Die, Weed, Die!

An alien moth munches an invasive fern into extinction

It quickly made its way into the wild. Nurserymen helped the spread after realizing they could save time by sowing the seeds in marshes rather than in planters. One early nurseryman chartered a plane and dumped cupfuls of seed onto the marshes. In the Thirties it was spread aerially over the Everglades in the mistaken belief it was a way of drying up "useless swampland." It grows extremely fast. Center recalls seeing seeds reach twenty feet in eighteen months in ideal growing conditions.

By the Fifties, melaleuca had become a problem tree. Once it catches hold in an area, the trees form a jam-packed canopy that shrouds the understory in near darkness. Nothing grows beneath the thickets -- except small melaleucas. An acquaintance of Center's recalls once seeing a spooked deer run toward a melaleuca stand, only to bounce off it because it was so dense.

The trees alter the landscape of the Everglades, especially when it comes to fire, which is a natural part of the ecology. Normally the wildfires are "cool," which doesn't kill the native plants. But when the flames hit melaleuca -- an invader that didn't evolve with the fires -- the flames become intense as they catch the paper-bark tinder and kill everything near it. If a melaleuca dies, it drops its seeds -- roughly 60 million in an average tree. Natural vegetation has a hard time competing with it.

Owen Richardson
The caterpillars of this moth, the Austromusotima camptonozal, eat 
Old World fern. It's the first insect released against the dreaded plant -- 
but not the last
Christine A. Bennett
The caterpillars of this moth, the Austromusotima camptonozal, eat Old World fern. It's the first insect released against the dreaded plant -- but not the last

Largely in response to the melaleuca and a handful of other invasives, the Florida Exotic Pest Plant Council was formed in 1984 by control agencies, plant nurseries, and environmentalists, and it soon held a symposium that included the Agricultural Research Service, which is the USDA's in-house research arm. "Most of the people involved originally had an environmental motivation," Center recalls. "Some economics followed because the only way they could control melaleuca was to cut a ring at the bottom of each individual tree and use full-strength herbicide. There could be 10,000 trees to an acre, and the cost was $4 a tree. So you were talking about $40,000 an acre."

The USDA had already established a lab in Australia as part of the hydrilla biocontrol effort. "Our funding was running out for that, and we had staff over there, so we just changed gears to look for bioagents for melaleuca," Center says. More than 400 species of insects feed on the tree in Australia, and the researchers began narrowing the field to insects that would eat melaleuca only. The tree contains the same kinds of oils as a eucalyptus tree, which are basically a defense against herbivores, so anything that eats it is usually specialized. That's good, because it reduces the chances the insects will be interested in Florida's native plants.

Far from being a crap shoot, it's quite predicable what insects will eat, Center asserts, as he opens a thick scientific treatise called Insects on Plants. "People get the idea that we're going to release an insect and it'll eat up the melaleuca and then it'll go eat everything else that's green. That's not the case. If it takes out the plant it feeds on, their populations usually crash." He estimates biocontrol scientists worldwide have released about 300 species of insects against roughly 150 species of plants -- with virtually no unintended consequences.

In 1997 a grayish weevil that looks a bit like a tiny horny toad was released against the melaleuca. A plant louse, which in its nymph form sucks the tree's juices and injects it with a toxic juice, was released in 2002. If you wonder how anything so small could ever raze hundreds of acres of 200-foot melaleucas, Thai Van and Min Rayamajhi have developed an outsize show-and-tell. Plant physiologists at the lab, they've been monitoring an experiment for eight years just west of Highway 27 in Broward County. A sweeping stand of melaleucas has overgrown scores of acres owned by a utility company, which has killed many trees with herbicide. New saplings are eagerly shooting up to replace them.

In the center of the property, however, lies a roughly twenty-acre section that's been left untreated. Deep inside is the test plot Van and Rayamajhi routinely monitor to measure the effects of the bioagents.

They weave their way about 50 feet into the thicket. "Look at this one," says Rayamajhi as he grasps a small melaleuca shrub with a rusty color on its leaves. "It got hit by disease, and it's trying to grow back. The rust is disease. This one got hit by weevil."

Van picks up what looks like a miniature brown cob of corn. This is the melaleuca seed fruit. Each of its hundreds of "kernels," which are the size of a needle head, can hold about 400 seeds.

Even the tallest trees are at risk from the good weevils. As trees grow bigger, they require more nutrients via photosynthesis. Thus a large canopy of leaves is essential. "The food production goes down because the insects feed on leaves and disease hits the trees," Rayamajhi says. "The tree diverts its energy to fight the insects, and then it doesn't have enough food to reproduce. Then they'll die. People wonder how little bugs can kill these big trees, but they're actually very vulnerable."

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