Fundamental to the effective management of codling moth in Washington State orchards has been the use of a simple predictive model to time the first spray of the season. Growers use traps to determine the start of moth activity (Biofix) and then accumulate 250 degree days (DD) to allow time for moths to mate, lay eggs, and for eggs to start to hatch. The first spray for the second generation is timed at 1250 degree days. Additional sprays are then usually timed based on the length of spray residue effectiveness. Twenty years ago growers sprayed Guthion at 21-day intervals; today, most sprays are applied at 10- to 14-day intervals.
This program has been in place since 1980 and has served growers remarkably well. But, today, that is no longer true for all growers. Many field horticulturists know that the model doesn’t seem to fit what they are seeing in the field. Instead of a single peak and a clear separation in moth flight between generations, moth counts continue to be high throughout June and increase again in July (see codling moth catches). More growers are getting into trouble due to unexpected fruit injury occurring when orchards are unprotected (see fruit injuries).
It is interesting that the codling moth model was originally developed at Michigan State in 1973 to 1974, soon after the sex pheromone of codling moth was identified. The cumulative curves for the timing of moth flight and egg hatch for each generation were measured in one unmanaged orchard for two years. This model was brought to Washington State and validated only by comparing its prediction with the observed start of first egg hatch. The cumulative curves for egg hatch were never compared between these two diverse geographical areas, sadly.
Knowing the start of the vulnerable pest period (egg hatch) is crucial for growers to time their management strategies. If growers could then maintain a toxic blanket of spray residue all season by repeated, calendar spraying, this would likely be sufficient. However, maintaining complete coverage of both flights of codling moth would require at least six sprays timed two weeks apart, whereas growers spray on average three to four times per season. Thus, opportune periods for codling moth survival exist during the season.
Unfortunately, a variety of factors can interplay to lengthen these periods, such as rain, overhead irrigation, poor spray coverage, and increasing tolerance of the pest to insecticides. Growers may further increase the number and/or length of these periods by stretching spray intervals due to economics, regulations, efforts to conserve natural enemies, low moth catches in monitoring traps, or failure to detect eggs or injured fruit in the orchard. Because growers must allocate a scarce resource (spray residue) over an entire season, it becomes very important to target the peak periods of pest abundance, not just the start of each period.
Field studies were conducted in ten Washington apple orchards from 2003 to 2006 to determine the periods of cumulative flight and egg hatch for codling moth so that a new model could be developed. The most significant difference between the old and the new models is the timing of egg hatch. The new model shows that cumulative egg hatch is much later during the first generation then previously described (see codling moth models).
This different seasonal phenology of codling moth has a strong impact on the effectiveness of current spray timing programs. While it remains critical to time the first cover spray at the start of the egg hatch period (Biofix + 250 degree days), accurate timing of later sprays is even more important because they affect a much greater proportion of egg hatch (see impact of programs). For example, failure to apply a third spray during the first generation could have a strong effect on successful management of codling moth.
A second point to consider is the timing for second generation codling moth management beginning at 1250 degree days after Biofix. The new degree-day model shows that there is potential for new fruit injuries throughout the season (see fruit injuries). Unfortunately, the current approach allows spray residues following the third spray to degrade for four to five weeks. More than 5% of all the codling moth eggs laid during the season can hatch during this largely unprotected period (see impact of programs). Growers may need to reconsider the timing of their spray program to protect orchards more effectively during this period.
Growers also need to consider their timing of ovicide sprays. At present, these materials are recommended at 100 degree days after Biofix to target the start of egg hatch and for control of leafrollers. However, use of this material at a second cover spray timing would target a much greater proportion of codling moth eggs laid during the first generation (see chart “Impact of programs”).
The principles of integrated pest management call for the effective use of diverse tactics to manage pest populations and to minimize the negative impacts on biological control. It is clear that the new insecticides available to growers can have serious impacts on natural enemies and cause disruption of some secondary pests. Thus, it remains important for growers to not overspray their orchards. Growers should continue to intensively monitor orchards for codling moth and to use the accepted thresholds based on moth catches and sampling for fruit injury in known hot spots.
Fortunately, for many growers, codling moth is well managed, and effective control can be achieved with a reduced spray program. The use of sex pheromones has been instrumental in allowing growers to maintain this sustainable program. The greatest value of the new degree-day model will be for growers who continue to face significant threats from codling moth and allow them to more effectively time their spray program.
A copy of a new look-up table for codling moth’s phenology has been prepared that looks similar to the table in the Orchard Pest Management book published by Good Fruit Grower. This can be downloaded at our Web site http://ars. usda.gov/pwa/yarl or requested from the author at email@example.com.