Graduate Thesis Or Dissertation
 

Arboreal arthropod predation on early instar Douglas-fir tussock moth

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  • Loss of early instar Douglas-fir tussock moth (Orgyia pseudotsugata McDunnough) (DFTM) has been found to constitute 66-92% of intra-generation mortality and to be a key factor in inter-generation population change. This death has been attributed to dispersal and to arthropod predation, two factors previously judged more important to an endemic than an outbreak population. Polyphagous arthropod predators are abundant in the forest canopy but their predaceous habits are difficult to document or quantify. The purpose of the study was to develop and test a serological assay, ELISA or enzyme-linked immunosorbent assay, for use as an indirect test of predation. Development of this assay involved production of an antiserum reactive with DFTM but not reactive with material from any coexisting lepidopteran larvae. Two-dimensional immunoelectrophoresis was used to select a minimally cross-reactive fraction of DFTM hemolymph as the antigen source so that a positive response from a field-collected predator would correlate unambiguously with predation on DFTM. Feeding trials using Podisus maculiventris Say (Hemiptera, Pentatomidae) and representative arboreal spiders established the rate of degredation of DFTM antigens ingested by these predators. An arbitrary threshold for deciding which specimens would be considered positive was established as the 95% confidence interval above the mean of controls. Half of the Podisus retained 0 reactivity for 3 days at a constant 24°C. Eighty percent of trial-fed spiders were positive in ELISA on the day on which they were fed; 80% were positive 12 days later when the test was concluded. Feeding on alternate prey decreased reactivity slightly. Fifty percent of the spiders initially fed a single first instar DFTM and subsequently fed 3 non-DFTM larvae tested positive after 0 being held 12 days at a constant 24°C. Field trials of this assay were conducted in 1979, 1980 and 1981 with arthropods collected in association with a native population of DFTM. Samples were taken at budburst from white fir from El Dorado National Forest, California. No cross-reactions with alternate prey were found in 1979 or 1980; phytophagous mirids did show false positive reactions in 1981. Cross-absorption of the conjugate prior to ELISA eliminated these heterologous reactions. All but rare species of arboreal arthropod predators were confirmed as having consumed first or second instar DFTM. A fortuitous 800-fold decline in the DFTM population occurred over these 3 study years. The proportion of predators of all guilds declined non-linearly with this decline in DFTM density. Spiders in each guild (diurnal active hunters, nocturnal active hunters, ambush hunters, larger web-spinners and minute web-spinners) consumed DFTM in linear proportion to the relative abundance of DFTM as one potential prey, i.e. were non-selective predators. Predaceous hemipterans, principally mirids, showed a strongly logarithmic response in the same analysis.
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