Evan Lue Codling Moth Resistance May 9 2005 Trade Offs Between Insecticide Resistance and Development Time in Codling Moth Evan Lue Abstract Resistance against organophosphates in the codling moth Cydia pomonella is suggested to increase the development time between life stages Such life history trade offs are not uncommon but may pose a threat to current pest management strategies Differences in development time for susceptible and resistant populations will not only result in two distinct emergence peaks but may also consequently force an alteration in the timing of pesticide applications The aim of this study is to confirm a difference in development times between susceptible and resistant codling moths A colony of each genotype was reared in a laboratory and 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 for the susceptible and resistant colonies respectively Degree days were calculated using a lower threshold of 50 F and eggs were examined for hatch every ten degree days Organophosphate resistant codling moths were found to have longer egg larva development times The bimodal emergence pattern observed between these two colonies is of particular concern at cooler temperatures spanning larger amounts of time between peaks These differences imply that identifying levels of pesticide resistance in codling moth populations is an important consideration in assessing the utility of current degree day models p 1 Evan Lue Codling Moth Resistance May 9 2005 Introduction Codling 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 et al 2002 During the summer over half of all insecticides used on apple orchards are used to control against codling moth Brunner et al 2002 Multiple generations of codling moth have been 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 of organophosphate insecticides Studies researching other forms of chemical pest management have indicated that removing organophosphates from current codling moth management programs will be difficult Knight et al 2000 Though intended for the codling moth these insecticides are also effective at indirectly controlling 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 pest outbreaks organophosphate applications are widely used in California and Washington Brunner et al 2002 Frequent application of insecticides can create selective pressures on pests forcing allelic substitutions that lead to genotype variation and insecticide resistance McKenzie and Batterham 1994 These anthropogenic perturbations can force a reallocation for different enzymatic activities Boivin et al 2003a Due to the wide use of insecticides some strains of codling moth have developed a resistance to the organophosphates Codling moths are becoming increasingly more 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 to detoxify a variety of different chemicals even when the mechanisms by which chemicals are toxic differ from one another Dunley and Welter 2000 While insecticide resistance poses a problem for pest management by making insecticides ineffective another indirect situation may develop as a result and could potentially pose a problem Resistance often comes with a cost to the pest Life history trade offs can occur in many forms including reduced fitness or size Particularly pleiotropy can result in a selective disadvantage in regard to development Boivin et al 2003a There is some evidence that organophosphate resistance in codling moth is correlated to a decrease in development rates Boivin et al 2001 More degree days are required for resistant p 2 Evan Lue Codling Moth Resistance May 9 2005 codling moths to complete their developmental stages The two distinct genotypes of resistant and susceptible codling moths differ not only in their susceptibility to insecticides but also in their 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 because of the large role that codling moth plays in many economically important crops Having multiple peaks in the development of codling moth could potentially limit the predictive capabilities of existing degree day models These models are of particular importance because choosing the period to apply insecticides has an effect on insecticide efficiency and the potential of pests to develop further resistance Frequent exposure of pests to sublethal doses of an insecticide could result in a gradual decrease in susceptibility Sauphanor et al 1998 Non overwintering populations are also observed to have slower developmental rates Boivin et al 2001 which will further limit the efficacy of degree day models Confirming and quantifying the difference in degree day requirements for these two strains of codling moth will help in identifying the developmental implications of organophosphate resistance Methods Adult 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 moth populations to insecticide The organophosphate resistant and organophosphate susceptible colonies were reared and kept in separate rooms to prevent contamination through crossbreeding Anywhere from 15 50 adult moths from each colony were placed in cylindrical mating chambers depending on the population sizes of the colony at the time of mating The chambers consisted of a tube of wax paper with an approximate area of 3 ft2 wrapped around an apparatus with wire mesh ends with a different chamber for each of the two genotypes The moths were given a window of 6 hours to lay and hatch eggs coinciding with a simulated dusk on a 16L 8D light cycle Eggs were laid on the wax paper which was processed recorded and replaced with clean wax paper after the 6 hour mating session Each egg sheet of wax paper was examined for eggs The eggs were
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