Bursaphelenchus xylophilus is an insect-vectored pathogen and the causal agent of the pine wilt disease (PWD). In Portugal, where B. xylophilus is a recent invasive pest, the native pine sawyer beetle, Monochamus galloprovincialis, is its only vector. Epidemics of PWD emerge from three-way interactions among B. xylophilus, M. galloprovincialis, and host pine trees. However, the basic ecology of these interactions is poorly known. We studied M. galloprovincialis with respect to its life history, host selection, and phoresy by B. xylophilus in an area severely affected by PWD in Portugal. We assessed: (1) seasonal patterns of phoresy related to the flight phenology of the vector; and (2) distribution of M. galloprovincialis within and among host pine trees. Monochamus galloprovincialis in Portugal flies from May through October, and is probably bivoltine, but phoresy by B. xylophilus only occurs within a short period in early summer, when infection of new pine hosts most likely occurs. Peaks of M. galloprovincialis emergence and B. xylophilus phoresy immediately precede onset of the Mediterranean dry season, favoring PWD because B. xylophilus succeeds best on trees suffering from severe water deficits. The abundance of phoretic nematodes was more evident when their vectors were trapped as free flying adults rather than emerging from dying trees. Bursaphelenchus xylophilus were highly aggregated on a subset of M. galloprovincialis, which were aggregated in a subset of dying trees. However none of the host parameters related to size and bark thickness, could explain host selection. Despite its importance for PWD, M. galloprovincialis is a minor component of the bark and wood-boring insects of pine trees in Portugal. Interactions within this community are likely important to the epidemiology of PWD.