Attack of the Killer Weed

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

Five living-room-sized greenhouses are attached to the general quarantine lab, which is where routine work is done. To reach the maximum security lab, which is for work on plant diseases, you must pass through yet more safety chambers. There are 13 doors between the outer world and this inner sanctum, with an eerie, muffled clank signaling you've earned the clearance to move through the next door.

"We can't work with viruses or bacteria, but we can work with fungal pathogens, which is mostly what we're interested in," Center says. "They [federal environmental authorities] will give us permission on a case-by-case basis."

The building is a huge leap forward for the dozen entomologists, plant pathologists, and ecologists working here on biocontrol. Until now, these researchers have used two small greenrooms at the University of Florida in Gainesville. "It's been incredibly productive," he says. "They've developed over 20 biocontrol agents in maybe the last 25 years. But it was small and too far away. With the invasive species problem getting bigger and bigger, we needed more space."

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.
Plant scientist Min Rayamajhi has helped develop a small army of insects in the battle against melaleuca.
Stephen Ausmus
Plant scientist Min Rayamajhi has helped develop a small army of insects in the battle against melaleuca.

They will now be able to test several, maybe up to a dozen, kinds of insects at the same time. "Before, we had to do it one at a time," Center says. "Our limitation had been space and funding; now, it's manpower."

Center opens a large glass door into a greenroom, where one of the first experiments is under way. A score of young melaleuca trees rise up a yard in small black pots. Clinched around their leaves are a series of screen cylinders. Inside is a particular breed of fly that carries within it minuscule worms. The two carry on a mutually beneficial relationship: The fly lays eggs inside the buds of the melaleuca, which is a source of food for the larvae when they hatch. But the fly also inserts the worms. This tricks the tree into believing that the hungry worms are part of its blossom system, and it steps up the amount of nutrients it's sending to the buds.

In essence, the tree wastes valuable photosynthesis energy on swelling buds that will never bloom because the fly larvae will consume the buds after birth.

The payoff for the enemies of the melaleuca is that the tree's ability to bear seeds is largely stymied.

"We recognized very early on that basically a dead tree and a nonreproducing tree are the same thing, from a biological standpoint," Center explains. "If it's not producing seed, it's not doing any harm. You don't really have to kill the tree to control it."

Biocontrol has a long and reliable history. Aquatic invasives were targeted in the 1960s, primarily because few herbicides are licensed for use in water. Alligator weed, a green, spaghetti-like plant, was a particular problem for boaters and fishermen. "Three insects were introduced to control it," Center says. "Within a few years, it was no longer a problem, although it's still a problem in the Mississippi Valley, where it gets too hot for these insects to survive. Alligator weed is still here at a low level, and so are the insects. When the alligator weed pops up, so do the insects."

Center's work with biocontrol began with aquatic nuisances, such as water hyacinth, a free-floating plant with beautiful flowers; and hydrilla, a submerged vine. One insect eventually released on hydrilla offered a hard lesson on just how specialized these herbivores are to their native terrain. In this case, the scientists, after years of testing and seeking government approval, released an Australian weevil that feeds on the stems of the hydrilla.

"This weevil looked really effective because it would burrow down the stems, mowing them off four or five feet below the surface," Center recalls. "We tested it; it was safe. We released it, never saw it again." That happened repeatedly. What they eventually surmised was that the weevil larvae continued down the stem to the lake bottom. The species had apparently adapted to extreme wet/dry seasons in its outback homeland, so that when its larvae become fully grown, they just wait for the water to dry out -- a seasonal extreme that doesn't occur in Florida. So it's back to the drawing board.

Still, even if you find the right bug for the right place, biocontrol takes patience. Although it's a very general rule of thumb, establishment in the wild is considered successful only after five generations have come and gone -- which is judged simply by the amount of time that's passed. It can take years to build up enough insects to reach a critical mass that can debilitate its host plant. And some insects just aren't as inclined to spread as others.

Melaleuca has long been recognized as a problem in South Florida, but it's now the poster boy for biocontrol success.

Like so many invasives, it was brought here from Australia as an ornamental lawn tree in the early 20th Century. With waxy green leaves the size of a small banana peel, it was fast-growing and made an excellent, full hedge. Nicknamed the paper-bark tree, it's far from attractive once it reaches maturity, however, when its whitish bark peels off in onion paper-like sheets.

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