Effects of elevated CO ₂ on feeding responses of biological control agents of Pontederia crassipes

Abstract Elevated carbon dioxide (eCO 2 ) and rising temperatures will have far‐reaching effects on global plant‐insect interactions, yet their implications for future biological control programs are not fully understood. Studies have shown that elevated CO 2 will affect insect feeding guilds differently and these responses can be predicted with some confidence. Water hyacinth, Pontederia crassipes Mart. (Pontederiaceae), is a native and representative species of the Del Plata wetlands (Argentina) that invades outside of its native environment. It is considered one of the world's worst aquatic weeds and a target for biological control. In this study, water hyacinth plants were grown under two CO 2 concentrations – current (400 p.p.m.) or elevated (800 p.p.m.) –, with and without two biocontrol agents representing different feeding guilds, the leaf‐chewing Cornops aquaticum Brüner (Orthoptera: Acrididae) and the phloem‐feeding Megamelus scutellaris Berg (Hemiptera: Delphacidae). Under eCO 2 concentration, photosynthetic rate, total dry weight, and relative growth rate of P. crassipes acclimated to eCO 2 conditions and plants showed very little CO 2 fertilization response in eutrophic water. Insect herbivory varied depending on feeding guilds at eCO 2 ; however, P. crassipes growth responses increased when exposed to insect herbivory. Chewing herbivory by C. aquaticum was consistent across CO 2 conditions, whereas the feeding by M. scutellaris increased substantially at eCO 2 . These results indicate that successful biological control of P. crassipes under conditions of elevated CO 2 might rely on phloem‐feeding insects, with chewers playing a lesser role.