Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/8/2000
Publication Date: N/A
Interpretive Summary: Natural mortality, that is death of boll weevil larvae, pupae, and adults caused by environmental conditions before adult emergence, is known to greatly influence boll weevil population growth. However, the conditions most responsible for this mortality are not well understood. Many investigators have considered high temperatures to be important causes of this mortality. Recent models assume drying of the fallen cotton flower bud (square) containing the weevil larvae is the most important factor. This drying results in deterioration of the larval food source and death from starvation. Effects of square drying are difficult to examine separately from those of high temperature, so we examined the effects of food deterioration by removing boll weevil larvae from the squares and holding them without food. The fates of these larvae (death or continued development) was monitored and evaluated in relation to larval weight. All larvae weighing less than 5 mg died without further development. Most larvae weighing more than 5 mg developed to adults regardless of larval weight at the time of removal from the square. Thus, our results indicate that larvae must weigh at least 5 mg to continue development in the absence of food. Because squares containing larvae usually remain on the plant until considerable larval development has occurred, our findings question the role of square drying in the natural mortality of boll weevils. These results should aid in designing additional experiments to better understand the causes and management implications of natural mortality.
Technical Abstract: Desiccation and high temperatures have been reported as major determinants of natural mortality of immature boll weevils. However, direct examination of these mechanisms is effectively precluded by the difficulty of manipulating temperature and desiccation independently. Thus, the mechanisms involved in natural mortality are not well understood. Desiccation is reported to act by rendering the square unfit for larval consumption, resulting in starvation. Therefore, we examined the impact of food removal on larval survival and development to indirectly assess the role of food deterioration in natural mortality. Four hundred twenty-four 3rd instars ranging in weight from 1.81 to 34.43 mg were removed from squares and held without food. No larvae weighing <5 mg survived to the pupal stage. A high proportion of larvae weighing >5 mg survived to the pupal and adult stages (86 percent and 81 percent, respectively). When only larvae >5 mg were considered, the proportion of larvae surviving to subsequent stages was not related to larval weight. Our results question the status of food deterioration as a primary mechanism of natural mortality, and provide insight to future efforts to investigation this important phenomenon.