Spruce budworm feeding and oviposition are stimulated by monoterpenes in white spruce epicuticular waxes

Abstract

Monoterpenes, source of the distinctive odor of conifers, are generally considered plant defensive compounds. However, they are also known to act as long-range insect attractants, as they are volatile and permeate forest airspaces. Moreover, they are lipid soluble and can be absorbed into plant epicuticular waxes. We test their role in short-range host plant choice by both adult females and larvae of a folivorous forest pest (Choristoneura fumiferana). We conducted laboratory assays testing the responses of Eastern spruce budworm to an artificial monoterpene mix (α-pinene, β-pinene, limonene, myrcene) and to white spruce (Picea glauca) epicuticular waxes in closed arenas. Ovipositing females preferred filter paper discs treated with P. glauca waxes to controls, and preferred the waxes + monoterpenes treatment to waxes alone. However, females showed no preference between the monoterpene-treated disc and the control when presented without waxes. Feeding larvae prefered wax discs to control discs. They also consumed discs treated with realistic monoterpene concentrations and wax preferentially over wax-only discs, but showed no preference between extremely high monoterpene concentrations and wax-only controls. In an insect-free assay, P. glauca epicuticular wax decreased monoterpene volatilization. These results suggest that P. glauca waxes and realistic concentrations of monoterpenes are stimulatory to both egg-laying females and feeding larvae, and that their effects are synergistic.

Continent‐wide population genomic structure and phylogeography of North America’s most destructive conifer defoliator, the spruce budworm (Choristoneura fumiferana)

“…White spruce has evolved some resistance to spruce budworm herbivory (Lamara et al, 2018;Mageroy et al, 2015), and this is reflected in the population genetic structure of the tree (Méndez-Espinoza et al, 2018;Parent et al, 2017). Spruce budworm behavior has changed in the presence of its host plants or some of their components (e.g., waxes and sugars) (Albert, 1991;Daoust et al, 2010;Ennis, Despland, Chen, Forgione, & Bauce, 2017), indicating that it has evolved adaptive, genetically driven traits associated with herbivory.…”

Simulated Herbivory: The Key to Disentangling Plant Defence Responses

“…Advantage 2: Eliminates the Effects of Tissue Quality It has been well documented that insects feed differentially based on the physical and chemical attributes of plant material [58][59][60][61], and therefore another major challenge associated with the use of true herbivory is the differential feeding patterns likely to be observed between treatments. Ryalls et al [58] showed that high concentrations of foliar silicon reduced herbivore feeding compared to leaves with lower amounts of silicon.…”

Do white spruce epicuticular wax monoterpenes follow foliar patterns?

“…Indeed, in our study, correlations between previous-year foliage wax in July 2010 and current-year needles in May 2011 show no significant values for any of the monoterpenes (data not shown). Previous work shows that ovipositing female budworm moths are attracted to monoterpene emissions (Grant et al 2007) and use wax monoterpenes as a cue for host identification (Ennis et al 2015). Similarly, ovipositing sawflies use wax monoterpenes to determine between different pine species that offer varying D r a f t quality foliage to offspring (Kazlauskas et al 2011).…”

“…It appears that both constitutive and induced monoterpenes are important in defense against several xylophages (Huber et al 2004, Keeling and Bohlmann 2006, Keeling 2016. Several monoterpenes, notably δ-3-carene and bornyl acetate, have been suggested to be toxic to the spruce budworm at high concentrations (Kumbasli andBauce 2013, Fuentealba andBauce 2012). Monoterpenes are also known to be active against fungi and gram-negative bacteria (Novak et al 2014), and could protect against pathogens both at the leaf surface and within leaf airspaces (Martin et al 2003).…”

“…Foliar epicuticular waxes can influence monoterpene emission (Steinbauer et al 2004), as monoterpenes are lipid soluble and can be absorbed into the waxes as they diffuse out through the stomata (Muller and Riederer 2005). Monoterpenes in epicuticular waxes can influence herbivore feeding or oviposition (Steinbauer et al 2004;Ennis et al 2015), as well as the invasion by plant pathogens and the growth of lichens, fungi and other epiphytic organisms (Martin et al 2003). Patterns of foliar monoterpene synthesis in response to various environmental cues have been considered in terms of their effect on foliar pools, on translocation to other tissues and on emission, but not on epicuticular wax content.…”