Graduate Thesis Or Dissertation
 

The effect of competition on the population dynamics of the Douglas-fir beetle, Dendroctonus pseudotsugae Hopk., in Oregon

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  • The effect of competition on the brood survival of the Douglas fir beetle Dendroctonus pseudotsugae Hopkins, in down or windthrown Douglas-fir Pseudotsuga menziesii (Mirb.) Franco was studied in the field and the laboratory. The investigation of the role of competition in the population dynamics of the beetle had four objectives: (1) to observe and record aspects of the beetle's biology and behavior which influence the intensity of competition; (2) to determine the phloem area necessary for normal development of an individual Douglas-fir beetle larva; (3) to assess the effects of intraspecific competition on the survival of the Douglas-fir beetle; (4) to assess the effects of interspecific competition on the survival of the Douglas-fir beetle. Peak periods of attack during the 1960 season followed periods of warm sunny weather and were distributed throughout the attack period which lasted from April through July. Attacks were recorded on all study trees from April through June. The pattern and density of infestation on down host trees were such that intraspecific competition occurred most frequently on the underside of the top half of these trees. Predators and parasites also tended to be slightly more abundant in this region. Studies of the gallery pattern revealed that the highest number of eggs per linear inch of egg gallery are laid within the first five inches of egg gallery. Alternate arrangement of the egg groups along the gallery serves to increase the possibility of successful larval development when overcrowding occurs. No difference was noted in the number of eggs laid at invasion densities ranging from one to three per square foot. Maximum brood survival in the laboratory was recorded when individual broods had 1.5 square feet of phloem in which to develop, or an individual larva had from 1.6 to 2.0 square inches of phloem available for feeding. Studies of intraspecific competition under caged conditions in the field revealed brood survival was highest when the larval mine density ranged from 50 to 100 per square foot. Survival decreased with progressive increases in density. A similar relationship existed between percent survival and attacks per square foot, and total egg gallery per square foot. Maximum brood survival in the field was recorded when each larva had 2.6 to 3.0 square inches of phloem in which to develop. The ratio of progeny to parent beetles was highest at larval mine densities of 51 to 100 per square foot of bark surface. The findings show that intraspecific competition is an important natural control factor, but that information on nutritional requirements of the beetle and food quality are needed before a more refined assessment of competition can be made. The lack of interspecific competition during the 1960 and 1961 field studies can be traced to different times of seasonal flight, different preferences for position on the tree bole, and low population levels of the Douglas-fir beetle and its competitors.
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file details SchmitzRichardF1965.pdf 2017-08-04 Pubblico Scaricare