The effects of toxic sugar bait on a natural predator, <i>Toxorhynchites rutilus rutilus</i> (Diptera: Culicidae)

Toxic sugar baits with boric acid have emerged as an alternative for reducing mosquito vector populations due to their low toxicity to nontarget organisms. We investigated the effects of boric acid-containing toxic sugar baits (1% w/w) on Toxorhynchites rutilus rutilus (Coquillett), a natural predator of mosquito larvae, and compared it to Aedes aegypti (L.). We determined the lethal dose of boric acid causing mortality in test mosquitoes. Additionally, the survival of Tx. r. rutilus exposed to toxic sugar baits applied to nonflowering and flowering plant foliage were compared. The results showed no significant difference in survival probability between females and males of Tx. r. rutilus after exposure to toxic sugar baits. However, female mortality occurred at a higher rate on days 1 and 2 compared to males. The amount of boric acid in the gut of Ae. aegypti was significantly lower (6×) than that in Tx. r. rutilus. The results may explain the rapid mortality of Ae. aegypti after exposure to toxic sugar bait compared to that of Tx. r. rutilus on day 1 and day 2. The flowering status of the plants to which the toxic sugar bait was applied significantly affected Tx. r. rutilus's survival. The survival in toxic sugar baits-treated flowering shaggy dwarf morning glory was higher than that of the nonflowering counterparts on the first 2 d. The results indicate low toxicity of boric acid toward Tx. r. rutilus compared to target Aedes mosquito. We suggested that the application timing during the flowering season may delay the effect of toxic sugar baits on Tx. r. rutilus.