Implications of methoprene resistance for managing Rhyzopertha dominica (F.) in stored grain

• We investigated methoprene resistance in Rhyzopertha dominica.
• Resistant populations reproduced in wheat treated with 30 mg kg−1 of s-methoprene.
• 93% of field samples collected from southeast Australia in 2009 were resistant.
• Methoprene resistance is a serious threat to the management of R. dominica.

Commercial formulations of methoprene have been used in a number of countries such as Australia and the USA to provide long-term protection to grain from a range of storage pests. The level of resistance in Rhyzopertha dominica (F.), the lesser grain borer, was investigated in laboratory experiments by direct exposure of adults on treated wheat. Adults of a reference homozygous resistant strain of R. dominica were exposed to treatments of 0, 2, 3, 5, 10, 20 and 40 kg−1 of s-methoprene for 7 days. Mortality and progeny production were dose dependent with 98.7% mortality and complete progeny suppression at 40 mg kg−1, which is 67 times the registered rate at which s-methoprene is applied as a grain protectant in Australia (0.6 mg kg−1) and eight times the rate which has been used in the USA (5 mg kg−1). This strain was also tested by adding adults to wheat treated at 0, 1, 3, 10 and 30 mg kg−1 and determining the number of adults (progeny plus original parental adults) after 6, 8, 10 or 12 weeks of continuous exposure. The effect of s-methoprene was consistent regardless of the number of weeks of continuous exposure and average population suppression was 99.5% at 30 mg kg−1. Screening of 162 field samples collected from southeast Australia in 2009 showed that 93% of samples reproduced when exposed to wheat treated with the Australian registered rate of 0.6 mg kg−1. When four unselected resistant field samples from this region were tested, progeny production but not mortality was dose dependent, and all four samples produced live progeny at the highest dose of 30 mg kg−1. Our results show that methoprene resistance is a serious threat to the management of R. dominica, and that strategies need to be developed to minimise the further development and spread of resistance.