Evaluating Rapid Bioassays to Assess Phosphine Resistance in the Red Flour Beetle, Tribolium castaneum (Herbst)

Registered grain fumigants are limited to just phosphine and sulfuryl fluoride for managing stored product insects. Phosphine resistance in grain pests was first documented in the U.S. in 1990 using a bioassay developed by the FAO in 1975. Although this method works well to identify the presence or absence of phosphine resistance, it takes a minimum of 15 days before accurate results can be received. Many researchers have attempted to create rapid bioassays that answer the same question as the FAO test within 24 hours, however, no one test has been commercially used or acknowledged. We evaluated a “knock-down” assay that uses the time for Tribolium castaneum adults to show a narcotic response to high levels of phosphine. Knock-down bioassays were analyzed and compared using 11 populations of T.c. with varying known levels of resistance. Variables evaluated were the time to knockdown of 100% of the test group, 50% of test group, and knock-down time for individual beetles. The results of these tests were then compared to data using the FAO test and the knockdown of 100% of the test group vs. the FAO % resistance was highly correlated (R² = 0.82). Additionally, bioassays were performed to assess whether physical disturbance to individual beetles (stimulus) would delay narcosis. Both a susceptible and a resistant population showed a significant difference in knock-down time between disturbed and un-disturbed insects in both groups (p<0.05). This research suggests that a rapid bioassay for evaluating phosphine resistance can be developed for commercial use.