- Fumigations with phosphine take time to work
- Silos used for fumigation need to be sealed to the Australian Standard
- Effective fumigations limit resistance development
Philip Burrill from the Queensland Department of Agriculture and Fisheries checks the pressure relief valve on a silo that meets the Australian Standard for gas tightness.
PHOTO: Queensland DAF
Most growers at some time will have used phosphine, sold as phostoxin or fumitoxin, to keep grain free of live insects as it is inexpensive, controls all pests (if used correctly) and is accepted by international markets.
However, some growers find themselves under pressure to prepare grain for sale quickly, tempting them to carry out a short fumigation.
Queensland Department of Agriculture and Fisheries (DAF) grain storage specialist Philip Burrill says he is often asked why fumigations with phosphine take so long. Scientists at Queensland DAF and the University of Queensland have the following explanation.
University of Queensland’s Associate Professor Paul Ebert says the direct toxicity of phosphine is relatively low. Phosphine kills insects by ‘tricking’ them into creating toxic compounds.
“In other words, phosphine forces the pests to create their own poison. The problem is, this process takes time, and that’s why fumigations take as long as they do,” Associate Professor Ebert says.
Research into the mode of action of phosphine, part-funded by the GRDC, has been underway for more than a decade and new DNA sequencing techniques have allowed researchers to identify the genes that confer resistance to the fumigant. The discovery of the resistance genes has provided insight into how phosphine kills.
Dr Ebert says understanding how the pests have become resistant to phosphine has allowed the research team to work backwards to conclude how the gas kills them.
Mode of action
Figure 1 Direct and indirect toxins.
SOURCE: Queensland DAF
Phosphine is a small molecule that makes its way into the cells of pests. Cells use oxygen to burn sugars for energy and phosphine disrupts this process.
Instead of producing energy, phosphine causes the cells to generate highly reactive and extremely toxic oxygen radicals.
The oxygen radicals damage other molecules in the cell, such as fats that become oxidised (rancid), which is extremely toxic. Phosphine makes the cells produce more oxygen radicals, which themselves trigger radical chain reactions in the cells as the fumigation progresses (Figure 1). The pest dies when damage becomes too great to sustain.
Resistant pests survive by slowing down the rate of toxin production and that means that phosphine can take much longer to work. Resistant insects can still be controlled with phosphine but the fumigation time required is much longer.
Question: Why do fumigations take a few days longer when grain is cold?
Answer: Chemical reactions are slower at cooler temperatures. Oxygen radicals are generated more slowly and radical chain reactions are also slowed. Decreased damage allows the insects to survive for longer. Fumigating in a gas-tight, sealed silo will extend the fumigation for complete pest control.
Most toxins, for example cyanide, have a direct effect on an organism. Cyanide binds directly to a protein that is critical for respiration and prevents it from working. This is the chemical equivalent of not allowing an insect to breathe. Cyanide kills quickly because an insect that is unable to breathe dies quickly.
However, phosphine is an indirect toxin. This means the gas itself does not cause death but it reacts with oxygen to form toxic by-products that cause death. Because phosphine is not consumed in the process it can act continuously, producing more toxins over time. In this way phosphine causes damage to build up like a toxic ‘snowball effect’. This is the power of indirect toxins. Small amounts can have great killing power if given a chance to work. For phosphine to be effective this means extending the fumigation time.
Resistant pests survive because the genes that cause phosphine resistance slow the rate of toxin production. As a result, phosphine in resistant insects takes a lot longer to work.
Increasing the concentration of gas will not result in resistant insects being reliably killed by short fumigations. However, a high enough concentration over a long fumigation will control resistant insects. This has been the case in the past when a label change led to an increase in the dose applied.
Dr Ebert says although it takes longer to kill resistant pests, phosphine attacks pests in such a way that it is very unlikely that pests will ever become completely immune.
“Phosphine will cease to be practical when the fumigation period needed to control pests exceeds industry logistical requirements,” he says.
Resistance to phosphine, although widespread geographically, is still infrequent among the beetles on a typical farm.
Although poor fumigation will kill some or most insects in a silo, when poor fumigations are repeated on the same parcel of infested grain, resistant insects become abundant and pest control is more difficult.
Phosphine is a small molecule and is roughly the same density as air. It is easy for phosphine to escape from an unsealed silo.
Mr Burrill says silo design and build quality are critical to obtaining a suitable seal to contain phosphine.
“Fortunately more silo manufacturers are recognising the importance of achieving a high level of gas tightness for fumigation,” he says. “There are several silos on the market that meet the Australian Standard for gas tightness (AS2628).”
Putting it together
Short fumigations are unlikely to kill resistant insects; so effective phosphine fumigations require a high concentration of gas to be maintained for the correct number of days. This requires a silo that meets the AS2628 standard.
Failing to use a gas-tight silo will mean the gas escapes (no matter how many tablets are used) before enough damage has been done to kill all of the pests. Mr Burrill says growers need to insist that any new silo used to fumigate insects meets the Australian Standard and this is noted in the sales contract.
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