Alternative insecticides to control grain aphids, Sitobion avenae, that are resistant to pyrethroids

Abstract

Cereal aphids, Sitobion avenae, have developed resistance to pyrethroids in the last few years, culminating in control failure at some locations in England 2011 and 2012, associated in the latter year with local epidemics of barley yellow dwarf virus (BYDV). Studies by Rothamsted Research and Syngenta have shown this to be due to target site resistance, which can be detected in populations by using a vial bioassay test, and in individual aphids using a DNA diagnostic test. Approximately 35-50% of S. avenae tested in the UK in 2012 and 2013 contained genes that confer resistance to pyrethroids, confirming other studies on this species caught in Rothamsted Insect Survey suction traps in those years.

Field trials in autumn 2012 in which plots were inoculated with a clone of S. avenae that was resistant to pyrethroids showed that cypermethrin at 25 g a.i./ha and deltamethrin at 7.5 g a.i./ha gave significantly poorer control than chlorpyrifos at 450 g a.i./ha, but not lambda-cyhalothrin at 7.5 g a.i./ha. There were no differences between these products when used against susceptible clones of either S. avenae or the bird cherry–oat aphid, Rhopalosiphum padi. The rate of lambda-cyhalothrin used was higher than the recommended rate, which might explain why it gave better control.

In two larger field trials to test alternative products against the resistant clone ofS. avenae, and the consequent spread of BYDV, the carbamate, pirimicarb at 120 g a.i./ha gave moderate control at one site, but good control at another; the neonicotioid, thiacloprid at 72 g a.i./ha gave good control at both sites, while acetamiprid at 50 g a.i./ha was relatively poor. Pymetrozine at 100 g a.i./ha plus adjuvant oil gave good control at the one site it was tested. Chlorpyrifos at 450 g a.i./ha gave best control at both sites.

Secondary spread of BYDV was low in untreated plots at both sites (<2%) in the winter of 2012-13, due to very low temperatures after spray applications, which prevented further primary colonisation by ‘wild’ aphids, and multiplication of those that survived the treatments. However, significant reductions in virus infections were still recorded the following spring, although not enough to affect yield at harvest. There were no significant differences between treatments at either site, but their comparative performance did reflect their efficacy against aphids.

None of the alternatives mentioned above are currently approved for use against aphids in cereals in the autumn, but these results will give regulators some evidence for their activity against resistant grain aphids, should alternatives be required.