Lab bioassay of insecticides for the control of Asian longhorned tick, 2023

Laboratory bioassays were conducted to evaluate the efficacy of 3 insecticides that are labeled for use on pasture grass to control adult Asian longhorned tick (ALT). Over 320 ALT were collected on 2 separate sampling dates (Aug 15 and 26) from established beef cattle pastures near Wytheville, VA. Of the 640 ALT collected, approximately 160 were nymphs, and 480 were adults. In the Aug 15 bioassay, in order to simulate spraying pesticide on pasture, 9-cm filter paper discs were dipped in 1 of 3 different insecticide treatments (Bifenthrin 2E, Lambda cyhalothrin, or Baythroid XL) or water (check); product concentrations shown in Table 1. There were 8 dipped discs for each treatment. The filter paper discs were placed outside in Blacksburg, VA, on tall fescue grass held in place by a window screen fastened to the ground for each of 2 wk, so that filter paper discs were exposed to the elements for 2 wk. On the morning of tick collection, new filter paper was again dipped in the appropriate treatments. These “freshly” treated filter papers offered a contrast to the treated filter paper that had weathered outside for 2 wk. This contrast was used to test the residual efficacy of the target pesticides. ALTs were collected via tick dragging and were placed directly into 50 ml Falcon tubes. Each treatment had 4 replicates, and each tube held 10 ALT along with its treated filter paper. ALTs were kept at room temperature in a climate-controlled laboratory benchtop and were checked at 24 h and 48 h to record mortality. ALT mortality was assessed by tapping ticks out of their tubes onto a Petri dish for observation. During this mortality check, a heat lamp shined on them to stimulate movement to accurately assess mortality. The Aug 26 bioassay was conducted following the same protocol except that filter paper discs were left out for only 1 d, rather than 2 wk. Mortality data were analyzed using ANOVA. Proportion mortality data were arcsine square-root transformed prior to analysis. Means were separated using Fisher’s Protected LSD at the P < 0.05 level of significance. Mortality data were analyzed using ANOVA. Means were separated using Fisher’s LSD at the 0.05 level of significance. In the Aug 15 bioassay (Table 1), there was a highly significant effect of treatment (P < 0.0001) on mortality of ticks at 24 and 48 h after exposure to the dipped filter paper. At 24 h, the highest mortality (>93%) occurred with bifenthrin (0 DAT), which was significantly higher than all other treatments except Lambda-cyhalothrin (0 DAT). Exposure to the elements for 2 wk reduced the efficacy of all insecticides. At 48 h, mortality increased slightly in all treatments, but the trends were similar to 0-d-old bifenthrin and lambda-cyhalothrin, having the highest mortality. In the Aug 26 bioassay (Table 2), there was a highly significant effect of treatment (P < 0.0001) on mortality of ticks at 24 and 48 h after exposure to the dipped filter paper. At 24 h, the highest mortality (90%) occurred with Baythroid XL (0 DAT), which was significantly higher than all other treatments except Lambda-cyhalothrin (0 DAT). Exposure to the elements for 1 d reduced the efficacy of all insecticides. At 48 h, mortality increased in all treatments, and exceeded 90% in all 3 0-d-old pyrethroid insecticide treatments, which were higher than all other treatments and the untreated check. These results confirm the efficacy of pyrethroid insecticides on ALT, but also show how quickly the residue breaks down when exposed to sunlight in an open pasture. Mortality data were analyzed using ANOVA. Means were separated using Fisher’s LSD at the 0.05 level of significance. This research was supported in part by the Virginia Cattle Industry Board. Grant number 25-4. The authors declare no conflicts of interest regarding this manuscript.