Scaling up the deployment of Psidium cattleyanum biocontrol in a Tropical Forest

It can be challenging to deploy and monitor biocontrol for invasive species, particularly in remote forest settings. One of the most abundant and disruptive invasive tree species in Hawai'i and across the Pacific is Psidium cattleyanum Sabine (strawberry guava), which replaces native vegetation and negatively impacts forest structure and ecosystem services. A Brazilian leaf-galling insect named Tectococcus ovatus Hempel (Homoptera: Eriococcidae) is an approved biocontrol for strawberry guava, but its introduction to date has been restricted to areas with established road and trail networks using ground-based methods. The aim of our study was to develop and test aerial methods of deploying T. ovatus within the larger goal of landscape-scale deployment. We also investigated using very high-resolution aerial imagery to detect T. ovatus galls on infected leaves to monitor post-deployment inoculation success and quantify spread. We found drone-based aerial deployment using a small 4-unit system was nearly 5 times faster than a ground-based approach in a heavily invaded lowland tropical forest setting and produced a statistically larger inoculation success rate one year after deployment, likely due to more central and higher placement within the targeted canopies. We found aerial imagery to be a useful tool to determine inoculation success and quantify spatiotemporal spread. We also found that larger-capacity drone platforms and conventional helicopters can successfully deploy T. ovatus onto individual targeted tree canopies and that landscape-scale deployment operations are feasible using these methods. Our findings are relevant to others working to deploy and monitor biocontrol in forested ecosystems worldwide.