Is monarch butterfly oviposition deterred by a biological control agent?

Previous studies have found that monarch butterflies lay up to 20% of their eggs on the invasive vines, black and pale swallow-worts (Vincetoxicum spp.), which may serve as an oviposition sink for monarch butterflies since their larvae cannot develop on these plants. As swallow-worts continue to spread throughout the eastern and midwestern United States, it may contribute to the monarch population declines. Hypena opulenta, a defoliating moth, has recently been approved for release as a biological control agent against swallow-worts. We tested whether the presence of H. opulenta on swallow-worts would deter oviposition by monarchs given the presence of another leaf feeder on the plants. In field experiments conducted in 2019 and 2020 we placed milkweed plants as a control and swallow-wort plants with and without H. opulenta at field sites. We found that monarchs laid a similar number of eggs on both swallow-wort treatments. In a laboratory choice and no choice test conducted in 2019, monarchs laid no eggs on any swallow-wort plants. Our data do not suggest that monarch oviposition is influenced by the presence of H. opulenta.