Weather can be your ally in managing codling moth

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.

A cool spring, accompanied by low catches in pheromone traps, once again has apple growers antsy with respect to managing this pivotal pest. The weather forecast is for more cool temperatures, at least through this week, a trend that will likely continue to limit codling moth activity. We have put the following article together to assist growers in making codling moth management decisions in light of the cool temperatures. It includes information on the negative impact of weather on codling moth biology and suggests some ways that growers can take advantage of the likely reduced pest pressure.

Cool temperatures reduce pest pressure

Codling moth mating and egg laying activities take place primarily during a four-hour period, beginning around dusk. Cool temperatures, as well as wind and rain, greatly limit flight during this critical period. Persistent cool temperatures at dusk can be an especially potent ally when it comes to managing codling moth. Experiments conducted this past season by the MSU tree fruit research team revealed that codling moth male behavior is greatly impeded when temperatures fall below 60ºF. Responsiveness of males to a female as measured by percentage of flights to a sex pheromone lure placed in a field wind tunnel was only zero and 4 percent on evenings where temperatures were below 60ºF for most of the flight period, while 41 and 31 percent of males responded on relatively warm evenings where temperatures exceeded 60ºF. In other words, there is little, if any, mating when temperatures fall below 60ºF by dusk. Thus, it is not surprising that moth captures in pheromone-baited traps are low during periods when evening temperatures are cool. The low catches are accurately assessing codling moth activity.

Although codling moth flight and mating are suppressed by low temperatures, some adults do emerge from their overwintering sites. This emergence occurs less rapidly than it would under warmer spring temperatures, but some adults are emerging. Male codling moth begin to emerge before female moths do, meaning that the first moths out are principally males. The emergence of moths despite cool temperatures is good news for growers, as it will likely assist them in controlling codling moth. How so? Consecutive low evening temperatures can substantially reduce the potential of a codling moth infestation as the pattern negatively impacts female fecundity, the quantity and viability of codling moth eggs laid. Fecundity is reduced approximately 25 percent for every 24 hours a codling moth female is delayed from mating after she has emerged from her cocoon, but is not mated. Thus, females that are emerging during the period of cool weather will likely produce very few offspring.

Codling moth management

The negative impacts of low temperatures on reproductive behavior provide an opportunity for some cost-effective management strategies. For the most part, growers who monitor codling moth with pheromone traps will see low catches. Catches will occur on the few evenings when temperatures are above the flight threshold. Significant moth captures will be recorded in a few orchards, but these most likely will be in blocks that suffered at least moderate infestations at harvest in 2009. Little to no activity should be the norm in orchards treated with pheromone-based mating disruption. Cool evening temperatures coupled with low moth captures provide a good foundation for deciding if a spray is necessary, and if so, when is the most cost-effective time to make the first application.

The vast majority of insecticides used for codling moth control are aimed at killing larvae. They primarily target newly emerging larvae at egg hatch, and thus are typically applied beginning at 250 GDD post biofix. However, temperatures have been below 60ºF on a majority of evenings since the first 2010 moths emerged in late April, a condition that should have substantially curtailed flight and mating. This reduced adult activity should limit the number of eggs deposited by females. The few eggs that were laid will likely hatch at the predicted 250 GDD due to a lower threshold of 50ºF driving embryo development. However, this early hatch will probably represent a very small fraction of the first generation activity. The extent to which these larvae are a threat to the crop is a decision that only the grower can make. A close look at the predictions of the GDD model, however, should be helpful in making a sound decision. Under good weather conditions for adult activity and larval development, the model predicts the start of egg hatch at 250 GDD, but only 12 percent of hatch occurs between 250 and 350 GDD. The majority of the hatch (68 percent) occurs between 350 and 650 GDD. Thus, even in a warm year an insecticide application at the start of egg hatch targets a relatively small portion of the generation’s larvae. Under the cool evening weather conditions occurring this season, an even smaller portion of egg hatch would be targeted with a 250 GDD spray. Delaying the first spray to 350 GDD, or even 450 GDD, would kill many more hatching larvae. Furthermore, it would allow a grower to cover well the entire second-generation hatch with two insecticide sprays.

The work of Dr. Gut and Dr. Wise is funded in part by MSU's AgBioResearch.

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