Specialist Herbivore Performance on Introduced Plants During Native Host Decline

Ash (Fraxinus spp.) is in rapid decline across the northeastern USA due to the invasive emerald ash borer (Agrilus planipennis Fairmaire). Three recently co-occurring confamilial species may serve as alternative larval host plants for ash-reliant Lepidoptera. These prospective hosts are nonnative shrubs often planted in managed suburban landscapes and are sometimes invasive or naturalized in North America. Given the imminent decline of ash trees, we considered potential downstream effects on insect herbivores historically specialized on ash foliage. We measured the performance of three ash-specialist hawkmoths (Lepidoptera: Sphingidae) on native white ash (Fraxinus americana L.) and alternative host plants: common lilac (Syringa vulgaris L.), weeping forsythia [Forsythia suspensa (Thunb.) Vahl], and European privet (Ligustrum vulgare L.). We found the nonnative host plants provided varied support for larval survival to pupation, with biomass and growth rate affected differently by both plant and insect identity. Nearly all caterpillars reared on one alternative host, European privet, exhibited distinct malformations of the wing buds at pupation. Given caterpillar presence on privet in the field, privet may constitute an ecological trap (i.e., when female moths select a sub-optimal host, offspring survival and fitness are reduced). This work demonstrates how performance testing can reveal species-specific effects of host plant loss on mono- or oligophagous insects. For some ash specialists, alternative nonnative host plants may be suboptimal, but some cultivated host plants may be able to support certain specialist insects during native host decline. We suggest that landscaping decisions can be tailored to support threatened insect species.

Automated summary

Ash trees in the northeastern USA are rapidly declining due to the invasive emerald ash borer. Three nonnative shrubs that co-occur with ash trees may serve as alternative larval host plants for ash-reliant Lepidoptera. In this study, the performance of ash-specialist hawkmoths on native white ash and alternative host plants was measured. The results showed that the nonnative host plants provided varied support for larval survival, with some caterpillars experiencing malformations when reared on European privet. These findings highlight the importance of considering alternative host plants to support threatened insect species during native host decline.