Pyrethroid Susceptibility in Stomoxys calcitrans and Stomoxys indicus (Diptera: Muscidae) Collected from Cattle Farms in Southern Thailand

SIMPLE SUMMARY: Stable flies (Stomoxys spp.) are important blood-sucking insect pests worldwide that cause problems in various animal production systems. The most common species found in Thailand are Stomoxys calcitrans (Linnaeus, 1758) and Stomoxys indicus Picard, 1908 (Diptera: Muscidae). Stable flies can be controlled by many methods, with the main approaches relying on insecticides. However, insecticide resistance has been widely reported in stable flies. This study addresses related gaps in knowledge, especially regarding pyrethroid insecticides, which are used in and around agricultural farms and in households. Adult stable flies of each species were collected from cattle farms in southern Thailand, and a susceptibility test was carried out using a World Health Organization (WHO) cone bioassay. We recorded the number of knockdowns at 30 and 60 min and mortality at 12 h and 24 h after exposure. The mortality range for S. calcitrans and S. indicus was up to 95.83% and 100%, respectively. Insecticide sensitivity in the pyrethroid phenotypes was demonstrated in the population of flies from the southern provinces of Thailand. Given the stable fly populations in Thailand, not only should insecticides currently used to protect livestock be continued, but novel pest management methods should also be introduced, considering additional semiochemical toxicity to improve the effectiveness of stable fly control. ABSTRACT: The susceptibility to six pyrethroid insecticides (permethrin, deltamethrin, alpha-cypermethrin, cypermethrin, lambda-cyhalothrin, and bifenthrin), each at the recommended concentration, was evaluated for two stable fly species—Stomoxys calcitrans (Linnaeus, 1758) and Stomoxys indicus Picard, 1908 (Diptera: Muscidae)—through tarsal contact using a World Health Organization (WHO) cone bioassay procedure. The field populations of S. calcitrans were collected from the Songkhla and Phattalung provinces, while S. indicus were collected from the Phattalung and Satun provinces in Thailand. The stable flies were exposed to insecticide-treated filter paper for 30 min, and their knockdown counts at 30 min and 60 min and mortality counts at 12 h and 24 h were recorded. The S. calcitrans and S. indicus Songkhla and Phattalung populations were moderately susceptible to pyrethroids, as indicated by the 24 h mortality. Nonetheless, the Satun population of S. indicus was completely susceptible to permethrin, with 100% mortality, and showed the lowest susceptibility to deltamethrin and bifenthrin. The results indicate the generally low susceptibility of stable flies to pyrethroids in the southern provinces of Thailand.