Assessment of pupal mortality in Hypena opulenta: an obstacle for establishment of a classical biological control agent against invasive swallow-worts.
Common obstacles for establishment of newly-introduced biological control agents include climate and the activity of native antagonists. In temperate climates, these obstacles can disproportionately affect overwintering life-stages because they are exposed to low winter temperatures, and may rely on passive defence from predators. We conducted a series of field exposure experiments with predator-exclusion treatments, in Ontario, Canada, to identify mortality factors for the pupae of Hypena opulenta, a biological control agent for invasive swallow-worts in North America. During two winters, predation rates in containers with large holes, that enabled predation, were relatively low (mean: 23.75%) but non-predation mortality in closed containers was high (mean: 66.25%), particularly during the colder of the two winters (87.5% vs. 52.5%). During the summer, non-predation mortality in closed containers was low (mean: 7.5%) but predation rates in containers with large holes were higher than during the winter (mean: 53.33%), increasing as the summer progressed. Predation in containers with large holes was 70% during late summer, compared with 25% during the spring. Across all seasons, pupal predation was dominated by large non-arthropod predators. Hypena opulenta can complete 2 generations per year. Photoperiods that induce diapause occur earlier in the introduced range than in the native range, however, and H. opulenta individuals in parts of the introduced range are likely to enter diapause early after a single generation. Our results highlight additional vulnerabilities encountered by such individuals, and can contribute to models predicting population dynamics of H. opulenta across its introduced range.