with the coming warmer weather aphids will become more noticeable in crops.
The Department of Agriculture and Food has been researching the effects of cabbage aphids on canola, looking at when they arrive, whether early or late sown canola crops are more at risk, and how long aphids can be allowed to feed before economic damage is evident.
Department development officer Dusty Severtson said research on these issues is important as canola aphids cause widespread damage in some years and pirimicarb is currently the only chemical registered for their control.
“Growers also have the added difficulty of potentially having to spray multiple times and combining aphid control with tactics for other likely pests, such as diamondback moth and native budworm caterpillars,” Mr Severtson said.
“Previous trials in WA have shown that canola in good growing conditions generally has a good ability to compensate from feeding by low to moderate levels of aphids.
“We have found when 20 per cent of plants have aphid colonies, growers need to decide if and when they should spray in order to protect their profit.”
Aphids have a greater impact on yield and seed size when plants are struggling for moisture.
“In this scenario, the feeding damage is more likely to cause wilting, flower abortion and reduced pod set,” he said.
“Hybrid varieties sown at low plant densities may be more at risk of yield loss, especially if they are late sown and aphids arrive at early canola growth stages (e.g. first flower) when plants are more vulnerable.
“We know growers sometimes apply insecticides as a preventative without monitoring aphid levels.
“Although this may be a cheap and sometimes effective option, the sustainability of this practice is being brought into question, with recent discoveries of redlegged earth mites, green peach aphids and diamondback moth caterpillars showing resistance to frequently used insecticides.”
Trials conducted by the department in 2011 using aphid-proof cages in field plots showed that 90 to100 per cent aphid infestation has a large impact on reducing canola yield and seed size.
“Large numbers of colonising aphids on early flowering canola had a larger impact on yield loss than at later growth stages, when aphid populations were left uncontrolled,” Mr Severtson said.
This year, the department is focusing on the effect of cabbage aphids in canola crops sown at recommended plant densities. Aphids will be introduced at different times during late winter and spring, but allowed to feed for shorter periods of time.
Mr Severtson said it is important to have confidence in what population levels of aphids are likely to affect yields. The use of economic thresholds to justify an insecticide application will become even more important as insecticide resistance issues arise and more expensive products are brought into the market.