Long-term pre-exposure of the pest mite Tyrophagus putrescentiae to sub-lethal residues of bifenthrin on rapeseed did not affect its susceptibility to bifenthrin

Tyrophagus putrescentiae on rapeseed was exposed to a sub-lethal residue of 0.15 μg of bifenthrin per 100 g of rapeseed over 147 days, i.e. 14 generations. Every 21 days, samples of mites were collected and mortality was evaluated using a bifenthrin-treated filter paper bioassay. The sampled mites were also introduced onto newly-treated seeds. The susceptibility of the mites to bifenthrin remained similar after pre-exposure for all intervals from 21 to 147 days. The, LD₅₀ ranged from 0.16 to 1.14 μg of bifenthrin per cm² of filter paper. After 147 days, the rates of population increase and respiration of mites pre-exposed to the sub-lethal dose of bifenthrin were evaluated on bifenthrin treated (sub-lethal dose) and untreated rapeseed. Pre-exposure to bifenthrin had no significant effect on respiration, but influenced the rate of increase of mites on rapeseed: The mites pre-exposed to the sub-lethal dose of exhibited a higher rate of increase on the bifenthrin-treated rapeseed than on untreated rapeseed. Thus a decrease in the susceptibility of T. putrescentiae to bifenthrin with selection on a sub-lethal bifenthrin dose on rapeseed during up to 147 days of exposure during which the mite completed approximately 14 generations was not observed. However, the sub-lethal doses of bifenthrin stimulated the population growth and metabolic activity of the mites. Under such conditions, the production of hazardous proteins will probably be higher than on untreated rapeseed.