Painting: Adam and Eve

The Astrologer (1538, Holbein)

8   Weather

8.1   Parameters of the Codling Moth Model

The parameters of the Growing Degree-Day model of codling moth development most commonly used throughout the world are:

  • Development Threshold = 50°F (10.0°C)
  • Development Cutoff = 88°F (31.1°C)

These are the parameters used to produce the table, above. Technically, the method of calculation is a single sine wave with horizontal cutoff.

8.2   Biofix

It has been noted that adult first flight occurs at about 175 cumulative Growing Degree-Days Fahrenheit from New Year's Day above 46°N latitude (Jones, New No-biofix Degree-Day Model). The fact that first flight occurs about the time of full bloom in all apple-growing regions is miraculous, of course, because codling moth pupates according to photoperiod (Oregon State University, Ecological Adaptations) whereas apple trees bloom according to winter and spring temperatures (Allen and Wann); nevertheless, this close synchronization between crop and pest seems to be more likely the rule from year to year than not. In any case, the time of full bloom is a matter of observation, and so is the time of first flight.

First flight typically is detected with pheromone traps, which are available commercially and are within the means of the backyard gardener. Traps are put out before full bloom and are checked weekly. Biofix is set at the end of the second week when at least five moths are captured after the first week that any moths are captured (Rothwell). On the next warm evening when sunset temperature is greater than 62°F, the cumulative Growing Degree-Days is the "biofix," and codling moth developmental stages are predicted by offsets to cumulative Growing Degree-Days from that point.

8.3   Critical Insect-Development Stages

Here is a list of critical stages of codling moth development, showing cumulative Growing Degree-Day Fahrenheit (GDDF) offsets from biofix (Wise et al.):

GDDF Stage
0 First Flight (Biofix)
100 Egg Laying Begins
250 Egg Hatching Begins
500 Peak Egg Hatching

In commercial orchards, achieving significant control of the first flight means the second flight may be so insignificant that no further controls are required. This is not true in the backyard orchard where the second flight originates from uncontrolled sources so close that they might as well be on the premises.

Pheromone traps must be maintained even after the first biofix. Chemical control in some parts of the world has tended to favor delaying pupation so that mating of first flight adults is at least a long, drawn-out affair. There may even be a noticeable second peak of the first flight showing up in pheromone traps (OMAFRA, Walgenbach, Knight). If this is the case, then chemicals may need to be reapplied for complete control of the first flight.

Pheromone traps must be maintained to detect the onset of the second flight, too. This is the second biofix. The second flight may be expected to peak at 1175 Growing Degree-Days, which is about 1000 Growing Degree-Days from the first biofix (Brunner).

The second flight follows the same pattern of development as the first, and chemical controls follow the same stages according to Growing Degree-Days. Of course, development is accelerated in the summer weather, and growth stages follow one another at a faster pace according to the calendar, so, even though development of the second flight is never as closely synchronized as the first flight, it still occurs over a relatively narrow time frame.

8.4   Tracking Growing Degree-Days

It is possible to determine the current cumulative growing Degree-Days at any location from the following Web page maintained by Oregon State University:

Select your location from the Google Map widget and click on the CALC/RUN buttons.

The Oregon site produces a graph similar to the following one from 2018:

Cumulative GDD — 2018
Sheboygan County Memorial Airport


8.5   Regional Differences

At the current state of the art, insect-development models do not take into account regional differences.

Significant differences in the phenology of codling moth are known to exist between different geographical areas, such as California versus Michigan, [and] the ... model developed in Michigan where codling moth has one to two generations [does] not fit the phenology of codling moth in North Carolina where ... it has two to four generations (Knight).

Year-to-year differences in rate of egg laying can blur the cumulative degree-day timings given above, and regional differences detract further from the efficacy of simplistic Growing Degree-Day models of insect development; nevertheless, the interval between the first-flight biofix and the start of egg hatching at about 250 GDDF remains fairly consistent from year to year and from place to place — consistent enough to form a basis for control strategies.

In the United States, your best regional guidance is provided by the County Cooperative Extension Agent of your state Land-Grant College.

8.6   What Can Go Wrong

But, Mousie, thou art no thy-lane,

In proving foresight may be vain;

The best-laid schemes o' mice an' men

Gang aft agley,

An' lea'e us nought but grief an' pain,

For promis'd joy!


Still thou art blest, compar'd wi' me

The present only toucheth thee:

But, Och! I backward cast my e'e.

On prospects drear!

An' forward, tho' I canna see,

I guess an' fear (1785, Burns)!

The Growing Degree-Day model of codling moth development described, above, is remarkably immune to small random errors in measuring daily maximum and minimum temperatures and can be counted on to give reasonable results with casual use, but a number of things still can go wrong.

8.6.1   Microclimate

One thing that can go wrong is consistent error in recording daily maximum and minimum temperatures. This is likely to occur if the orchard has been planted in a microclimate where prevailing temperatures may diverge significantly and consistently from those reported by the nearest weather station.

For example, an offshore breeze near a large inland lake will not change temperatures drastically, but an onshore breeze lowers them as much as 10°F in the spring. Thus, even if wind direction were completely random, the actual daily temperatures would be biased much lower overall than those reported only a few city blocks further inland. Growing Degree-Days calculated with reported temperatures would thus outrun those calculated with actual temperatures.

Fig. N

Fig. N shows how microclimate affects cumulative Growing Degree-Days. In 2018, I was able to capture daily maximum and minimum temperatures in my backyard, using WeeWX software to manage an Ambient Weather WS-2095 Wireless Home Weather Station. The red curve is is calculated from these readings taken two blocks from Lake Michigan. The Sheboygan County Memorial Airport (KSBM) is located seven miles inland from Lake Michigan. The orange curve is calculated from automated instrumental temperature observations taken there. The cumulative Growing Degree-Days at the airport outran the cumulative Growing Degree-Days at my home beginning in late May. The predicted date of the codling moth second flight in July was four or five days earlier using airport data.

Although the first flight predicted by the airport data and that predicted by the backyard data were within a day of one another, the airport reached the egg-hatching stage by accumulating 250 Growing Degree-Days in 16 days, whereas my backyard weather station required 20 days.

The cumulative Growing Degree-Days at my home outran the cumulative Growing Degree-Days at the airport beginning in August. There are obvious gaps in the airport records, which cause the cumulative Growing Degree-Days to be lower than they ought to be. Another justification for the backyard data's outrunning the airport data may be that the lake exerts a warming effect in the fall. It is often remarked that, although we do without spring in Sheboygan, our autumn goes on forever. It may also be that the heat-island effect produced by the city is more prominent in my backyard in the fall as the relatively warmer water of the lake draws a persistent off-shore breeze.

The solution to microclimate bias at public weather-reporting stations is to use actual temperatures recorded by a miniature backyard weather station.

There are other kinds of microclimates. Downhill and uphill breezes in mountainous locations may cool or heat areas exposed to them while protected areas are not so susceptible. Even the slope of an orchard toward or away from the sun affects the prevailing temperature in hilly locations.

Urban heat islands are a prevalent type of microclimate. Modification of land surfaces in cities captures solar energy, which warms the air at ground level particularly after dark (Wikipedia, Urban Heat Island). Waste heat from human activity is also a factor. The combined effect can be as much as 5°F. Official weather reporting stations may or may not be located within urban heat islands. Nearly all reports of prevailing temperature are affected by urban heat islands because most gardeners either live in an urban heat island or rely on reports from inside one. Certainly wind direction plays a part with actual temperatures biased much higher overall.

To repeat: Daily maximum and minimum temperatures used in the model to predict codling moth development and plan control strategies must be taken as close to the orchard as possible — preferably within it — in order to eliminate microclimate bias.

8.6.2   Miscalibration

Another source of consistent error is miscalibration of temperature-measuring equipment. Growing Degree-Day models are based on accumulated environmental heating, and consistent errors in measurement build up during the hundred or so days in a season.

8.6.3   Fuzzy Biofix

The attractive aspect of the codling moth development model is its objectivity. Development seems to depend completely and only upon Growing Degree-Days. This is an illusion, however. The critical decision is where to place the biofix because this affects the timing of all the subsequent applications of chemical controls. Determining the biofix becomes somewhat subjective when inclement weather splits the first flight into two or more "cohorts."

A cohort in this situation is a group of moths that are flying on a warm night and are captured in the trap. This group of moths is capable of mating and laying eggs, and the hatching larvae pose a threat to the fruit.

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 caught no moths on June 7, we would not consider that first cohort a threat to fruit with such a low a trap catch and not set a Growing Degree-Day (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 it is to control the population (Rothwell).

8.6.4   Unfavorable Weather

Even when weather is favorable for first flight and after biofix is set, conditions may still be unfavorable for egg laying. The Growing Degree-Day models are good at predicting the beginning of egg laying, but they do not do so well to forecast the rate of egg laying nor the consequent end of egg laying. Peak egg laying is somewhere between, and estimates of when that occurs are less determinate than estimates of the beginning of egg laying because of the possibility of intervening adverse weather.

Predicting the first egg hatch from first male moth catch has proved to be rather straightforward and consistent. The length of this interval for codling moth is caused by the occurrence of a male protandry, a preovipositional period for the mated female, and the Degree-Days required for completion of egg development. The shape of the cumulative curve of egg hatch for each generation, however, is more variable and is likely influenced by several factors impacting the occurrence and rate of several important activities of adult codling moth: sex pheromone release, mating, and egg laying. First, the lower temperature threshold for full expression of mating or oviposition, 15.6°C [60°F], is substantially higher than for physiological development of immature stages, 10°C [50°F]. Thus, Degree-Days can accumulate that drive egg development in a predictive model even though few to no females were mated or a limited number of eggs were deposited under poor field conditions. Second, codling moth's sexual behaviors can be strongly affected by climatic conditions in addition to temperatures below physiological thresholds, such as wind, rain, and relative humidity, that can occur during the restricted time periods of codling moth activity, i.e., dusk. Thus, the intercept and slopes of the observed cumulative curves for oviposition and subsequent egg hatch plotted on a degree-day scale could vary significantly from predictive models based only on physiological development (Knight).

Egg laying and egg hatching can occur over a more or less brief period or not. The best that the models can do is predict when to begin treatment. More than one chemical application will then likely be necessary to span the entire time when larvae may be hatching.

8.6.5   Missed Biofix

Depending on traps to detect the first flight is all well and good, but what if they don't catch anything? In these cases it is best to rely on one of the following:

  • Experience with the recorded first flight according to the calendar
  • Experience with the cumulative Growing Degree-Days at which biofix was established in previous years
  • Up-to-date advice of your state's Cooperative Extension Service
Then proceed with chemical controls anyway. Odds are that first flight has already occurred unless weather has been noticeably unfavorable. By the time damage is obvious, it will be too late to do anything about it.

Traps must be fresh. Also, they must be stored and handled according to directions. Relying on ineffective traps is much worse than not relying on traps at all.