Secure Your Hive Supply This Almond Season
RESEARCH & EVENTS
So, can the absorption be overcome by increasing the phosphine concentration? This is where the story becomes slightly more complicated. Firstly, rates of absorption differ with almond variety. Of the three varieties tested, Nonpareil were the lowest absorbers and Carina the highest( Figure 1). This could be overcome by basing rates on the highest known absorbing variety. However, since Nonpareil is most at risk from insect damage and fumigated most often, it may be possible to calculate a rate based on this variety. Determining a new phosphine rate is complicated further because the rate of absorption increases with higher rates( Figure 1). Field trials using double the label rate have yielded mixed results. In one trial the phosphine concentrations throughout the stockpile fell below the required level in under 48 hours and were at 0 ppm by day four. However, this phosphine loss was exacerbated by leakage as rainfall during the fumigation permeated the stockpile. In the most effective field trial, phosphine concentrations well above 300 ppm were achieved in one section of the stockpile for the duration of the fumigation. However, this was not replicated throughout the rest of the stockpile. This implies that factors other than phosphine dose, such as climatic conditions, also play a role in fumigation success. Laboratory studies suggest there is a phosphine concentration where a maximum rate of absorption is obtained( Figure 2). This commenced around 1800 ppm and equated to around four times the current label rate. The higher the dose, the longer the phosphine concentration remained stable. The volume of nuts also appears to be a factor, with a lower volume of nuts increasing the duration of stability. Consequently, this may suggest that it is only possible to maintain phosphine concentrations in stockpiles containing small volumes of nuts with an extremely high dose of phosphine. An issue that needs to be considered regarding the delivery of a higher phosphine rate concerns worker safety. Safe Work Australia is reducing the exposure limits for phosphine. Time weighted average will be reduced from 0.3 ppm to 0.05 ppm, while the short-term exposure limit will drop from 1 ppm to 0.15 ppm. These changes will come into effect on 1 December 2026. By increasing the phosphine rate, the risk of worker exposure also increases, and it will be a critical factor for consideration of any label rate changes.
Conclusion: Based on work within the laboratory it appears that phosphine label rates need to be at least four times the current label dose to maintain effective concentrations for seven days. This could be confirmed with field trials, however with changes to the worker safety limits and risks of exposure, it is unlikely that this rate would be approved by regulatory bodies. Consequently, it is recommended that phosphine fumigations are fully monitored and topped up as required when phosphine concentrations drop below 400 ppm. This will ensure that all life stages of any insect pests present in the stockpile are fully controlled.
Secure Your Hive Supply This Almond Season
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