Efficacy of two thiamethoxam pre-germination seed treatments and a phorate side-dressing against neonicotinoid- and pirimicarb-resistant cotton aphid, Aphis gossypii (Hemiptera: Aphididae)

In a glasshouse trial with potted cotton plants grown from thiamethoxam-treated seed, neither 2.76 g a.i./kg seed (Cruiser((R))) nor 5.52 g a.i./kg seed (Cruiser Extreme((R))) protected plants from neonicotinoid-resistant Aphis gossypii Glover, 1877. Against susceptible A. gossypii each treatment was highly effective, providing control of >90% for 42 days. Continued use of either thiamethoxam treatment against resistant A. gossypii will select for resistant phenotypes and probably restrict the useful life of the neonicotinoid insecticides against this pest. In a separate trial, side-dressing of cotton seed with phorate 200 g/kg (Thimet((R))) effectively provided plants with protection from susceptible A. gossypii. The insecticidal activity of phorate-treated plants against pirimicarb-resistant A. gossypii was not statistically different to untreated plants (P>0.05). To maintain the effectiveness of pirimicarb in the Australian cotton integrated pest management strategy, the use of phorate as an alternative pre-germination treatment to thiamethoxam for aphid control must be managed. We recommend that the first foliar spray applied to cotton treated with phorate at planting should not be pirimicarb or any other insecticide affected by insensitive cholinesterase (ACE1) type resistance.