Rain needed to flush out aphids
WELL, I am back in the country and my early paddock inspections for our wheat and barley suggest we could do with an inch of rain in the near future.
I am finding a lot of aphids in our wheat and barley crops.
So far, I have been able to identify oat aphids and corn aphids even with my jet-lagged eyes.
Don't let the names fool you as they can all affect any winter cereals.
I understand it has been fairly cold here and perhaps that is depressing our natural predators, parasitoids and pathogens against this infrequent pest, whose mere presence does not necessarily mean spray, by the way.
No, a few sequential inspections should occur to ascertain if aphid numbers are going up or down.
Then you get to the rubbery stage of threshold numbers to commence insecticide control measures.
I guess we have to go by past years' experience and even include potential yields as part of the calculations.
So, without wanting to start a prophylactic war on these sporadic pests, my own thresholds around the tillering stage, which is fairly early, is that 10-15 aphids per tiller for 50% of the crop.
You only need a few virus infected aphids that are fairly mobile to infect vascular tissue or phloem of your plant to result in this extensive yellow tipping of the tiller leaves.
This yellow tipping is not reversible and if any of these sucking insects do have the barley yellow dwarf virus (BYDV) in their mouth pieces, then this disease is transmitted to the cereal plant.
So the first problem aphids create is a moisture-sapping issue for our cereal plants and then, of course, the spectre of BYDV or even CYDV as a possibility due to any infected sap-suckers.
This moisture-sucking capability can lower yield and even impact on grain size but the biggest detrimental impact on yield and quality is with a virus being introduced to the cereal plants' system.
Their life cycle is slow and steady as, once they mature, they deliver live babies every few hours without the need to mate.
Once a decision to spray has been made, you go on to the agonising problem of what insecticidal to use.
The main options will be dimethoate, pirimor or synthetic pyrethroids.
The most beneficial and pest-friendly will be the pirimor product and I have inspected some effective post-spray jobs in past years with this product, that appear to have ability to travel both directions in the plants' vascular tissue.
Of course, dimethoate and synthetic pyrethroid are not very IPM-friendly but will have the benefit of being available in the retail system.
None have a long-lasting residual in the plants' sap stream, so in bad years re-treatment may be necessary.
So some sequential inspections are my first step to determine if aphids could be a problem and, if my inch or so of rain appears, then that may be enough to wash a lot of these pests off the leaves and upper root system where they congregate.
At best it may delay the onset of these tiny destructive pests but your regular inspections will be critical in the overall strategy of aphid control in what may be a bad year for them.
Paul McIntosh is a former manager of Landmark Emerald now based on the Darling Downs.
