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 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 at sites throughout the state of Hawai'i. 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 hundreds of thousands of acres invaded by guava statewide.
Cut stems or individual leaves with mature galls can be harvested and placed in new locations to accelerate spread of Tectococcus. To ensure the galls are mature, leaves should be fully expanded, green and stiff - no longer red, thin and soft. Browning indicates galls may have aged to a point that the females are old or dead.
Initial releases of Tectococcus (2012) were conducted in demonstration plots on Hawai’i Island, where we closely monitor its spread and impact on three varieties of strawberry guava. Galling by wind borne insects has been observed 10-20 meters from their likely origin, with spread to untreated and control trees increasing steadily.
All three varieties of strawberry guava are susceptible to Tectococcus, but galling develops most rapidly on yellow fruited trees. 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.
Galling at the Waiakea site has increased to a level that is beginning to reduce plant growth for two varieties (red- and yellow-fruited).
Guava fruit production is variable at the Waiakea site, with weak trends toward lower fruiting on heavily galled trees.