Browsing by Subject "Threshold"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Fish growth and degree-days: Advice for selecting base temperatures in both within- and among-lake studies
    (2013-08) Chezik, Kyle August
    Ectotherm growth and development are largely a function of temperature. Within the range of thermal tolerance, growth and development increases linearly over a mid-range of temperatures. Degree-days (DD; °C;days) are a method of quantifying the thermal experience of an organism over the linear range and are an increasingly popular method for describing growth and development in fish. To limit degree-day calculations to temperatures within the linear range, a lower temperature threshold (To) is incorporated that defines the onset of growth. However, there is currently no convention for choosing To and the implications of choosing an inappropriate To are largely unknown. This uncertainty has resulted in a wide range of To values currently in use both among- and within-species. In this thesis I explore i) how To affects the ability of DD to explain within-population variation in fish growth, and ii) the effect of To on apparent growth among populations in thermally dissimilar environments. To address these objectives, I first use simulated data to determine the theoretical relationship between To and growth. I then demonstrate these theoretical relationships using immature length-at-age data from 8 species and 85 waterbodies in Minnesota USA and Ontario Canada. My results show that DD at low To are highly correlated and that these highly correlated low To values explain variation in growth equally well within a single population. Furthermore, growth rates among populations of thermally dissimilar habitat become increasingly dissimilar as To deviates from the To that minimizes the variation in growth rate among populations. This effect is especially apparent when the among-population range in thermal habitats is large. These findings suggest that, although To matters little when accounting for variation in growth within a single population, an inappropriate To can lead to the appearance of among-population differences in growth. When choosing To, I recommend estimating To using the Charnov and Gillooly (2003) “10°C rule” and then rounding to the nearest To standard (0, 5, 10 and 15°C). Choosing a To standard in this way will minimize the effect of To error on growth bias in among-population studies. In general, standardization i) simplifies To identification, ii) facilitates comparative studies, and ii) promotes the use of DD in future studies.
  • Thumbnail Image
    Item
    Utility of Scouting with Sticky Traps for Integrated Pest Management of Corn Rootworm
    (2017-12) Leaf, Trisha
    Corn rootworms (Diabrotica virgifera virgifera LeConte and Diabrotica barberi Smith and Lawrence) have been pests of economic importance on corn for over a century due to the injury they inflict on corn roots. Injury primarily takes place following larval feeding and occurs in the form of root pruning, stalk lodging, reduced nutrient up-take, and secondary attack by pathogens, but may also occur from adults feeding on silks and pollen. For the past decade, growers have been relying primarily upon corn rootworm- active Bacillus thuringiensis Berliner (Bt) transgenic hybrids to control corn rootworms, and thereby disregarding classic IPM practices due to the reliability of these hybrids alone. In 2009, however, field-evolved resistance to corn rootworm- active Bt hybrids was found in Iowa. Since then, cases of problem fields (i.e., fields where excessive lodging and root pruning occur despite the use of transgenic hybrids) are becoming more prevalent across the Corn Belt where corn-on-corn is common, causing IPM to become a renewed focus in the effort to control corn rootworms. The frequency of problem fields and high cost of corn rootworm-resistant traits is leading growers to use outdated integrated pest management (IPM) decisions to control corn rootworms and hopefully reduce the injury in inflict on the corn plant. Integrated pest management plans with corn rootworm-active Bt hybrids are a cornerstone for successful use of refuge-based Insect Resistance Management (IRM) plans as well. IRM plans combine corn rootworm biology with agronomic practices to delay population resistance to current Bt hybrids containing corn rootworm-active events. While corn rootworm phenology has been extensively reviewed in the past, the influence of Bt hybrids on current phenology is not fully understood although developmental have been observed. A delay in adult emergence has been observed in beetles emerging from Bt hybrids in comparison to non-Bt (refuge) hybrids. The delay in emergence may result in reduced efficacy of refuge corn, which is essential to the IRM plan by increasing mating between the beetles from the refuge and Bt corn. Emergence differences may cause Bt tolerance to develop more quickly. Planting date is known to play an important role in controlling the amount of damage that may occur on the corn plants as well but often coming at the expense of yield. Late planting results in a lack of food availability for corn rootworm larvae, which results in increased mortality, reduced injury, and delayed adult beetle emergence. The combination of planting date and Bt hybrid on corn rootworm adult emergence is largely unknown. This has implications for determining adult scouting windows in order to determine insect pressure the following season. Current Bt hybrids exhibit superior genetics when compared to past inbred lines; in fact, no current data exists for adult beetle thresholds in relation to injury on Bt hybrids. With the new threat of Bt-resistant corn rootworm populations becoming more prevalent, recommendations on how and when to scout for resistant populations are needed. Updated beetle thresholds and scouting windows will also help prevent the overuse of pesticides and control beetle populations the following year. Studies which try to establish economic injury levels with adult beetles as well as economic thresholds in order to aid grower’s management decisions are important for extending the shelf life of corn rootworm-active Bt hybrids as well. This study aimed to evaluate sticky traps as a scouting tool on current corn hybrids, establish precision and required sample sizes, define scouting windows based on planting date and transgenic hybrid-induced emergence delays, and determine the relationship to root injury the following season.