Submitted to: International Symposium on Biological Control of Weeds
Publication Type: Proceedings
Publication Acceptance Date: November 1, 2003
Publication Date: April 1, 2004
Citation: Moran, P.J. 2004. Opportunistic and plant-mediated interactions between neochetina spp weevils and the fungal pathogen cercospora piaropi fungi on waterhyacinth (eichhornia crassipes). International Symposium on Biological Control of Weeds. (Proceedings) Interpretive Summary: Waterhyacinth is a non-native floating plant imported from South America over 100 years ago that is now causing serious weed problems in the Rio Grande, irrigation canals and reservoirs in the Lower Rio Grande Valley of Texas and throughout the southeastern U.S. USDA scientists released two waterhyacinth weevils as biological control agents in the 1970s. The weevils feed on the leaves and can reduce plant growth, flowering and reproduction. A fungal disease native to the U.S. causes brown spotting on leaves and also reduces the ability of waterhyacinth to cause problems. How do the weevils and the disease interact when they attack the plant at the same time? This study found that high levels of spotting on old leaves lead to more beetles feeding scars on tender young leaves. However, levels of spotting and scarring vary a lot between four field sites and within sites. Might there be an explanation inside the plant? This study found that waterhyacinth plants with high levels of spotting also had high levels of peroxidase, an antioxidant enzyme in the plant, and higher levels of the mineral, potassium. Phenolics, a type of chemical the plant makes that reacts with peroxidase, were lso higher in concentration in plants with lots of spots. A properly timed human release or a natural infestation with a combination of the fungus that causes spotting and the weevils will likely improve biological control of waterhyacinth.
Technical Abstract: Insect biological control agents of weeds may aid infection by plant pathogens by generating wounds or vectoring. Pathogen infection may lead to plant biochemical changes that alter host suitability for insects. In waterhyacinth (Eichhornia crassipes), adult Neochetina bruchi and N. eichhorniae weevils feed mostly on immature leaves, while symptoms of infection by Cercospora piaropi, a fungal pathogen, occur mostly on old leaves. This study examined associations between weevil scarring and spotting (necrosis) on leaves, and determined if necrosis is associated with levels of biochemical factors that may influence weevil feeding. Scarring and necrosis scores were positively correlated across four field sites sampled at four times. Two of the four sites tended to have higher scarring and necrosis scores, but scores were not correlated at individual times. Total available carbohydrate (TAC), potassium and phenolic contents did not vary across sites in a manner consistent with necrosis scores. Peroxidase activities and potassium levels in furled leaves were positively correlated to necrosis scores in oldest nonsenescent leaves. Phenolic content in late-season samples was also correlated to necrosis. Prior C. piaropi symptom production on old leaves of cultivated plants did not influence weevil feeding on young leaves. Infection and symptom production does not alter the feeding of weevils by changing plant biochemical components.