sealHawai'i Invasive Species Council

 "Slowing Down Strawberry Guava"

HISC eNews  

July 16, 2008

Featured Update
Strawberry Guava
The U.S. Forest Service is preparing to release a Brazilian scale insect to attack strawberry guava, a threat to native forests and watersheds.

Invasive species are altering the lush landscape of native Hawaiian forests on a staggering scale. The native forest, important to Hawai'i's cultural heritage and ecology, is a complex ecosystem that supports myriad life forms. However, it is being overrun by invaders that shade out native species from sunlight, smother roots and slow growth, keep rainwater from penetrating the earth, impact habitats for native birds and insects, and impede access to traditional Hawaiian resources, as well as recreation.
The slow growing 'ōhi'a lehua and the majestic koa, both found only in Hawai'i, are examples of unique native plant species at risk in the forest. Hundreds of thousands of acres of native forests across the state are being replaced by the competitive strawberry guava which lacks natural enemies that might keep it under control and whose dense stands crowd out other species.
While the strawberry guava has become a favorite in the islands for its fruit and wood, its destructive spread is now beyond manual control in the forest. Each tree produces dozens of fruit, each producing about 30 seeds which are spread by birds and pigs. 
Researchers are proposing biocontrol as a viable control method. Biocontrol is based on the principle that the specialized insects or diseases that keep the plant in balance in its native range (Brazil) can be used, after careful testing to check their safety, to help restore balance in Hawai'i, without causing harm to other species.
Tectococcus ovatus is a scale insect that naturally occurs with strawberry guava in Brazil, where both are native. In Brazil, these and other natural enemies keep the strawberry guava population growth low. Tectococcus larvae feed and develop on growing leaves of strawberry guava, causing the plant to form a gall around them. The gall production means less energy available for fruit production and growth, resulting in slower growth and fewer seeds, but the trees would continue to produce new leaves and some fruit. Introduction of this insect to Hawai'i is intended to reduce the invasiveness of strawberry guava. 
"The most common concerns are that people don't want the strawberry guava killed or other plants affected," said Tracy Johnson, U.S. Forest Service biocontrol researcher. "I explain that the biology of these insects makes them so dependant on strawberry guava as a host that they cannot live without it. So the insects are highly specific, they do not kill strawberry guava, and we have 15 years of tests showing the insects will not move to other Hawaiian plants. Tectococcus would reduce the growth and reproduction of strawberry guava, which would help prevent further destruction of native forests, and allow slower-growing native plants like 'ōhi'a and koa a chance to compete."
The USDA Forest Service plans a series of statewide public information and listening sessions about the proposed release of the insect Tectococcus to slow the spread of strawberry guava. Listening sessions begin this September and will help the public to become familiar with the proposed biocontrol insect, to ask questions, and to provide input prior to completion of a new draft Environmental Assessment (EA).

On O'ahu, one only needs to take a hike up the Wa'ahila Ridge trail to see strawberry guava and its dense, nearly impassible stands. Much of the first mile of the hike is dominated by thickets of nothing but strawberry guava to the left and the right of the path. Here and there, a stately koa or 'ōhi'a stand out from the strawberry guava but there are few if any 'ōhi'a or koa keiki at their feet, only more strawberry guava. This scenario is played out across the state where strawberry guava is spreading over thousands of acres of watershed forests, replacing all native plants and the native animals that depended on them. 
Studies have also shown that strawberry guava forests are half as effective at holding and cycling water into the watershed, when compared to 'ōhi'a forests. This HISC eNews will highlight some of the work being done to control this aggressive weed. In particular, this is an update on the upcoming revised Environmental Assessment for the release of a strawberry guava specific biocontrol. Granted, biocontrol is almost never the complete answer to an invasive weed problem, but in the case of strawberry guava, it may reduce the aggressive spread enough to give native plants a chance to compete and allow other control efforts to be more effective over time.

PC Chee signature
Patrick Chee
Hawai'i Invasive Species Coordinator
Strawberry Guava Thicket 
A dense thicket of strawberry guava in a Hawaiian forest. Photo by Don Gardner
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