Toxicity of phosphine to the diapausing stages of Ephestia elutella, Plodia interpunctella and other lepidoptera

Diapausing larvae of the stored product moths Ephestia elutella (Hübner) and Plodia interpunctella (Hübner) were tolerant of phosphine only in short exposures, the tolerance of E. elutella being at least twice that of P. interpunctella. The tolerance of a freshly collected wild stock of each species was, at some temperatures, about two or three times that in a laboratory reared stock. The wild stock of E. elutella required concentration-time (CT) products of 36 mg hr/l in 4-day exposures at 10° and 15°C, and 18 mg hr/l in 2-day exposures at 20°C and above for 100% kill. The tolerance of E. elutella larvae in diapause was not dependent on the manner in which diapause was induced, maintained or subsequently terminated. Longer exposures to phosphine were more effective than shorter ones of similar CT product because an initial period of about a day in each exposure was required for insects in diapause to be killed. At 25°C, a concentration of 0.03 mg/l was below the threshold level for mortality of some larvae, long exposures at such very low concentrations proving ineffective.

Diapausing pupae of an outdoor species, Pieris brassicae (L.), and eggs of the univoltine race of Bombyx mori (L.) were highly tolerant of phosphine at 10°C.