MSU Extension

With the current temperature fluctuations, Michigan State University Extension has seen inconsistent activity of pest insects in orchard systems. Since May 1, 2013, we have seen more than our fair share of two or three warm, 75 degree Fahrenheit days followed by days that topped out in the mid-50s; it has not been usual to see temperature swings of 25 degrees this season. Because all insect development is based on temperature, we have seen bursts of activity during these warm periods, and little to no activity during the cold snaps.

Codling moths, like all insects, are dependent on temperature for development, but for this insect to fly, mate and lay eggs in fruit, a series of factors needs to align. We know from experience that codling moth adults emerge earlier and females lay more eggs during warm springs. Under these conditions, we have a higher codling moth population, which in turn, is more likely to result in a higher proportion of larvae in the fruit at harvest unless a solid insecticide or mating disruption program is used. As with various insect species, a smaller population is easier to control than a large one. We have also seen a third or a partial third generation of codling moths in years with warm springs because of the accelerated development of the first and second generations.

In contrast, cool springs are not favorable for codling moth development and nature helps out when it comes to codling moth management. Cooler dusks mean less activity and mating flights and for each day with cold temperatures, female codling moth fecundity or egglaying potential is reduced. Males do not fly and seek mates when dusk temperatures are below 60 F. Furthermore, few eggs are laid at temperatures below 62 F, but as temperatures approach 70 F, egglaying increases greatly. After four days of temperatures not conducive to mating and egglaying, a codling moth female’s reproductive capacity is reduced by at least 75 percent. The bottom line is cooler temperatures are a grower’s ally when it comes to codling moths, helping growers naturally keep codling moth populations in check.

The current codling moth model on Enviro-weather is designed to help growers best time insecticides to prevent entry of larvae into fruit. The model is initiated on March 1. Using 50 F as a base, growing degree days (GDD) for codling moth activity are as follows:

First generation

• 0 GDD – first adult emergence (biofix 1)
• 250 GDD – first eggs hatch β treat*
• 350 GDD – 20 percent egg hatch
• 550 GDD – peak egglaying

Second generation

• 1,060 GDD – first emergence of second generation adults (biofix 2)
• 1,250 GDD – first egg hatch β treat*
• 1,600 GDD – peak emergence of second generation adults
• 1,700 GDD – peak egglaying by second generation adults

The challenge for a season like we are experiencing in 2013, one with high and low temperatures, is determining a “biofix” date. For the past few years, we have been setting a biofix for each orchard based on monitoring with pheromone traps. We put pheromone traps into the orchard prior to bloom and we begin accumulating growing degree days (base 50 F) on the day (biofix) at which the first moths (more than five) are trapped after moths are captured on two successive trapping dates.

For example, if we caught our first moths (more than five) on Monday, June 3, and we catch moths again on Monday, June 10 (more than five), then the biofix date would be June 3 and we would start accumulating growing degree days from that date. The difficulty in setting a biofix arises when moths are caught, but then temperatures drop, resulting in little or no flight and no catch the following week.

A possible solution to this quandary is to replace thinking of trap captures in terms of a single biofix but rather as a “cohort” of the population. A cohort in this situation is a group of moths that are flying on a warm nights and are captured in the trap — this group of moths is capable of mating, laying eggs and the hatching larvae pose a threat to the fruit. When this first cohort is caught in the traps, this is the date to begin accumulating GDD to time insecticide sprays with respect to the portion of the population that has emerged and has the potential to infest fruit. So, in a warmer season, these cohorts fly on the first warm evenings and we catch the moths in our pheromone traps; when we come back the following week, we see additional moths flying and set the date at the first trap catch to begin GDD accumulations (this is like the biofix).

However, when we do not see a successive catch of moths two weeks in a row (i.e., growers catch moths the first week and not on the second week), should a grower set a date for determining spray timing on that first flight, even if he or she did not get moths in traps during that second week? The answer is only if enough moths were caught and temperatures were high enough to promote egglaying. This situation of highly variable moth captures is typical in a year like this one where we have had a few warm evenings sprinkled in with cold temperatures — we caught moths Friday (May 31) and Saturday (June 1) night, but with cold temperatures for the remainder of this week, we will not likely have moths in the traps on this coming Friday and Saturday night (June 14-15) — should we set the degree-day accumulation date for May 31?

With this new cohort thinking, it becomes a numbers’ game. If a grower caught about 20 moths on May 31 and there were no moths in traps last Friday (June 7), we would consider that first cohort a threat to fruit with higher traps counts of 20 moths per trap and set the accumulation date on May 31. On the other hand, if we caught only three moths on May 31 and catch no moths on June 7, and with a trap catch so low on that first catch (three moths per trap), we would not consider that first cohort a threat to fruit and would not set a GDD accumulation date on May 31.

The situation becomes more complicated if a grower catches five to 10 moths on May 31 and nothing on June 7 — is that first cohort a threat? At this point, a grower will have to take into account other factors to decide if this first cohort of minimal catch poses a potential problem: early evening temperatures, past history of codling moths, spray program, mating disruption, etc.

In short, the biofix or cohort strategy is only a guiding principle, and in a year with temperature fluctuations, it is a challenge to decide when to set the biofix or start of GDD accumulation for a cohort approach, and to use this date to best time insecticide applications for codling moths. In addition, population size influences trap catches: the higher the population, the higher the trap catches, the potentially more difficult to control the population. Therefore, growers should be trapping their own blocks rather than relying on the Northwest Michigan Horticultural Research Station, the Trevor Nichols Research Station, or a neighbor’s trap counts. For instance, the codling moth populations are low here at the Northwest Michigan Horticultural Research Station and we have not yet set a biofix date, but growers that have higher populations set their biofix date two weeks ago.

Despite this confusion with a biofix date, this date is important because all insecticide recommendation timings are based on this trap catch, so whether growers use a strict biofix or the cohort strategy, he or she will need to know this date to apply insecticide applications at the best timing.

Using Enviro-weather’s codling moth model. Locate the Biofix Date (first date of sustained catch) on the top row. Follow that column down to determine the Base 50F Growing Degree Days (GDD) that have accumulated between the biofix date and the date listed at the left side of that row.

* May cause mite flaring in combination with carbaryl or pythrethroids that kill
predacious mites.

Dr. Gut's work is funded in part by MSU's AgBioResearch.