Antiherbivore Defense Mutualism Under Elevated Carbon Dioxide Levels: A Fungal Endophyte and Grass

Abstract

Previous studies have shown that insects commonly consume more when fed leaf tissue grown under CO2 enrichment, but with few negative effects on growth. However, lepidopteran larvae fed tissue infected with Balansiae fungal endophytes (which produce toxic alkaloids) typically eat less but also suffer negative effects on growth and survival. This study was carried out to understand how these 2 factors may interact to affect larval consumption and growth in fall armyworm, Spodoptera frugiperda . (J. E. Smith) (Lepidoptera: Noctuidae). Infected and uninfected ramets of a single genotype of tall fescue, Festuca arundinacea Schreb., were grown under CO2 concentrations of 400 and 700 μl CO2/liter of air. Relative consumption of leaf tissue by larvae was 32% greater in the high CO2 treatment compared with leaves grown under low CO2 concentrations, but was not influenced by infection. As expected, larvae had significantly decreased relative growth rates when fed infected tissue, with their growth rates somewhat increased under high CO2 levels. Increased CO2 level and infection both led to significantly reduced efficiency of conversion of ingested food. These 2 factors also interacted so that the lowest efficiency of conversion of ingested food was seen when both infection and an enriched atmospheric CO2 environment were present. As global atmospheric CO2 levels continue to increase, it appears that fungal endophytes will continue to be important in turfgrasses as protection against insect herbivores and may lead to increased fitness for infected plant genotypes.