Studies on the toxicity of phosphine and methyl bromide to four moth pests of stored products with special emphasis of diapause
Moth pests of stored products included in this work were Ephestia elutella (Hubner), Ephestia kuehniella Zeller, Ephestia cautella (Walker), and Plodia interpunctella (Hubner). Information was gathered on the physical limits of the four species, on the properties of cultures set up in the laboratory, and on the factors involved in the induction and termination of diapause. Diapause was induced by low temperature and short daylength in fully grown larvae of E. elutella and P. interpunctella. Light intensities below 1 lux affected the induction of diapause in both species. In P. interpunctella, another factor inducing diapauses was high population density. Attempts to produce an arrest in the development of E. cautella were unsuccessful. Termination of diapause was hastened by such factors as long photoperiod, high temperature, chilling periods, and fumigation. Diapausing larvae of E. elutella were tolerant to methyl bromide, requiring concentration time (CT) products for 99% kill of 281 and 164 mg h/1 at 15 and 25 ºC respectively. Diapausing larvae of P. interpunctella required CT products of 225 and 67 mg h/1 at 10 and 25°C respectively for 99% kill. In the absence of diapause all stages were susceptible to methyl bromide, succumbing to a CT product of 64 mg h/1 at 15°C. With phosphine, the egg stage of all four species was highly tolerant for the first 40% of the developmental period. Eggs exposed beyond this period showed a marked increase in susceptibility, and were controlled by less than 0.04 mg/l in 4-day exposures at 25ºC. Longer exposures of phosphine were more efficient than shorter ones of similar CT product, and Haber's rule could not be applied. Factors controlling diapause in laboratory and field stocks were different, and a correlation was observed between diapause intensity and tolerance to fumigation. Further information was gained by fumigating the high intensity diapausing stages of Pieris brassicae (L) and Bombyx mori (L).
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