Use of Degree Days and Plant Phenology: A Reliable Tool for Predicting Insect Pest Activity Under Climate Change Conditions

Insects are poikilothermic, i.e., cold-blooded, and hence the environment temperature has the greatest effect on insect development rates. In general, the development rate increases as temperature increases. However, upper and lower limits of these thresholds vary with the species. Phenology is the study of recurring biological phenomena and their relationship to weather. Bird migration, blooming of wildflowers and trees, and the seasonal appearance of insects are examples of phenological events that have been recorded for centuries. Plants bloom earlier in warm springs. Insects also emerge earlier when it is warm than in cooler seasons. Because the development of both plants and insects is temperature dependent, plants can accurately track the environmental factors that determine when insects are active. For this reason, plant phenology can be used to predict insect emergence. Plants bloom and insects emerge in virtually the same order every year, no matter what kind of weather occurred that winter or spring. Hence, the flowering sequence of plants can be used as a biological calendar to predict insect activity and to time other gardening practices that are dependent on a particular stage of plant development, such as propagation or weed control. In this chapter, methods of calculating degree days and how it is useful for prediction of plant phenology vis-a-vis insect activity are presented.