Physical, Chemical and Biological Behaviours of Fumigants

Amy Xiao

The annual loss of grain, beans, and oil crops caused by insects in stored products is about 5–10% of the total reserves, and it is as high as 40% in some developing countries. Insect pests of stored cereals not only damage and consume the seed itself but also contaminate seed with live insects (eggs, larvae and adults), insect fragments, faeces, webs, metabolic products, and enable invasion by fungi and other microflora, thereby reducing commercial value. The three main insect pests of stored cereals in Australia are the beetles Rhyzopertha dominica (F.), Sitophilus oryzae (L.) and Tribolium castaneum Herbst, presumably because these species can survive on the generally dry grain under arid conditions. Fumigation is the only feasible method for controlling all developmental stages of the main insect pest species in granaries, warehouses, and other large-scale grain-storage structures. Currently, phosphine (PH3) is the only fumigant accepted by many of Australia’s international markets for insect disinfestation of grains and oilseeds. Concerns about the health implications of fumigant phosphine for non-target species, including humans, means quantification of fumigant residues in and on cereal grains is critically important. Three approaches (1) solvent extraction of whole seed, (2) grinding the seed followed by solvent extraction, and (3) microwave irradiation to quantification of phosphine residues on canola seed and cottonseed were compared. Recoveries of phosphine from fortified whole canola and cottonseed samples were 94.8±5.7% to 96.3±5.5% for the grinding plus solvent method and 95.8±6.1% to 96.4±7.1% for the microwave method. Limits of quantification were < 0.001 ng/g. Multi fumigants were evaluated for protection of cotton seed in storage and pre-shipment from insect pests and/or microorganisms. Cottonseed strongly absorbs carbon disulfide (CS2), ethanedinitrile (C2N2), ethyl formate (EF), methyl bromide (MB) and PH3 (>85% disappeared within 5 hours of exposure), but carbonyl sulphide (COS) still maintained at >20 mg/L (<60% disappeared) during 24 h exposure. After 1 day of aeration, 75-85% of the absorbed COS and MB and 20-40% of the absorbed CS2, EF and PH3 were released from treated cottonseed. The fumigant residues were reduced by 80% for COS, 50% for EF or MB and 25% for CS2 after 1 day aeration. Therefore, from occupational health and safety point, and, environment and consumer safety, MB should not allow to use for the QPS on cottonseed. However, COS dissipated quickly and does not negatively impact germination, it has potential as a fumigant for control of insect pests in cottonseed. For protection of seed in store from insect pests, a semi sealed seed store loaded with newly harvested seeds of barley, wheat, oats and canola, was treated with liquid EF + nitrogen achieve a relatively constant EF concentration of about 16-28 g/m3 for slightly more than two days. All tested mixed aged (egg, larva, Papua and adult) insects of Oryzaephilus surinamensis, T. castaneum, Cryptolestes ferrugineus, R. dominica, S. oryzae and Psocids were controlled. Ethyl formate had no effect on the germination and plumule length of the stored barley, wheat and oats and canola seeds. Therefore, ethyl formate has promised an ideal fumigant for management of seed store insect pests. For protection of stored grain, management of phosphine resistance and development of environmentally friendly fumigant, ozone was tested for all life stages of Rhyzopertha dominica (lesser grain borer) and Tribolium castaneum (red flour beetle) in wheat were completely killed at 700ppm ozone and 25±1C and 55-60% r.h. for 16 h treatment. Ozone can penetrate and evenly distribute through bulk wheat silo in short purging period with vacuum sucking technology. There was no effect of vigour and germination, soluble protein, gluten, moisture content and hardness of wheat. Two new chemical compounds nonanal and nonanoic acid were generated after ozone treatment, which are naturally produced in plant and contribute ross, coconut and fresh grain odour. Therefore, ozone can be an environmentally friendly alternative fumigant to offer fast (one day) fumigation and management of phosphine resistance.

Physical, Chemical and Biological Behaviours of Fumigants

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