Susceptibility of Brazilian Populations of Chrysodeixis includens (Lepidoptera: Noctuidae) to Selected Insecticides.

Soybean looper (SBL), Chrysodeixis includens (Walker) (Lepidoptera: Noctuidae), is an important pest of soybean and cotton in Brazil. The use of insecticides is one of the main control tactics against this pest. To support Integrated Pest Management (IPM) and Insect Resistance Management (IRM) programs, we characterized the susceptibility of Brazilian populations of SBL to insecticides. Field populations were collected from soybean fields during the 2016-2017 and 2017-2018 crop seasons. In the laboratory, late L2/early L3 larvae were exposed to insecticides in diet-overlay or topical bioassays. Field populations of SBL showed high susceptibility to spinetoram (LC50 = 0.074-0.25 µg a.i. per cm2), indoxacarb (LC50 = 0.46-0.94 µg a.i. per cm2), thiodicarb (LC50 = 9.14-36.61 µg a.i. per cm2), chlorantraniliprole (LC50 = 0.15-0.57 µg a.i. per cm2), flubendiamide (LC50 = 0.45-2.01 µg a.i. per cm2), and chlorfenapyr (LC50 = 0.15-0.25 µg a.i. per cm2); the resistance ratios were less than 16-fold. In contrast, SBL populations have reduced susceptibility to lambda-cyhalothrin (LC50 = 3.71-9.54 µg a.i./cm2), methoxyfenozide (LC50 = 0.67-4.23 µg a.i. per cm2), novaluron (LC50 = 27.52-77.63 µg a.i. per cm2), and teflubenzuron (LC50 = 13.41-73.02 µg a.i. per cm2). The resistance ratios relative to a Lab population (susceptible of reference) was up to 38-, 63-, 1,553-, and 5,215-fold, respectively. These high resistance ratios can be associated with resistance evolution. Implications of these findings to IPM and IRM programs are discussed.

Baseline Susceptibility of Brazilian Populations of Chrysodeixis includens (Lepidoptera: Noctuidae) to C. includens Nucleopolyhedrovirus and Diagnostic Concentration for Resistance Monitoring.

The Chrysodeixis includens nucleopolyhedrovirus (ChinNPV: Baculoviridae: Alphabaculovirus) is a registered insecticide for the management of soybean looper, Chrysodeixis includens (Walker, [1858]) in Brazil. We conducted studies of baseline susceptibility of Brazilian populations of C. includens to the ChinNPV (Chrysogen, AgBiTech, Fort Worth, TX) as valuable knowledge in support of Integrated Pest Management and Insect Resistance Management programs. In bioassays, neonates were infected with different concentrations of ChinNPV using the droplet feeding bioassay method. Larvae were then transferred to artificial diet and mortality was assessed at 7 d. Results confirm that neonates from Brazilian populations of C. includens are susceptible to ChinNPV. Concentrations from 1.0 × 103 to 1.0 × 108 occlusion bodies (OBs) per ml caused mortality from 1.5 to 99%, respectively. The LC50 ranged from 1.4 × 105 to 7.7 × 105 OBs per ml for populations of C. includens (5.5-fold variation). Similar variation was detected for the LC90 which ranged from 1.6 × 107 to 7.7 × 107 OBs per ml (4.8-fold variation). Importantly, the field-collected populations showed equivalent susceptibility to the reference susceptible population. This indicates a low interpopulation variation in susceptibility of Brazilian populations of C. includens to ChinNPV, representing natural geographic variation and not variation caused by previous selection pressure. The candidate diagnostic concentration of 2.9 × 108 OBs per ml was estimated based on the pooled data and caused mortality ranging from 98.6 to 100%. This concentration will be used in proactive resistance monitoring programs. The Chrysogen will be a valuable tool as a new mode of action in C. includens resistance management in Brazil.

Susceptibility of the Lesser Mealworm, Alphitobius diaperinus (Coleoptera: Tenebrionidae), From Broiler Farms of Southern Brazil to Insecticides.

The lesser mealworm, Alphitobius diaperinus (Panzer, 1797) (Coleoptera: Tenebrionidae), is considered the primary insect pest in broiler farms in Brazil. In this study, we characterized the susceptibility of A. diaperinus populations from broiler farms of southern Brazil to cypermethrin and chlorpyrifos. Larvae and adults of A. diaperinus were exposed to these technical insecticides diluted in acetone in residual bioassays. A geographic variation in the susceptibility of larvae and adults of A. diaperinus to both insecticides was detected. The larval LC50 for cypermethrin ranged from 0.43 to 7.33 µg a.i./cm2. Two populations from Santa Catarina state presented higher resistance ratios of 13.6- and 17-fold. When adults were exposed to cypermethrin, the LC50 ranged from 0.46 to 4.93 µg a.i./cm2, with population SC-3 from Santa Catarina having lower susceptibility (resistance ratio of 10.7-fold). When exposed to chlorpyrifos, A. diaperinus larvae present LC50 values ranging from 0.21 to 4.30 µg a.i./cm2. Larvae from Paraná and Santa Catarina (SC-1 population) presented the highest resistance ratios, ranging from 10- to 20-fold. In adults, the LC50 of chlorpyrifos ranged from 0.17 to 5.30 µg a.i./cm2, showing a maximum resistance ratio of 31-fold in a population from Paraná state. Based on LC99 values, candidate diagnostic concentrations of 15 and 12 µg a.i./cm2 of cypermethrin and chlorpyrifos, respectively, were also estimated for the resistance monitoring of A. diaperinus in Brazil. The implications of these results in Insect Resistance Management are discussed.