Biological Control of Weeds: Invasive Plant Research Laboratory

Gary Buckingham, Gainesville.USDA-ARS


The IPRL has two locations, Ft. Lauderdale and Gainesville. Ft. Lauderdale is the main location where research on the release and evaluation of biological control agents is conducted. Research on safety testing in support of permission for release is conducted in Gainesville at the Florida Division of Plant Industry's quarantine facility. Currently there are two main target weeds, melaleuca or paperbark tree and Old World climbing fern. Additional target weeds are waterhyacinth, hydrilla, giant salvinia, and skunkvine.

The Australian melaleuca tree, Melaleuca quinquenervia, has invaded the Everglades region and other parts of south Florida north to about Sarasota. Vast monocultures grow in wetlands and along their margins. The tree eventually grows to 50 ft or more. Two biocontrol agents, both insects, have been released in Florida to control melaleuca. Both were tested against a wide array of melaleuca relatives and cultivated plants.

The first insect released against melaleuca was the melaleuca snoutbeetle, or weevil, Oxyops vitiosa. It was released in 1997 and is now found throughout much of the range of melaleuca. Although it spreads on its own, the Ft. Lauderdale researchers redistributed it widely to speed up the spread. Adult snoutbeetles feed mostly on the young leaves where females lay their eggs. Although adults do damage the leaves, their damage is not as important as damage by the leaf-feeding larvae. The blackish larvae scrape tissue off young leaves so that only a thin transparent leaf layer remains. These damaged leaves die. Although mostly young leaves are eaten, eventually trees are defoliated because as the leaves age and fall off, there are no young leaves to replace them. Extensive snoutbeetle damage is now visible in many melaleuca stands, especially on the west coast of Florida. In a field that has been infested for 5-6 years and in which the young plants were occasionally mowed for pasture maintenance, Ted Center, Research Leader of IPRL, has reported that the plant population is about 20% of the original. The mowing helped produce young leaves, coppice growth, which was the preferred food for the weevil. However, even large trees on the margin of the pasture are dying.

The second insect released against melaleuca was a sap-sucking psyllid, Boreioglycaspis melaleucae, released in winter/spring 2002. This small yellowish psyllid, 2 mm, is also from Australia. Adults and young feed on the phloem and consequently produce abundant honeydew. Most of the plant damage is caused by the psyllid nymphs that kill the leaves and stems. The young psyllids live in small groups and produce a white wax that protects them from weather and natural enemies. These psyllids have established large populations at most release sites. Estimates of numbers per acre are in the multimillions. The Ft. Lauderdale researchers are now redistributing the psyllids and they are also spreading by themselves.

A third melaleuca insect, a gall-forming fly, Fergusonina sp., also from Australia, was tested in quarantine (Susan Wineriter) and a petition for its release was submitted in autumn 2002. TAG has reviewed it and we are awaiting news from APHIS. The fly galls the buds of melaleuca where multiple fly larvae feed and develop. This bud damage stunts the growth of melaleuca and should reduce seed production. Along with the eggs, females deposit a symbiotic nematode, Fegusobia sp., into the tissue. Gall initiation appears to be stimulated by the nematode. When fly larvae hatch, the tissue forms a chamber around each larva. Nematode females penetrate only female fly larvae and the juveniles enter the oviducts and the cycle begins again.

Old World climbing fern, Lygodium microphyllum, is practically exploding in the Everglades and throughout south Florida. The minute spores are windblown like dust, and plants are being found on almost all tree islands in some parks and in many habitats. This weed has the potential to dwarf many of the other major weed problems.

No insects have been released yet against Old World climbing fern, but one Australian moth has been tested in quarantine and another is in process (Chris Bennett). A petition for permission to release the moth, "Cataclysta" camptozonale, was submitted in March 2003 to the federal Technical Advisory Group. The moth did not feed on Florida's native and cultivated ferns, but it did develop on a temperate congener, Lygodium palmatum. However, the native range of L. palmatum is far outside the potential range of the moth.

Other weed projects of IPRL are surveys in South America for additional insects to control waterhyacinth (Ted Center), release and evaluation of weevils to control giant salvinia or waterfern in the western US (Phil Tipping), and surveys in Asia and quarantine testing in Hawaii for skunkvine insects (Bob Pemberton and Paul Pratt). An area wide TAME project at IPRL is coordinating use of the melaleuca insects (Paul Pratt and Cressida Silvers). Surveys and testing of plant pathogens are conducted on various plants (Min Rayamahji) and basic research is conducted on the insects and plants (Greg Wheeler, Thai Van, and Allen Dray).