Hungry mite may quell old world climbing fern

Agricultural Research, July, 2004 by Marcia Wood

Though you might not expect so, some tiny mites that feed on plants are, apparently, exceedingly particular about exactly which ones they choose to eat. That seems to be the case with the eight-legged, leaf-curling Floracarus perrepae mites that ARS scientists in Florida and Australia would like to recruit. These small, tan-colored mites are, the scientists' studies have shown, top candidates for the big job of fighting Old World climbing fern.

This fast-growing, dark-green weed, Lygodium microphyllum, is on the march in Florida's unique and fragile Everglades ecosystem and other wetlands. In tact, climbing fern is spreading so rapidly that it's now the state's worst invasive weed.

First detected growing outdoors as an "escaped ornamental" in 1965, the fern now infests more than 100,000 acres in the Sunshine State.

Old World climbing fern smothers native plants by forming dense mats along the ground and by extending--vine-like--up shrub stems and tree trunks, forming massive green walls of vegetation. These curtains greatly increase fire danger by providing a flammable ladder stretching from ground to canopy. Making matters worse, fern leaves, called fronds, that catch fire often break off and sail away on the fire's updrafts, serving like torches to spread the blaze even further.

A Mite-y Adversary?

The F. perrepae mite acts as a natural enemy of the fern. That's why ARS entomologists John A. Goolsby at the agency's Australian Biological Control Laboratory at Indooroopilly, near Brisbane, and Robert W. Pemberton of the ARS Invasive Plant Research Laboratory, Fort Lauderdale, Florida, want to put the mite to work in Florida's fern-ridden wetlands.

The chances for their strategy to succeed have been greatly improved by the recent discovery of mite populations that may be a perfect match for the particular kind of climbing fern that is choking vegetation in Florida. Not all F. perrepae mites will feed on and make a home in the Florida fern, the scientists have learned.

"But we think we've found what, so far, is the right mite for the right fern," Goolsby points out.

Matchmaking Makes a Difference

The quest for the perfect pairing of mite and fern was spurred by Goolsby and cohorts' finding that F. perrepae mites collected in some regions of Australia wouldn't feed and reproduce on climbing fern collected from other locales, such as Florida.

The scientists combed climbing fern's native range, hunting for the plant and the critters that keep it in check in rainforests, dry seeps, coastal marshes, bay swamps, and tree islands. These treks took them to more than a dozen nations and territories, including Benin, China. Ghana, India, Indonesia, Malaysia, New Caledonia, Papua New Guinea, Singapore, South Africa, Sri Lanka, Thailand, and Vietnam, scouting more than 100 different sites. These exotic venues not only presented a significantly good chance of harboring important natural enemies of climbing fern, but were also home to adversaries of the scientists--including disgruntled elephants, fierce tigers, hungry crocodiles, and an array of poisonous snakes.

Goolsby and teammates from Australia's Commonwealth Scientific and Industrial Research Organization made it home unscathed--with a key discovery, too: They are the first to separate climbing fern into genetically distinct local races, or genotypes. "By comparing fern genetic material, we learned that the fern occurs in different genetic forms," says Goolsby. "We looked at the same stretch of DNA--a region known as trnF through trnL--in climbing ferns, and found an exact genetic match to the Florida genotype. It was growing alongside a stream in the Cape York area, a rugged, remote part of northern Australia where the fern wasn't previously known to exist.

"And a fern we collected in a rainforest in Thailand is nearly an exact match."

Miniature Fern Grotto Hastens Search

To speed the search for mites that were in sync with the Florida genotype, Goolsby developed the tactic of taking small plantlets, or sporelings, of the Florida variant along on expeditions so that local mites could be immediately exposed to this target plant within the confines of a mobile laboratory. (Of course, taking the miniature fern forest into foreign countries was done only with approval from the host nations.) This cafeteria-on-wheels approach to screening mites saved weeks of time that might otherwise have been spent on mites that don't favor the Florida fern.

Not surprisingly, the F. perrepae mites that Goolsby and colleagues plucked from the Cape York and Thailand ferns are today the best prospects for clobbering climbing fern in Florida.

In tests with climbing ferns grown in garden plots at the Indooroopilly laboratory for 24 months, F. perrepae mites were able to blunt growth of fern fronds, stems, roots, and other plant parts by half, in contrast to ferns kept mite-free with a miticide.

Goolsby and colleagues are the first to discover the potential of F. perrepae as a natural foe of climbing fern. The Indooroopilly team is now rearing millions of mites indoors, awaiting approvals necessary before the mites can be exported to the United States and released outdoors.