HERE ON THE BIG ISLAND: Battling Those Weed Trees


By Kelly Moran

Battling Those Weed Trees

A few months ago, I wrote about a tree that was brought here from Brazil

and that has gone terribly wild. It’s officially psidium cattleianum, but commonly called “strawberry guava”or waiawi (“vy-vee”), and it’s extraordinarily invasive: seeds from the fruit sprout easily wherever they fall, and are spread by birds and pigs; if the tree is cut down, it quickly regenerates from stumps and fallen branches, ultimately forming a dense thicket in which nothing else grows.

Researchers estimate that waiawi is now entrenched in more than 800,000 acres on the Big Island, and though its range may ultimately be limited by drier microclimates and higher alititudes, it is still in-filling where it’s already established, especially in Hamakua and Puna, where it squeezes out practically everything else, especially native and endemic species. It also draws fruit-flies, expanding their range, which frustrates efforts to cultivate more desireable fruit.

To fight this weed tree, the Hawaii Dept. of Land and Natural Resources, the Hawaii Dept. of Agrictulture, and the Forest Service of the U.S. Dept. of Agriculture propose to introduce a Brazilian insect called tectococcus ovatus,which severely weakens – but doesn’t kill – waiawi. It tunnels into the leaves, forcing the tree to make “galls” that contain the pest, instead of making new leaves. This is expected to slow the spread of waiwai, allowing people more time to cut thickets down and keep them down. The insect has no wings, and can move to adjacent trees only on the breeze; moreover, tests prove that it can live only on strawberry guava and on no other plant; so the release of this biological control agent is considered very safe.

Waiawi does have some practical uses. The fresh fruit, being in the guava family, are easily made into tasty jams and jellies; the wood, like other fruit-woods, makes an excellent smoke for curing meat and fish; and the trunks – if thick and straight enough – can turned into hardwood poles. So there is a small vocal contingent here, mainly in Puna, that objects to introducing tectococcus, in the name of “saving” the waiawi.

But, the USDA counters this misguided effort by pointing out that, if anyone actually wants to cultivate waiawi, or keep wild stands from being infected, they can do what farmers do for any other orchard crop: i.e., protect it with ordinary (preferably organic) insecticidal spray.

There is another invasive weed tree here that was introduced about the same time as waiawi; but it is currently being decimated without human intervention. The rose-apple (syzygium jambos), though not quite as aggressive as waiawi, tends to spread out more, and to form dark “tree-tunnel” arches over back-country roads. The fruit is rather dry: its “rose” being more of a scent than a flavor.

But rose apple trees are being attacked by a “rust fungus” disease that kills new growth and thereby starves the tree of energy. In a couple of years, many stands of rose apple will be bare and dead – and likely will be overtaken by waiawi, which is often found in the same areas.

There is a small but real danger that this rust could spread to other trees in the same (myrtle) family. The worst-case scenario would be a jump to native ohia. So Hawaii forest managers are urging the state to restrict new imports of nursery trees and other plant material that can harbor the rust. For more information about the rust,
click here