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4 WeatherChanging 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 PhenologyNatural 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:
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-DaysGrowing 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 to be linearly proportional to the amount of daily development of insects. 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 TableHere 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 ModelThe parameters of the growing degree-day model of codling moth development most commonly used throughout the world are:
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 BiofixIt 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 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. 4.6 Critical Insect-Development StagesHere is a list of critical stages of codling moth development, showing cumulative growing degree-day Fahrenheit (GDDF) offsets from biofix (Wise et al.):
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. 4.7 Tracking Growing Degree-DaysIt is possible to determine the current cumulative growing degree-days at any location from the following Web page maintained by Cornell University: See also the 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
4.8 Regional DifferencesAt the current state of the art, insect-development models do not take into account regional differences.
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. |