Phosphine resistance one in ten

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Photo of a saw-toothed grain beetle

Phosphine-resistant insects: the saw-toothed grain beetle has developed some resistance to several grain insecticides.

PHOTO: Queensland DAF

Monitoring of stored-grain insects across Australia reveals that the number of populations with insects highly resistant to the fumigant phosphine has increased from about five per cent in 2010 to 10 per cent in 2015.

While the incidence is increasing, the monitoring program, which now has data for more than 30 years, has been crucial in extending the effective life of phosphine as a fumigant following the first discovery of strong resistance in 1997.

Overseas, the emergence of strong resistance has made phosphine ineffective for insect control in many areas. Principal entomologist with the Queensland Department of Agriculture and Fisheries (DAF) Dr Pat Collins says data is limited, but strong resistance to phosphine has been identified in 99 per cent of grain insect populations sampled in India.

In Vietnam and Brazil, strong resistance has been identified in the lesser grain borer in 97 per cent and 74 per cent of samples, respectively. Although there have been few formal surveys in the US, “resistance is easily found there”, Dr Collins says.

Australia is the only country to systematically monitor for and manage phosphine resistance. The program has been operating more than 30 years, and it has successfully provided an early warning system that has allowed the grains industry to develop and implement management strategies.

The strategy, developed through the National Working Party on Grain Protection, has a two-pronged approach:

  • to reduce selection pressure by limiting the number of fumigations to three per parcel of grain, where possible, and to use non-chemical alternatives such as cooling and good hygiene to remove insect populations from the system
  • to destroy resistant insects by making every fumigation count. This includes developing new fumigation protocols where necessary and monitoring gas concentrations during fumigation to ensure effectiveness.

For most of the past decade the stored-grain monitoring program, funded by the GRDC, has been coordinated through the Plant Biosecurity Cooperative Research Centre (CRC) and its predecessor the CRC for National Plant Biosecurity and led by Dr Manoj Nayak, principal research entomologist, also from Queensland DAF. Dr Nayak reported on the findings of monitoring for the 2014-15 season at the Australian Grain Storage and Protection Conference held earlier this year.

Of the 1150 insect populations tested (Figure 1), strong resistance was found in 116 populations, with the rusty grain beetle the most common problem in central storages and the lesser grain borer the most common problem on-farm.

In eastern Australia, Plant Biosecurity CRC project staff collect insects from farms, merchants and central storages, although some bulk handlers send their samples to the relevant laboratories, particularly if they suspect resistance.

In Western Australia, samples are collected from farms by government biosecurity inspectors and CBH sends all insects found for testing.

“Sampling by region allows us to identify trends and efficacy of treatments over time. Where we find strongly resistant populations we visit the property to advise on eradication strategies and later revisit and resample those properties to ensure the successful management of resistant pests,” Dr Nayak says.

For bulk grain handlers, provision of same-day advice on the resistance status of a pest population using a recently developed rapid resistance test has allowed them to employ higher phosphine concentrations or alternative treatments when strong resistance is confirmed.

Where rapid testing identified strong resistance to phosphine in the rusty grain beetle, one application of sulfuryl fluoride significantly reduced the incidence of resistance in the following two years.

Dr Nayak says emerging challenges include the potential for more widespread occurrence of strongly resistant rusty grain beetles on-farm, and the development of strong resistance in rice weevil populations in the southern grain-growing region.

Graphic showing the stored grain insect populations diagnosed with strong phosphine resistance

Figure 1 Stored grain insect populations diagnosed with strong phosphine resistance in on-farm and central storage, 2014-15.

SOURCE: Queensland DAF

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GRDC Project Code NPB00013 Plant Biosecurity CRC Code 3035

Region National, North, Overseas, West