Are outbreaks of cambium-feeding beetles generated by nutritionally enhanced phloem of drought-stressed trees?

Abstract

Outbreaks of cambium-feeding beetles follow droughts, becoming most severe in overmature trees and those growing on stressful sites or planted beyond their natural range. Outbreaks develop from small endemic populations confined to a few senescing or damaged trees and expand into neighbouring trees that become newly stressed as a drought intensifies, eventually coalescing as a full-blown outbreak. The beetles' larvae normally face a severe nutritional challenge feeding on the slow outflow of dilute nutrients in the phloem of damaged or senescing trees. This is reflected in their slow rate of growth. But when drought stresses trees, they senesce more quickly, releasing a faster flow of more concentrated nutrients that enhances the survival of the insects. The increased survival and faster rate of growth of larvae feeding in this enriched phloem can generate an outbreak. Both the dieback of the trees and the outbreaks of these insects are generated by drought. This hypothesis presents a parsimonious explanation for outbreaks of cambium-feeding insects. Apart from providing a focus for future research, it has the added benefit that it replaces non-explanatory descriptions of these outbreaks with a testable biological explanation based on known plant and insect physiology.