Observations on the entomopathogenic fungus Hirsutella citriformis attacking adult Diaphorina citri (Hemiptera: Psyllidae) in a managed citrus grove

Abstract

A two-year field study was conducted in an orange grove in the United States (Florida) to characterize the phenology of the entomopathogen Hirsutella citriformis Speare infecting adults of the Asian citrus psyllid, Diaphorina citri Kuwayama. On the average, 23% of adults observed on mature leaves were killed by H. citriformis. These dead psyllids were characterized as being mummified and covered to various extents by synnemata produced by the fungus. Mummified cadavers were most abundant on citrus leaves during the fall and winter months, with the maximal percentage of mummified psyllids sometimes exceeding 75% of the total number of adults observed. Mummified cadavers were nearly absent each spring, presumably because relative humidity levels were suboptimal for the fungus at this time. Based on dispersion analyses, a monitoring plan for mummified cadavers would best include multiple samples in individual trees as well as multiple tree samples throughout a grove. Mummified cadavers with synnemata, which serve as point sources for new infections of the fungus in psyllids, were observed to remain on leaves for a mean of 68 days (one cadaver remained on a leaf for 168 days). Rainfall was positively correlated with the number of days mummies remained on leaves while mean daily air temperature was negatively correlated. Mummified cadavers were abundant in the summer during 2006 but not during 2007. This may have been a density-dependent consequence of low psyllid host populations in the grove in 2007. Alternatively, combination sprays of oil and copper applied during 2007 may have suppressed the fungus. This latter possibility prompted a laboratory investigation into the toxicity to H. citriformis of six chemicals commonly used in citrus. Copper hydroxide, petroleum oil, and elemental sulfur at maximum label rates each significantly reduced the infectivity of a laboratory culture of H. citriformis while copper sulfate pentahydrate, aluminum tris and alpha-keto/humic acids did not. This finding indicates that citrus growers interested in capitalizing on H. citriformis as a biological control agent of D. citri should avoid applying high rates of copper hydroxide, oil or sulfur.