Efficacy of diflubenzuron plus methoprene against Sitophilus oryzae and Rhyzopertha dominica in stored sorghum

Abstract

The efficacy of diflubenzuron (1 mg kg⁻¹)+methoprene (1 mg kg⁻¹) against Sitophilus oryzae (L.) and Rhyzopertha dominica (F.) in sorghum was evaluated in a silo-scale trial in southeast Queensland, Australia. Sorghum is normally protected from a wide range of insects by mixtures of grain protectants. The chitin synthesis inhibitor diflubenzuron was evaluated as a potential new protectant for S. oryzae in combination with the juvenile hormone analogue methoprene, which is already registered for control of R. dominica. Sorghum (ca 200 t) was treated after harvest in 2000 and assessed for treatment efficacy and residue decline during 6.5 months storage. The reproductive capacity of S. oryzae and R. dominica was greatly reduced in bioassays of treated sorghum throughout the trial, and efficacy remained relatively stable during the trial. An initial exposure of S. oryzae adults to treated sorghum for 2 weeks reduced F₁ progeny production of all strains by 80.8–98.8%, but a second exposure of 4 weeks reduced F₁ progeny production by 98.5–100%. In addition, the reproductive capacity of any S. oryzae progeny produced was greatly reduced. Exposure of R. dominica adults to treated sorghum for 2 weeks reduced F₁ progeny production of all strains by 99.6–100%, including a methoprene-resistant strain. The results indicate that S. oryzae or R. dominica adults invading sorghum treated with diflubenzuron (1 mg kg⁻¹)+methoprene (1 mg kg⁻¹) would be incapable of producing sustainable populations.

Introduction

Insect growth regulators have been investigated for several decades for their potential to control insect pests of stored grain (Oberlander et al., 1997), but only methoprene, a juvenile hormone analogue, has been registered as a grain protectant (Daglish et al., 1995; Arthur, 2004). In general, methoprene applied to grain at the rate of 1 mg kg⁻¹ should give extended protection against prevalent strains of Rhyzopertha dominica (F.) and Tribolium castaneum (Herbst) (Daglish, 1998), and probably also Oryzaephilus surinamensis (L.) (Collins and Wilson, 1987). It does not control the methoprene-resistant strains of R. dominica which are becoming more common in Australia (Wallbank and Collins, 2003), however, and it is ineffective against Sitophilus species (Edwards and Short, 1984). Therefore, in order to control the full range of major grain insects, storage managers and farmers need to apply methoprene together with another registered protectant such as chlorpyrifos-methyl (Daglish et al., 1995).

Several studies have demonstrated the potential of using the chitin synthesis inhibitor, diflubenzuron, as a grain protectant against Sitophilus species (Carter, 1975; McGregor and Kramer, 1976; Desmarchelier and Allen, 1992; Elek and Longstaff, 1994). Application rates as high as 10 mg kg⁻¹ of diflubenzuron gave high, but incomplete control of Sitophilus oryzae (L.) and S. granarius (L.) in standard bioassay tests of wheat stored for up to 44 weeks (Carter, 1975). Desmarchelier and Allen (1992) noted the observation of McGregor and Kramer (1976) that diflubenzuron had a delayed effect on the reproduction of adults of some species, and used modified bioassays designed to detect such delayed effects in S. oryzae and S. granarius. They found that when parental adults were exposed to wheat treated with diflubenzuron at 0.2 mg kg⁻¹ or greater, the F₁ progeny were produced from eggs laid in the first 1–2 weeks exposure only. Furthermore, these F₁ adults failed to reproduce when transferred to treated wheat. They suggested, therefore, that diflubenzuron could be used to prevent population increase in lightly infested grain.

Sorghum is prone to attack from a range of insect pests during storage including S. oryzae, R. dominica, T. castaneum and O. surinamensis. Methoprene is registered for control of the latter three species in Australia, hence a mixture with diflubenzuron could be expected to overcome weaknesses caused by using methoprene alone. This paper reports the results of a field trial undertaken to evaluate the efficacy of diflubenzuron (1 mg kg⁻¹)+methoprene (1 mg kg⁻¹) on stored sorghum against S. oryzae and strains of R. dominica possessing or lacking resistance to methoprene.