Evan Lue Codling Moth Resistance May 9 2005p. 1Trade-Offs Between Insecticide Resistance and Development Time in Codling MothEvan LueAbstract Resistance against organophosphates in the codling moth, Cydia pomonella, issuggested to increase the development time between life stages. Such life history trade-offs arenot uncommon, but may pose a threat to current pest management strategies. Differences indevelopment time for susceptible and resistant populations will not only result in two distinctemergence peaks, but may also consequently force an alteration in the timing of pesticideapplications. The aim of this study is to confirm a difference in development times betweensusceptible and resistant codling moths. A colony of each genotype was reared in a laboratoryand adults were mated to produce eggs. Freshly laid eggs were stored in growth chambers at 55°,70°, 80°, and 90°F. Lower developmental thresholds were found to be 50.41°F and 51.74°F forthe susceptible and resistant colonies, respectively. Degree-days were calculated using a lowerthreshold of 50°F, and eggs were examined for hatch every ten degree-days. Organophosphateresistant codling moths were found to have longer egg-larva development times. The bimodalemergence pattern observed between these two colonies is of particular concern at coolertemperatures, spanning larger amounts of time between peaks. These differences imply thatidentifying levels of pesticide resistance in codling moth populations is an importantconsideration in assessing the utility of current degree-day models.Evan Lue Codling Moth Resistance May 9 2005p. 2IntroductionCodling moth, Cydia pomonella (Lepidoptera: Tortricidae), is a major pest in apple, pear,and walnut orchards across the world, causing huge economic losses to agriculture (Bouvier etal. 2002). During the summer, over half of all insecticides used on apple orchards are used tocontrol against codling moth (Brunner et al. 2002). Multiple generations of codling moth havebeen observed to attack orchards continuously for the entire duration of a harvest (Bouvier et al.2002). Among the many pest management methods employed by crop-growers is the use oforganophosphate insecticides.Studies researching other forms of chemical pest management have indicated that removingorganophosphates from current codling moth management programs will be difficult (Knight etal. 2000). Though intended for the codling moth, these insecticides are also effective at indirectlycontrolling many secondary pests associated with the codling moth, such as the leafroller(Walker and Welter 2001). Because of their effectiveness and their limitation of secondary pestoutbreaks, organophosphate applications are widely used in California and Washington (Brunneret al. 2002).Frequent application of insecticides can create selective pressures on pests, forcing allelicsubstitutions that lead to genotype variation and insecticide resistance (McKenzie and Batterham1994). These anthropogenic perturbations can force a reallocation for different enzymaticactivities (Boivin et al. 2003a). Due to the wide use of insecticides, some strains of codling mothhave developed a resistance to the organophosphates. Codling moths are becoming increasinglymore resistant to benzophenylureas (BPUs) such as diflubenzuron (Sauphanor et al. 1995,Bouvier et al. 2002). Selective pressures can result in cross-resistance, enabling codling moth todetoxify a variety of different chemicals, even when the mechanisms by which chemicals aretoxic differ from one another (Dunley and Welter 2000).While insecticide resistance poses a problem for pest management by making insecticidesineffective, another indirect situation may develop as a result, and could potentially pose aproblem. Resistance often comes with a cost to the pest. Life history trade-offs can occur inmany forms, including reduced fitness or size. Particularly, pleiotropy can result in a selectivedisadvantage in regard to development (Boivin et al. 2003a).There is some evidence that organophosphate resistance in codling moth is correlated to adecrease in development rates (Boivin et al. 2001). More degree-days are required for resistantEvan Lue Codling Moth Resistance May 9 2005p. 3codling moths to complete their developmental stages. The two distinct genotypes of resistantand susceptible codling moths differ not only in their susceptibility to insecticides, but also intheir peak emergence times. Development plays a key role in timing of pest management(Sauphanor et al. 1997).Crop growers’ ability to control codling moth populations is particularly important becauseof the large role that codling moth plays in many economically important crops. Having multiplepeaks in the development of codling moth could potentially limit the predictive capabilities ofexisting degree-day models. These models are of particular importance because choosing theperiod to apply insecticides has an effect on insecticide efficiency and the potential of pests todevelop further resistance. Frequent exposure of pests to sublethal doses of an insecticide couldresult in a gradual decrease in susceptibility (Sauphanor et al. 1998). Non-overwinteringpopulations are also observed to have slower developmental rates (Boivin et al. 2001), whichwill further limit the efficacy of degree-day models. Confirming and quantifying the differencein degree-day requirements for these two strains of codling moth will help in identifying thedevelopmental implications of organophosphate resistance.MethodsAdult moths were obtained from apple orchards in northern California by Welter and Cave(2004). Bioassays for a prior experiment were conducted to determine the susceptibility of mothpopulations to insecticide. The organophosphate-resistant and organophosphate-susceptiblecolonies were reared and kept in separate rooms to prevent contamination through cross-breeding. Anywhere from 15-50 adult moths from each colony were placed in cylindrical matingchambers, depending on the population sizes of the colony at the time of mating. The chambersconsisted of a tube of wax paper with an approximate area of 3 ft2 wrapped around an apparatuswith wire mesh ends, with a different chamber for each of the two genotypes. The moths weregiven a window of 6 hours to lay and hatch eggs, coinciding with a simulated dusk on a 16L:8Dlight cycle. Eggs were laid on the wax paper, which was processed, recorded, and replaced withclean wax paper after the 6 hour mating
View Full Document
Unlocking...