WITH THE recent loss of registration of the grain protectant bioresmethrin (BRM), Australian growers must now rely on mixes of the insect growth regulator methoprene and organophosphate(OP) protectants to protect grain in unsealed storages.
However, there is a big hole in this shield, with populations of lesser grain borer resistant to methoprene.
Barry Wallbank, from NSW Agriculture in Wagga Wagga, told delegates at the recent Post-harvest. Technical Conference, "strong resistance to methoprene has arrived and is increasing fast.
"Continuous surveys in the last five years indicate that strong resistance is now present in about 25 per cent of samples taken in northern NSW. Most of the increase has occurred in the last two years. Frequencies of resistance have also increased in Queensland, but only to about half that detected in northern NSW. The intensity of the resistance in the strongest strains renders the protectant almost useless against this species," he said.
To make matters worse, rice weevil with strong resistance to chlorpyrifos-methyl has been identified at six sites in the same period, mostly in western and northern NSW. On the brighter side, no new detections have been made since 2000 and no resistance been found as yet in Victoria or South Australia.
What's it mean to growers?
There are significant implications for Australian growers who use protectants, since methoprene was particularly effective against borers and the saw-toothed grain beetle. Where there is high-level resistance to methoprene, treated grain will be subject to attack from both species, and grain is likely to become infested within three months. One option is to use phosphine - itself under attack from resistance - but this is effective only where sealed storage is available.
Apart from aeration cooling of grain, few options are left to growers with unsealed storages for complete control of all insects.
Methoprene is typically mixed with one of three OP grain protectants - chlorpyrifos-methyl (Reldan®), fenitrothion or pirimiphos-methyl (Actellic®). The OP is needed because methoprene is not effective in controlling rice weevil at any growth stage, and has no activity on adult growth stages of other insect pests. Resistance in rice weevil has not yet made its mark because this species is much less common in the drier-than-normal grain stored in recent years. The real test will come with a wet harvest.
Growers and the Queensland and NSW governments are investing in research that could provide options in the future. Researchers are searching for potential new grain treatments from chemicals with low human and environmental impacts.
One chemical, spinosad, produced from naturally occurring bacteria, has already been identified, with activity against all populations of the lesser grain borer, including methoprenehas resistant strains. The research is at an early stage and further work is needed to determine its long-term efficacy before its full potential as a grain protectant can be assessed.
Program 3 Contact: Dr Joanne Holloway, NSW Agric 02 6938 1605 Dr Pat Collins or Dr Greg Daglish. QDPI 07 3896 9433
North, South, West