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While milkweed native to southern California (such as narrowleaf milkweed [Asclepias fascicularis]) die back in the winter, tropical milkweed provides a year-round food source for monarchs. This induces monarchs to breed in winter and disrupts their natural migration patterns by causing them to remain at an artificially available food source instead of migrating. If monarch butterflies do not migrate, prolonged feeding on tropical milkweed results in an accumulation of a debilitating protozoan parasite (Ophryocystis elektroscirrha, aka OE) in their bodies. At high densities, these parasites decrease the survival rate of caterpillars and shorten the life span of adult butterflies.
Pesticides, which are often used on milkweed purchased from nurseries that supply garden and box stores, are widely believed to contribute to the overall decline in monarch butterflies.
It has also been found that warmer temperatures increase the concentration of a toxin found in tropical milkweed called cardenolides, to levels that are largely intolerable to monarch butterflies. This, in combination with the additional stressor of warming temperatures predicted with climate change, will likely result in a dramatic decline in their overall survival. Scientists therefore recommend that native milkweed species be preferentially planted over tropical milkweed in anticipation of warming temperatures.
Learn more about native narrowleaf milkweed in San Diego County: