Resistance Risk Assessment In Field-Collected Strains Of The German Cockroach (Dictyoptera: Blattellidae) To Fipronil And Indoxacarb Baits

Abstract:
The German cockroach, Blattella germanica (L.) is an important urban insect pest world-wide. Heavy reliance and frequent usage of residual insecticides have led to the development of insecticide resistance in the German cockroach. Bait formulations containing fipronil and indoxacarb are excellent resistance management tools against insecticide-resistant cockroaches. Despite that they have been used for nearly a decade, there are limited studies on the quantitative assessment of the development of fipronil and indoxacarb resistance in B. germanica. This study was carried out to assess the risk of resistance development towards fipronil and indoxacarb baits in three field-collected German cockroach populations (Boat Quay, Cavenagh and Ghimmoh strains) from Singapore, based on laboratory bait selection. These strains were each divided into two groups, one group selected with fipronil bait, while the other one selected with indoxacarb baits. After continuous selection for 7 generations, there were 9–11.2 fold increase in the LD50 of fipronil, and 24.1–68.7 fold increase in LD50 of indoxacarb, when compared to their parental generations. The realized heritability (h2) of fipronil resistance ranged from 0.336 – 0.600, whereas indoxacarb resistance was in range of 0.197–0.475. The estimated slopes and h2 were not correlated and all the possible combinations of these estimates provided prediction that it will take between 3.2 and 21.3 generations, and between 1.5 and 16.9 generations to achieve 10-fold increase in resistance to fipronil and indoxacarb, respectively, under a selection pressure of 50–90% mortality. This is the first study on resistance risk assessment in the German cockroach to fipronil and indoxacarb baits using multiple field strains. The outcome of this study provides useful information to the pest management industry towards preventing or delaying insecticide resistance development to baits in B. germanica.