Probability of occurrence and phenology of pine wilt disease transmission by insect vectors in the Rocky Mountains.
Pine wilt disease, caused by pinewood nematode (Bursaphelenchus xylophilus; abbreviated 'PWN'), is a damaging and globally distributed insect-vectored forest pathogen. Native forest tree mortality associated with PWN is newly reported from the Front Range of Colorado, but there is no regional information on PWN frequency or biology of local insect vectors, limiting management options. . A sampling array was established to survey PWN in native pines (Pinus ponderosa) and longhorn beetles (Monochamus clamator and M. scutellatus) over 2 years and across natural and urban forest landscapes. We developed flight phenology models and evaluated effects of landscape factors on vector abundance and probability of infection. Flight phenology was similar for vectors; Monochamus flight initiated in mid-July and continued into October for both species. We report the first M. clamator-PWN association in the United States. PWN was distributed in the region at rates lower than reported from its putative native range: 3.6 and 4.2% of sampled pines and beetles, respectively, tested positive for PWN. Many host trees were outwardly asymptomatic; infection frequency in tree populations varied considerably and four epicentres of vector infectivity were identified. Epicentres varied in timing of anomalous infective vector frequency - some epicentres had high abundances of infected beetles early in the growing season whereas others had high abundances of infected beetles late in the growing season, though PWN-positive beetles were captured at all sites. Monochamus populations were found primarily in natural forest stands but migrated to urban areas late in the growing season. The only landscape factor positively correlated with abundances of both Monochamus species was distance to previous wildfire. PWN epicentres in the southern Rocky Mountains exhibit specific temporal windows of vector activity that differ from proximal sites. Urban forests, where the disease was initially observed in the region, do not support vector populations. Our results suggest that natural forest landscapes in the region are important reservoirs of PWN, and vector populations are especially abundant near burned stands. Collectively, our findings are important for timing disease management activities appropriately and help to distinguish priority areas for mitigation efforts.