Flight periodicities, phoresy rates, and levels of Pseudopityophthorus minutissimus branch colonization in oak wilt centers

Oak bark beetles, Pseudopityophthorus minutissimus and P. pruinosus, are considered important vectors of the oak wilt fungus, Ceratocystis fagacearum, in Missouri and Ohio. However, the frequency of the species' association with diseased oaks in Minnesota and their relative importance in pathogen spread in the state are unclear. Window traps were placed in canopies of recently killed northern pin oaks to determine seasonal flight periodicities of dispersing oak bark beetles and their phoresy (pathogen presence) rates. Branch samples were collected from diseased northern pin and northern red oak canopies in May and Aug., life history data obtained, and oak bark beetles emerging from the branches assayed for the pathogen. Only P. minutissimus was found in the study areas. In 2003, peak flight of P. minutissimus in Minnesota occurred 12 May to 19 May, with 869 beetles being trapped. Dispersing beetles carried viable pathogen propagules at low frequencies (4 to 13 per 1,000) in May and June. Branches of oak wilt-killed trees were commonly colonized by the beetle. More beetles emerged from branch samples collected in May than in Aug., but none yielded C. fagacearum. These results support the hypothesis that the relative importance of P. minutissimus in the overland transmission of the pathogen in red oak species in Minnesota is minor.