Partial third generation of codling moth means keep monitoring
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Michigan growers will need to keep their codling moth trapping programs in place for a bit longer this fall. High summer temperatures have resulted in potential for a third generation of codling moth. Over 60 percent of second generation larvae are predicted to develop towards adulthood, rather than entering diapause until next spring. Growers will need to continue their monitoring programs to determine whether additional codling moth control sprays are needed to protect their crop through harvest. Low second generation activity and reduced crops on many farms across the state need to be factored in to any decision to apply additional insecticides.
Every year, the amount of codling moth larvae that enter diapause during the summer is determined by a combination of the daily photoperiod (the duration of an organism's daily exposure to light) and environmental temperature experienced by developing larvae in the fruit. During a temperate summer, most individuals in the second larval generation infest fruit later in August when day length is shortening. Larvae experiencing the shorter days are induced into diapause and remain in this stage until the next spring when they resume development. However, during years when summer temperatures consistently remain above 80°F, a large proportion of those individuals will continue their normal development. In other words, the normal photoperiodic induction of diapause normally seen in Michigan at the end of second generation has been offset by the high temperatures experienced by the larvae.
In 2010, temperatures have been extremely warm throughout the summer. As a result, development of many crops is advanced. Insects are responding in a similar way to the very warm conditions, and the codling moth is no exception. According to the predictive model for diapause induction (Garcia-Salazar et al. 1988), the Critical Photoperiod (photoperiod at which 50 percent of a larval population enters diapause) has increased more than half hour due to the hot temperatures. The population segment that continues to develop under these conditions is large enough to give origin to a third adult generation that will mate and oviposit on mature apples at a time close to harvest when many growers are not expecting codling moth injury to their fruit. According to the predictive model, only 37-38 percent of larvae have entered diapause induction this year. Consequently, 62-63 percent of the summer larval generation will continue development to adult stage. The damage caused by third generation larvae will be noticed too late, at harvest time or during the packing process. Thus, codling moth trapping programs should be continued into September to monitor codling moth activity and to determine if additional control measures are necessary to control codling moth injury to your apple crop.
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