Relationship between ambient and leaf litter temperatures in overwintering habitats of boll weevil (Coleoptera: Curculionidae)
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Temperature data are unavailable for most boll weevil overwintering habitats. As a result, ambient data often are used to estimate overwintering survival and emergence. In this study, relationship of ambient and overwintering habitat temperatures was determined for 6 habitats in the Rolling Plains of Texas. The amplitude of the daily temperature cycle in the leaf litter of each habitat was dampened compared with that recorded for ambient. Average minimum daily leaf litter temperature was 3.19C higher in the shelterbelt habitat, followed by mesquito - grass pasture (3.54C), tall shinnery motte (3.64C), low shinnery brush (3.81C), lovegrass (4.01C), and pecan (6.01C). Likewise, the average maximum daily leaf litter temperature was 3.77C lower in shelterbelt, followed by pecan (3.86C), lovegrass (3.87C) mesquite-grass pasture (4.07C), low shinnery brush (5.51C), and tall shinnery motte (6.12C). Stepwise multiple regression of hourly leaf litter temperatures explained 93% of the variability, with hourly ambient temperature and week of year being significant variables. Similarly, multiple regressions of daily leaf litter minimum temperatures and daily leaf litter maximum temperatures explained 96 and 90% of the variability, respectively, with ambient temperatures and week of year being significant variables for both regressions. The effect of temperature quality (ambient versus leaf litter) and sampling frequency (hourly versus daily) on positive (>6.1C) and negative (<0.0C) degree-days experienced by overwintering boll weevils was quantified for each of the 6 habitats for 4 yr of independent data, representing 2 mild and 2 severe winters. Degree-day calculations based on fitting minimum - maximum data to an asymmetrical double sine curve overestimated positive degree-day accumulation by 2.5%. Ambient temperature positive degree-days were similar to that for mesquite - grass pasture habitat, but the ambient temperatures overestimated positive degree-days experienced by overwintering weevils in the pecan by 6.5%, followed by low shinnery brush (6.7%), lovegrass (8.7%), tall shinnery motte (9.2%), and shelterbelt (15.7%). All habitats had significantly fewer cumulative negative degree-days compared with that calculated for ambient. Pecan leaf litter accumulated 99% fewer negative degree-days, followed by shelterbelt (95%), tall shinnery motte (88%), low shinnery brush (82%), lovegrass (71%), and mesquite - grass pasture (50%).
