Monarch Alert, in collaboration with MJV and the Xerces Society, researched patterns of tree use at five overwintering locations along the California coast and prepared a document describing overwintering site managment recommendations based on their findings. The goal of this research was to determine whether or not monarch butterflies have a preference between overwintering in non-native eucalyptus trees and native trees, like Monterey cypress or Monterey pine. Each of the five sites that were monitored over the course of three different overwintering seasons contained some mixture of native and non-native tree species. To determine whether or not monarchs exhibited a preference for one species over another, analysis was conducted looking at monarch use of a particular species based on the proportion of the canopy that the species represented. Results showed that while there was variation within a given site between years and seasonal timing, monarchs clustered disproportionately on native conifers in years when the overwintering population was relatively high, in most years and at most sites, they clustered less than expected on eucalyptus and more than expected on native trees, and in several instances, monarchs switched from clustering on eucalyptus at the beginning of the season to clustering on native conifers later in the season.
These findings suggests that based on patterns of tree use in California overwintering sites, a long term management outlook for these sites should include a mixture of tree species. Monarch Alert recommends planting native native conifers such as Monterey cypress or Monterey pine around the perimeter of overwintering sites and in any areas where trees are down or are likely to fall.
For more details regarding this research and recommendations for management, read the report which can be found on our publications page.