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

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. Also, they have endeavored to predict seasonality using astronomical observation. Here is a well-known literary reference:

Priam saw him first, with his old man's eyes,

A single point of light on Troy's dusty plain.

Sirius rises late in the dark, liquid sky

On summer nights, star of stars,

Orion's Dog they call it, brightest

Of all, but an evil portent, bringing heat

And fevers to suffering humanity.

Achilles' bronze gleamed like this as he ran.

Presumably around 850 BC, Homer compared the sudden appearance of Achilles before Troy to the heliacal rising of the star Sirius. This same annual astronomical event signaled to Pagan Egyptians the anticipated rising of the flood waters of the Nile River in summer. Sirius, while evidently a blue-white star, is referred to historically and poetically as bloody because of the ominous effects the weather has upon the affairs of men after it first appears in the morning sky. No doubt this is largely in homage to Homer, but, in fact, when perceived in the mist of dawn, it may indeed seem to have a brassy hue.

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.

The problem with calendars is that they are based in astronomy, whose affects on weather are mostly indirect. While we adjust our calendars to keep them in sync with the seasons, we now note the long-term drift of astronomical observations against our calendars due to various kinds of precession. The Nile started to rise every year at the beginning of July for the ancient Egyptians and continued to do so until the closing of the Aswan High Dam in the 1960s, but, whereas Sirius rose at the beginning of July in Memphis in classical times, nowadays, it rises at the beginning of August (King).

Furthermore, succeeding seasons might be early or late relative to calendars, so calendars were never a definite guide to when crops and pests might develop. Because crops and pests all differ in their response to temperatures during their development, anticipating their stages of development was historically based on concurrent observations of many living organisms in addition to calendars.

This field of study was ancient in ancient times. Today, it is called phenology. The term dates from the 1850s, but it was René-Antoine Ferchault de Réaumur, a pioneer in the scientific measurement of temperature, 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 (Jones, New No-biofix Degree-Day Model).

4.2   Growing Degree-Days

Growing degree-days is a measure of temperature duration. Specifically it is the persistence of temperature in excess of a threshold amount that is necessary for daily biological processes such as insect development to begin. It is also the excess of average temperature over the threshold, which influences the speed of development. The threshold can be peculiar to an insect or plant species of interest although most insect species have closely equivalent thresholds. Growing degree-days are calculated so that the amount of daily development of insects is linearly proportional. Growing degree-days is capped by a temperature ceiling (the cutoff temperature), too, above which the amount of daily development does not depend on temperature.

The terminology is confusing. The term "growing degree-days" as applied to phenology of insects (in this paper) is distinct from the terms "degree-days," "heating degree-days," and "cooling degree-days" common in heating, ventilation, and air-conditioning (HVAC) contexts such as laymen's summaries of architectural and construction engineering documents. "Degree-days" accounts for day-to-day differences in energy consumption for heating and cooling buildings, but the terms "degree-days" and "growing degree-days" are used interchangeably in biological contexts, too. Please be aware that academic papers may and frequently do use their own terminology.

In 1969, Baskerville and Emin published refinements to the calculation of growing degree-days based on high-speed digital computing, which adhere more closely to models of insect development than conventional cooling degree-days does. Theirs are the calculations that produce growing degree-day tables for controlling various species of insects.

4.3   Growing Degree-Day Table

Here is such a table for codling moth:

Growing Degree-Day (°F) Table

For Codling Moth (Cydia pomonella)
Threshold =  50°F
Cutoff =  88°F
Method:  gdd_single_sine_horizontal_cutoff

  \ Min Daily Temp
Max  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 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 50   0   0   0   0 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 52   0   1   1   1   2 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 54   1   1   2   2   3   4 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 56   2   2   2   3   4   5   6 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 58   3   3   3   4   5   6   7   8 *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 60   4   4   4   5   6   7   8   9  10 *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 62   5   5   5   6   7   8   9  10  11  12 *** *** *** *** *** *** *** *** *** *** *** *** *** ***
 64   5   6   6   7   8   9  10  11  12  13  14 *** *** *** *** *** *** *** *** *** *** *** *** ***
 66   6   7   7   8   9  10  11  12  13  14  15  16 *** *** *** *** *** *** *** *** *** *** *** ***
 68   7   8   8   9  10  11  12  13  14  15  16  17  18 *** *** *** *** *** *** *** *** *** *** ***
 70   8   9   9  10  11  12  13  14  15  16  17  18  19  20 *** *** *** *** *** *** *** *** *** ***
 72   9  10  10  11  12  13  14  15  16  17  18  19  20  21  22 *** *** *** *** *** *** *** *** ***
 74  10  11  11  12  13  14  15  16  17  18  19  20  21  22  23  24 *** *** *** *** *** *** *** ***
 76  11  12  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26 *** *** *** *** *** *** ***
 78  12  13  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28 *** *** *** *** *** ***
 80  13  14  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30 *** *** *** *** ***
 82  14  15  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32 *** *** *** ***
 84  15  16  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34 *** *** ***
 86  16  17  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36 *** ***
 88  17  18  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38 ***
 90  18  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  36  37  38  38
 92  18  19  20  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  37  38  38
 94  19  20  20  21  22  23  24  25  26  27  28  29  30  31  32  33  33  34  35  36  37  38  38  38
 96  20  20  21  22  23  23  24  25  26  27  28  29  30  31  32  33  34  35  35  36  37  38  38  38
 98  20  21  21  22  23  24  25  26  27  28  29  30  30  31  32  33  34  35  36  36  37  38  38  38
100  20  21  22  22  23  24  25  26  27  28  29  30  31  32  32  33  34  35  36  36  37  38  38  38
102  21  21  22  23  24  25  26  27  27  28  29  30  31  32  33  34  34  35  36  37  37  38  38  38
104  21  22  22  23  24  25  26  27  28  29  30  30  31  32  33  34  34  35  36  37  37  38  38  38
106  22  22  23  24  24  25  26  27  28  29  30  31  31  32  33  34  35  35  36  37  37  38  38  38
108  22  22  23  24  25  26  27  27  28  29  30  31  32  32  33  34  35  35  36  37  37  38  38  38
110  22  23  23  24  25  26  27  28  28  29  30  31  32  33  33  34  35  36  36  37  37  38  38  38
112  22  23  24  24  25  26  27  28  29  30  30  31  32  33  34  34  35  36  36  37  37  38  38  38
114  23  23  24  25  26  26  27  28  29  30  31  31  32  33  34  34  35  36  36  37  37  38  38  38
116  23  24  24  25  26  27  27  28  29  30  31  31  32  33  34  34  35  36  36  37  37  38  38  38
118  23  24  24  25  26  27  28  28  29  30  31  32  32  33  34  35  35  36  36  37  37  38  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. 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 growing degree-days where the column and row intersect are transcribed to yesterday's cell on the calendar. Cumulative growing degree-days for today is the sum of growing degree-days from all the previous cells on the calendar. Using the paper table is fairly simple even though the calculations that produce the table are arcane.

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

The published growing degree-day requirements for various species have been experimentally verified.

U. C. Agriculture and Natural Resources has a list of insect "Research Models" and the papers that purport to validate them, but see also Coop's "Library of Degree-Day Models."

Looking at the summaries of these papers, it is obvious that methods of approximating the actual growing degree-days vary as do recommended threshold and cutoff temperatures for most species. The marvel is that insect-development models function well enough to be of real-world value in pest treatment, so do not be seduced by the precision of all the figures. It is not possible to practice the concepts without adopting one or another of the thresholds and cutoffs and explicit methods, of course; nevertheless, you should still feel free to make your own accommodation to the range of choice that the research presents.

4.4   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.

4.5   Biofix

It has been noted that adult first flight occurs at about 175 cumulative growing degree-days Fahrenheit from New Year's Day in the Northern Hemisphere (Jones, New No-biofix Degree-Day Model), 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 pheromone traps, which are available commercially and are within the means of the backyard gardener. 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 (Rothwell). If the sunset temperature is greater than 62°F, the cumulative growing degree-days on that date is the "biofix," and codling moth developmental stages are predicted by offsets to cumulative growing degree-days from that point.

4.6   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). 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.