Fipronil seed treatments for the control of chironomid larvae (Diptera: Chironomidae) in aerially-sown rice crops

Abstract

The phenyl pyrazole insecticide fipronil was bioassayed against final instar larvae of Chironomus tepperi Skuse, and found to be highly toxic (LC 50 0.43 μg l −1, LC 90 1.05 μg l −1) relative to conventional organophosphorus insecticides. A comparison of fipronil and malathion (the standard bloodworm seed treatment in use in New South Wales) under glasshouse conditions using model soil/water systems showed that fipronil at 0.005 mg l −1 gave superior residual control to malathion at concentrations of up to 0.6 mg l −1. In small plot trials fipronil applied to pregerminated seed provided control of Chironominae at 12.5 and 25 g active ha −1 at least equivalent to that provided by the standard 300 g active ha −1 malathion treatment. The 12.5 g active ha −1 rate provided between 9 and 14 days residual control of Chironominae, whilst raising the application rate to 50 or 100 g active ha −1 increased residual control to between 14 and 19 days and provided significantly better control than malathion. Other chironomids (predominantly Tanypodinae) were strongly suppressed by all rates of fipronil for all or most of the 34-day trial period. Plant establishment and shoot length were significantly greater in all fipronil treatments relative to both the malathion standard and the untreated control. There was also evidence of stronger root system development in the fipronil treated plants. At an initial application rate of 12.5 g active ha −1, fipronil levels in the water column declined from 2.1 μg l −1 1 day after sowing to 0.01 μg l −1 14 days after sowing (DAS). At 22 DAS the parent compound was below detection limits (0.005 μg l −1). Four unreplicated commercial scale trials of fipronil were conducted in which three treatments or treatment combinations were evaluated. In general, fipronil (12.5–15.75 g active ha −1) either alone or followed by a post-sowing chlorpyrifos treatment (75 g active ha −1 10–15 DAS) provided a level of control equivalent to paired chlorpyrifos applications (75 g active ha −1 at 2–5 and 10–17 DAS), but at one site fipronil alone was inadequate without a post-sowing chlorpyrifos treatment. Our results indicate that fipronil applied to seed at 12.5 g active ha −1 is a more efficacious seed treatment than malathion at 300 g active ha −1 and provides protection against chironomids for 9–14 DAS. An additional post-sowing insecticide application may still be required in some seasons, but should only be applied in response to high larval populations and evidence of crop damage.