Resistance levels of Brazilian populations of Brevipalpus yothersi (Acari: Tenuipalpidae) to hexythiazox: monitoring and population performance.

Abstract

The citrus leprosis mite (CLM), Brevipalpus yothersi Baker (Acari: Tenuipalpidae), is an important mite pest in Brazilian citrus orchards, with control mainly dependent on synthetic acaricides. However, frequent applications can lead to resistance development, compromising long-term efficacy. The objective of this study was to estimate resistance levels to the acaricide hexythiazox in different CLM populations from Brazil's citrus belt and evaluate potential fitness costs associated with resistance. Populations were collected from commercial sweet orange orchards, and direct-contact bioassays were conducted by exposing eggs to hexythiazox for 10 d to determine resistance levels, based on resistance ratios (RRs) estimated at LC50 and LC95. Additionally, biological parameters were evaluated, including egg incubation period, duration of developmental stages, preoviposition period, and demographic parameters (R₀, rm, T, and Dt). Populations showed resistance levels ranging from low to very high, with LC50 RRs between 0.33- and 17.15-folds and LC95 between 0.41- and 643.38-folds. The resistant population exhibited a significantly longer egg incubation and deutochrysalis period, while deutonymph duration and preoviposition period were significantly shorter. No significant differences in demographic parameters were observed between the resistant and susceptible population. Overall, hexythiazox resistance remains low in most populations, with only one showing moderate resistance based on RR50, which may be associated with higher selection pressure from hexythiazox. To preserve the efficacy of hexythiazox and ensure continued control of CLM in Brazil's citrus belt, rotation of synthetic acaricides with different modes of action is recommended.