Can Insects Adapt to Non-Native Plants?

 

This is a question often posed to native plant enthusiasts: is it possible that over time Lepidoptera (butterflies and moths) will come to use imported plants as their host plants? It’s a hopeful question for gardeners who like non-native plants, but it’s also a question that scientists are interested in answering in case native host plants decline or disappear.

It’s the subject of a paper just recently published [Specialist Herbivore Performance on Introduced Plants During Native Host Decline] that examined ash (Fraxinus) relatives in the olive family (Oleaceae) that are already here as ornamentals: privet (Ligustrum), Forsythia, and lilac (Syringa). With the decline of ash species due to the emerald ash borer (EAB), it’s a very real possibility that moths and butterflies that use it as a host plant may need to find an alternative host. There is some usage of these plants already by specific insects. The question the paper posed to answer was how productive of a host plant could they be (would the larvae survive to adulthood on these plants or would the plants be ecological traps?).  

Green ash (Fraxinus pennsylvanica)

I appreciate that such research is available to the public. Also included are all of their references, of course, and those are interesting as well. I had no idea, for example, that 5 species of Lepidoptera are considered extinct because of the dramatic decline in American chestnut trees (Castanea dentata) (reference: Wagner, D. L., and R. G. van Driesche.2010. Threats posed to rare or endangered insects by invasions of nonnative species. Annu. Rev. Entomol. 55: 547–568.) Certainly that makes sense and it could foretell the fate of some ash-dependent insects.

The results of this study were mixed. Yes, the studied moths did lay eggs on the non-native plants to some degree Yes, some insects did develop into caterpillars and lived to pupate (form a cocoon). A quote from the paper:

“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).”

The article is a fascinating read and it contains many interesting references to other studies (like the note about chestnut-dependent insects) as well as mentioning that they were able to use some iNaturalist observations in their research (go, citizen scientists!).

What can we do about EAB? Learn to recognize the emerald ash borer and signs of affected trees so that you can recognize if you have it (and report it to iNaturalist). It is in Georgia. Look for D-shaped beetle exit holes in ash trees and wilting foliage. Don't move firewood as that can contribute to the spread.

Photo: https://www.newhopemn.gov/city_hall/public_works/forestry/emerald_ash_borer

If you want to take a chance on native members of the olive family, plant ash tree species native to your area. In Georgia, that is primarily green ash (Fraxinus pennsylvanica) and white ash (Fraxinus americana). Other members include fringetree (Chionanthus virginicus) and American olive (Cartrema americana).