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Bagous affinis Hustache (Coleoptera: Curculionidae) is a biological control agent of Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae; hydrilla), a weed in ponds, lakes, and waterways. Adults feed on hydrilla stems and leaves that are left exposed as waters recede during the dry season or following a drawdown (Baloch et al. 1980). The adults oviposit in moist organic matter found in and among the exposed hydrilla stems (Bennett & Buckingham 1991). The larvae burrow in the soil beneath the exposed stems and feed on subterranean vegetative propagules called tubers (Baloch et al. 1980). The basic life history and host range of B. affinis has been studied (Buckingham 1988, Bennett & Buckingham 1991). However, other aspects of the life history of B. affinis need to be determined to assess its potential and optimize its use as a biological control agent. For example, information on the rate of development of each life stage can be used to predict the best time for release (i.e., when air and soil temperatures are above developmental thresholds). In addition, the time of egg hatch or completion of the larval or pupal stage in the field after release and the number of generations possible at a site in a given period of time can be estimated. Therefore, in this study we measured the rate of development of the egg, larval, and pupal stages at six constant temperatures and constructed simple linear thermal unit models. The range of temperatures used represent those typical of northern California from late spring through fall, the dry season when aquatic systems undergo natural drawdowns (NOAA 1985). The rates of development of the egg, larval, and pupal stages were estimated at 18, 21, 25, 28, 30, and 32?C (? 1?C) constant temperature regimes. The weevils used in these experiments were maintained in laboratory culture for 8 to 14 generations and were from the same colony as that described by Godfrey et al. (1994). B. affinis were reared individually in small, plastic containers (5.5 x 5.5 x 6.5 cm) to measure development at each temperature. Each container was filled with moist soil (a fine sandy loam), and five dioecious hydrilla tubers were buried 3 cm beneath the soil surface. Eggs were dissected from water-soaked wood (an oviposition substrate) that had been placed in colony cages for less than 24 hours. Each egg was placed on a piece of moist filter paper on the soil surface. The container was covered with aluminum foil to maintain humidity. The rearing containers were placed in a growth chamber (Model CEC 36-1OHLE, Rheem Sherer, Weaverville, NC) at the appropriate temperature regime, and checked once daily for egg hatch. The time of day that the eggs were checked and the condition of the eggs (i.e., hatched or not hatched) were recorded. The following number of eggs were placed at each temperature regime: 180 at 18?C; 145 at 21?C; 181 at 25?C; 135 at 28?C; 130 at 30?C; and 293 at 320C. Differing numbers of eggs were used because the survivorship of B. affinis from egg to adult varied with temperature. In addition, measurement of developmental rate was attempted at 150C (n=20) and 35?C (n=60), but no B. affinis survived to the adult stage. |
