The pathogenicity of Fusarium spp. to Wheat Stem Sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae)
The wheat stem sawfly is the most destructive insect pest in both winter wheat and spring wheat production in the northern Great Plains. The sawfly is univoltine, and spends all immature stages within protective wheat stems, which explains the difficulty in controlling populations. However, the almost continuous inhabitation of stems also makes larvae more vulnerable to invasion by microorganisms colonizing both living stems and postharvest stubble. Fusarium spp. were frequently isolated from fungalcolonized larval cadavers, and were found to be the major lethal factors for overwintering larvae in both laboratory emergence experiments and field surveys over three years. The pathogenicity of single isolates of three Fusarium spp., including F. pseudograminearum, F. culmorum, and F. acuminatum, was evaluated against overwintering larvae in sawflycut stems and against actively-feeding larvae in growing winter wheat plants over two years. The tested Fusarium isolates caused twenty to sixty percent mortality in overwintering larvae, and caused forty to eighty percent mortality in actively-feeding larvae. The Fusarium isolates also caused decomposition of sawfly-cut stems and disease in wheat plants, which reflected the versatility of Fusarium isolates acting as saprophytes, entomopathogens, and phytopathogens. Deoxynivalenol was detected in wheat stem tissues colonized by the Fusarium isolates from two years of field experiments, and deoxynivalenol caused toxicity and inhibited the growth of second and third-instar actively-feeding larvae in laboratory bioassays. Wheat grower observations of greater sawfly infestation in dryland wheat fields led to assessment of larval mortality from Fusarium infection in both dryland and irrigated wheat fields. This was studied using cage experiments or field surveys at different locations for three years. Parasitoid attack and fungal infection, mainly by Fusarium spp., were found to be the major lethal factors for developing and overwintering larvae, respectively. There was no difference in sawfly survival in dryland or irrigated wheat fields. As ubiquitous soil microorganisms and plant pathogens, Fusarium spp. impact wheat stem sawfly populations in the field each year.