A note on the insecticide sensitivity status of Meligethes species (Coleoptera: Nitidulidae) in Luxembourg

Abstract

Adult pollen beetles, Meligethes spp. (Coleoptera: Nitidulidae), were sampled from 27 and 35 oilseed rape (Brassica napus subsp. oleifera) fields scattered over Luxembourg in 2008 and 2009, respectively. Ten well-developed and active individuals from each field were transferred into 40-ml glass vials (4 replicates) with their interior surface being coated with the pyrethroid deltamethrin at rates corresponding to 0%, 0.8%, 4%, 20%, 100%, or 500% of the recommended field application rate. The percentage of dead insects was recorded at 0 h, 1 h, 5 h, and 24 h, and increased with time to a maximum. The application rate required to obtain a mortality of 50% (LC₅₀) after 24 h of exposure was estimated by logistic regression models. The overall median LC₅₀ calculated for all locations and years was 1.15 ± 0.29% of the field rate. The median LC₅₀ for 2008 was 0.79 ± 0.40% of the field rate and 1.80 ± 0.40% in 2009. In 2008, no population with an LC₅₀ below the overall median LC₅₀ was found in the north-eastern part of the country, whereas an approximately homogeneous spatial distribution of populations with LC₅₀s below the overall median was found in 2009, suggesting a spread of less sensitive Meligethes spp. from southwest to northeast within the period of observation. Pollen beetles from four locations were additionally tested for their sensitivity towards bifenthrin and chlorpyri- fos-methyl. The period of time until 50% of the individuals were affected (LT₅₀) decreased exponentially with rate and differed significantly among insecticides (P = 0.005), but not among locations (P = 0.753). The average LT₅₀ at 4% of the field rate was 9.3 ± 2.2 h, 5.4 ± 0.3 h, and 2.9 ± 0.1 h for deltamethrin, bifenthrin, and the organophosphate chlorpyrifos-methyl, respectively.