Hydrilla verticillataThe fight for control

Carey Minteer

Department of Biological Sciences

University of Arkansas

Methods

• Mechanical

• Chemical

• Biological

Mechanical

• Can be used for smaller populations

• Clear boat ways• Obvious problems

Chemical

• Flouridone- only chemical approved by the USEPA.

• Inhibits the phytoene desaturase (PDS) enzyme

• Rate limiting enzyme in carotenoid biosynthesis

Effects

• Causes photobleaching of green tissues (Böger and Sandman, 1998)

• H. verticillata is controlled in large bodies of water by sustaining a concentration of 4-12 µg/L for several weeks

Problems

• Expensive

• Hard to maintain proper concentration for proper amount of time

• Resistance

Resistance

• It is considered unlikely in higher plants that resistance can occur in the absence of sexual reproduction (Hill, 1982).

• However, three biotypes of resistant H. verticillata have been found (Michel et. al, 2004).

BiologicalThe Fungus and the Fly

Mycoleptodiscus terrestris (Gerd.) Ostazeki

Hydrellia pakistanae Deonier

The Fungus

• Mycoleptodisus terrestris (Gerd.) Ostazeski

• Collected from hydrilla growing in Lake Houston, TX in 1987 (Joye, 1990 and Shearer, 1996)

• Currently undergoing evaluation

Effects

• Fungus can greatly reduce hydrilla biomass.

• After inoculation disease symptoms appear in 5-7 days.

• Interveinal chlorosis is followed by a complete loss of color.

• Plants begin to disintegrate within 10-14 days. (Shearer, 1996)

The Fly

• Hydrellia pakistanae Doenier- Asian hydrilla leaf mining fly

• First released in the US on Lake Patrick, FL in 1987 (Buckingham et al, 1989)

• 750,000 larvae have been released in Lake Ouachita.

• 250,000 larvae have been released in Degray lake.

Biology

• Small, about 2 mm in length.• Live almost exclusively on or near H. verticillata

infestations.• Not strong flyers and appear to “hop” along the

water surface (Deonier, 1971)• Eggs are laid on H. verticillata• Females lay eggs one at a time, but can produce

several hundred eggs during her reproductive period (Buckingham et al, 1989)

Biology

• Eggs hatch in 3-4 days depending on temperature.

• Larvae tunnel or mine hydrilla leaves, where they feed and destroy 9-12 leaves during the three larval stages.

• Total development takes 20-35 days (Buckingham et al, 1989)

Effects

• Reduction in leaf area

• Reduces photosynthetic area and therfore reduces vigor and competitiveness (Doyle et al, 2002)

• Damage to canopy leaves approached 100% when infested with 4000 larva per m2 (Wheeler and Center, 2001)

Effects

• Under field conditions 20% leaf damage was shown when larval levels reached 2320 larvae per kg of Hydrilla tissue (Grodwitz et al, 1997).

• 50-75% surface canopy leaf damage when immature fly levels were greater than 6000 immature flies per kg of H. verticillata.

• Leads to a 30% reduction in biomass, relative to controls.

• 15-40% damage with 2000-4000 immature flies per kg (Doyle et al, 2002)

Problems

• Parasitic wasp, Trichopria columbiana, which could keep H. pakistanae from establishing.

• Field populations have rarely achieved the densities necessary to produce extensive damage to H. verticillata.

• Biocontrol agents are unlikely to eradicate invasive pests.

• Biological control agents reduce plant vigor and competitiveness, rather than cause direct mortality (Crawley, 1989)

Literature Cited• Böger, P. and G. Sandman.1998 Carotenoid biosynthesis inhibitor herbicides-

mode of action and resistance mechanisms. Pesticide Outlook, 9:29-35.• Crawley,M.J.,1989. Insect herbiours and plant population dynamics. Annual

Review. Entomol. 34:531-564.• Doyle,R.D., M.J. Grodowitz, R.M. Smart, and C. Owens. 2002. Impact of

herbivory byHydrellia pakistanae on growth and photosynthetic potential of Hydrilla verticillata.

• Grodowitz,M.J., T.D. Center, A.F. Confrancesco, J.E. Freeman. 1997. Release and establishment of Hydrellia pakistanea for the biological control of the submersed aquatic plant Hydrilla verticillata in the United States. Bio. Control. 9:15-23.

• Hill, RJ. 1982. Taxonomy and biological considerations of herbicide- resistant and herbicide-tolerant biotypes. In: Herbicide resistance in Plants. Pp 81-98. Wiley and Sons, Inc., New York, NY

• Joye, G.F. 1990. Biocontrol of Hydrilla verticillata with the endemic fungus Macrophomina phaseolina. Plant Disease 74:1035-1036.

• Shearer,J.F. 1996. Field and laboratory studies of the fungus Mycolptodiscus terrestris as a potential agent for management of the submersed aquatic macrophyte Hydrilla erticillata. Technical reportA-96-3. U.S. Army Engineer Waterways Experiment Station. Vicksburg, MS.