How sweet is the extrafloral nectar secreted by the invasive alien Tree of Heaven, Ailanthus altissima Mill.?

Authors: BON, MARIE-CLAUDE - European Biological Control Laboratory (EBCL); GUERMACHE, FATIHA - European Biological Control Laboratory (EBCL); KASHEFI, JAVID - European Biological Control Laboratory (EBCL); SFORZA, RENE - European Biological Control Laboratory (EBCL)

Submitted to: International Journal of Plant Biology and Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/9/2023
Publication Date: 9/11/2023
Citation: Bon, M., Guermache, F., Kashefi, J., Sforza, R. 2023. How sweet is the extrafloral nectar secreted by the invasive alien Tree of Heaven, Ailanthus altissima Mill.?. International Journal of Plant Biology and Research. 11(1): 1135. https://doi.org/10.47739/2333-6668/1135.
DOI: https://doi.org/10.47739/2333-6668/1135

Interpretive Summary: The Tree of Heaven (ToH),(Ailanthus altissima) is a rapidly growing deciduous tree native to China and Taiwan that has become a widespread invasive species worldwide. Introduced from China to Europe and the United States in the late 1700s, Tree of Heaven valued for its fast-growing ability and its resistance to insect infestation and damage, was widely planted as a shade tree for parks and public promenades in these two regions of the world. ToH has since invaded urban and natural ecosystems crowding out native plants. ToH has also helped advance the spread of the spotted lanternfly, one of the most prolific and detrimental invasive pests in North America, and also originally from China. As an invasive, ToH owes its competitive advantage to several attributes among which is its large pinnated leaves. ToH leaves harbor specialized glands named extrafloral nectaries which produce extrafloral nectar (EFN). EFN represents a significant energy source, owing to its sugar-rich content to insects visiting the plants. There is mounting evidence from the literature that EFN is involved in so-called indirect defense by attracting ants, parasitoids and other predatory arthropods such as spiders that prey on herbivores or that physically deter herbivores from feeding on the plants. In spite of the growing knowledge on the biology of ToH, neither the role of EFN nor the sugar composition of EFN have been investigated. The sugar composition of EFN was analyzed in ToH plants from three sites in France. Our findings rank Tree of Heaven as a sucrose rich species. As it is generally admitted from the literature that nectars with high sucrose content attract insect parasitoids and generalist ants, our findings represent a timely and a potentially fruitful avenue for future research addressing the ecological functions of EFN in ToH, including indirect defense in relation to the assemblages of natural enemies associated with ToH. By understanding the role of EFN in repelling or attracting selected natural enemies, this study may also lead to an interesting implementation of a biological control management.

Technical Abstract: Tree of Heaven (ToH), Ailanthus altissima is a fast-growing tree native to China and Taiwan, but invasive in Europe and the U.S. where it disrupts urban and natural biocenoses. This invasive brings together successful strategies or traits including defense mechanisms and extrafloral nectaries. The Extrafloral Nectar (EFN) represents a significant energy source, owing to its carbohydrate-rich content, for visiting insects. The core ecological function of EFN is an indirect defense to plants by attracting insects (generally ants) that prey on herbivores or that deter herbivores from feeding on the plants. Assemblages of natural enemies associated to ToH were shown to include a large proportion of defoliators. Despite the importance of EFN, information on the sugar composition of EFN in ToH remains sparse. We analyzed by enzymatic assays the sugar composition of EFN in ToH plants from three locations in France. The total sugar concentration of EFN was not significantly different between locations unlike the hexose ratio. The ratio of sucrose to hexose, (r) ranged from 0.501 to 0.640, ranking ToH as a sucrose rich species. As it is generally admitted that sucrose rich nectars attract generalist ants and parasitoids, this finding represents a timely and a potentially fruitful avenue for future research addressing the ecological functions of EFN in ToH including indirect defenses in relation to the assemblages of natural enemies associated with ToH in both native and invasive ranges.