Transport of fungal symbionts by mountain pine beetles

Abstract

The perpetuation of symbiotic associations between bark beetles (Coleoptera: Curculionidae: Scolytinae) and ophiostomatoid fungi requires the consistent transport of fungi by successive beetle generations to new host trees. We used scanning electron microscopy and culture methods to investigate fungal transport by the mountain pine beetle (MPB), Dendroctonus ponderosae Hopkins. MPB transports its two main fungal associates, Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield and Ophiostoma montium (Rumbold) von Arx, in sac-like mycangia on the maxillary cardines as well as on the exoskeleton. Although spores of both species of fungi were observed on MPB exoskeletons, often in pits, O. montium spores were generally more abundant than G. clavigera spores. However, a general scarcity of spores of either species on MPB exoskeletons compared with numbers on scolytines that lack sac-like mycangia indicates that fungal transport exteriorly on MPBs is incidental rather than adaptive. Conidia were the dominant spore type transported regardless of location or species; however, our results suggest that once acquired in mycangia, conidia may reproduce in a yeast-like form and even produce hypha-like strands and compact conidiophore-like structures. Fungi that propagate in mycangia may provide beetles with a continual source of inocula during the extended egg-laying period.

Predicting sapstain and degrade in fallen trees following storm damage in a Pinus radiata forest

“…Such insects can carry fungal spores on their surface or in specialised structures such as mycangia which evolved in bark beetles as a means of transporting spores of mutualistic fungi (Bleiker et al, 2009;Grebennikov and Leschen, 2010;Masuya et al, 2009;Reay et al, 2005Reay et al, , 2006b. However, in this study, although the incidence of beetle colonisation increased with time, no difference was found in the proportion of sapstain present in discs as a function of beetle colonisation until the last sample time, 67 weeks after the storms in rooted trees.…”

Assessing forest vulnerability and the potential distribution of pine beetles under current and future climate scenarios in the Interior West of the US

“…Other studies suggest that infestations are a function of more complex abiotic and biotic relationships between bark beetles and their hosts. These include reproductive and dispersal opportunities for bark beetles (Aukema et al, 2005;Roberston et al, 2009), transport and infection by fungi and other pathogens (Adams et al, 2008;Bleiker et al, 2009), predation and competition of bark beetles Schlyter and Anderbrant, 1993), fire (Breece et al, 2008;Fettig et al, 2008), and management practices (Waring and Pitman, 1985;Hayes et al, 2008). Although the results of these studies help us better understand local dynamics between bark beetles and their hosts, they are difficult to apply across large spatial or temporal scales.…”

Fungal Frequency and Mite Load Trends Interact with a Declining Mountain Pine Beetle Population

“…As an example, Ophiostoma montium, which is more tolerant of warmer temperatures, has been found predominantly in MPB populations attacking during warmer periods [17]. It has been suggested that under a warming climate scenario, O. montium could displace G. clavigera [29] as the most important blue-stain fungal beetle associate; however, both species are found in most MPB populations [10,12,16,17,20,30]. Analyses of the effects of temperature on the stability of the above two blue-stain fungi have shown that beetle movement between warm and cold habitats, as well as variations in their attack densities, can help explain the prevalence of both species in the symbiosis [31].…”

“…Analyses of the effects of temperature on the stability of the above two blue-stain fungi have shown that beetle movement between warm and cold habitats, as well as variations in their attack densities, can help explain the prevalence of both species in the symbiosis [31]. Thus, a fungal redundancy in the system may occur [10,21], but studies of fungal community trends during changing MPB populations have not been performed.…”

“…These include unicellular yeasts and multicellular ascomycete fungi such as the blue-stain causing species. Yeasts are the most prevalent ascomycetes on many bark beetles [7][8][9], including the MPB [10][11][12]. Yeasts perform essential functions that impact MPB success in the newly attacked tree such as metabolizing alpha-pinene into verbenone, a semiochemical acting as an anti-aggregation pheromone during MPB mass-attacks and signaling attack saturation [13].…”