Strawberry guava displaces native forest, Wao Kele O Puna, Hawai’i
(Carnegie Airborne Observatory photo)
Strawberry guava (Psidium cattleianum; Myrtaceae), native to Brazil, commonly dominates low and mid elevation landscapes in Hawai’i and is expanding into high elevation rainforest watersheds. Spread by pigs and alien birds, its fruit also serves as a host for pest fruit flies. Strawberry guava grows rapidly, creating dense thickets and squeezing out native trees and numerous rare species. Conventional methods of control are too costly for widespread use.
A highly specific leaf-galling insect from Brazil, Tectococcus ovatus (Hemiptera: Eriococcidae), was introduced to Hawai’i in 2012 to slow the spread of strawberry guava. Tectococcus females within galls produce eggs and hatched nymphs that spread to new foliage, carried by wind and by crawling. The crawling nymphs settle on young leaves and establish new galls that grow as the leaves expand. Galls shelter the insects from weather and enemies and, at high density, sap the plant’s energy for growth and reproduction.
Tectococcus has established successfully everywhere released, although repeated effort occasionally is needed to find appropriate conditions. It has spread gradually on its own from release sites, moving 10-20 meters per year, sometimes further under ideal circumstances. Methods for efficient dispersal of the biocontrol are critical for its use as a management tool across the hundreds of thousands of acres of forests invaded by strawberry guava statewide.
Cut stems or individual leaves with mature galls can be placed to accelerate spread of Tectococcus to new locations. Leaves should be fully expanded and no longer red, thin and soft. Browning indicates galls may have aged to a point that the females are old or dead.
“Tectobolas” – novel method to distribute Tectococcus to guava canopy
Initial releases of Tectococcus (2012) in demonstration plots on Hawai’i Island allow us to monitor its spread and impact on three varieties of strawberry guava. Galling by wind borne insects was observed 10-20 meters from their likely origin, with spread to untreated and control trees increasing steadily. Galling at the Waiakea site increased to a level that is beginning to reduce guava fruiting. Gall development was slower at high elevation in Volcano due to cool temperatures and vog-induced defoliation. Long term data from these and other sites help us understand impacts of biocontrol.