Response of Suidasia pontifica (Acaridida: Suidasiidae) to phosphine fumigation.

The present study evaluated the level of phosphine toxicity of various strains of Suidasia pontifica, infesting stored agricultural products in the Philippines. The bioassays for the efficacy of phosphine fumigation followed the FAO standard method. A full assay comprised eight concentrations ranging from 0.005 to 1.00 mg/L with 20, 72 and 144-h exposure periods. A total of 15 strains were collected, representing 15 provinces in the country. Results revealed that the most susceptible strain (MR12gsn) was recorded from a private owned warehouse in General Santos City-LC50 and LC99 were 0.009 and 0.025 mg/L, respectively. On the other hand, the most resistant strain (LR3tar) was collected from a private owned warehouse in Tarlac City-LC50 and LC99 were 1.501 and 2.407 mg/L, based on discrimination dose tests. Eggs were more tolerant than the adults; complete mortality of eggs was achieved at 0.50 mg/L for 72 h and 0.35 mg/L for 144 h, whereas complete mortality of adults was obtained with 0.10 and 0.20 mg/L for 144 and 72 h exposure, respectively. The estimated minimum effective concentration (MEC) of phosphine to give complete kill of S. pontifica at various exposure periods must exceed 0.50 mg/L for 72 and 144 h. These results simply illustrate the increased efficiency (i.e., increased mortality) of phosphine fumigation of longer exposure of S. pontifica to lower concentrations.