The Mortality Response of Sitophilus oryzae (L.) Eggs to Diurnal Interrupted Doses of Phosphine (PH3)

Fumigant distribution in grain is inherently variable with movement occurring by gas expansion and contraction, convection and diffusion. Diurnal changes in temperature and wind velocity, and weather changes more broadly, drive movement which is further influenced by the type of storage facility and the degree of gas tightness. Near boundaries, in particular, this can prevent lethal concentrations being maintained. Such conditions may have implications for resistance selection in stored grain insects. However, there is limited information on insect mortality responses to variable concentrations similar to those often confronting industry. This paper reports on initial results from a study that investigates the impact of repeated sub-lethal doses of phosphine on the mortality of eggs from three strains of Sitophilus oryzae that are susceptible, slightly resistant and moderately resistant to the fumigant. The treatments used are designed to represent fumigant environments undergoing simple diurnal fluctuations. Results are compared to those obtained from equivalent continuous treatments and discussed in reference to phosphine toxicology and with regard to the practical implications for successful fumigation.