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.

Emergent insects, pathogens and drought shape changing patterns in oak decline in North America and Europe

“…In theory, insects could benefit if drought resulted in reduced tree resistance, improved tree nutrition, or both (White, 1969(White, , 1984(White, , 2015Rhoades, 1979Rhoades, , 1985Mattson and Haack, 1987a,b). Reduced resistance could occur by way of impaired water relations, C balance dysfunction, or both.…”

An alternative hypothesis explains outbreaks of conifer‐feeding budworms of the genus Choristoneura (Lepidoptera: Tortricidae) in Canada

“…More generally, it will show that budworms and their predators are not innately any different from other forest insects (White, , , , ). They are reacting in the same way as other animals to changes in the availability of food that will support survival and growth of their young (White, , ).…”

“…All these sites are “refuges” (also variously described as “hot spots,” “foci” or “epicentres”), where the foliage of trees prematurely stressed on these sites provides food that is more nutritious than in the surrounding nutritional desert and therefore supports improved survival of isolated endemic populations. These refuges need not be as clearly defined and readily identifiable as those of, for example, desert‐inhabiting rodents (Pavey et al., ) or bark beetles (White, ), but their function is the same; they are places in an otherwise hostile environment where small endemic populations can survive and persist.…”

Effects of drought stress on oviposition preference and offspring performance of the lace bugCorythucha marmorataon its goldenrod host,Solidago altissima

“…Both stressed and vigorous plants can be favorable resources for various trophic groups of phytophagous insects (Price, 1991;De Bruyn, 1995;Schowalter et al, 1999). White's (1976White's ( , 1978White's ( , 1984 plant stress hypothesis and Price's (1991) plant vigor hypothesis have both been supported by many plant-insect pairs (Pickford, 1966;Craig et al, 1989;De Bruyn, 1995;Fritz et al, 2000;Inbar et al, 2003;White, 2014), with the plant vigor hypothesis accounting for the majority of observations (Cornelissen et al, 2008). The two hypotheses are valid for plant-insect pairs with different insect feeding strategies (Larsson, 1989;Price, 1991;Koricheva et al, 1998;Schowalter et al, 1999).…”