Attack of the Killer Weed

Can a hardy little moth stop the fern that wants to eat the Everglades?

It quickly made its way into the wild. Nurserymen helped the spread after realizing that they could save a lot of 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 1930s, it was spread aerially over the Everglades in the mistaken belief that it was a way of drying up "useless swampland." It grows extremely fast. Center recalls seeing the trees reach 20 feet in 18 months in ideal growing conditions.

By the 1950s, 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 for small melaleucas. An acquaintance of Center's recalls once seeing a spooked deer run toward a melaleuca stand only to bounce off 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 -- which didn't evolve with the fires -- the flames become intense as they catch the paper-bark tinder and kill everything near it. The charred melaleuca drops its seeds -- roughly 60 million from an average tree. Native vegetation has a hard time competing with it.

FIU Professor John Volin has studied Old World  fern for six years and holds a grudging respect for the plant's amazing adaptability.
Colby Katz
FIU Professor John Volin has studied Old World fern for six years and holds a grudging respect for the plant's amazing adaptability.

Largely in response to the melaleuca and a handful of other invasives, the Florida Exotic Pest Plant Council was formed by control agencies, plant nurseries, and environmentalists in 1984, and it soon held a symposium that included the Agricultural Research Service, which is the U.S. Department of Agriculture'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 down the field to insects that would eat only melaleuca. The tree contains the same kinds of oils as a eucalyptus tree; these oils are basically a defense against herbivores, so anything that eats it is usually quite specialized. That's good, because it reduces the chances that 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 that 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 minuscule horny toad was released against the melaleuca. A plant lice, 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 lay low hundreds of acres of 200-foot melaleucas, Thai Van and Min Rayamajhi have developed an outsized show-and-tell. Plant physiologists with the lab, they've been monitoring an experiment for eight years just west of Highway 27 in Broward County. A sweeping stand of melaleuca 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 20-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," Rayamajhi says as he grasps a small melaleuca shrub with a rusty color on the 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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