Why So Few Monarchs in California This Season?
by David F. Marriott
Executive Director, The Monarch Program
The extremely low overwintering monarch populations along the coast of California during the 2002/03 season have been declared the lowest since at least the 1960’s, especially south of Point Conception. The main culprit for this phenomenon is the five year drought in the Southwest and into central California. The lack of precipitation in the late summer of 2002 had a great impact on milkweed species in the Southwest, especially the three most common: Asclepias fascicularis (narrow leaf milkweed), A. eriocarpa (Indian milkweed), and A. subverticillata (poison milkweed). These plants have been fairly healthy in the spring months when there is some precipitation. However, during the past several years, the plants have been so stressed from lack of water in August and September that they often cannot provide a proper diet for monarch larvae.
Monarchs that visit our overwintering sites were in their larval stage in August and September. When milkweed is absent, there are no caterpillars or monarchs. When plants are stressed, plant predators have a heyday and cause future problems. The drought and poor condition of milkweeds in the late summer is probably the leading cause for low populations this season, but we must also remind ourselves that these are insects and their survival is dependent on population fluctuations.
The scarcity of Santa Ana winds from the Great Basin in the autumn also affects population numbers. These strong high pressure wind currents boost the monarchs’ migration to the coast in greater numbers from mid October to early November. We have had fewer Santa Ana conditions during this window of time for the past several years.
These days, we cannot leave out Global Warming as a contributor to lower populations. The constant rise above normal temperatures is, according to scientists, creating changes in weather patterns; from droughts, to flooding, heavy snow storms, and glaciers melting. Since monarchs are climate indicators in association with their host plant and their migration pattern, a few degrees in temperature may affect their behavior someday. These are insects and their populations fluctuate much like a stock market graph. We look forward to the mysteries of next season.