Geographical variation in seasonality and life history of pine sawyer beetles Monochamus spp: its relationship with phoresy by the pinewood nematode Bursaphelenchus xylophilus.
Bursaphelenchus xylophilus (Steiner & Buhrer) (Nematoda: Aphelenchoididae), the pinewood nematode and the causal agent of the pine wilt disease, is a globally important invasive pathogen of pine forests. It is phoretic in woodborer beetles of the genus Monochamus (Megerle) (Coleoptera, Cerambycidae) and has been able to exploit novel indigenous species of Monochamus (but only Monochamus) in newly-invaded areas. North America (NA) is the continent of origin for the B. xylophilus/Monochamus spp. phoretic system. NA also contains the largest number of Monochamus species known to act as vectors for B. xylophilus. Understanding this phoretic system in its native geographical area helps to explain the evolutionary ecology of pine wilt disease. In the present study, we measured the flight phenology, size, sex ratios and species identity of Monochamus species in five geographically distant forests in NA. We also measured phoresy by B. xylophilus. We found the nematode to be abundant across eastern NA but rare or absent in western NA. In eastern forests, nematode phoresy was highest on the Monochamus species that flew earliest in the year. However, in the southeast, where Monochamus is most likely multivoltine with a long flight season, we found vectors with high nematode loads throughout the season, indicating that B. xylophilus can be transmitted to new hosts during most part of the year. The frequency distribution of nematode dauers on Monochamus was highly aggregated. Bursaphelenchus xylophilus in NA appears to be able to use all available Monochamus species as vectors. In native NA pine forests, the pinewood nematode appears to have an ecology that is sufficiently flexible to exploit different species (and both genders) of Monochamus, and disperse at different times of the year. This flexibility may contribute to its recent success in invading Eurasian pine forests.