Function of Sitka spruce stone cells as a physical defence against white pine weevil

Abstract

Stone cells are a physical defence of conifers against stem feeding insects such as weevils and bark beetles. In Sitka spruce, abundance of stone cells in the cortex of apical shoot tips is associated with resistance to white pine weevil. However, the mode of action by which stone cells interfere with growth and development of weevil larvae is unknown. We developed a bioassay system for testing potential effects of stone cells, which were isolated from resistant trees, on weevil larvae. Bioassays using artificial diet and controlled amounts of stone cells focused on physical defence. We evaluated the effects of stone cells on establishment of neonate larvae, mandible wear and changes in relative growth rates of third instar larvae. Establishment of neonates and relative growth rates of third instars were significantly reduced by stone cells. Stone cells appeared to be indigestible by weevil larvae. Our results suggest that stone cells affect weevil establishment and development by forming a physical feeding barrier against neonate larvae at the site of oviposition, and by reducing access to nutrients in the cortex of resistant trees, which contain an abundance of stone cells in place of a more nutrient rich tissue in susceptible trees.

Successful reproduction and pheromone production by the spruce bark beetle in evolutionary naïve spruce hosts with familiar terpenoid defences

“…Because terpenoids have negative effects on attacking bark beetles (Erbilgin et al 2006;Wallin and Raffa 2000;Zhao et al 2011b), these results suggest that Sitka and Lutz spruce are equally or more suitable hosts for the beetles than Norway spruce. However, host suitability to the spruce bark beetle is probably also influenced by physical or chemical characteristics of the bark that we did not quantify, such as the abundance of stone cells and the concentration of nutrients and secondary metabolites other than terpenoids (Schiebe et al 2012;Whitehill et al 2016). …”

Functions of stone cells and oleoresin terpenes in the conifer defense syndrome

“…Stone cells have a high predictive power of weevil resistance in Sitka spruce (King et al ., ). Stone cells are hypothesized to be a durable quantitative defense that provides resistance to several spruce species against bark beetles and weevils (Wainhouse et al ., ; King et al ., ; Whitehill et al ., ,b). Experiments with weevils on an artificial diet showed that SC function as a feeding barrier against larvae, disrupting their establishment, development and survival (Whitehill et al ., ,b).…”

“…Additional oleoresin terpene biosynthesis is induced by wounding or insect attack as part of the development of traumatic resin ducts in the cambium and xylem (Franceschi et al ., ; Martin et al ., ; Miller et al ., ; Zulak et al ., ). Physical defenses of conifers include bark texture and thickness, calcium oxalate crystals, and stone cells (SC, sclereids) (Franceschi et al ., ; King et al ., ; Whitehill et al ., ,b). The contribution of a specific defense mechanism to insect resistance and possible interactions of different defenses, are difficult to dissect in conifers when genetic mutants are not available.…”

“…Naturally occurring, contrasting genotypes of weevil‐resistant (R) and weevil‐susceptible (S) Sitka spruce (King & Alfaro, ; King et al ., ) have previously been explored to identify components of the chemical and physical defense system. Specifically, the clonally propagated Sitka spruce R genotype H898 and the S genotype Q903 were used to describe terpene and SC defenses (Hall et al ., ; Whitehill et al ., ,b). The H898 R genotype originates from an area where Sitka spruce has historically been exposed to the weevil (King et al ., ), while the Q903 S genotype is from the weevil‐free Haida Gwaii Islands (King & Alfaro, ; Robert et al ., ).…”

“…In the present study we first validated our previous observations of SC affecting weevil performance gleaned from in vitro studies with artificial diets (Whitehill et al ., ) by using a set of newly designed in planta oviposition experiments. Based on some unexpected results, we then assessed the hypothesis that SC may interact with other components of the Sitka spruce defense system, specifically terpene oleoresin.…”