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From page 39 Sufficient time is therefore needed for the fumigant to disperse evenly throughout the entire stockpile and infiltrate every kernel . Fumigation protocols take all these factors into account to ensure that when applied correctly , the fumigation will kill all life stages of the pests throughout the entire stockpile .
Undoubtedly , the best — if not only — way to ensure that the correct dose is applied for the right duration , is to conduct fumigations in gas-tight enclosures . Gases generally follow the path of least resistance . If there is a leak such as a hole in a tarpaulin or gaps in seals , the fumigant will readily flow through these outlets rather than penetrate the stockpile . This is particularly true for phosphine as it is so mobile . Gas-tight enclosures allow fumigations to be applied and monitored correctly , to safely and effectively disinfest the crop .
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Almond stockpile fumigation Fumigation of stockpiles with phosphine is currently the most common method for treating insects in harvested unprocessed almonds . Freshly harvested almonds are stockpiled on a cleared area of ground , and then covered with tarpaulins . Several tarpaulins are generally required for each stockpile . To minimise loss of gas , the tarpaulins are positioned with large overlaps and the edges weighted down , often with tyres ( Figure 1 ). Phosphine dispensers , in the form of blankets or chains of bags , are placed regularly along the top of the stockpile , under the tarpaulins . After the required fumigation time , the stockpile is aired by removing the tarpaulins and dispensers . The target dose and duration for effective fumigation is 300 ppm of phosphine for a minimum of seven days when the product temperature is above 25 ° C , or 200 ppm for ten days at 15-25 ° C . Phosphine is a relatively slow-acting toxin and increasing the dose will not shorten the time required to control all life stages of the pests .
Current fumigation practices are falling short We assessed the efficacy of several commercial almond stockpile fumigations by placing sensors strategically within the stockpiles to monitor phosphine concentrations . In all cases the fumigations were managed by the operators as per their usual procedure . None of the fumigations achieved the critical combination of fumigant dose and duration required for insect control ( Figure 2 ). Peak phosphine concentrations ranged from around 110-300 ppm and were maintained for only a few hours , not days . The period over which phosphine was detected within the stockpiles often fell short of the target , and in all fumigations the gas had completely disappeared several days prior to removal of the tarpaulins for airing . Underdosing of the fumigant may be part of the problem . Dosage appears to be determined by a standard ‘ rule of thumb ’, rather than by a calculation based on stockpile volume . Even if the correct amount of phosphine was not applied ( an easy factor to fix ), the problem of diminishing gas remains . Our initial laboratory trials indicate that whole almonds may absorb phosphine at a greater rate than wheat , which was used to develop generic fumigation protocols . If this is confirmed , new fumigation protocols for whole almonds will need to be developed .
Figure 2 . Typical example of phosphine concentrations from a commercial bunker fumigation of freshly harvested , unprocessed , whole nonpareil almonds , in 2021 . The dashed box indicates the dosage and duration required for effective fumigation .
Sealed storages for fumigation As mentioned previously , phosphine is a highly mobile gas that will escape through holes and leaky tarpaulin edges . Any such gas leakage would also result in underdosing . A sure way to avoid this problem is to fumigate almonds in purpose-built , gas-tight enclosures . There are many advantages to using such enclosures :
• Efficacious fumigations ( provided gas-tight seals are maintained )
• Gas concentrations maintained for required period
• No influence of winds on gas distribution
• Use of fans or thermosiphons to ensure fast and even distribution
• Fast ventilation of phosphine post-fumigation
• Ability to monitor fumigation concentrations more easily
• Use of forced aeration to maintain almond quality by maintaining even temperature and moisture levels throughout the bulk
• Product is sheltered from rain and extreme temperatures
• Reduced insect pressure as the bulk is not exposed to the environment and colonisation by dispersing insects .
Conclusion Almonds are a valuable crop , but that value can be swiftly eroded by pests along the entire chain of production . For post-harvest pests , management starts in the orchard and needs to be backed up by sound post-harvest fumigation practices that apply and maintain the correct dose for the required duration . Our monitoring has shown that a high proportion of stockpile fumigations do not meet the standards for effective disinfestation . This highlights the need for improvements in almond fumigation , such as a review of dose rates and investment in sealed , gas-tight storages which also offer additional benefits to maintain the quality of the almond crop .
38 In A Nutshell - Winter 2022 Vol 23 Issue 2