Field tests of egg and larval release methods of biological control agents (Archanara neurica, Lenisa geminipuncta) for introduced Phragmites australis australis (Cav.) trin. Ex Steud

Abstract

Field releases of the stem-boring moths Archanara neurica and Lenisa geminipuncta (Lepidoptera: Noctuidae) as classical biological control agents for introduced Phragmites australis australis (Cav.) Trin. ex Steud. began in Canada during 2019. Since then, we have developed and tested release methods for eggs and larvae of both agents. Experiments here include comparisons of: (1) egg survival from exposed “egg card” and protected “egg cup” release methods to assess the risk of agent predation and test mitigation strategies; (2) performance of egg cups overwintered at the release site and those placed shortly before emergence in spring to test for any potential phenological asynchrony between biological control agents and the target weed; and (3) effectiveness of egg cup and larval-inoculated stem releases at a large geographic scale across 15 field sites in southern Ontario. Egg card releases experienced high egg loss likely due to predation (∼90 % of eggs), but egg cups were successful at reducing these losses to near zero. Releasing eggs in cups during the spring made it easier to place releases near phenologically synchronized microsites, minimized the risk of damage to the egg cups, and avoided a 16 % reduction in egg hatch likely associated with desiccation. Both larval-inoculated stems and egg cups produced highly encouraging feeding damage in the first season of release, with 7.7-fold higher damage from stem larvae compared to egg cups. Overall, we recommend a combination of larval-inoculated stems and spring egg cups for the primary release strategies of A. neurica and L. geminipuncta as biological control agents of introduced Phragmites. Both methods offer protection from predation and varying degrees of control over phenological mismatches. The release of larval-inoculated stems is a more labour-intensive but highly effective method in terms of initial feeding damage whereas egg cups are a more efficient mass-release method than stems with a lower amount of initial feeding damage per agent released.