- The purpose of this study was to examine various host and microclimatic influences affecting the survival and development of the flatheaded fir borer, Melanophila drummondi (Kirby) in Douglas fir, Pseudotsuga menziesii (Mirbel) Franco residues. It also expands current knowledge of the biology, bionomics, life history, ecology, and biotic potential for decomposition of forest residues by this insect.
The primary study objective was to determine the optimum host and microclimatic conditions for development of the flatheaded fir borer by measuring the rate of beetle development with an index employed in other studies on forest insects. The monthly rates of development of larvae from clearcut and partial cut (shelterwood system) logs and from 15.6°, 21.1°, and 26.7°C controlled-temperature rearings were determined over the development season. Inner phloem moisture content and pH were monitored monthly from four quadrants (south-top, south-bottom, north-bottom, and north-top) on logs from each treatment throughout the season. Measurements of ambient air temperature and precipitation for the two field sites were used to correlate the microclimate of the host to these physical factors of weather. In addition, various other studies examined the relationship between inner phloem temperature and ambient air temperature on cloudy and clear days and also between logs of different bark thicknesses, and at the top and bottom of a log.
Forest residues are attacked immediately after they become available from timber cutting or tree mortality in the spring and summer. Tops down to 7.62 cm in diameter are utilized by this beetle, although thinner bark of small diameter residues afford less protection from temperature extremes, parasites and avian predators than thick-barked residues.
Eggs are typically deposited on the tops or upper sides of logs and residues by woodborers, and they also prefer clearcuts over partial cuts. Furthermore, larval densities are higher on the tops and sides of residues and on clearcuts, compared to other locations. The top portion and upper sides of residues are characterized by warmer inner phloem temperatures, lower moisture contents, and less acidity than the bottom half of logs and residues on the ground. In addition, clearcut logs are generally characterized by these qualities. Temperature and moisture differences around the logs
are believed to affect the distribution of the flatheaded woodborers within logs.
Although the flatheaded fir borer invades residues on both clearcuts and partial cuts, the clearcuts are preferred because of warmer temperatures. The rate of development is also greatest on clearcuts, and maturity to adult is reached sooner due to faster accumulation of heat units on these sites. The clearcut represents optimal conditions for development and survival of this insect.
Larvae reared at three different constant temperatures in the laboratory failed to develop past the 3rd instar (pre-pupa), presumably because of the lack of a cold period which is required to break diapause in this resting stage.
Parasitism accounts for the greatest proportion of woodborer mortality. Other mortality factors include resinoisis, predation, and unknown causes. Various parasites were reared and identified as mortality factors, but Atanycolus longifemoralis Shenefelt was most frequently observed. Other parasites and predators are discussed in their role as natural enemies of the flatheaded fir borer.
An accurate and precise linear regression method is described which enables the estimation of inner phloem surface area of larval galleries from measurements of the gallery length. Its use in estimating woodborer-caused deterioration in forest residue surveys is emphasized.