Insect Growth and Development
Insect growth is affected by two major factors, time and temperature. Insects are unable to maintain a constant body temperature. Because they are cold-blooded, their body temperature varies with the temperature of their surrounding environment. Insects require a certain amount of heat to develop from one stage in their life cycle to another (eggs to larvae to pupae to adults). Insect growth only occurs within a certain range of temperatures, the upper and lower developmental thresholds. The temperature below which no growth occurs is the minimum or lower developmental threshold. The temperature must be at or above the minimum developmental threshold in order for insect growth to occur. Growth increases with higher temperatures up to a maximum temperature known as the upper or maximum developmental threshold. Once the upper threshold is surpassed, no additional growth occurs. Developmental thresholds are different for all insect species.
The amount of heat required by an organism to complete its development is known as physiological time. Physiological time is usually expressed in units called degree-days. Degree-days measure insect growth and development in response to daily temperatures. Degree-days are the accumulation of heat units above some temperature (the lower threshold) for a 24-hour time period. One degree day results when the average temperature for a day is one degree over the minimum threshold. The accumulation of degree-days can be added over a period of time and used to estimate growth and predict insect development. The accumulation of degree-days usually begins with either an arbitrary starting point such as a calendar date (many insect pests use January 1 or March 1) or a biofix. Click here to learn more about degree-day calculations.
Using Phenology Models in an Integrated Pest Management Program
Phenology models help predict the timing of events in an organism's development using degree-days. Degree-days allow us to predict when significant biological events such as the appearance of insect pests may occur. Determining when an insect pest will appear is often a difficult task. Depending on the variation in weather patterns, insect development may vary by a couple of weeks each year. This makes it difficult to predict insect growth stages using a calendar. Determining when an insect will appear should be based on some kind of temperature-based function, such as a phenology model instead of a calendar. Scouting for an insect may begin too early or too late if using calendar dates alone, resulting in wasted time and missed damage.
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