Species‐specific plant‐mediated effects between herbivores converge at high damage intensity

Abstract

Plants are often exposed to multiple herbivores and densities of these attackers (or corresponding damage intensities) often fluctuate greatly in the field. Plant-mediated interactions vary among herbivore species and with changing feeding intensity, but little is known about how herbivore identity and density interact to determine plant responses and herbivore fitness. Here, we investigated this question using Triadica sebifera (tallow) and two common and abundant specialist insect herbivores, Bikasha collaris (flea beetle) and Heterapoderopsis bicallosicollis (weevil). By manipulating densities of leaffeeding adults of these two herbivore species, we tested how variations in the intensity of leaf damage caused by flea beetle or weevil adults affected the performance of root-feeding flea beetle larvae and evaluated the potential of induced tallow root traits to predict flea beetle larval performance. We found that weevil adults consistently decreased the survival of flea beetle larvae with increasing leaf damage intensities. In contrast, conspecific flea beetle adults increased their larval survival at low damage then decreased larval survival at high damage, resulting in a unimodal pattern. Chemical analyses showed that increasing leaf damage from weevil adults linearly decreased root carbohydrates and increased root tannin, whereas flea beetle adults had opposite effects as weevil adults at low damage and similar effects as them at high damage. Furthermore, across all feeding treatments, flea beetle larval survival correlated positively with concentrations of carbohydrates and negatively with concentration of tannin, suggesting that root primary and secondary metabolism might underlie the observed effects on flea beetle larvae. Our study demonstrates that herbivore identity and density interact to determine systemic plant responses and plant-mediated effects on herbivores. In particular, effects are species-specific at low densities, but converge at high densities. These findings emphasize the importance of considering herbivore identity and density simultaneously when investigating factors driving plant-mediated interactions

Undercover operation: Belowground insect herbivory modifies systemic plant defense and repels aboveground foraging insect herbivores

“…Future research should investigate changes in other aspects of larval performance as well, such as rate of development or growth over longer periods of time. Additionally, recent evidence from Chinese tallow showed aboveground herbivore effects on belowground root nutrient content, as well as herbivore performance, were dependent on the identity and density of aboveground herbivores (Wan et al, 2022). Future research should investigate different densities of aboveground striped cucumber beetles and squash bugs to discern if there are any densitydependent effects on belowground striped cucumber beetle larval performance.…”

Intensity of herbivory correlates with stronger constitutive and weaker induced defenses for non-native plant species -- another mechanism for EICA?

“…For example, a common garden survey found that different non-native species suffered from 0% to more than 40% leaf herbivory from generalists (Agrawal & Kotanen, 2003). Variation in herbivore pressure should lead to variation in defense traits (Coverdale & Agrawal, 2022;Wan et al, 2022) and less intense and less frequent herbivory is theorized to select for allocation to induced defenses (Bixenmann et al, 2016). However, how this variation in generalist herbivore pressure affects any potential tradeoffs in constitutivevs.…”

Trade‐offs in non‐native plant herbivore defences enhance performance

“…Different non‐native species often experience very different degrees of generalist herbivore pressure (Agrawal & Kotanen, 2003; Sims‐Chilton et al, 2009). Such variation in herbivore pressure should lead to variation in defence traits (Coverdale & Agrawal, 2022; Wan et al, 2022) and less intense and frequent herbivory should select for allocation to induced defences (Bixenmann et al, 2016). However, how this variation in generalist herbivore pressure affects potential trade‐offs in constitutive versus induced defensive strategies among non‐native species is unknown.…”