High Dose Strategies for Managing Phosphine-Resistant Populations of Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae)

Abstract

BACKGROUND Rhyzopertha dominica is a serious pest of stored grains and many populations have resistance to the fumigant phosphine. Some populations contain beetles with a “strong resistance” phenotype. Recent work found the LC50 values for two strong-resistant populations recently studied in North America, Belle Glade and Minneapolis, were 100 and 595-fold higher, respectively, compared to LC50 of a lab-susceptible strain. Populations with “weak resistant” phenotypes had LC50 values 5 to 10-fold that of a susceptible strain. The work reported below aimed to determine the minimum phosphine concentrations and number of days of exposure needed to effectively control all life stages of representative weak and strong resistant strains, and then to recommend treatment conditions needed to control strongly phosphine-resistant R. dominica in pest populations. RESULTS A dose-mortality assay estimated that phosphine fumigation over 48 hours using 730-870 ppm at 25° C would control adults of both strongly resistant R. dominica populations. Fumigations with mixed life stage cultures found 200 ppm killed all susceptible and weak resistant beetles in 2 days, but the strong-resistant Minneapolis and Belle Glade strains had substantial survivors at 200 ppm. Further, the Belle Glade strain had beetles survive 1000 ppm in 2-day fumigations. The strong-resistant Belle Glade strain needed nearly 10 days at over 400 ppm to have acceptable levels of control. CONCLUSION This study recommends protocols to manage strongly resistant R. dominica populations provided that a minimum phosphine concentration of 400 ppm be maintained at 25° C or higher for up to 10 days. This article is protected by copyright. All rights reserved.