Functional roles of nematodes associated with Pityophthorus juglandis Blackman (Coleoptera: Curculionidae) and Juglans nigra L. (Fagales: Juglandaceae) in the etiology of Thousand Cankers Disease

Bark beetles (Curculionidae: Scolytinae) form intimate associations with a diverse suite nematodes. Despite their ubiquity, the ecological significance of nematodes in the lifecycles of economically important bark and ambrosia beetle species and their associated plant diseases remains largely unknown and unexplored. Thousand cankers disease (TCD) caused by Pityophthorus juglandis Blackman and the fungus Geosmithia morbida Kolařík (Sordariomycetes: Hypocreales) leads to mortality of eastern black walnut (Juglans nigra L.) throughout the western U.S., but has been less severe in the east. To investigate the role of phoretic and free-living nematodes in disease etiology, we recovered nematodes from P. juglandis and J. nigra, made taxonomic assignments based on molecular barcodes, and co-inoculated J. nigra with nematodes and G. morbida. A phoretic Bursaphelenchus sp. (Aphelenchoidoididae) found on P. juglandis and Panagrolaimus spp. (Panagrolaimidae) found in walnut bark decreased canker area when co-inoculated with G. morbida in seedlings and tree branches, respectively. Crown deterioration was observed in seedlings co-inoculated with Bursaphelenchus sp. and G. morbidaunder shade conditions.
To investigate the role of entomoparasitic nematodes in regional differences in TCD severity, we fit a climate-based model to a historical dataset of nematodes recovered from 15,000+ beetles collected from 76 locations throughout the US over a 30-year timespan. Our model revealed that parasitism increased with summer precipitation and decreased with winter precipitation and summer temperatures. This result is consistent with parasite biology and geographical differences in TCD severity, and suggests that nematodes may influence the etiology of beetle-fungal forest disease complexes.