Red alert on rusty beetle resistance
From minor pest to high-risk species, strong resistance to phosphine has propelled the rusty grain beetle to the top of the stored grain research priorities
Not so long ago the rusty grain beetle (Cryptolestes ferruginaeus) was just one of three flat grain beetles commonly found in stored grain. Considered to be a relatively minor pest, little effort was made to distinguish it from similar Cryptolestes species.
However, that was before it developed strong resistance to the most commonly used fumigant of stored grain, phosphine. Strong resistance in the rusty grain beetle is more than double that of strong resistance in any other stored-grain insect.
Researcher Dr Manoj Nayak says that under the current label rates, phosphine applied at 720 parts per million, at 25ºC, will kill all other species of strongly resistant stored-grain insects within eight days.
However, in laboratory trials it takes 18 days (nearly double the current highest label rate at this dose) to kill all strongly resistant (SR) rusty grain beetle (Figure 1). Label changes would be required before phosphine could be used at this rate.
Moreover, it would be difficult to maintain temperatures and the required gas concentrations for such an extended time, in addition to logistical difficulties of tying up storages and grain for long periods.
Dr Nayak, who works with the Queensland Department of Agriculture and Fisheries (DAF), has been leading research on phosphine resistance for the past nine years for the Plant Biosecurity Cooperative Research Centre (CRC) and the GRDC.
He also leads the national phosphine-resistance monitoring project.
Monitoring the issue
Dr Nayak says although the rusty grain beetle is found in all grain-growing regions, it is far less prevalent than the lesser grain borer
(Rhyzopertha dominica) or the rust-red flour beetle (Tribolium castaneum). Strong resistance on-farm remains rare.
However, it is a significant concern for central grain storages in the grainbelt of eastern Australia.
In 2014-15, SR populations were identified at three farms in the northern region and three in the southern region. The farms affected were able to use alternative treatments – usually a contact protectant such as chlorpyrifos-methyl (Reldan®) and follow-up surveys found the resistant populations had been eradicated.
In 2014-15 there were also six SR populations identified in northern region central storages and 53 in southern region central storages.
The only effective alternative fumigant available for export-bound grain is sulfuryl fluoride, following the phase out of methyl bromide. Dr Nayak says although contact protectants are effective, residue issues limit the potential markets for the grain.
Sulfuryl fluoride was first used as a ‘resistance breaker’ in central storages in 2010, and resistant insect populations declined in the following two years. As populations rebuilt it was again deployed in 2013.
“We are urging bulk handlers to use sulfuryl fluoride only when they really need to,” Dr Nayak says. “So far there is no cross resistance between phosphine and sulfuryl fluoride, and we need to prevent this from developing for as long as possible, as there are no other options at this point.”
Sulfuryl fluoride is also not recommended for on-farm use. If resistance is occurring as a result of selection from poor phosphine fumigations, the same issues are likely to apply to sulfuryl fluoride fumigations. Dr Nayak says sulfuryl fluoride is also expensive – as high as $8 per tonne of grain to be treated. Phosphine costs up to $0.35/t.
Testing new protocols
The cost comparison highlights the importance of maintaining phosphine as an effective control tool. Dr Nayak is working on new protocols for phosphine to be applied specifically where the SR rusty grain beetles are found. The new protocols have tested phosphine at rates of 1080 and 1440ppm at 25ºC to 29ºC, which killed all stages of SR rusty grain beetles in 10 and six days, respectively. Field validation of these rates and proposed label changes is still underway.
Since strong resistance was first discovered in the rusty grain beetle in 2006 the industry has been on high alert to learn as much as possible about the insect. The presence of resistant insects is usually first discovered when fumigation fails.
Molecular tests that can identify resistance in dead insects have been developed for the lesser grain borer and the rust-red flour beetle, but are still being developed for the rusty grain beetle. A quick test is available for live insects, which can diagnose strong resistance in 5.5 hours and provide storage operators with same-day advice on the resistance status of a pest population. This allows bulk handlers to adjust their grain treatment strategies.
A new lure has also recently been developed by Queensland-based company Research Directions, which has synthesised the aggregation pheromones that attract the rusty grain beetle. While Canadian researchers identified the pheromones in the 1970s, manufacturing them has previously been too difficult and expensive. This work was GRDC-funded through the Plant Biosecurity CRC to help to more effectively identify the presence of the insects. Bulk handlers currently rely on sieving grain to find insects.
The lures are available for research and commercial use for the first time this year, following successful field trials in Australia assisted by GrainCorp. Kansas State University, a partner in the Plant Biosecurity CRC, is also conducting field trials with the lure in the US.
Know your enemy
Meanwhile, Queensland DAF entomologist Dr Greg Daglish has been undertaking ecological research to learn whether the rusty grain beetle is limited to storages, how far they travel and how to slow or prevent infestations. This GRDC-supported work through the Plant Biosecurity CRC is in collaboration with scientists from the University of Queensland and the NSW and Western Australian governments.
The research team has identified a strong pattern of flight in the insects once daytime temperatures reach 20ºC. Late afternoon and early evening are the periods of greatest movement. And despite their tiny size, at only two millimetres, the insects are also capable of travelling considerable distances to infest clean grain bulks – up to two kilometres from the original point of infestation.
The insects are long-lived – up to six months – and fecund. Dr Daglish says the newly developed lures were used to trap beetles in a trial in Queensland, and the insects were then raised in optimal conditions to test their longevity. The lures trapped equal numbers of male and female adult beetles, and all females had been fertilised.
All insects survived for 33 days, and more than half survived for 118 days after collection. Females continued to produce offspring for up to 89 days without mating again.
Dr Daglish says the fact that the lure was attracting young, fertilised and fecund insects suggests it has potential to form part of an integrated control strategy. Every insect trapped reduces the potential population in grain facilities.
A study of the insect population genetics has identified two distinct genetic lineages within the rusty grain beetle species. He says at a molecular level the distinction was almost significant enough to consider them different species, but the two groups do interbreed, which simplifies identification and control strategies.
The distribution of DNA markers in insects sampled from grain-growing regions in all states also indicates that rusty grain beetle populations are moving and interbreeding across the landscape, potentially spreading resistance genes as they do so.
Dr Daglish says although it was not possible to screen for the resistance genes themselves, the gene flow indicated more widespread beetle movement than had previously been considered. The study also shows there are separate populations between eastern and western Australia; in other pest species gene flow can occur between eastern and western populations.
Dr Daglish says the spreading of resistance appears to be more than a matter of local selection based on unsuccessful fumigations and appears to be assisted on a larger scale by more general human movement and the insects’ own capacity to travel.
“This means that what growers do on their own property in terms of stored-grain insect control has broader implications across districts and regions that we haven’t really considered previously,” he says.
More information:Dr Manoj Nayak
0421 225 906
Dr Greg Daglish
07 3255 4438,
GRDC Project Code NPB00013, PBCRC3035, PBCRC3036, PBCRC3039, PBCRC5073
Region North, Overseas