Life history of the Obscure Root Weevil, Sciopithes obscurus (Horn), on rhododendron Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rn301504p

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  • The life history of the Obscure Root Weevil, Sciopithes obscurus (Horn), on Rhododendron was studied at four sites in the Willamette Valley. This research was conducted in an effort to provide information necessary to develop adequate methods to control this Curculionid pest on Rhododendron in the Willamette Valley. Five larval instars were reared on potted strawberry roots in the laboratory. Teneral adults were obtained after 288 days of growth. Soil samples indicated that in the field S.obscurus larvae are contagiously dispersed among Rhododendrons and apparently move toward the soil surface prior to pupation. Adults emerge from the soil between late May and September, although some emergence may occur as late as October. The adults are parthenogenetic and their ovaries develop in four distinct stages over a period of approximately 21 days at 20°C and a 12-hour photophase. A fifth state appears among older individuals as the germarium becomes inactive. A low percentage of S.obscurus adults successfully overwinter. The state of ovariole development of these adults was shown to be of two classes: incomplete development without oviposition prior to overwintering; completed development with oviposition at least once prior to overwintering. Between June and August, the number of active adults on Rhododendron increases dramatically as a result of adult emergence. From the middle of August to the first of October the rate at which individuals enter and leave the population through emergence and mortality reaches an equilibrium. The number of active adults on Rhododendron declines through October as a quiescent period, during which the adults attempt to overwinter, is initiated by the approach of winter. Adult activity then resumes by the end of the following April. Egg clusters are deposited in Rhododendron flowers and in leaf fragments which result from adult feeding. Oviposition occurs in the field between March and November. The intensity of oviposition is highest from August through October, ceases during the winter months, and resumes by the overwintered adults in April. When they are active, the adults exhibit a feeding preference among Rhododendron species. Younger foliage is preferred as a food source. The results of a detachedleaf host preference experiment indicated the possibility of host resistance against feeding by the adult weevils.
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