Plant Protection Treatment by Drones: Risks for Residents?

Orchard crop spraying using unmanned aerial spraying systems commonly referred to as drones can lead to drift, posing a risk to residents and bystanders. The study shows that the risks arising from this are taken into account by the current registration process.

Risks and Regulation

Drones (unmanned aerial spraying system, UASS) are widely used for plant-protection treatments in Asia and are eliciting increasing interest in other regions of the world. Despite this, there are currently very few studies on direct resident and bystander exposure to drift during crop spraying with drones. In Switzerland, crop spraying with drones has been authorised since 2019 and drone usage has been increasing year on year, particularly in steeply sloping vineyards. The Swiss authorisation process was developed on the basis of different Agroscope studies encompassing drift measurements with an initial evaluation of resident and bystander exposure. To supplement these data, fluorescent marker treatments were carried out with a DJI AGRAS T30 UASS in an orchard in the Valais region. Figure 1 shows the experimental device with the different sample collectors used.

Drift and exposure

Drift is strongly affected by wind speed and direction. Aerial drift is higher when close to ground level and decreases with height. This can be explained by the strong downward-directed air current generated by the drone in flight. The further away one moves from the sprayed area, the more the extent of ground deposition due to drift decreases. The amount of marker deposited on the mannequins is variable. Regarding adult mannequins, it is the legs that are most exposed. Child mannequins that are 110 cm tall are relatively exposed, since they are in the area with the highest drift.
Inhalation exposure is very low or even negligible compared to dermal exposure.

Application by drones vs. tractor-assisted application

To check whether the risk of an application by drones is covered by the risk assessment within the context of plant-protection product registration in Switzerland, the drift and exposure values obtained in this study are compared to the reference values for a tractor-assisted orchard application. For ground sedimentation (Fig. 2A), Drone values are always lower than tractor values, even if the sedimentation generated by the two techniques seems to be similar for the late treatments and for treatments at distances of 3 m and 5 m. For exposure via the dermal route (Fig. 2B), the drone values are markedly lower than the reference.