Western Australian trial work has revealed that cabbage aphids – a common pest of canola crops – tend to be distributed along crop edges.
As well as the outside edges of paddocks, they may also aggregate on canola plants around dams, patches of natural vegetation or contour banks.
Researcher and PhD student Dusty Severtson said the information about the aggregation of cabbage aphid infestations provided improved and targeted guidelines about where to inspect and potentially detect emerging infestations.
“In addition, it indicates that a border spray may be just as effective at controlling cabbage aphids as a full paddock spray,” he said.
The Grains Research and Development Corporation (GRDC) funded research also found that cabbage aphids tend to colonise the underside of leaves in the lower portion of the crop canopy first, rather than the racemes (flowering spikes), which are traditionally inspected as part of canola crop monitoring.
“The new research clearly suggests that infestation of racemes occurs later in the growing season, and that in the weeks prior, the same canola plants host cabbage aphids feeding on leaves in the bottom of the canopy,” Mr Severtson said.
“Our field-based and experimental results indicate that inspection of leaves in the bottom of the canola canopy is important to enable early detection of cabbage aphid infestations.”
The research investigating cabbage aphid distribution in canola is being conducted by Mr Severtson, of The University of Western Australia (UWA) and Department of Agriculture and Food (DAFWA), under the supervision of UWA Associate Professor in Applied Entomology Christian Nansen.
Mr Severtson said that in some years, cabbage aphids and other species such as turnip aphids could cause widespread damage in late winter and spring by feeding on growing canola shoot tips, causing wilting, flower abortion and reduced pod set.
“They may also cover the plants with sticky honeydew, encouraging the growth of black sooty mould, reducing the plants’ ability to photosynthesize and generally decreasing plant vigour,” he said.
Two commercial canola crops were sampled at York and New Norcia in late August 2013.
The sampling of 10 adjacent plants was done at 60 sampling points at the York site, and at 100 points at the New Norcia site.
“The paddocks were chosen because of their differing environmental characteristics, with the York site containing contour banks and the New Norcia site containing tree patches,” Mr Severtson said.
“Both crops contained plants which ranged from budding to podding growth stages.”
Mr Severtson said the sampling results revealed an ‘edge effect’ for cabbage aphid infestation at both the York and New Norcia sites.
“Cabbage aphids were most commonly found within 20 to 30 metres of the crop edge and were rarely detected further inwards,” he said.
“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.
“It highlights the importance of weed control in and around canola crops, especially of wild radish which is a host for canola aphids.”
Mr Severtson said the research revealing the ‘edge effect’ implied that a border spray might be just as effective at controlling cabbage aphids as a full paddock spray.
He said one of the benefits to growers from spraying only part of a canola paddock was reduced pesticide costs and less time spent spraying.
“Growers may also be less reluctant to use a more expensive chemical control option if they know that only a border spray is required,” he said.
Mr Severtson said there were additional benefits from spraying only part of a paddock.
“A small number of cabbage aphids are likely to survive if a paddock is partially sprayed,” he said.
“This is actually a good thing as these remaining pests are likely to kick-start populations of ‘beneficial insects’ - such as parasitoid wasps, ladybird beetles and hoverfly larvae - which feed on aphids and help to keep populations low.
“These beneficial insects might prevent a secondary flare-up of aphids in the paddock or adjacent areas.”
Mr Severtson said spraying part of a paddock might also reduce the likelihood of insecticide resistance developing in aphids.
As part of glasshouse trial work, cabbage aphids were released into tents containing canola plants and the position of cabbage aphids on each plant was assessed after one week.
“Aphids consistently colonised the lower portion of the canopy, even when racemes were present,” Mr Severtson said.
“In all the glasshouse trials, all aphids were found on the underside of leaves.”
Mr Severtson said the trials suggested that it might take weeks from when cabbage aphids first colonised a canola crop, to when they became detectable on racemes.
“Cabbage aphids can reach relatively high numbers in the lower canopy before they will move on to the racemes,” he said.
“We had previously assumed that winged cabbage aphids flew on to racemes, but this research has shown this is actually not the case.”
Mr Severtson said these findings could help growers detect cabbage aphids earlier, and assist them to prevent the pests from reaching damaging levels in crops.
“To find aphids early, canola growers should inspect the under-side of leaves in the lower canopy,” he said.
Mr Severtson plans to develop a cabbage aphid sampling plan to fine-tune where and how many samples should be taken in a canola crop.
“Research is also being conducted into remote sensing work to improve detection of the pests,” he said.
For more information about the cabbage aphid research, contact Mr Severtson on email@example.com or 0427 196 656.
Information about aphid management in canola crops is available via the DAFWA Farmnote 440 available at http://archive.agric.wa.gov.au/objtwr/imported_assets/content/pw/ins/pp/gc/fn_canola_aphid.pdf
Growers and others in the industry are encouraged to send reports of pests and diseases to the DAFWA PestFax service at firstname.lastname@example.org.
GRDC Project Code