Development and Mortality of Ips calligraphus (Coleoptera: Scolytidae) at Constant Temperatures

Abstract

We describe effects of constant temperature, beetle density, and time of oviposition on aspects of development, mortality, sex ratio, and size of Ips calligraphus (Germar). Emphasis is placed on individual life stages. Relationships of median development times to constant temperatures were similar for eggs, larvae, and pupae in the 12.5–37.5°C range. These relationships appeared as backward J-shaped curves. Reciprocal development times versus temperatures for these life stages were described by the six-parameter rate function of Sharpe & DeMichele (1977, J. Theor. Biol. 64: 649–670). Development times and foraging distances of larvae increased with each instar. The third instar required 61% of total larval time and excavated 73% of the total larval gallery. Optimum development temperature of teneral adults was not identified, indicating greater tolerance to high temperatures of this stage compared with the others. The four-parameter Sharpe & DeMichele (1977) model (without high-temperature inhibition) described teneral adult rate as a function of temperature. Timing of oviposition influenced development time of larvae (increased development time with late oviposition) and teneral adults (decreased), but not development times of eggs or pupae. Reversed patterns of longer development times for larvae and shorter times for teneral adults canceled any effects of sequential oviposition on length of life cycle. Distributions of development times were generally skewed toward the longer times, and variation around mean time tended to increase with successive life stage. A single, temperature-independent distribution of normalized development times is presented for each life stage and is described by a three-parameter cumulative Weibull function. Stage-specific mortality (resulting from unknown causes) formed partial or full U-shaped patterns when plotted against temperature. Density-dependent mortality resulting from cannibalism is discussed, with a Weibull function describing influence of egg density on probability of being cannibalized. Timing of oviposition influenced percentage of mortality of all life stages, primarily due to increased cannibalism of later-developing brood. An overall sex ratio of 1:1 was observed, but proportion of males to total I. calligraphus increased from 0.374 in the first quarter of the emergence period to 0.569 in the fourth quarter. Beetles reared at 15–25°C were somewhat larger than those reared at 30–35°C. Generally, males were slightly larger than females when size was examined relative to time of emergence. An inverse relationship of beetle size to time of emergence was observed for each sex.