For Ash-Dependent Insects, Some Plants Make Good Alternatives—But Others Don’t
By Andrew Porterfield
Across the northeastern United States, the ash tree has been heading toward extinction since 2002. That year, the emerald ash borer (Agrilus planipennis), native to northeastern Asia, was first identified in Michigan. Since then, landscape managers, horticulturalists, and entomologists have been looking at alternative plants to support species of Lepidoptera (moths and butterflies) that have evolved to depend on the ash.
But finding the right plant can be challenging, as evidenced in a study published in January in Environmental Entomology, which found that, at least for three species of ash-dependent moths, alternative non-native plants vary significantly in their effects on the moths’ larval development. The study was led by Grace Horne, as part of her undergraduate thesis at Colby College in Maine, in collaboration with researchers at The Caterpillar Lab in New Hampshire and the Oak Spring Garden Foundation in Virginia. Horne is now a Ph.D. student in entomology at the University of California, Davis.
Most insect herbivores have evolved to feed on just a small number of plant types. “Compared to many regionally native plants capable of hosting large communities of locally adapted insect herbivores, introduced and invasive plants generally support less biodiverse food webs and host fewer insect populations … than native congeners,” the authors write.
To see how that adaptation could work among ash-dependent insects, Horne and her team tested three types of hawkmoths on native ash. These insects represent just three of nearly 300 arthropods (and 100 lepidopterans) that associate with ash. They also tested their developmental reactions to three alternative plants: lilac (Syringa vulgaris), weeping forsythia (Forsythia suspensa), and European privet (Ligustrum vulgare).
The researchers reared three ash-specialist moth species—Ceratomia undulosa, Sphinx kalmiae, and Sphinx chersis (known as the great ash sphinx)—from hatching to pupa stage. During the summer of 2020 in southern New Hampshire, they raised 154 C. undulosa, 123 S. kalmiae, and 166 S. chersis caterpillars on the host plants in a laboratory. To confirm their laboratory results, the researchers also placed six C. undulosa caterpillars on field trees. They also conducted field surveys along suburban roads of alternative host plants, identifying caterpillar species on those trees.
Larval growth for all three species varied greatly by host plant. “Perhaps the biggest surprise for us was just how variable the responses were,” Horne says. “For example, Sphinx kalmiae did quite well on lilac, but Sphinx chersis perished entirely on the same plant. We expected that these congeners might have similar tolerances of the non-native plants, but this was not the case.”
Specifically, caterpillars raised on non-native plants took longer to reach pupation than those raised on ash. Ceratomia undulosa and Sphinx chersis showed higher mortality rates on non-native plants than on native ash. In addition, despite normal caterpillar growth, most pupae of all three species reared on privet exhibited malformed wing buds and were probably not viable. “This malformation … was another major surprise,” Horne says. “We attributed this to some sort of nutrient deficiency, but further work will have to be done to elucidate the specific mechanism causing this deformity.”
The findings counter anecdotal accounts of success in raising C. undulosa and S. kalmiae on privet, an invasive plant in eastern North America (and especially invasive in the southeastern U.S.). Privet, the authors write, “may constitute an ecological trap for some ash-feeding insects in North America,” but they warn that further verification is needed.
The study underscored the need to continue preserving ash trees but also to remove privet, all in order to support ash-dependent lepidopterans. “The choices made by zoning commissions, city planners, landscape architects, and homeowners determine integral components of food webs, and the size of the native insect community that can be supported is becoming increasingly relevant when making landscaping decisions,” the researchers write.
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.