Yellow toadflax is an introduced ornamental perennial that has become a serious weed in Wisconsin peppermint production. Glyphosate is the only labeled herbicide that controls yellow toadflax in peppermint. Other control strategies, either mechanical or cultural, have little effect on the weed in solid stands of peppermint. Previous research has suggested that the root mining moth Eteobalea serratella Tr. (Lepidoptera: Cosmopterigidae) may provide effective biological control of yellow toadflax. Therefore, experiments were conducted to determine the host specificity of E. serratella and to quantify E. serratella root mining of yellow toadflax. In larval and adult no-choice tests, E. serratella attacked only yellow toadflax. Other plant species collected from E. serratella's natural range were not attacked, neither those closely related to yellow toadflax in the family Scrophulariaceae, nor those closely related to peppermint in the family Labiatae. Root mining by E. serratella reduced root biomass an average of 20% compared to nontreated plants. Peppermint competition alone or in conjunction with root mining by E. serratella reduced root biomass 52 and 65%, respectively, compared to nontreated plants under noncompetitive conditions. E. serratella is host specific for yellow toadflax and demonstrates potential as a biological control agent by contributing to the reduced competitive ability of yellow toadflax in peppermint.
