Author
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Reardon, Brendon |
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Spurgeon, Dale |
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Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 7/8/2002 Publication Date: N/A Citation: N/A Interpretive Summary: Natural mortality (death caused by exposure to unfavorable environmental conditions) is an important factor limiting the growth of boll weevil populations, but the factors most responsible for this mortality are poorly known. Recent models have assumed that starvation of the immature weevils (larvae), caused by drying of the infested flower buds (squares), is a primary cause of natural mortality. We examined the effects of starvation on weevil development and survival by denying food to larvae of different sizes, and estimated the size distribution of larvae in newly- fallen squares in the field. Starved larvae weighing about 5 mg did not develop into adults. Half of the larvae weighing about 9 mg, and about 85 percent of larvae weighing more than 17 mg, became adults. About 19 percent of larvae in newly-fallen squares were large enough to become adults without additional feeding. Because larvae grow rapidly under field conditions, square drying would play a major role in natural mortality only if most of the squares dried within one or two days, whereas temperatures commonly occurring in the field are capable of killing weevil larvae within hours. Our results question the contribution of square drying to natural mortality. These results allow a better understanding of boll weevil ecology and should aid efforts to develop cultural practices that maximize the impacts of natural mortality. Technical Abstract: Starvation of immature boll weevils resulting from square desiccation is considered a major determinant of natural mortality. However, the critical weight below which a larva cannot complete development without further feeding has not been determined. Critical weights of second and third instars were investigated using food-removal techniques, and the age and size distributions of larvae in newly abscised squares were estimated from field collections. Second instars examined ranged in weight from 0.23 to 5.55 mg. About 80 percent of second instars weighing more than/equal to 1.67 mg molted, but none pupated. The estimated critical weight for 50 percent of unfed second instars to survive to third instar was 2.49 mg. Third instars examined ranged in weight from 1.81 to 34.43 mg. About 64 percent of third instars weighing more than/equal to 5.29 mg developed into adults, ranging in weight from 1.61 to 21.49 mg. Estimated critical weights for 50 percent of unfed third instars to survive to the pupal and adult stages were 6.63 and 8.89 mg, respectively. The estimated critical weight for 50 percent of pupae to survive to adulthood was 4.52 mg. Larvae collected from newly abscised squares were predominantly second (56 percent) and third instars (39 percent). Further, an estimated 19 percent of these larvae were capable of development to adulthood without further feeding. In light of the rapid rate of larval growth and development, our results suggest that square desiccation sufficient to deter feeding by larvae must occur within 1-3 d of square abscission to produce a high proportion of starvation-induced mortality. |
