Across the state and out in the Waikoloa Dry Forest Preserve, a small wasp is causing huge problems for our beloved wiliwili. By now, many of us have heard of the invasive pest that has been badly damaging Hawai`i’s wiliwili tree, but for those of us that haven’t- that pest is the Erythrina Gall Wasp (EGW). After first being observed in Hawai`i in 2005, this tiny wasp (smaller than an ant) began laying its eggs into the new stems, leaves, and inflorescence of both our native and non-native coral tree species. A substantial amount of Waikoloa’s wiliwili were killed by these infestations within the first few years, but luckily the Hawai`i Department of Agriculture was able to quickly identify, locate, research, and release a known predator to the gall wasp- the Eurytoma Parasitoid Wasp. Yes- a wasp that attacks and destroys another wasp!
The eurytoma parasitoid wasp seeks out the galls on wiliwili and lays its eggs into said galls so that its larvae can feed upon the EGW larvae before they can emerge as a fully formed wasp. Eurytoma were first released in 2008, and provide an excellent example of what a successful biocontrol is meant to be. The parasitoid wasps quickly began finding galls and laying eggs and a dramatic decrease in gall wasp infestation was seen. Over the past 10 years since the original biocontrol release, and the rate at which wiliwili were dying due to EGW infestation was significantly reduced and a new equilibrium was attained. We still observe both gall wasp and the eurytoma, but what we now see is an ebb and flow in which the EGW populations begin to rise, shortly followed by an increase in eurytoma population, and the eventual tapering off of the wasp populations as eurytoma eradicates the EGW and food sources become scarce.
However, the gall wasps continue to be a problem- especially during the flowering time for our wiliwili. Currently, as the wiliwili begin to send out their flower buds, the EGW begin to infest the new tissues of the inflorescence, and the Eurytoma just can’t ramp up their numbers before the harm is done. Although the wiliwili are no longer dying, the EGW are still causing significant damage to the inflorescence- resulting in aborted flower buds, improperly formed fruit, and ultimately lower than expected seed production.
This year, the staff at the Waikoloa Dry Forest Initiative has decided to take the next step in the battle against the gall wasps and have undertaken a new and potentially groundbreaking project to raise and release Eurytoma during this flowering period to curtail the gall wasps and maximize the available wiliwili seed set to ensure lasting wiliwili for generations to come. We’re finishing up this year’s project and will be sharing the results soon!
-Contributed by Robert Yagi, Preserve Manager for Waikoloa Forest Initiative