Urban microclimate warming improves overwintering survival of evergreen bagworms

In the northernmost latitude of North America, the evergreen bagworm, Thyridopteryx ephemeraeformis (Haworth), distribution is limited by overwintering temperatures. Urban impervious surfaces such as roads, buildings and parking lots can warm microclimates and create ecological temperature gradients that have the potential to increase the winter survival of insects. To test this hypothesis, we evaluated survival of bagworms over gradients of microclimatic conditions. Bagworms live within spindle-shaped bags constructed from fragments of foliage. In late summer, adult male bagworms fly to bags containing wingless adult females. Mated neotenous females lay eggs within their pupal case. These eggs hatch into larvae during the late spring of the following year and disperse to hosts by ballooning. A total of 2255 bagworm bags were collected from 119 sites in Indiana and Illinois prior to egg hatch in the spring of 2018 and 2019. The maximum temperature during the coldest days of winter was recorded at each site. Up to 25 bagworms were removed from each host plant to assess the overwintering survival of eggs. Survivorship rose as estimates of impervious surface within a 20-m radius increased. Specifically, 50% of bagworm eggs survived at maximum daily temperatures of −19.4°C, −20°C and −20.6°C when plants were surrounded by 25.7%, 48.39% and 50.75% impervious surface, respectively. Egg mortality was not buffered by impervious surfaces at temperatures at or below −21.67°C. Our findings provide insights about how impervious surface in urban areas can provide refugia for marginally hardy insects and improve their chances of surviving the cold of winter.