Key points
– Strong resistance to phosphine in stored grain insects has both increased and spread over the past decade, with more detections in eastern Australia than Western Australia
– Misuse of phosphine, particularly in unsealed storages, appears to be the main reason for the development of strong resistance
Key points
– Successful pest control in storages depends on combining regular grain monitoring, good hygiene, aeration cooling and correct fumigation practices
– Fumigation results rely on a gas-tight sealable storage to achieve the required gas concentrations and exposure times (C x T) to kill all insect pest life stages
– In larger silos (150 to 2000 tonnes) recirculating gases using a small fan during the fumigation ensures rapid, uniform distribution of phosphine gas
Resistance monitoring of key insect pests has shown that combined grain protectants are a valuable integrated pest management tool for stored grain. Up to 30 per cent of freshly harvested grain is treated with contact insecticides – or grain protectants – before being placed into storage.
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Using an alternate fumigation chemical to break the development of insect pest resistance will help protect the efficacy of phosphine fumigation
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Resistant rusty grain beetles, one of the flat grain beetles, have been found throughout the grain supply chain, suggesting the need for adoption of area-wide integrated pest and resistance management.
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Australian growers have relied heavily on phosphine fumigation to disinfest stored grain and meet market expectations for high-quality grain with nil tolerance to insects. However, reliance on one chemical over a long period of time has led to resistance.
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The total time required for effective fumigation ranges from 10–17 days, accounting for the minimum exposure period, ventilation and withholding period. This highlights the importance of monitoring grain regularly and at least 17 days before out-loading to allow sufficient time to fumigate if required.
When determining how much phosphine to apply, it is important to treat the entire storage volume, regardless of how much grain is contained inside. For example, a 100 tonne silo full of grain requires 200 phosphine tablets. If that same 100t silo is only half full of grain, it still requires 200 phosphine tablets for effective fumigation.
Phosphine is a highly toxic gas with potentially fatal consequences if handled incorrectly. As a minimum requirement, the label directs the use of cotton overalls buttoned at the neck and wrist, eye protection, elbow-length PVC gloves and a breathing respirator with combined dust and gas cartridge.
Arrange the tablets where as much surface area as possible is exposed to air, so the gas can disperse freely throughout the grain stack. Spread phosphine tablets evenly across trays before hanging them in the head space or placing them level on the grain surface inside a gas-tight, sealed silo. Hang bag chains in the head space or roll out flat on the top of the grain so air can freely pass around them as the gas dissipates. Bottom-application facilities must have a passive or active air circulation system to carry the phosphine gas out of the confined space as it evolves. Without air movement, phosphine can reach explosive levels if left to evolve in a confined space.
To control pests at all life stages and prevent insect resistance, phosphine gas concentration needs to reach 300 parts per million (ppm) for seven days (when grain is above 25°C) or 200ppm for 10 days (between 15–25°C). Insect activity is slower in cooler grain temperatures so require longer exposer to the gas to receive a lethal dose.
Table 1 – Application rates for phosphine tablets in storage
Following fumigation, ventilate silos so grain can be delivered free from harmful gas residues. With tablet residue or bag chains removed, leave silos open for no less than five days, or no less than one day with aeration fans
operating. The final step is to hold grain for a further two days after ventilation before using for human consumption or stockfeed
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CAUTION: RESEARCH ON UNREGISTERED PESTICIDE USE
Any research with unregistered pesticides or of unregistered products reported in this document does not constitute a recommendation for that particular use by the authors or the authors’ organisations. All pesticide applications must accord with the currently registered label for that particular pesticide, crop, pest and region. Copyright © All material published in this Fact Sheet is copyright protected and may not be reproduced in any form without written permission from the GRDC.
This GRDC GroundCover feature brings together two articles exploring the benefits of aeration cooling in grain storage, including solar-powered aeration systems and research showing how cooler grain temperatures can reduce insect pressure and help slow phosphine resistance development.
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For more than 40 years Phosphine has been a safe and reliable means of controlling grain storage insects. Over 80% of stored grain is treated with this chemical but resistance to Phosphine is building amongst insect populations. So when a previously resistant strain of Rust Red Flour Beetle in Western Australia was eradicated by Phosphine it was a remarkable result.