Writer: Geire Kami Published: The West Australian Countryman 16 October 2014
A mild winter coupled with a warm dry spring can be factors which impact canola crops in Western Australia.
Invertebrate pests like aphids can be hard to control and require a high level of visual and biological monitoring.
Despite best efforts they can appear to descend on canola crops virtually over night.
But farmers find an unusual ally in the battle against these soft bodied armies in the transverse ladybird (Coccinella transversalis).
Department of Agriculture and Food entomologist Svetlana Micic said surveys have found that the transverse ladybird is the most common species found in canola crops.
In recent years, and with corresponding weather conditions, canola farmers in Western Australia have seen an astronomical rise in the presence of aphids at the time of flowing and podding, impacting bud formation and delaying the onset of flowering, wilting stems, reducing pod sets, and setting off flower abortion.
Typically dense on growing tips, aphids suck the sap out of plants and leave crop yields compromised if unmanaged.
Three aphid species are common in the state. Ms Mimic said the cabbage aphid and turnip aphid are found on the flowering spikes of canola.
"The feeding damage from these aphids can cause yield loss in moisture stressed canola. The green peach aphid is known to be a vector for viruses that affect yield only when seedling crops are infested. Green peach aphids can be readily seen on the underside of leaves in mature crops but do not cause yield loss,” Ms Micic said.
Infestations occur when winged aphids from autumn weeds fly into crops.
They can be hard to spot in autumn and early winter, as they subsist in low numbers, but in late winter the sightly warmer weather encourages a burst of activity, and aphid populations can explode.
As well as devouring the sap of canola plants, aphids can be responsible for the unfolding of other bad outcomes for canola: covering the plants with sticky ‘aphid honeydew’, which is the precursor to black sooty mould.
Black sooty mould in turn reduces photosynthesis. Ms Micic said she had read about this, but had never seen it.
Replicated field trials have shown a 33% reduction in canola yields in Western Australia in moisture stressed crops where aphids were introduced.
Wetter winters which produce stronger canola plants seem to stave off aphids more readily than drier years, where compromised, drought stressed plants don’t seem to be strong enough to stem the tide of the tiny bugs.
Farmers often get a bad wrap for unavoidable chemical controls (however low the active ingredient per hectare), but in the fight against aphids, farmers are often pleased to reassess their spray programs for a biological solution.
Recognisable by its striking lateral three-lobed wing marks, on a bright red/orange body, it is one of very few natural predators to the aphid: full grown ladybirds and larvae alike feed on them.
They also have a curious way of stopping their own predation: they bleed a smelly liquid out of their leg joints if threatened.
Ms Micic said ladybirds could be difficult to find in crops and he first sign that ladybirds were present in crops and feeding on aphids was their appearance in a grain sample.
"As temperatures heat up, trials have shown that ladybirds prefer to shelter inside swaths rather than in the open,” she said.
Dustin Severston (PhD Candidate at UWA’s School of Animal Biology) believes aphids to already be present in some canola across WA.
“These aphids are beginning to be seen in canola crops now and reports will increase over the next month, however not all canola crops are infested every year to damaging levels. If you would like a good snapshot of WA's situation, you should contact DAFWA,” said Severston.
The only issue farmers face after harvesting a crop full of ladybird covered canola, is cleaning them out of the seed.
© Geire Kami. All Rights Reserved. Australia 2017.