Dramatic decline of Russian wheat aphids

However, populations of beneficials like parasitic wasps and ladybirds are on the rise.

Research entomologist at cesar, Dr Elia Pirtle said they had carried out four field surveys around Griffith, Deniliquin, Hay, Balranald, Condobolin and Forbes since October last year.

Dr Pirtle said they saw a dramatic decline in aphids over last summer.

"In our January 2019 survey we didn't pick up any Russian Wheat Aphids in our regular NSW sites, we did find them in a couple of places with some extra searching but it was a dramatic decline so we were interested to see how they were going to recover," Dr Pirtle said.

"On our spring survey, in the last couple of weeks, it looked like the aphids had a very recent recovery.

"At about a quarter of our sites we visited, we did find them but they were generally very small populations and I think that was a sign that they were only recently starting to migrate, at quite a low level.

"The exception was a few reports from growers in Eugowra, Oaklands and Condobolin, over the winter, where populations did manage to start up around July/August.

Populations in the green bridge were much smaller, and one of the only large populations detected in weeds was found near Balranald.

Dr Pirtle said the low numbers meant it was unlikely yield loss due to aphids was experienced this season, in the areas they surveyed.

"Overall it's too little, too late to damage crops," Dr Pirtle said.

"Trial site data collected by SARDI and knowledge of Russian wheat aphid biology gained from overseas work suggests that once the crops have matured in the spring they're at a very low risk of infestation."

She explained crops are most at risk from aphid damage during the autumn.

"Aphids do best on young, actively growing crops, that's their number one preference and that's when they could actually cause damage that persists in the plant," Dr Pirtle said.

She said they believed climate was one of the reasons Russian wheat aphids struggled to survive over last summer.

"The high temperatures would not have been good for aphids," Dr Pirtle said.

"But also the fact that they don't have a high preference for summer grasses.

"In summer we were still finding green summer grasses around irrigation channels, like barnyard grass, but they won't establish colonies on these.

"All the preferred grasses, including barley and brome grasses, were already dried out and dead and without those grasses around in the summer there was a real restriction on their range."

Dr Pirtle said growers' insect friendly choices were also paying off for them.

"Despite not finding high numbers of aphids, there's already been really high levels of parasitic wasps attacking aphids this spring and we found a lot of ladybirds and lacewings, so that's been really great," Dr Pirtle said.

Dr Pirtle said the next stage of the study involved predictive modelling to forecast aphid risk.

"We will be comparing the sort of rainfall and climate patterns across the sites we visited, with where we did or did not find aphids in the hope we can tie their activity to a climate predictor," Dr Pirtle said.

Ag n Vet agronomist at Henty, Nathan Soulsby said they had seen some species of aphids this year but few Russian Wheat aphids.

He said despite aphid numbers building during spring, they had not had to spray for aphids in their cereals because of the high number of beneficials present.

"Numbers were building but once it warmed up the predators came in as well and pretty much cleaned them up," Mr Soulsby said.

He said the predators included ladybirds, hoverflies and parasitic wasps.

Mr Soulsby said his clients were becoming more aware of beneficials and more targeted with their spraying.

"We're trying to get away from calendar spraying and we're doing more monitoring," Mr Soulsby said.