Silverleaf whitefly (SLW), Bemisia tabaci B biotype (Gennadius), is a major insect pest of cotton and horticultural industries. In cotton it is a pest because it produces sugary exudates (honeydew) that contaminate cotton lint and cause problems during textile processing. Honeydew contaminated lint may receive price penalties or in extreme cases may be rejected from sale.

Insecticides are an important SLW management tool, but SLW has the propensity to rapidly develop resistance to many insecticide groups including synthetic pyrethroids, organophosphates and carbamates. Pyriproxyfen (Admiral®) is currently considered the most important insecticide for SLW management because it has excellent efficacy against high density infestations. Overseas, resistance to Admiral® has been reported for SLW.

Resistance monitoring for Admiral® between 2007 and 2010 indicated there was no evidence of resistance to pyriproxyfen developing in cotton dominated regions. Resistance factors in cotton dominant regions were generally not significantly different to the susceptible strain. In 2007-08 elevated resistance factors were recorded for Silverleaf whitefly collected in cotton against pyriproxyfen in St George and the Burdekin. In St George, subsequent sampling in more recent years indicated that resistance factors had returned to a susceptible level and there was no evidence of resistance developing.

In the mixed cropping zone of the Burdekin, no pyriproxyfen usage occured in cotton as part of an area wide resistance management strategy to give priority products to the dominant cropping industry. It is likely that the elevated resistance factors recorded in the Burdekin from cotton are due to close proximity between cotton and horticulture.

Elevated resistance factors were recorded for pyriproxyfen for silverleaf whitefly collected from Ayr and Gumlu, North QLD in 2008-09. Resistance factors 100 times higher then the susceptible strain were recorded for these two field collections. These elevated resistance factors are a warning to the cotton industry of the risk of resistance developing to pyriproxyfen. The cotton Insecticide Resistance Management Strategy of a maximum of one application of Admiral® per season is a valuable insecticide resistance management tool.

Elevated resistance factors were recorded for diafenthiuron and bifenthrin between 2007-10. The increased resistance factors indicate that resistant genes are present in the populations however at this stage there is no evidence that the resistance factors have increased over the three years of monitoring. Overseas, diafenthiuron is thought to be stable to resistance however it has not be used widely due to phytotoxicity issues. Bifenthrin is not recommended for use in cotton due to poor efficacy and high toxicity to beneficial insects.