The Strawberry Guava Threat to Hawaii’s Native Forests
The strawberry guava, or waiawī, is a Brazilian tree that was brought to Hawai'i in the 1820s. People in Hawai'i today are most familiar with the plant because of the popular jellies and sauces made from the strawberry guava fruit. Since its introduction nearly 200 years ago, strawberry guava has encroached steadily into Hawaii’s native wet forests, entirely taking over vast swaths of the landscape and threatening the continued survival of native 'ōhi'a and koa forest ecosystems. As strawberry guava encroaches further and further into Hawaii’s forests, native trees and plants are crowded out by the dense thickets and cannot reestablish themselves, which in turn means that natural habitat and food for native birds and insects are destroyed.
The strawberry guava fruit also poses a problem as it leads to an increase in fruit flies (which threaten local agriculture) and facilitates the spread of avian malaria that is a major threat to the survival of Hawaiian native forest birds (the fruit attracts feral pigs that in turn create holes and wallows that mosquitoes use for breeding; it is the mosquitoes that infect birds with malaria).
Additionally, Hawaii’s water supply is put at risk by strawberry guava. Research at Hawai'i Volcanoes National Park conducted by University of Hawaii’s Professor Thomas Giambelluca shows that strawberry guava evapotranspires 27% more water than trees and plants native to Hawai'i. What this means is, as native forests are replaced by strawberry guava thickets, less water is entering into our stream and groundwater systems, which translates into less water for human uses such as drinking water and irrigation of agricultural crops.
Why Strawberry Guava Biocontrol Will Help With Native Forest Restoration
The proposed strawberry guava biocontrol effort involves using the plant’s natural enemy, a scale insect called Tectococcus, to slow down the spread of this invasive species, thereby making it easier to stem the tide of invasion. Biocontrol does not replace the need to remove strawberry guava thickets by mechanical means (hand cutting or logging) and careful use of herbicides. The scale insect occurs naturally in Brazil and has been rigorously tested for many years as a biocontrol agent. It is not expected that the scale insect will kill strawberry guava plants outright but rather slow their spread and reduce the amount of fruit produced. It will likely take many years before we start to see the benefits of strawberry guava biocontrol, which is why it’s so important that we start this work now.
By slowing down the spread of strawberry guava, land managers can focus efforts on protecting the remaining intact native forests from this and other invasive species and restoring those forests that have been impacted by invasive species for the benefit of all in Hawai'i.
