Facilitation between invasive herbivores: hemlock woolly adelgid increases gypsy moth preference for and performance on eastern hemlock

Abstract

1. Interactions between invertebrate herbivores with different feeding modes are common on long-lived woody plants. In cases where one herbivore facilitates the success of another, the consequences for their shared host plant may be severe. Eastern hemlock (Tsuga canadensis), a canopy-dominant conifer native to the eastern U.S., is currently threatened with extirpation by the invasive stylet-feeding hemlock woolly adelgid (Adelges tsugae). The effect of adelgid on invasive hemlock-feeding folivores remains unknown.2. This study evaluated the impact of feeding by hemlock woolly adelgid on gypsy moth (Lymantria dispar) larval preference for, and performance on, eastern hemlock. To assess preference, 245 field-grown hemlocks were surveyed for gypsy moth herbivory damage and laboratory paired-choice bioassays were conducted. To assess performance, gypsy moth larvae were reared to pupation on adelgid-infested or uninfested hemlock foliage, and pupal weight, proportional weight gain, and larval period were analysed.3. Adelgid-infested hemlocks experienced more gypsy moth herbivory than did uninfested control trees, and laboratory tests confirmed that gypsy moth larvae preferentially feed on adelgid-infested hemlock foliage. Gypsy moth larvae reared to pupation on adelgid-infested foliage gained more weight than larvae reared on uninfested control foliage.4. These results suggest that the synergistic effect of adelgid and gypsy moth poses an additional threat to eastern hemlock that may increase extirpation risk and ecological impact throughout most of its range.

Forest management, site characteristics and climate change affect multiple biotic threats in riparian forests

“…The positive correlation between fungus-and insectinduced leaf damage suggests facilitation between both types of defoliators, with each decreasing the overall vitality of the tree, attracting the other damaging agent. Such facilitation has previously been shown between the hemlock woolly adelgid and the gypsy moth (Kinahan et al 2020), although no general rule appears to be established for these types of interactions (Eberl et al 2019). Another explanation for the positive correlation between both threats could be a more generally reduced vitality of single trees, where a limited defense makes them attractive for various biotic threats.…”

A Four-Year, Seven-State Reforestation Trial with Eastern Hemlocks (Tsuga canadensis) Resistant to Hemlock Woolly Adelgid (Adelges tsugae)

“…Regardless of mechanism, lower densities of sap-feeding herbivores may also indirectly provide protection against other consumers. Results of field and laboratory research indicate that gypsy moth (Lymantria dispar) and hemlock looper (Lambdina fiscellaria) preferentially consume and have higher fitness on HWA-infested eastern hemlocks [38,39]. Although we did not assess folivore densities, outplantings of HWA-resistant hemlocks may thus also be more likely to survive outbreaks of other pest species.…”

Impact of early insect herbivory on the invasive oak lace bug (Corythucha arcuataSay, 1832) in different oak species

“…For instance, Rigsby et al, (2021) study showed that the positive effect of previous hemlock wolly adelgid ( Adelge tsugae ) on later-instar spongy moth ( Lymantria dispar ) attack on eastern hemlock ( Tsuga canadiensis ) was mediated by an increase on SA pathway and consequent reduction in JA pathway as a result of hemlock wolly adelgid attack. However, most of previous studies have been performed under experimental conditions, making it difficult to assess the relative importance of this phenomenon in the real world (but see Hernandez-Cumplido et al, 2016; Kinahan et al, 2020).…”

“…Biological invasions create new biotic interactions between insects and their host plants, as well as among insects co-occurring on the same plant (Kinahan et al 2020; Rigsby et al 2021). The introduction of insect herbivores in new areas is a major cause of disturbance in natural and managed ecosystems.…”