Journal of Taiwan Agricultural Research

Author:Wen-Bin Feng、Chi-Yang Lee1、Me-Chi Yao*

Abstract:

The lesser grain borer, Rhyzopertha dominica (Fabricius), is a major pest of stored paddy rice in Taiwan. Deltamethrin has been the primary insecticide recommended for rice-mixing applications in warehouses since 2002. However, its long-term use has led to widespread resistance in field populations, particularly in central and southern Taiwan. To identify potential alternative insecticides, this study evaluated the susceptibility of 20 field-collected R. dominica populations with varying levels of deltamethrin resistance to six insecticides representing four different modes of action: 1B organophosphates- malathion and phoxim; 3A pyrethroids- permethrin and phenothrin; 5 spinosynsspinosad; and 6 avermectins & milbemycins- abamectin. Laboratory bioassays using the rice-mixing method revealed that most populations remained susceptible to malathion, phoxim, spinosad, and abamectin compared to the laboratory population. In contrast, all tested populations showed high resistance to phenothrin. Although some populations showed only moderate resistance to permethrin compared to the laboratory population, they demonstrated extremely high LC50 values exceeding 10,000 μg mL-1, indicating limited field utility. These findings suggest that malathion, phoxim, spinosad, and abamectin show potential as alternative insecticides for managing deltamethrin-resistant R. dominica in paddy rice warehouses. Field trials are required to validate their efficacy, determine appropriate operational dosages under practical conditions to support future insecticide registration, as well as to establish a science-based insecticide rotation strategy in paddy rice warehouses in Taiwan.

Key words:Rhyzopertha dominica, Alternative insecticides, Lethal concentration 50%, Resistance ratio, Level of insecticide resistance.

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