The effects of release site hydrology on biological control agents for introduced Phragmites in Canada

• Archarana neurica and Lenisa geminipuncta eggs can survive periods of submersion. • Predation rates of eggs in the field were low in both wet and dry locations. • Releases of Archanara neurica performed equally well in wet and dry habitats. • Archanara neurica significantly affected Phragmites growth in the year of release. • Phragmites biocontrol can be effective across a range of hydrological conditions. Local hydrological conditions play a major role in shaping insect assemblages and can drive geographical and localized variation in the success of biological control agents. Archanara neurica and Lenisa geminipuncta , two insects for Phragmites australis spp. australis (introduced Phragmites ), are being released in Canada, but it is unknown whether agent releases will work in sites with standing water. We addressed this knowledge gap by (1) assessing the submergence tolerance of A. neurica and L. geminipuncta eggs in laboratory conditions, (2) comparing predation of A. neurica eggs in wet and dry habitats, and (3) examining the effects of hydrological conditions on the success of A. neurica releases and subsequent population persistence. Submersion of A. neurica and L. geminipuncta eggs for up to six weeks in the laboratory had no negative effect on egg viability. In the field, egg mortality due to predation between July 2024 and May 2025 was not affected by hydrological conditions. Finally, releases of A. neurica eggs and larvae performed equally well in wet and dry locations, and both significantly affected Phragmites stand characteristics (stem density or panicle formation) within release plots. The success of A. neurica releases in flooded environments suggests that the moths can be used in wet release sites that might previously have been avoided. The ability to release the moths over standing water will be crucial for the integration of biological control with conventional management tools, the use of which can be restricted in flooded environments.