Charles J. Mason, Caterina Villari, Ken Keefover-Ring, Stephanie Jagemann, Jun Zhu, Pierluigi Bonello, Kenneth F. Raffa

Plants face multiple biological threats that can adversely affect their growth, reproduction and survival. They defend themselves from herbivores and pathogens by integrated chemical and physical defenses. These defenses can be constitutively present, heightened upon injury, or in some cases primed for subsequent attack. The major mortality agents of pines are native bark beetles and their symbiotic Ophiostomatioid fungi. Conifers defend themselves against these insects and symbiotic fungi by pronounced alterations in subcortical terpenoid and phenolic chemistry.

We used red pine (Pinus resinosa) and its principal insect mortality agent, the pine engraver (Ips pini)  – microbial complex, to investigate how tree perception of an initial bark beetle attack stimulus influences local and tissue-wide phloem chemistry, and subsequent responses to attack. Concentrations of defense chemicals increased rapidly at the site of simulated attack, but these changes did not extend throughout the tree’s trunk. The defense responses to additional attacks were not different from those to an initial attack.

These localized, immediate responses do not seem to be due to an inability to increase defense chemical concentrations tissue-wide, as necrotrophic pathogens (that infect and kill host tissue and extract nutrients from the dead cells) can induce systemic, tissue-wide defenses in pine species. Instead, it likely relates to the lethal consequences of an initial beetle attack being successful and eliciting mass attacks via pheromones. In other words, our study shows that conifers do not hedge their bets when defending against bark beetles, but rather commit all available resources to defeat the first arriving insects in an attempt to prevent subsequent attacks.

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Image caption: Localized conifer response to bark beetles attack. Accumulation of resin and lesion formation at the site of bark beetle attack.