Assessment of pupal mortality in Hypena opulenta: An obstacle for establishment of a classical biological control agent against invasive swallow‐worts

Abstract

1. 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.2. 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.3. 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%).4. 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.5. Across all seasons, pupal predation was dominated by large non-arthropod predators.6. 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.

The effects of warm and cold periods on resource depletion and emergence synchrony in diapausing Hypena opulenta: Implications for biological control of invasive swallow‐worts in North America

“…Rearing efficiency can be maximized by storing diapausing pupae at 5 °C for 6 months, but cold storage can safely be reduced to 3 months if the rearing season needs to be extended. Our results also address key obstacles for establishment that have been identified for H. opulenta in North American release sites, such as the effects of early diapause on resource depletion, and the impacts of longer, colder winters (Jones et al, 2022). The effects of early diapause in H. opulenta on winter survival and adult fitness in regards to energy depletion may not be as detrimental as expected.…”

“…Indeed, outdoor overwintering experiments using laboratory-reared H. opulenta pupae found that mortality due to cold stress was quite high (ca. 30% in the colder of two winters) (Jones et al, 2022). Finally, resource depletion in response to warm diapause conditions can take the form of accelerated consumption of carbohydrates and proteins, rather than lipids (Zhao & Wang, 2021).…”

“…Second, the absence of active H. opulenta larvae later in the season will allow invasive swallow-worts to recover and compensate for early season damage (Milbrath et al, 2016). Third, early diapausing H. opulenta will be exposed, in their most vulnerable stage, to predators that become increasingly active later in the summer (Jones et al, 2022). One consequence of early diapause that has yet to be studied in H. opulenta is the metabolic costs associated with undergoing diapause through periods of relative warmth, such as at the beginning of an early diapause.…”

Biology of Invasive Plants 3. Vincetoxicum nigrum (L.) Moench and Vincetoxicum rossicum (Kleopow) Barbarich

“…However, the threshold photoperiod may result in only one generation per summer at more southern latitudes in North America, reducing the potential impact of H. opulenta on Vincetoxicum infestations. Other factors influencing establishment and population increase of the moth may include substantial pupal mortality, resulting from a combination of late-summer predation and overwintering mortality, and the availability of adult food sources (Jones et al 2022; Seehausen et al 2019).…”