Terence L. Wagner

A biophysical model of southern pine beetle, Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae), development☆

Abstract Predicting the population dynamics of the southern pine beetle, Dendroctonus frontalis Zimmermann, in forests of the U.S.A. requires predictive models of cohort development. The cryptic nature of the insect, however, prevents direct observations of its development, and undefined host requirements of early stage larvae prevent rearing it in the laboratory. Consequently, obtaining the information needed to formulate predictive models is difficult. This study describes experimental, analytical, and modeling techniques used to obtain information on beetle development times, the distribution of those times, and percent mortality over a full range of constant temperatures. Our data indicate that D. frontalis is highly sensitive to temperature. Cohort development occurs over extended intervals of time with the durations of development influenced by temperature in a nonlinear fashion. Information on development rates and the distributions of development times were used to formulate biophysical models of life stage development and development from eggs to adult emergence. Tests of the latter model here and elsewhere (Feldman et al., 1981a) showed good agreement with data from several field populations. These tests provided a basis for incorporating the model into a larger population dynamics model for this insect (Feldman et al., 1981b).

Impact of foraging byMonochamus titillator [Col.: Cerambycidae] on within-tree populations ofDendroctonus frontalis [Col.: Scolytidae]

The impact of foraging byMonochamus titillator (Fab.) on within-tree populations ofDendroctonus frontalisZimm. was described. Total population estimates forD. frontalis and inner bark area foraged byM. titillator were computed for 89 loblolly pine,Pinus taeda L., sampled over a 3 year period. Frequency histograms were prepared for the surface area of habitat infested, area foraged byM. titillator, and within-tree populations ofD. frontalis. These data were used to calculate the proportion of area foraged byM. titillator, the proportionalD. frontalis mortality for the entire tree, and the proportional mortality occurring in the foraged area. Histograms showing the variation of these components were prepared and described using nonlinear mathematical models. Ca. 20% of the infested surface area was foraged byM. titillator. Mortality toD. frontalis on a per tree basis was ca. 14%. Mortality in the area foraged byM. titillator was ca. 70%. These estimates were highly variable between individual trees and a procedure for predicting the probability of a given level of foraging and mortality was described. Sources of variation influencing foraging byM. titillator and mortality toD. frontalis were investigated. Variation between years and season followed similar trends with both foraging and mortality increasing from the base to the top of the infested bole. Variation between tree size-class was highly significant. Larger size-class trees had substantially greater foraging and mortality than did the smaller size-classes. Mortality within the foraged area was also found to be greater at the extremes of the infested bole.RésuméL’impact de l’activité deMonochamus titillator (Fab.) sur les populations à l’intérieur de l’arbre deDendroctonus frontalisZimm. est décrit. Le total des évaluations de population deD. frontalis dans des zones à l’intérieur du tronc attaquées parM. titillator a été soumis à l’ordinateur pour 89 pins,Pinus taeda L., échantillonnés pendant 3 ans. Les histogrammes de fréquence ont été établis pour la surface de l’habitat infesté, la surface explorée parM. titillator et pour les populations deD. frontalis présentes dans l’arbre. A partir de ces données on a calculé la part de surface utilisée parM. titillator, la mortalité deD. frontalis extrapolée pour l’arbre entier et la mortalité se produisant dans la surface exploitée. Les histogrammes traduisant les variations de ces composantes ont été dressés en ayant recours à des modèles mathématiques non linéaires. Environ 20% de la surface infestée a été exploitée parM. titillator; la mortalité du scolyte fut d’environ 14% dans l’arbre et de 70% dans la zone attaquée parM. titillator. Ces évaluations sont très variables d’un arbre à l’autre et on a mis au point un procédé pour prévoir la probabilité d’un taux donné d’activité alimentaire du cerambycide et de mortalité du scolyte. Les causes de cette variabilité ont été étudiées. La variation suivant les années et les saisons suit les mêmes tendances à savoir l’augmentation à la fois de l’activité alimentaire et de la mortalité de la base au sommet du pin attaqué. La variation selon les classes de taille des arbres est hautement significative: les arbres les plus gros présentent une exploitation et une mortalité beaucoup plus importantes que les arbres plus petits. La mortalité du scolyte dans la zone attaquée par le cerambycide est également plus élevée aux extrémités du pin infesté.

Within-tree life process models of the southern pine beetle, dendroctonus frontalis

Abstract This paper develops models of gallery construction, emergence and re-emergence for use in a general population dynamics model of the southern pine beetle, Dendroctonuc frontalis Zimmermann. Models of these processes were originally developed from laboratory data, and are extended here to account for fluctuating temperatures and variable attack densities under field conditions. The resulting models were tested using data from three natural populations (infestations) from east Texas. These tests reveal that the laboratory-derived models closely predict the timing and length of gallery, and the timing of emergence and re-emergence in the field.

Factors influencing the growth of multiple-tree infestations ofDendroctonus frontalis

The southern pine beetle, Dendroctonusfrontalis ZIMMERMANN, is a significant mortality agent of pine (Pinus spp.) forests in the southern USA. Outbreaks of the insect occur periodically throughout the region. The severity of an outbreak is a function of two principal variables: the rate of initiation of new infestations and the subsequent growth rate of these infestations (CouLSON et al., 1983, 1984). Growth of infestations occurs as a result of colonization of host pines by adult beetles. There is a characteristic growth pattern for D. frontalis infestations. Attacks are concentrated on new hosts proximal to previously colonized trees. This behavior results in the enlargement of infestations by an accumulation of newly attacked trees along a front. In some instances infestation growth lasts for several months. In other cases infestations become inactive and do not increase in size beyond the initial conditions. There are a number of factors known to influence infestation growth. Among the most important and documented factors are the semiochemical communication system utilized by D.frontalis to locate hosts and assemble mates (PAYNE and COULSON, 1984), stand age (LoRIO, 1980), tree geometry (spatial arrangement) of the stand (ScHOWALTER et al., 1981), and the interaction of stand geometry and meteorological conditions with the semiochemical communication system (VITE et al., 1964; FARES et al., 1980). Previous studies conducted on the dynamics of infestation growth were limited because quantitative estimates of populations were not available for the description and analysis of the process. Our objectives in this study were to investigate: (1) the relationships of D. frontalis population and host parameters on infestation growth, i.e., the final size attained in D.frontalis infestations, and (2) the daily number of new attacks occurring within infestations as related to population and host parameters and meteorological conditions.