Anti-insect defenses of Achnatherum robustum (sleepygrass) provided by two Epichloë endophyte species

Abstract

Many pooid grasses (Poaceae) harbor Epichloë species (Hypocreales), endophytic fungi that often produce toxic alkaloids which may provide anti-insect protection for their hosts. Two natural populations of Achnatherum robustum (Vasey) (sleepygrass), in the Lincoln National Forest, Cloudcroft, and Weed (NM, USA), are infected with the endophyte species Epichloë funkii (KD Craven & Schardl) JF White and Epichloë sp. nov. We tested whether: (1) these endophytes affect survival, growth, and development of the insect herbivore Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae) (fall armyworm), (2) larval diets alter adult fecundity (assessed as number of larvae or eggs produced by females and number of spermatophores that males transfer to females when enclosed in pairs within each feeding group), and (3) infections affect leaf consumption in larval no-choice and choice experiments. Individual larvae were reared on Epichloë infected vs. uninfected clipped leaves from the Cloudcroft and Weed population plants. Overall, armyworm survival was not affected when fed infected sleepygrass from either population. However, larvae that fed on Weed-infected plants were smaller and had longer development than larvae that fed on uninfected and Cloudcroft-infected plants. Males fed on Weed-infected leaves had reduced mating success. Interestingly, pupal mass increased when larvae fed on either the infected leaf types. However, heavier females from both infected diets did not lay more eggs than lighter females from uninfected diets. In a no-choice test, larvae on Weed-infected plants diet consumed more leaf biomass than larvae from three other groups. In choice tests, larvae avoided feeding on leaves infected with either of the endophytes relative to uninfected leaves. Thus, the two Epichloë may provide direct protection to sleepygrass from insect herbivory by deterrence. The Weed population endophyte may provide stronger indirect protection than the Cloudcroft endophyte by reducing insect fitness or increasing risks of predation and parasitism through delayed development, even though larvae may consume more leaf biomass.