Stimulation of tree defenses by Ophiostomatoid fungi can explain attack success of bark beetles on conifers

Abstract

Keywords:pathogen / forest tree / physiological stress / blue stain fungi / host resistance Abstract • Our aim is to present why the hypothesis, that Ophiostomatoid fungi play an important role in the establishment of most bark beetle species on living conifers, is valuable.• After summarizing knowledge about the relationships of bark beetles with conifers and fungi, we conclude that controversy results from misinterpretations when using fungal pathogenicity to demonstrate the role of Ophiostomatoid fungi in beetle establishment on host trees.• We demonstrate that fungal pathogenicity is not the right parameter to appreciate the role of fungus in beetle establishment on host trees. We argue that artificial low density inoculations that allow the appreciation of fungus ability to stimulate tree defenses and thus to help beetles in overcoming tree resistance must be used in complement to mass inoculations. In both cases, results must be expressed in terms of tree defense stimulation rather than in terms of tree killing.(i) Fungal species stimulating tree defenses are generally not those that grow the best in the sapwood.(ii) We argue that beetle development in the phloem, fungal invasion of the sapwood and phloem, and tree death, occur after tree defenses are exhausted, and that any fungus present in the beetle gallery could thus potentially invade the sapwood after defense exhaustion.• We conclude that stimulation of the tree defense reactions in both the phloem and the superficial sapwood is a real benefit brought by fungi to the beetles during the first phase of establishment (overcoming tree resistance).• Considering the origin of the bark beetle fungus associations attacking living trees and their general functioning based on stimulation of tree defenses, we develop three hypotheses:(i) any beetle species would be helped in its establishment in a given tree species by developing an association, even loosely, with a fungus species belonging to the Ophiostomatoid flora of that tree species; (ii) the necessity of a considerably low level of tree resistance for fungus extension into the tree is the selection pressure that has led fungi to develop their intrinsic ability to stimulate tree defenses, through their ability to grow into the phloem. This association can be completed by antagonistic fungal species controlling extension of the previous fungal species in the tree tissues; (iii) Beetle species using the strategy of overcoming tree resistance are associated with a fungal complex, of which species could assume three roles regarding relationships between beetles and trees: 1-to stimulate tree defenses in the phloem and superficial sapwood, 2-to grow into the sapwood after tree resistance is overcome, and 3-to control phloem extension of the first other two categories. Bringing nutrients to the beetle progeny can be a fourth role.• We propose that bark beetle -Ophiostomatoid associations can be categorized, based on associations' frequency and complexity while taking into account beetle aggressiveness. We show...

Defence reactions of mature Norway spruce (Picea abies) before and after inoculation of the blue-stain fungus Endoconidiophora polonica in a drought stress experiment

“…We assume that the increased resin flow in September 2014 observed in fully covered spruce trees of the Rosalia roof experiment was also due to the formation of traumatic resin ducts, stimulated by wounding and necrosis. Strong stimulation of tree defences can prevent trees from being successfully mass attacked by bark beetles (Zhao et al 2011), but might also speed up the exhaustion of carbon resources, as argued by Lieutier et al (2009).…”

“…E. polonica speeds up tree death through impairing and destroying the trees' vital functions of water and nutrient transport, by killing phloem and cambium, and causing occlusion of the tracheids (Kirisits 2004;Francheschi et al 2005). The scolytines profit from a more rapid exhaustion of constitutive defences and the suppression or exhaustion of induced resistance of the host trees supported by the fungus (Christiansen et al 1987;Rosner and Hannrup 2004;Lieutier 2004;Lieutier et al 2009). E. polonica can be more easily manipulated for experiments on live trees than I. typographus beetles themselves.…”

Aftermath of Mountain Pine Beetle Outbreak in British Columbia: Stand Dynamics, Management Response and Ecosystem Resilience

“…Davidson, Ophiostoma montium (Rumbold) von Arx., and Leptographium longiclavatum Lee, Kim and Breuil), which contribute to beetle success [4][5][6]. Although MPB broods can be reared without their symbiotic fungi [7], the fungi have a positive influence on the survival of beetle broods by (i) exhausting tree defenses during mass-attack [8]; (ii) expediting the development of immature beetles [9]; and (iii) increasing phloem nutrients [10][11][12].…”

Bark and ambrosia beetles (Curculionidae: Scolytinae), their phoretic mites (Acari) and associated Geosmithia species (Ascomycota: Hypocreales) from Virgilia trees in South Africa

“…However, many scolytine beetles attack only trees that are weakened and/or stressed, or dead (Avtzis et al 2012;Paine et al 1997;Raffa et al 1993;Six & Wingfield 2011). Despite their ecological importance in, for example, initiating nutrient cycling (Christiansen et al 1987;Stark 1982), they have not attracted much research interest, as they seldom cause economic losses, except for those few that vector detrimental fungi (Lieutier et al 2009). …”