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| Using weather and climate information to predict pests and diseases | |||
| Excerpt from Fruit
Crop Ecology and Management, Chapter 2: Managing the Community of Pests
and Beneficials by Larry Gut, Annemiek Schilder, Rufus Isaacs and Patricia
McManus.
We can forecast pest development using mathematical models because pest development is closely linked to weather conditions. Because of the close relationship between temperature and insect growth, crop managers can predict when many insects will be active by recording temperature data. Sometimes more than one environmental variable is needed to create an effective model. When predicting fungal infection periods, temperature, humidity, and leaf wetness need to be considered. Models are designed to be simple to use but accurate at predicting pest events under most environmental conditions. Growing degree day models for insect informationBecause insects are cold-blooded, the growth of adults, larvae, and eggs is driven by temperature. As the temperature fluctuates within these limits, development speed changes, with faster growth typically occurring at warmer temperatures. For all insect pests, diseases, and their natural enemies, there are also low and high temperature thresholds for development. |
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| Since temperatures
may vary widely from year to year, pest management strategies may not
be effective if control measures are based on calendar dates rather than
on insect development. Often the stage of insect development can be tied
to the growth stage of the crop because plant growth is also driven by
temperature. A widely used tool for predicting insect growth is the growing
degree day (GDD) model. These models calculate the number of GDDs or heat
units that are accumulated between the minimum (base) temperature threshold
and the upper threshold. At the end of every day the GDD total for that
day is added to the previous total to create a cumulative number of GDDs.
Pest managers can use the GDD total to predict emergence, egg laying,
and other important events based on the amount of heat accumulated in
the vineyard, field, bed, or orchard.
Models have been developed that link GDDs to the stage of development of some key pests and, to a lesser extent, beneficial insects. Using a maximum-minimum thermometer, preferably placed in or near the crop, growers can track the development of insects on their farm. One degree day is accumulated when the average temperature for a day is one degree over the lower limit (base temperature) needed for development. A base temperature for each organism is used in the calculation because very little growth occurs below the base temperature. Growth rates also are reduced when temperatures exceed the upper threshold, and so the maximum is set to this value if temperatures become hotter. There are many sophisticated methods for estimating GDDs based on more than maximum and minimum temperatures. For example, computer programs can be used to keep track of GDD accumulation based on hourly temperature data. However, for most uses, the method below provides sufficient ability to predict major events in insect development. |
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| The start
point for accumulating GDDs can be decided in two different ways. Either
GDDs are counted from a set date, such as March 1, or they are counted from
a specific biological event, called a biofix -- the date when the
first adult is captured in a pheromone or other trap, provided additional
adults are captured on two successive trapping dates. This may be called
the first sustained capture. Using a biofix is usually more accurate and
means that the GDDs have to be counted for a shorter period. Often optimal
timing for an insecticide application is during egg hatch, because this
is when the insect is most vulnerable. At a set number of GDDs after biofix,
sprays aimed at the pest can be applied to target the appropriate stage
of the insect. For many of the apple and cherry pest insects, the number
of GDDs from first sustained catch to egg-hatch is well known. By keeping
track of the number of GDDs after biofix, the optimal timing for control
measures can be determined. It is therefore important to check traps often
near the start of adult emergence.
Above all, remember that degree day accumulations should be used only as a guide for making management decisions. Ultimate decisions should also be based heavily on frequent scouting of the crop for the presence of insects or insect damage. Disease prediction modelsDisease prediction models usually predict one or more critical phases in epidemic development such as the presence of primary or secondary inoculum or an infection period. Infection periods are times when the minimum environmental conditions have been met for infection to take place. Environmental data such as temperature, relative humidity, and leaf wetness are typically needed to run disease prediction models. Research in Michigan indicates that leaf wetness can vary significantly over short distances and heights in a crop, especially late in the growing season when the canopy is most dense. The use of models such as for apple scab that predict disease severity based on leaf wetness may be complicated by the variations in microclimate. For models that predict infection periods, fungicides are generally applied "after the fact," which means that growers have to use curative, systemic fungicides. Models and curative fungicides are not available for all diseases. Most models are based on current or past weather data, but can also be run using predicted weather data, however, those based on predictions are less accurate. |
in collaboration with MSU Extension and the Michigan Agricultural Experiment Station.
Updated 11/28/07 Contact: J.N. Landis.