Unmanned aerial systems (UAS) are increasing in popularity for recreation, commercial, and scientific purposes. However, UAS can have unexpected direct effects on animals, inducing stress responses and avoidance behaviors. Because all animals rely on interactions with other species, direct effects of UAS could have indirect effects throughout a community. The most popular UAS are multi-rotor designs, which generate a large (>25km/h) downward force of wind to remain airborne. I tested the hypothesis that UAS disturbance directly affects predators and indirectly affects lower trophic levels using a system of ladybeetles (Harmonia axyridis) and aphids (Aphis glycines) on soybean plants (Glycine max). Previous work has shown that strong winds decrease ladybeetle abundance and predation rates, suggesting that UAS could decrease the ecosystem service ladybeetles provide by controlling aphids.
I tested the behavioral response of aphids and ladybeetles to UAS in a series of controlled experiments. I simulated UAS-caused wind using fans in a greenhouse experiment and measured the effects on aphid growth rates and ladybeetle predation rates. I evaluated behavioral avoidance of aphids and ladybeetles to UAS by flying a commercially available UAS (DJI Phantom 3) over potted plants and measuring emigration from plants.
Results/Conclusions
Simulated UAS disturbance in the form of pulses of wind from a fan had no direct effect on aphid abundance (n = 12, t-test, p >0.05) probably because soybean aphids exist within the boundary layer of the leaves. However, fans did reduce ladybeetle predation on soybean aphids in feeding trials (n = 5, t-test, p < 0.001). Flying a UAS over potted soybean plants containing foraging ladybeetles did not directly affect soybean aphid abundance (predator-free treatments; n = 5, t-test, p > 0.05), but did reduce ladybeetle abundance (more ladybeetles left plants that were exposed to UAS; n = 5, X2 = 6.66, p = 0.010).
These results demonstrate that disturbance from UAS can generate indirect effects within a community. Ladybeetles and aphids provide a useful model system to demonstrate how UAS can generate indirect effects, but these results have broad implications for other species and communities. While this technology provides exciting recreation and unique research opportunities, caution must be exercised to minimize their ecological consequences. Ongoing research efforts will evaluate the effects on soybean plant production to quantify the effects of UAS on the strength of trophic cascades.