Relation of Lightning to Herbivory by the Southern Pine Bark Beetle Guild (Coleoptera: Scolytidae)

The goal of this paper was to examine the premise that lightning is a disturbance to pine forests that occurs with sufficient frequency to provide reliable habitat for the persistence of bark beetles. The study was conducted in the pine forest region of East Texas. Using Texas Forest Service maps, this 55,400-km2 area was partitioned into systematically numbered grid cells (5 by 5 min) of latitude and longitude (units ≍64 km2 in area). There were 607 cells in the study. Data for the study were collected for the years 1975, 1976, 1977, 1982, and 1983 and consisted of forest cover, bark beetle infestations, and lightning strikes. Cumulative bark beetle herbivory (number of infestations) in relation to cumulative lightning strikes and the rate of change in numbers of bark beetle infestations relative to lightning strikes were examined. In 4 of the 5 yr of the study, cumulative infestations increased as a function of cumulative lightning strikes. As population levels increased (i.e., changed from enzootic to epizootic levels), a greater proportion of lightning-struck hosts were exploited by the bark beetles. At enzootic population levels, refuges in the form of lightning-struck trees were judged to be particularly significant to the persistence of the insects. Conversely, at epizootic population levels, initiation of a substantial portion of the observed infestations was due to factors other than lightning. Examination of the relation of rate of change in infestations relative to cumulative lightning strikes indicated that the availability of lightning-struck hosts for colonization was not a limiting factor in regional landscape epidemiology. Sufficient lightning-struck trees are available for colonization across a range of enzootic to epizootic population levels.