Environmental factors affecting efficacy of bifenthrin-treated vegetation for mosquito control

The use of pesticide-treated vegetation as a barrier for control of nuisance and disease-bearing mosquitoes has become an option for mosquito management for home owners and public health and mosquito control professionals. Potted wax myrtle and azalea plants were treated with bifenthrin (0.79% AI) at maximum label rate using backpack and electrostatic sprayers and exposed to various treatments that could affect the residual degradation of the applied pesticides. Treatments included leaf aspect, simulated rainfall, shade, and natural sun exposure with the residual effectiveness of leaves examined in tarsal contact Petri dish assays using laboratory-reared Aedes aegypti. There was no significant difference in efficacy between the adaxial (top) or abaxial (bottom) surfaces of electrostatically or backpack-treated leaves. Significant differences existed between application method, plant species, and exposure with most significant effects between weeks 1 and 4. Simulated heavy rainfalls applied 3 times weekly reduced knockdown by leaves treated with electrostatic and backpack methods with reductions seen as soon as 1 wk after treatment. Reductions were seen with both wax myrtle and azalea leaves and after 1, 4, and 24 h contact of mosquitoes to leaves. Placement of plants with full exposure to sunlight also significantly reduced efficacy compared to plants placed in the shade. Differences were observed most often for 4 and 24 h knockdown counts, and significant decreases were seen from week 4 onwards. Clearly factors such as rain and exposure to sun impact degradation of efficacy of bifenthrin-treated vegetation in the field. Degradation of bifenthrin efficacy was slowest in sites protected from rain and sun, which coincide with preferred resting site locations for many mosquito species.