Graduate Thesis Or Dissertation

Biology and feeding habits of pleocoma larvae (Coleoptera : Scarabaeidae) in western Oregon coniferous forest

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  • Food habits and biology of five species of Pleocoma larvae were studied at a dozen forested sites in western Oregon between May 1960 and December 1961. First instar Pleocoma hatch in late summer and moult to second instars in early fall. Second and subsequent instars moult annually between mid-summer and early fall. Larvae appear to go through more than nine instars pupating after the seventh, in the upper 20 inches of soil, in mid-summer. Male larvae outnumber females by about 30 percent. Larvae move through the soil primarily by use of the mandibles. This movement can exceed a rate of four inches a day. Larval populations varied from none to 4.4 larvae per square foot and were distributed between two and 44 inches in depth. Soil temperatures and soil moisture influenced most larvae at some sites to leave the upper 16 inches of soil during the summer. At other sites, however, a shallow silicate clay hardpan influenced larvae to remain at shallow depths throughout the year. A fungus disease killed from five to more than 20 percent of the larvae in some areas. Dipterous predators killed some larvae. Coniferous roots comprise the major part of the larval diet throughout most of the year, being found exclusively in 86 percent of the larvae. Thirty percent of the roots in the guts were definitely identified as Douglas-fir, the predominant conifer at the sites of the collections. Larvae preferred smaller roots, mostly smaller than 2mm, many of them mycorrhizal rootlets. A few larvae merely girdled the roots, stripping the bark leaving the xylem in the soil. Most larvae, however, severed and consumed the entire root. Larvae either severed and ingested intact root segments or gnawed on root ends masticating them into very fine pieces before ingestion. Except for last instar larvae, usually at shallower depths, and first stage larvae, neither of which appeared to feed on roots, all other larval stages at all depths were feeding on roots throughout the year, except during the moulting period. The cessation of root feeding extended over about a four-month moulting period from June to September, inclusive. The moulting period varied somewhat between species. Most larvae consumed the exuvia. Some soil, probably less than five percent by volume, was ingested. Other material tentatively identified in larval guts was: remains of fungal hyphae, cast ventricular epithelium, gregarine parasites, and bacteria. Pleocoma larvae do not appear to be serious forest pests in old-growth and advanced second-growth coniferous forests because of the generally low and scattered larval populations. In newly-established forests, however, Pleocoma larvae are a potential pest as one or two feeding larvae can kill a small tree. Only further studies in regenerated areas will determine their role in these cut-over and reestablished forests.
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