Efficacy of diflubenzuron plus methoprene against Sitophilus oryzae and Rhyzopertha dominica in stored sorghum
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.
Section snippets
Grain treatment, storage and sampling
The trial took place in a vertical concrete silo at Baigin in southeast Queensland, Australia. A single mass of sorghum (200 t, grade 1 Red) was treated by spraying grain on a conveyor before being elevated into a silo cell (22.0 m high×6.4 m diameter). The sorghum was harvested from February to April 2000, treated on 17 July 2000 and sampled after 0, 3, 4.5 and 6.5 months storage. The target treatment was diflubenzuron (1 mg kg−1)+methroprene (1 mg kg−1). An experimental formulation of diflubenzuron
Grain conditions and chemical residues
Grain moisture remained relatively constant during 6.5 months storage, but grain temperature near the bulk surface increased in response to the seasonal change in ambient temperature (Table 1). Residues of both insecticides were relatively stable during the trial and showed no clear decay trend with increasing times of storage. Diflubenzuron and methoprene residue levels were usually higher than the nominal target application rate by about 20% or 10%, respectively, reflecting a slightly lower
Discussion
The results showed that the combination of diflubenzuron (1 mg kg−1)+methoprene (1 mg kg−1) was highly effective in protecting stored sorghum from S. oryzae and R. dominica despite low mortality of adults. The treatment resulted in 99–100% reduction in F1 progeny of R. dominica throughout the trial, and also greatly reduced the reproductive capacity of S. oryzae. Invading adults would be incapable of producing a sustainable population. Although the treated sorghum was not sampled for insects during
Acknowledgements
We are grateful to V.S. Byrne, P. Charnock and M. McCurdy for conducting the bioassays, to M. Bengston and J.M. Desmarchelier for advice on bioassay methods, and to F.H. Arthur and M.K. Nayak for comments on the manuscript. The trial was conducted with the cooperation of the Grainco Australia Ltd, residue analyses were supplied by the Queensland Department of Natural Resources, and Aventis Ltd provided an experimental formulation of diflubenzuron. This research was supported financially by the
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