The decline in overwintering monarchs (1995-2015) may be attributed to habitat loss, climate change, pesticides, and diseases/parasites, emphasizing the need for conservation efforts to address these multifaceted threats.
The graph shows a decreasing trend in the total area occupied by overwintering monarchs from 1995 to 2015. Some possible reasons for this trend are:
Habitat loss: Monarchs need milkweed plants to lay their eggs and feed their caterpillars. However, the use of herbicides and agricultural expansion have reduced the availability of milkweed in the US and Canada.
Climate change: Extreme weather events, such as droughts, floods, and storms, can affect the survival and migration of monarchs. Climate change can also alter the timing and availability of nectar sources for monarchs.
Pesticides: Some pesticides, such as neonicotinoids, can harm monarchs by affecting their nervous system, growth, and reproduction. Pesticides can also contaminate the water and soil where monarchs live.
Diseases and parasites: Monarchs can be infected by various pathogens, such as the protozoan Ophryocystis elektroscirrha (OE), which can reduce their lifespan and ability to fly. Parasites, such as tachinid flies and wasps, can also attack monarchs and their larvae.