Submitted to: Biocontrol
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/5/2004
Publication Date: 6/1/2005
Citation: Moran, P.J. 2005. Leaf scarring by waterhyacinth weevils (Neochetina eichhorniae and N.bruchi) enhances infection by the fungus Cercospora piaropi on waterhyacinth, Eichhornia crassipes. Biocontrol. 50(3):511-524. Interpretive Summary: Insects and diseases attack weeds just as they do crop plants. Because they damage plants and reduce their ability to grow, insects and diseases can act as natural enemies, providing biological weed control in a way that reduces chemical herbicide use and economic costs. A key question is the ability of different insects and diseases to influence each other when they attack the same plant. Waterhyacinth, originally from South America, is a fast-growing, economically-important floating weed in canals, ponds, lakes,and rivers in the Southern USA. Two beetle species brought to the U.S. from Argentina make scars on leaf surfaces. A fungus from the U.S. infects leaves and kills patches of tissue. Might the beetles and the fungus in combination provide more control than either enemy by itself? I grew waterhyacinth plants in confined plots in a reservoir in south Texas and exposed them to these enemies. The fungal symptom developed more quickly and reached a higher level when I allowed beetles to scar the leaves before I infected the plants with the fungus. The combination of beetles and fungus reduced the ability of the plants to reproduce and make new leaves. Beetles mostly didn't care if they were feeding on infected or healthy waterhyacinth, and the plants were not able to defend themselves against infection by changing their chemical contents. So, the beetles don't mind having the fungus around, since the fungus doesn't make it tougher for the beetles to feed. These findings can be used to change beetle release strategies to match times when the fungus is abundant, to improve biological control.
Technical Abstract: Biological control of waterhyacinth (Eichhornia crassipes) could depend on interactions among insects and plant pathogens. In field plots infested with adult Neoechetina bruchi and Neochetina eichhorniae waterhyacinth weevils, leaf laminae were scarred by weevil feeding. Plots were inoculated with the fungal pathogen Cercospora piaropi, which causes necrotic spotting on leaves. The rate of development of this disease symptom was elevated 7.5-fold or more in plots previously augmented with weevils compared to plots receiving only C. piaropi. Seventeen days after inoculation, the percent laminar area on youngest and second or third youngest leaves covered by necrotic lesions was 2.4-fold or higher in plots containing weevils. Scar density and necrosis coverage on young leaf laminae were positively correlated. Plots subjected to combined weevil and fungal infestation had 32% lower shoot densities and 18% fewer leaves compared to plots receiving no augmentation. In no-choice bioassays, weevil feeding was reduced 44% on oldest leaves on inoculated compared to uninoculated plants, but did not differ in younger leaves. Protein content and soluble peroxidase activity in young and old leaves were not influenced by C. piaropi inoculation. Additive or synergistic control by waterhyacinth weevils and the fungus is possible but depends on plant and environmental suitability for infestation and infection.