Strategies for the use of phosphine to combat infestation problems affecting the quality of bulk grain
About this project
A rapid diagnostic test for resistance to phosphine, based on the mobility of adults during exposure, has been developed for a number of grain pest species. Field strains of S. oryzae were tested with the rapid test, alongside the laboratory susceptible strain, and the results agreed with those obtained with the standard F.A.O. resistance test for six of the seven strains tested. The other strain was diagnosed as susceptible by the F.A.O. test but found to be resistant by the rapid test. The disagreement was attributed to the fact that the rapid test is able to detect heterozygotes whilst the F.A.O. test was never designed to do this, detecting only homozygous insects. Since the F.A.O. test relies upon the presence of homozygous insects to detect resistance, it cannot provide an early warning of an emerging resistance problem.
Efforts to select a homozygous resistant strain of S. granarius were unsuccessful but field strains diagnosed from the F.A.O. test as susceptible all gave responses in the rapid test which agreed with the diagnoses from the F.A.O. test.
A homozygous strain of O. surinamensis was produced from a strain collected from Palmital in Brazil and heterozygotes were then produced from this strain and the laboratory susceptible strain. Homozygous resistant, susceptible and heterozygote insects were tested with the rapid test and there was no overlap in the response lines of the homozygous resistant insects and either of the other two genotypes. The responses of the heterozygotes could still be distinguished from susceptible insects. When field strains of this species were tested, the raping test agreed with the diagnoses of the F.A.O. test in 10 out of 13 strains tested. For the other three stains, the rapid test found evidence of resistance but the F.A.O test did not. As with one strain of S. oryzae, this was attributed to the rapid test's ability to detect the major resistance gene in its heterozygous form.
The discriminating knockdown times for each species were as follows:
S. oryzae 141 min (2 hr, 21 min), O. surinamensis 225 min (3hr, 45 min) and for S. granarius it is estimated to be at least 1065 min (17hr, 45 min).
The immature stages of resistant strains of the above species were controlled by exposure to high concentrations of phosphine at 15°C. The most tolerant species was S. oryzae which required a 24-day exposure at a concentration of 2 g m-³. At 10°C a single individual of this species survived a four week exposure at this concentration.
An automated dosing system for cylinder based phosphine was developed and tested by the Central Science Laboratory. The performance of the dosing system was compared with the Sirofloâ system that has been developed by CSIRO in Australia. Both systems were tested in a silo containing 700 tonnes of feed wheat and in a floor store containing 150 tonnes of feed wheat. Concentrations of 0.05 g m-³ or greater were achieved in all positions using the Sirofloâ system but the automated dosing system required gas re-circulation with a centrifugal pump to obtain the same result.
When re-circulation was used, higher concentrations and CTPs were achieved using the automated dosing system compared with the Sirofloâ system in both the grain silo and the floor store. In the case of the silo this was achieved using less gas. In the floor store the Sirofloâ used only 55 % of the amount of gas used by the automated dosing system with re-circulation but the average concentration was only 28 % of that obtained using the automated dosing system.
The automated dosing system was also used in an experimental fumigation of a part of a large bulk of infested grain. In the infested area concentrations of over 0.05 g m-³ were reached everywhere by 5 days and remained above this value until the system was switched off.
Methyl phosphine has been shown to be more effective against immature stages and adults of phosphine-resistant strains in comparison with susceptibles.
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