Performance of phosphine in fumigation of bagged paddy rice in indoor and outdoor stores

Phosphine fumigation trials were carried out on bag-stacks of paddy rice to study the differences in gas loss rates and concentration–time (Ct) products achieved during the treatment of indoor and outdoor stacks. Stacks (89–132 t) were fumigated singly under 250 μm thick polyethylene sheeting, which was sealed with a double layer of sand-snakes to the concrete floor. Phosphine was applied as an aluminium phosphide formulation and the fumigations continued for 7 days. In the first experiment, stacks of paddy rice with moisture contents ranging from 12.2 to 13.7% were held in either indoor or in outdoor storage and subjected to fumigation at the rate of 2, 3 or 4 g of phosphine/tonne. The outdoor stacks held relatively low levels of phosphine with Ct products for the indoor stacks of 135, 171 and 294 g h/m³, respectively, whilst the corresponding values for the outdoor stacks were 70, 85 and 166 g h/m³ only. The average gas loss rate was 14.5% per day for the indoor stacks and 29.5% for the outdoor stacks. In the second experiment, old stacks of paddy rice inside a godown, one each with grains at 8.8 and 9.8% moisture content, were fumigated at 3 g phosphine/tonne. Release of phosphine was delayed and fumigant sorption was less and therefore higher Ct products of 204 and 216 g h/m³ were achieved. In the stacks built outdoors, the resident infestations of Rhyzopertha dominica, Cryptolestes sp. and Oryzaephilus surinamensis were completely controlled despite lower Ct products. On the other hand, in the stacks of old paddy, R. dominica survived the treatment. Subsequent testing showed that the population had a degree of resistance to phosphine.     

Phosphine resistance in saw-toothed grain beetle,Oryzaephilus surinamensis in the United States

Phosphine (PH₃) fumigation resulting in sub-lethal exposure has led to the development of phosphine resistance in many stored-product insect species worldwide and is a major challenge to the continued effective use of phosphine. In 2016 phosphine resistance was found in Tribolium castaneum (Herbst) and Plodia interpunctella (Hübner) collected from California dried fruit and nut processing facilities. Although Oryzaephilus surinamensis (L.) infests grain, dried fruit, and nuts in storage and processing facilities, phosphine resistance in this species has not been studied in the United States. In this study, the discriminating dose of phosphine for O. surinamensis eggs was estimated using a laboratory susceptible strain; it was found to be 28.4 ppm over a 72-h fumigation period (1 mg/L of phosphine = 714.18 ppm or 1 ppm = 0.0014 mg/L). Discriminating dose bioassays were used to determine phosphine resistance in both eggs and adults of 14 different populations collected from California and Oklahoma. Resistance to phosphine was detected in four out of 14 populations in adults and nine out of 14 populations in eggs and ranged from 2 to 100%. Phosphine percent survival values in both adults and eggs of three populations, namely, Box BR, Box BF, and OKWat were >90%. Lethal concentration values required to kill 99% of individuals in samples for adults of these three populations were predicted as 320.5, 290.7, and 263 ppm, respectively, and those for eggs were 1030.7, 1055.9, and 564.5 ppm, respectively, over a 72-h fumigation period. This study confirms that phosphine resistance is present in O. surinamensis in the United States.     

高大平房仓磷化氢熏蒸及散气作业过程安全防护距离研究

近年来由于熏蒸散气而产生的仓内安全事故引起了行业乃至社会各方的高度关注,但熏蒸散气阶段的作业安全研究较少,相关隐患尚未被深入研究以及重视。在实仓实验中,对高大平房仓磷化氢熏蒸和散气过程仓房内外环境空气中磷化氢浓度进行现场检测,探讨熏蒸、散气作业过程的有效安全防护距离以及防护措施,研究结果为防范熏蒸作业安全事故、减少或避免发生人身伤害和经济损失提供依据。研究结果表明,安全防护距离与仓内(粮堆内)磷化氢气体浓度大小有关,不能完全以固定的安全防护距离作为判定是否安全的依据,接近熏蒸散气区域前必须检测磷化氢浓度。     

Modeling post-fumigation desorption of phosphine in bulk stored grain

Phosphine is a dangerous gas commonly used in fumigations of stored grains throughout the world. Grain that has not fully released the phosphine it absorbed during fumigation may continue to desorb phosphine into the headspace of a shipping container or storage. USOSHA standards for handling phosphine state the acceptable Threshold Limit Value (TLV) of 0.3 ppm. In many cases, during grain transport and handling, the level of phosphine in work environments can exceed the 0.3 ppm TLV which resulted from desorption of phosphine from fumigated grains. It is also important to note that desorbing grain, with low concentrations of phosphine, can facilitate insect resistance. Therefore, it is important to understand the process of phosphine venting and desorption in order to ensure safe handling of fumigated grain. In order to achieve this, the venting and release of phosphine was studied on location in a well-sealed grain silo in Lake Grace, Western Australia. The data set served for verification of a 3D finite element ecosystem model and were compared to the predicted results. Results were calculated using two different fumigant desorption models based on previous literature, i.e., a reversed sorption model and an air-grain equilibrium model. Simulations reproduced accurate trends of desorption but did not accurately reproduce the quantity of fumigant, with 55.5% error for the model based on reversed sorption equations, and 86.3% error for the air-grain equilibrium based model. For both models, additional simulations were conducted to compare the effectiveness of existing grain venting regulations at producing grain that is within post-fumigation safe handling limits. Results revealed that current U.S. standards may be insufficient to guarantee safety based on minimum venting times needed. These results highlight the necessity for continued research into phosphine desorption and the importance for establishing realistic post-fumigation venting guidelines.     

Phosphine fumigation of silo bags

Fumigation with phosphine has the potential to disinfest grain stored in silo bags but only limited research has been conducted on whether phosphine fumigation can be undertaken effectively and safely in this form of storage. Fumigation with phosphine was tested on two (70 m) replicate silo bags each containing 240 t of wheat (9.9 and 9.2% m.c.). The target application rate of phosphine was 1.5 g m⁻³ with a fumigation period of 17 days. Aluminium phosphide tablets were inserted into each bag at ten release points spaced at 7 m intervals starting 3.5 m from either end of the bag. A total of 14 bioassay cages containing mixed age populations of strongly phosphine resistant Rhyzopertha dominica (F.) were inserted into each fumigated silo bag. Complete control of all life stages of R. dominica was achieved at all locations in the fumigated silo bags. Phosphine concentrations at release points increased rapidly and remained high for the duration of the fumigation. Concentrations at midway points were always lower than at the release points but exceeded 215 ppm for ten days. The diffusion coefficient of available phosphine averaged over the first three full days of the fumigation for both fumigated silo bags was 2.8 × 10⁻⁷. Venting the silo bag with an aeration fan reduced the phosphine concentration by 99% after 12 h. Relatively small amounts of phosphine continued to desorb after the venting period. Although grain temperature at the core of the silo bags remained stable at 29 °C for 17 days, grain at the surface of the silo bags fluctuated daily with a mean of 29 °C. The results demonstrate that silo bags can be fumigated with phosphine for complete control of infestations of strongly phosphine resistant R. dominica and potentially other species.     

高大平房仓不同熏蒸方式浓度分布研究

本研究通过膜下环流熏蒸、氮气气调、氮气与磷化氢混合熏蒸三种熏蒸方式,来分析在高大平方仓中这三种方式施药后,熏蒸气体的空间浓度分布情况。结果表明,覆膜环流熏蒸仓房底部磷化氢浓度在第3 d达到最大,较常规熏蒸快2～4 d。氮气和磷化氢混合熏蒸在环流6 h后各层粮堆内气体分布基本达到均匀,整个熏蒸过程中磷化氢最低浓度与最高浓度的比值范围为0.37～0.67,氮气浓度始终保持在83%～87%之间。混合熏蒸与膜下环流熏蒸相比,可以减少用药量10.8%;同时,解决了氮气气调对气密性要求严苛以及运营成本高的难题,供粮食仓储企业在实际熏蒸杀虫工作中参考。     

Loss of phosphine from unsealed bins of wheat at six combinations of grain temperature and grain moisture content

Hard red winter wheat (1.4 t) at 11.1 or 13.5% moisture content (wet basis) and 20, 25, or 30 degrees C was fumigated with tablets of an aluminum phosphide formulation in unsealed, cylindrical grain bins of corrugated metal. The fumigant leakage rate was manipulated to approximate that commonly encountered in farm and commercial-scale bins of this type. Phosphine concentration profiles were recorded and phosphine loss and sorption were characterized to determine which conditions provided the greatest probability of successful fumigation in these bins. Phosphine leakage and sorption were both positively related to grain temperature and moisture content. The fumigant concentration profiles were compared with previously-published data relating temperature to the developmental rate and fumigant susceptibility of lesser grain borer eggs, which are phosphine-resistant but become less resistant as they age. The mean phosphine concentration observed at the time corresponding to one-half of the calculated egg development time was compared to the lethal concentration (LC(99)) for a 2-day exposure at each temperature-moisture combination. In the low-moisture grain at 20 degrees C, the observed fumigant concentration was below the lethal concentration, due to the long development time under these conditions. At 25 and 30 degrees C in the low-moisture wheat, the likelihood of complete kill appeared more favorable because the fumigant concentration remained above the published LC(99) for more than half of the egg development time. In the wheat with 13.5% moisture content, rapid fumigant sorption and loss resulted in phosphine concentrations below the LC(99) at one-half of the development time at 20 or 25 degrees C. At 30 degrees C, due to the very rapid development rate, the observed phosphine concentration exceeded the LC(99) half-way through the egg development period despite the rapid rate of fumigant sorption and loss. Repeated fumigation of the same grain reduced the rate at which phosphine sorbed into the grain.     

储烟害虫防治过程中减少磷化氢使用量和排放量的措施

为了减少烟草仓贮害虫防治过程中磷化氢的使用量和向大气的直接排放量,考察了不同清洁隔离措施对仓内虫口数量的影响,开展了不同温度条件下低剂量磷化铝熏蒸试验及熏蒸余气磷化氢的化学吸收处理试验。结果发现:1)隔离仓间内的虫口数量远低于普通仓间;2)在夏季较高温度条件下,磷化铝熏蒸的用药量可以由常规的6.0 g/m3左右降低至4.0 g/m3左右;3)饱和漂粉精溶液能有效吸收磷化氢,但在大规模试验中难以实现一次性的彻底净化。研究表明,减少磷化氢的使用量和排放量,应当从加强清洁隔离、适当降低磷化铝熏蒸的单位体积用药量、对熏蒸余气进行净化处理等三个方面采取综合措施。     

PERSISTENCE OF PHOSPHINE GAS IN FUMIGATED IRAQI DATES

Phosphine gas (PH₃) is a widely used fumigant for stored fruits and grains. Previous studies indicated that all residues of this gas are eliminated by normal aeration but the exact time has never been determined.
The objective of this work was to determine the time required for desorption of phosphine from fumigated dates relative to aeration time and temperature. Flame photometric detection by gas chromatography was used for the detection of sorbed phosphine.
Results indicate that the initial residue level of PH₃ falls rapidly within 24 h after fumigation but residues persist for at least nine days. Higher residue levels were found in dates stored at low temperatures (4°C) than at 28°C. It is recommend that fumigated dates be marketed after longer periods of aeration, especially those aerated at temperatures of 4°C or less.     

Evaluation of dosimeter tubes for monitoring phosphine fumigations

Within integrated pest management options, fumigation of stored products is one method to help control post-harvest insect infestations in our food and agricultural products. Fumigant gas concentration monitoring is important to confirm that the treatment was adequate to achieve the desired insect control, but monitoring can be relatively expensive and labor intensive. This study evaluated how accurately dosimeter tubes could monitor phosphine fumigation treatments. The dosimeter tube is designed to continuously react with phosphine gas during the fumigation period and yields a measurement in terms of concentration 1 time product or CT, which can be interpreted as cumulative exposure. Two models of dosimeter tubes were evaluated (high range and low range). The reference method for these trials were wireless phosphine monitoring sensors, which recorded gas concentrations at hourly intervals during an exposure, and from this a CT product was also calculated. Model LPG-1, high-range dosimeter tube, measured within ± 25% of the phosphine monitoring sensors for CT dosages less the 70,000 ppm1hr. Model LPG-2, low-range tube, tended to significantly over-estimate phosphine CT dosage by 50%–100% of the phosphine monitoring sensor references. Secondly, bioassays of fumigant efficacy were performed using susceptible and resistant adult Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae), lesser grain borers, and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), red flour beetle, for estimating insect control at the varied fumigation CT treatments. For the susceptible strains, CT dosages ∼5000 ppm1hr controlled both species. However, the insect control varied from 60% to 100% for resistant adults at CT dosages of ∼20,000 ppm1hr. The dosimeter tubes function in these ranges of dosages where each insect species are controlled and the dosimeter tube model LPG-1 provides reasonable estimates of the fumigation dosage for a given treatment level.     

Developing and verifying a fumigant loss model for bulk stored grain to predict phosphine concentrations by taking into account fumigant leakage and sorption

To ensure fumigation effectiveness and address phosphine resistance concerns, fumigant concentrations and movement in a grain storage silo need to be understood. A mathematically accurate fumigation model was developed that is capable of predicting fumigant concentration and movement throughout a grain storage silo that takes into account fumigant loss from leakage and sorption, and was verified with experimental fumigation data. Equations estimating fumigant leakage and sorption were developed based on literature values and added to an existing finite element model. Fumigation data was used from a fumigation conducted on an Australian made silo filled with 45.5 tonnes of maize in Manhattan, Kansas. Two verifications were conducted based on phosphine concentration release times of 24 h and 30 h, with both verifications demonstrating accurate prediction of phosphine fumigant values and trends. The two verifications resulted in concentration-time products that were within 0.9% and 4.3% of the experimental values, respectively. The fumigation model is most accurate during the times of highest phosphine concentration. However, the model under predicted phosphine concentrations during the first 12 h of fumigation and over predicted phosphine concentrations beyond the first six days of fumigation. This fumigation model was found to be sufficiently accurate to allow for future experimentation on predicting fumigant concentrations as a function of environmental conditions and operational variable.     

Reproduction of phosphine resistant Rhyzopertha dominica (F.) following sublethal exposure to phosphine

Phosphine fumigation is commonly used to disinfest grain of insect pests. In fumigations which allow insect survival the question of whether sublethal exposure to phosphine affects reproduction is important for predicting population recovery and the spread of resistance. Two laboratory experiments addressed this question using strongly phosphine resistant lesser grain borer, Rhyzopertha dominica (F.). Offspring production was examined in individual females which had been allowed to mate before being fumigated for 48 h at 0.25 mg L^?1. Surviving females produced offspring but at a reduced rate during a two-week period post fumigation compared to unfumigated controls. Cumulative fecundity of fumigated females from 4 weeks of oviposition post fumigation was 25% lower than the cumulative fecundity of unfumigated females. Mating potential post fumigation was examined when virgin adults (either or both sexes) were fumigated individually (48 h at 0.25 mg L^?1) and the survivors were allowed to mate and reproduce in wheat. All mating combinations produced offspring but production in the first week post fumigation was significantly suppressed compared to the unfumigated controls. Offspring suppression was greatest when both sexes were exposed to phosphine followed by the pairing of fumigated females with unfumigated males and the least suppression was observed when males only were fumigated. Cumulative fecundity from 4 weeks oviposition post fumigation of fumigated females paired with fumigated males was 17% lower than the fecundity of unfumigated adult pairings. Both of these experiments confirmed that sublethal exposure to phosphine can reduce fecundity in R. dominica.     

A continuous, cumulative procedure for analysis of phosphine

A modification of the White-Bushey method of phosphine analysis is described, designed for study of the release characteristics of phosphine-generating fumigant formulations and their residues. Phosphine was estimated indirectly from the acid produced when the gas, entrained in nitrogen, was absorbed in mercuric chloride solution. The acid production was measured, continuously and cumulatively, by titration using a pH-stat to maintain pH 3.0. The low pH minimised the pH-dependent reactions that can lead to erroneous phosphine estimations and enabled the analysis to be carried out at room temperature. The method was found to be accurate within 1%, when checked against known quantities of phosphine, and with a standard deviation of ± 1.4% as found by comparison with indirect analysis of phosphine by oxidation of the phosphine-mercuric chloride reaction product with bromine and subsequent colorimetric estimation of the phosphate produced. The presence of ethanol, normally used for preference in the aqueous absorbing solution, was found to interfere with the colorimetric estimation. Examples are given of the use of the titrimetric system to study release of phosphine from a fumigant formulation and a formulation residue.     

Developmental and population growth rates of phosphine-resistant and -susceptible populations of stored-product insect pests

Phosphine resistance positively contributes towards an individual's fitness under phosphine fumigation. However, phosphine resistance may place resistant individuals at a fitness disadvantage in the absence of this fumigant, which can be exploited to halt or slow down the spread of resistance. This study aimed to determine if there is a fitness cost associated with phosphine resistance in populations of the red flour beetle (Tribolium castaneum (Herbst)), the lesser grain borer (Rhyzopertha dominica (F.)) and the sawtoothed grain beetle (Oryzaephilus surinamensis (L.)). The developmental rate and population growth of phosphine-resistant and -susceptible populations of these three species of stored-product insects were therefore determined under phosphine-free environment. The majority of the phosphine-resistant populations exhibited lower developmental and population growth rates than the susceptible populations indicating that phosphine resistance is associated with fitness cost in all three species, which can potentially compromise the fixation and dispersal of the resistant genotypes. Nonetheless, some phosphine-resistant populations did not show a fitness cost. Therefore, resistance management strategies based on suppression of phosphine use aiming at eventual reestablishment of phosphine susceptibility and subsequent reintroduction of this fumigant will be useful only for insect populations exhibiting a fitness cost associated with phosphine resistance. Therefore recognition of the prevailing phosphine-resistant genotypes in a region is important to direct the management tactics to be adopted.     

Using immobilization as a quick diagnostic indicator for resistance to phosphine

In the present study, we evaluated a quick diagnostic test for resistance to phosphine in stored-product beetle species. We collected different populations of thirteen species, obtained from different laboratories in different counties, i.e., USA, Greece, Australia, Germany and Spain. There were also tested populations that have been sampled from different facilities (field populations). We used the Detia Degesch Phosphine Tolerance Test Kit (DDPTTK), which is based on the exposure of the insects on a high concentration of phosphine for shorter exposure periods. The tested concentrations to phosphine were 1000 and 3000 ppm. Briefly, 20 adults of the tested populations were placed in a 100 ml plastic syringe. The observations were taken every 2 min and the exposed adults were classified as either walking normally or being immobilized (knocked down), i.e., not walking normally. In light of our findings, the time to reach knockdown of all adults was notably reduced at 3000 ppm in comparison with 1000 ppm. For the majority of the species and laboratory populations tested, at 3000 ppm, the time required for knockdown ranged between 8 and 14 min. In contrast, for some of the field populations, knockdown did not reach 100% even after 300 min of exposure, at either 1000 or 3000 ppm. Based on the results of the present study, we recommend that the DDPTTK can be operated at 3000 ppm, and we provide the critical threshold times per species for the characterization of tolerance/resistance.     

不同施药方式硫酰氟熏蒸高大平房仓效果比较研究（网络首发、推荐阅读）

通过比较分析3种不同实仓施药方式硫酰氟熏蒸高大平房仓的穿透力、均匀性和杀虫效果,探索硫酰氟在实仓熏蒸的最佳工艺。结果表明,硫酰氟气体可在2.5 h内自然扩散穿透粮堆,24 h左右达到整仓基本均匀。环流熏蒸可以加速气体均匀,但此次实验环流过程中导致了一定药剂损失。在整仓熏蒸、膜下上行环流和膜下下行环流的3种熏蒸方式中,整仓熏蒸操作最为简便,可用于快速杀灭成虫;膜下上行环流方式,药剂损耗量较大;膜下下行环流,浓度均匀速度快,用药量较整仓小,药剂浓度保持时间长,杀虫效果好。     

Optimising indoor phosphine fumigation of paddy rice bag-stacks under sheeting for control of resistant insects

Three indoor, sheeted bag-stack fumigations of paddy rice using aluminium phosphide were undertaken in Guangdong Province, southern China. We measured the effect of two types of sheeting (polyvinylchloride [PVC] or polyethylene [PE]) and two types of floor sealing (clips or fixing into a slot with a rubber pipe) on phosphine concentration and retention. The aim was to test the feasibility of retaining fumigant at a sufficient concentration for long enough to control known resistant insect pests. Each stack was pressure tested and phosphine concentrations measured daily during the fumigation. Cages of test insects in culture medium, including resistant and susceptible strains, were placed inside each stack and could be observed through the clear sheeting. Highest concentrations for the longest period were obtained in a PVC-covered stack that included a ground sheet and wall sheets sealed to the floor with rubber pipes. A similar PVC-covered stack sealed to the floor with clips instead of pipe did not retain gas as efficiently and required re-dosing. A PE-covered stack, with no ground sheet but also with wall sheets sealed to the floor with pipe, produced an acceptable fumigation. Susceptible Rhyzopertha dominica were controlled in 2 days and the most resistant strain in 15 days. Resistant Cryptolestes ferrugineus survived until day 21. The paddy was still free of insect infestation 7 months later when the bag-stack was opened to mill the rice. Pressure half-lives correlated with gas concentration and retention. Sorption appeared to be a major limiting factor, reducing potential fumigant dosage by about 50%. The trials demonstrated the feasibility of sealing bag-stacks to a standard high enough to control all known resistant strains.     

Evaluation of the Factors Affecting the Phosphine Residue in the Fumigation–Aeration Process of Stored Wheat Using Orthogonal Array Design

Factors that affect the phosphine residue in the fumigation–aeration process were studied. Simulated experiments were carried out on the samples in nine different conditions (nine treatments using L₉ orthogonal array on four factors: amount of aluminum phosphide, fumigation time, aeration time, and fumigation temperature). The extracted phosphine residue of grains was quantified by UV/visible spectrophotometry at 410 nm. The results revealed that the most important factors are the aeration and fumigation time. The fumigation temperature has negligible effect on the phosphine residue, and there is no significant effect from the amount of aluminum phosphide. Under optimum conditions, the phosphine residue content was 0.0067 mg/kg of wheat grain, which is very less than the reported amounts by related organizations.     

浅埋大型房式仓磷化氢环流熏蒸试验研究

根据我库粮食储量大,土地紧张等特点,2002年由郑州院负责设计完成6栋浅埋大型房式仓(单仓存量8600t),当年设计、施工、当年入粮。为确保储粮安全,采取环流熏蒸及常规熏蒸两种熏蒸方法进行对比试验,通过对所测数据分析、研究,提出对该种仓型采取磷化氢环流熏蒸杀虫的方法,取得了良好的杀虫效果。     

Real time monitoring of phosphine and insect mortality in different storage facilities

In this study, we evaluated wireless phosphine sensors to quantify and depict spatio-temporal dynamics and distribution of this gas within different types of facilities and commodities. The use of wireless sensors has certain advantages over the use of traditional monitoring techniques (e.g. tubes etc.), as any measurements with these traditional techniques correspond to the specific time and location of monitoring and are not transferable to additional intervals and locations, which leads fumigators to either overestimate or underestimate the concentrations and outcomes of a given fumigation. In fact, in light of our findings, the distribution of phosphine in large warehouses was not usually adequate for a satisfactory level of insect control, and gas concentrations varied remarkably through time and space. In contrast, commercial treatments at containers were sufficient to control the insects tested, even on stored-product insects which were found to be resistant to phosphine. Furthermore, in the case of silos and ship holds, our work indicated that the use of forced recirculation systems for phosphine is essential to increase concentration and, as a result, insect mortality. Overall, our tests clearly suggested that the sensors were very effective in measuring phosphine and are generally expected to play important role in the near future in IPM-based programs at the post-harvest stages of agricultural commodities. At the same time, real-time monitoring can be used with success for the prediction of insect mortality in the treated facilities.