4   Weather

Changing the weather is not considered an appropriate way to control codling moth populations in backyard orchards, because, as it has spread to various climates around the world, codling moth has evolved to survive under most weather conditions. Tracking increasing temperatures in the spring, however, can predict the synchronized pupation and emergence of adults from overwintering cocoons as well as the succeeding developmental stages (egg laying and egg hatching). This section presents the technique for predicting these stages. The resulting predictions may then be used to conserve chemical controls for use at the most effective times.

4.1   Phenology

Such techniques are not new. Natural philosophers have long noted that development of desirable plants such as crops and undesirable pests such as insects is tied to signs and portents of advancing seasons such as earliest flowering of certain plants, and the development of calendars in order to track such events is implicated with the development of systems for numbering and writing these things down, which emerged with the dawn of human history.

This field of study was ancient in ancient times. Today, it is called phenology. The term dates from 1857, but it was René-Antoine Ferchault de Réaumur who first suggested in 1738 that year-to-year differences in the dates of grape harvest could be attributed to temperature and was able to quantify how the differences in dates followed the daily sum of degrees above freezing (Demarée).

Modern efforts to improve phenological models of insect development began in the 1920s.

4.2   Degree-Days

Degree-days is a measure of temperature duration. Specificially it is the persistence of temperature in excess of a threshold amount that is necessary for daily insect development to begin. It is also the excess of average temperature over the threshold. The threshold can be peculiar to an insect species of interest although most species have closely equivalent thresholds. The amount of daily development of insects is proportional to degree-days. Degree-days is capped by a temperature ceiling, too, above which the amount of daily development does not depend on temperature.

The term degree-days as applied to phenology of insects is distinct from the term heating-degree-days, which accounts for day-to-day differences in fuel consumption for heating buildings.

In 1969, Baskerville and Emin published refinements to the calculation of degree-days, which adhere more closely to models of insect development than heating-degree-days does. Theirs is the calculation that produces degree-day tables for controlling various species of insects.

4.3   Degree-Day Table

Here is such a table for codling moth:

Degree Day (°F) Table

  \ Min Daily Temp
Max  34  36  38  40  42  44  46  48  50  52  54  56  58  60  62  64  66  68  70  72  74  76  78  80  82  84  86  88  90
 48   0   0   0   0   0   0   0   0 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 50   0   0   0   0   0   0   0   0   0 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 52   0   0   0   0   0   0   0   0   1   2 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 54   0   0   0   0   1   1   1   1   2   3   4 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 56   1   1   1   1   1   1   2   2   3   4   5   6 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 58   2   2   2   2   2   2   3   3   4   5   6   7   8 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 60   2   2   3   3   3   3   3   4   5   6   7   8   9  10 *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 62   3   3   3   4   4   4   4   5   6   7   8   9  10  11  12 *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 64   4   4   4   4   5   5   5   6   7   8   9  10  11  12  13  14 *** *** *** *** *** *** *** *** *** *** *** *** ***
 66   5   5   5   5   6   6   6   7   8   9  10  11  12  13  14  15  16 *** *** *** *** *** *** *** *** *** *** *** ***
 68   5   6   6   6   6   7   7   8   9  10  11  12  13  14  15  16  17  18 *** *** *** *** *** *** *** *** *** *** ***
 70   6   6   7   7   7   8   8   9  10  11  12  13  14  15  16  17  18  19  20 *** *** *** *** *** *** *** *** *** ***
 72   7   7   8   8   8   9   9  10  10  12  13  14  15  16  17  18  19  20  21  22 *** *** *** *** *** *** *** *** ***
 74   8   8   9   9   9  10  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24 *** *** *** *** *** *** *** ***
 76   9   9   9  10  10  11  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26 *** *** *** *** *** *** ***
 78  10  10  10  11  11  12  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28 *** *** *** *** *** ***
 80  11  11  11  12  12  13  13  14  14  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30 *** *** *** *** ***
 82  12  12  12  13  13  14  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32 *** *** *** ***
 84  12  13  13  14  14  15  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34 *** *** ***
 86  13  14  14  15  15  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36 *** ***
 88  14  15  15  15  16  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38 ***
 90  15  15  16  16  17  17  18  19  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  38
 92  16  16  16  17  17  18  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  34  35  36  37  38  38
 94  16  17  17  18  18  19  19  20  21  22  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  36  37  38  38
 96  17  17  18  18  19  19  20  20  21  22  23  24  25  26  27  28  29  30  31  31  32  33  34  35  36  36  37  38  38
 98  17  18  18  19  19  19  20  21  22  22  23  24  25  26  27  28  29  30  31  32  33  33  34  35  36  37  37  38  38
100  18  18  19  19  19  20  21  21  22  23  24  25  26  27  28  28  29  30  31  32  33  34  34  35  36  37  37  38  38
102  18  18  19  19  20  20  21  22  22  23  24  25  26  27  28  29  30  31  31  32  33  34  35  35  36  37  37  38  38
104  19  19  19  20  20  21  21  22  23  24  25  25  26  27  28  29  30  31  32  32  33  34  35  35  36  37  37  38  38
106  19  19  20  20  21  21  22  22  23  24  25  26  27  28  28  29  30  31  32  33  33  34  35  36  36  37  37  38  38
108  19  20  20  20  21  21  22  23  23  24  25  26  27  28  29  29  30  31  32  33  34  34  35  36  36  37  37  38  38
110  20  20  20  21  21  22  22  23  24  24  25  26  27  28  29  30  30  31  32  33  34  34  35  36  36  37  37  38  38
112  20  20  21  21  21  22  23  23  24  25  26  26  27  28  29  30  31  31  32  33  34  34  35  36  36  37  37  38  38
114  20  20  21  21  22  22  23  23  24  25  26  27  28  28  29  30  31  32  32  33  34  35  35  36  36  37  37  38  38
116  20  21  21  22  22  22  23  24  24  25  26  27  28  29  29  30  31  32  33  33  34  35  35  36  36  37  37  38  38
118  21  21  21  22  22  23  23  24  25  25  26  27  28  29  30  30  31  32  33  33  34  35  35  36  36  37  37  38  38

The asterisks fill cells representing the nonsense situation where minimum temperature would exceed the maximum.

Using the paper table requires a paper calendar or at least a list. Each morning, the minimum and maximum temperature of the preceding day are recorded. The minimum temperature is located along the top of the table, the maximum temperature is located along the side, and the degree-days where the column and row intersect are transcribed to yesterday's cell on the calendar or to the list. Cumulative degree-days for today is the sum of degree-days from all the previous cells on the calendar or list. Using the paper table is pretty simple even though the calculations that produce the table are arcane. Needless to say, the whole process has been automated. More about that, below.

Predicting the developmental stage of an insect is then a straightforward matter of comparing today's cumulative degree-days with published degree-day requirements for each developmental stage of the species of interest.

UCANR has a list of insect "Research Models" and the papers that purport to validate them.

4.4   Parameters of the Codling Moth Model

The parameters of the degree-day model of codling moth development most commonly used throughout the world are:

  • Development Threshold = 50°F (10.0°C)
  • Development Ceiling = 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.

4.5   Biofix

It has been noted that adult first flight occurs at about 175 cumulative degree-days Fahrenheit from New Year's Day in the Northern Hemisphere (Jones), which is about the time of full bloom in the orchard (OMAFRA), but there are alternate methods of determining the date.

First flight typically is detected with phermone traps, which are available commercially and are within the means of the backyard orchardist. Traps are checked weekly. The date is set at the end of the second week when least five moths are captured after the first week that any moths are captured while the sunset temperature is greater than 62°F (Rothwell). The cumulative degree-days on that date is the "biofix," and codling moth developmental stages are predicted by offsets to cumulative degree-days from that point.

4.6   Critical Insect Development Stages

Here is a list of critical stages of codling mode development, showing cumulative degree-day Fahrenheit (DDF) offsets from biofix (Wise et al.):

DDF Stage
0 First Flight (Biofix)
100 Egg Laying Begins
250 Egg Hatching Begins
500 Peak Egg Hatching
1060 Beginning of Second Flight

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 noticable second peak of the first flight showing up in pheromone traps (OMAFRA). 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 follows the same pattern of development as the first, and chemical controls follow the same stages according to 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.

4.7   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 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.

4.7.1   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. Degree-days calculated with reported temperatures would thus outrun those calculated with actual temperatures.

The obvious solution 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 and 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. 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 orchardists 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 stragegies must be taken as close to the orchard as possible -- preferably within it -- in order to eliminate microclimate bias.

4.7.2   Fuzzy Biofix

The attractive aspect of the codling moth development model is its objectivity. Development seems to depend completely and only upon 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).

4.7.3   Early Treatment

If chemical controls are applied too early, their effectiveness degrades before the moth population reaches the targeted stage of development. If they are not reapplied, the moth population may be able to complete its development.

4.7.4   Late Treatment

If chemical controls are applied too late, the moth population may have passed the targeted stage of development, and the treatment may not be effective at all.