Hungry mite may quell old world climbing fern

Agricultural Research, July, 2004 by Marcia Wood

To make sure the mite will attack just climbing fern but not fern relatives, crop plants, or trees and flowers in someone's backyard, the Indooroopilly researchers tested it with more than a dozen plants, including fern relatives from Cuba and other parts of the Caribbean and South America. Florida's proximity to those parts of the world makes this aspect of the mite's background check especially important.

Mites' Meals Balance Ecosystem

Mites damage ferns by puncturing the edges of fern fronds with their tube-like stylets and other mouthparts, sucking up the nutritious contents of frond cells. If the mite attack is successful, in about 3 days the fern leaf will curl downward and inward, rolling over itself two or three times. This reduces the amount of leaf space available to capture light, which the plant needs to make its food. Eventually, the afflicted frond tissue dries up and fails off. Until that happens, however, the swollen, curled edge makes a cozy shelter for the mites. There, they can feed, lay their eggs, and hide from their enemies. As many as 20 adults can crowd into a curl. And there's enough room for female mites to fit hundreds of their little eggs into this refuge.

When the injured leaf edge falls off, the adults merely migrate to another frond, beginning the cycle again. Notes Goolsby, "In the fern's native range, this damage is both subtle and significant. It causes the plant to grow slowly and to stay in balance with other plants."

As much as the researchers have learned, there's still a piece missing in the puzzle. "We had tools of modern biotechnology to help us differentiate the look-alike fern genotypes," Goolsby says, "but we still don't know how mites tell the difference." (See box, page 13.)

Coming to America?

In Florida, the mite may eventually be joined by two other natural enemies of climbing fern. They are small moths, each no more than a half-inch from wingtip to wingtip. In their caterpillar stage, they feed on fern fronds.

Austromusotima camptozonale, collected from climbing fern in Australia, is bright white with a few black and brown spots and stripes on its wings. Neomusotima conspurcatalis, from Australia, India, Malaysia, and Thailand, has dark-brown wings, edged in white and dotted with white, boomerang-shaped flecks.

Perhaps this trio of Old World climbing fern's natural enemies can turn the tide against this pernicious Everglades invader.

Microscopic Look at Mite Mouthparts

It's already known that when mites successfully attack fern plants, fronds form tight curls that provide perfect food and housing for the little attackers.

But the mystery of why mites of certain genotypes attack certain Old World climbing fern genotypes--but not others--remains unsolved. Scientists have recently ruled out the possibility that the stylets of certain mites are too short to puncture cells on the front surfaces of some ferns.

Thomas Freeman of North Dakota State University, Fargo, used scanning electron microscopy to determine that mite stylets are 10 micrometers long, certainly long enough to penetrate frond surface cells, which he showed are only 1 or 2 micrometers thick.