Plant-mediated interactions between Neochetina spp. weevils and the fungal pathogen Cercospora piaropi on Eichhornia crassipes (water hyacinth)*

Insect biological control agents of weeds may aid infection by plant pathogens by generating wounds or by vectoring. Pathogen infection may lead to plant biochemical changes that alter host suitability for insects. In Eichhornia crassipes (water hyacinth), Neochetina bruchi and N. eichhorniae adult 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 fungal 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. At individual sampling times, two of the four sites tended to have higher scarring and necrosis scores, but scores were not correlated. Total available carbohydrate, potassium and phenolic contents did not vary across sites in the same manner as did necrosis scores. Peroxidase activities and potassium levels in furled leaves were positively correlated to necrosis scores in oldest non-senescent 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. Leaf scarring and necrosis were related, but fungal infection did not alter the feeding of E. crassipes weevils by changing plant biochemical components.