PHOSPHINE RESISTANCE IN THE CIGARETTE BEETLE LASIODERMA-SERRICORNE (COLEOPTERA, ANOBIIDAE) AND OVERCOMING CONTROL FAILURES DURING FUMIGATION OF STORED TOBACCO

Survivalship of the life stages of the cigarette beetle, Lasioderma serricorne F. (Coleoptera: Anobiidae), was recorded in tobacco bales that were fumigated under a gas-proof cover with phosphine at the rate of 1-1.25 g/m3 for 7 or 8 days with the terminal concentration at greater-than-or-equal-to 0.1 g/m3 in warehouses at Ongole, Andhra Pradesh, India. Screening tests at the discriminating concentration of 0.07 g/m3 with a 24-h exposure period confirmed the occurrence of phosphine-resistant strains in the premises. Repeated phosphine treatments under leaky situations, especially with tarpaulins of poor gas retention, resulted in a high level of resistance. Field trials proved that a dosage of 2 g/m3 with 10 days' exposure and a terminal concentration exceeding 0.5 g/m3 was effective against all stages of the phosphine-resistant strains. During fumigation of tobacco packed in gunny bales with wooden bale boards, phosphine concentrations between the freespace in the stack and inside the bales equilibrated in 24 h, whilst it took more than 120 h for equilibration of the gas between the freespace and tobacco packed in corrugated fibreboard cartons with internal polythene liners of 0.1 mm thickness. PVC fumigation sheets of 0.15 mm thickness were better at retaining the phosphine fumigant than the polyethylene sheets which are currently used.