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.

► Bifenthrin is an effective pesticide for control of Tyrophagus putrescentiae in rapeseed. ► The LD50 ranged from 0.16 to 1.14 μg of bifenthrin per cm2 of filter paper in a biotest. ► Bifenthrin suppressed the rate of increase of the mite. The ED50 was 2.7 μg of bifenthrin per 100 g of rapeseed. ► Mites pre-exposed to a sub-lethal dose of bifenthrin exhibited a higher rate of increase on the bifenthrin-treated rapeseed than on untreated rapeseed.