Cabbage aphids living on the edge
The latest research results show that cabbage aphids are most abundant along crop edges in canola paddocks.As well as outer crop edges, they may also aggregate inside a paddock on canola plants around dams, patches of natural vegetation or contour banks harbouring weeds.
Researcher and PhD student Dusty Severtson said improved knowledge about the distribution of cabbage aphids, one of the primary aphid species in canola crops, could be used to optimise crop inspections and precision-targeted spraying to control the pest.
The results also show that the insects prefer to colonise the underside of leaves in the bottom portion of the canola canopy first, rather than the racemes (flowering spikes).
“Therefore, scouting for early and emerging cabbage aphid infestations should focus on inspections of lower leaves,” Mr Severtson said.
The Grains Research and Development Corporation (GRDC) funded research is being conducted by Mr Severtson, of The University of WA (UWA) and Department of Agriculture and Food (DAFWA), under the supervision of UWA Associate Professor in Applied Entomology, Christian Nansen.
“Field trials at York and New Norcia in 2013 showed that cabbage aphids – which can cause feeding damage to canola crops in late winter and spring – tend to be distributed along crop edges,” Mr Severtson said.
“Cabbage aphids were most commonly found within 20 to 30 metres of the crop edge and were rarely detected further inwards.
“Where significant infestation of cabbage aphids was detected further into the crop, these locations were either near a tree line or contour bank harbouring weeds.
“This shows that the probability of detecting cabbage aphids is highest along crop edges and non-crop regions within a paddock, especially where wild radish is, or has been present.”
Mr Severtson said that as well as highlighting the importance of weed control, the results implied that a border spray might be just as effective at controlling cabbage aphids as a full paddock spray, saving growers time and money.
“Additional benefits from partially spraying a paddock include the retention of beneficial insects, which can prevent secondary flare-ups of aphids, and a reduced likelihood of aphids developing resistance to insecticides,” he said.
Mr Severtson said 2013 glasshouse trials involving experimental infestations of cabbage aphids on canola plants clearly showed that the insects spread quite slowly from an initial point of infestation.
“Therefore, it probably takes several weeks from when cabbage aphids first colonise a canola crop to when they become detectable on racemes – where the insects have traditionally been sampled from,” he said.
“We had previously assumed that winged cabbage aphids flew directly onto racemes, but this is actually not the case.
“Cabbage aphids can reach relatively high numbers in the lower canopy before they will move on to the racemes.
“To find aphids early, canola growers should inspect the underside of leaves in the lower canopy.”
Growers and others in the industry are encouraged to send reports of diseases and pests including cabbage aphids to the DAFWA PestFax service at email@example.com.
Dusty Severtson, UWA and DAFWA
0422 157 769
Natalie Lee, Cox Inall Communications
08 9864 2034, 0427 189 827
GRDC Project Code UWA00144, UWA00158