In order to be effective, pesticide applications need to be done when the pest being treated is present and vulnerable on the tree. Most foliar diseases, for example, are treated early in spring when diseases are moving around via wind and rain and are able to attack emerging leaves. Applications for foliar diseases are therefore most effective during and immediately following bud break. For many insect pests, we use phenology to determine the correct timing of pesticide applications. Phenology is the study of plant and animal life cycles and their interaction with annual seasonal and climatic variations. For example, we treat eastern tent caterpillars each year when buds on magnolia trees turn pink, even if the timing is slightly different in relation to the calendar year due to fluctuations in temperature and sunlight.