Over the last 20 years, the population of Monarch Butterflies (Danaus Plexippus) in North America has declined by approximately 90% (Moore, 2019). This reason for this decline has been identified to be the loss of their wintering grounds in Mexico and the increased used of herbicide, which is toxic weed control, in the Midwest. Monarch butterflies are herbivorous and are very fond of milkweeds, which are a type of toxic flowering plant which is native to North America. To be clear, the toxin has no effect on the butterfly’s but, can be harmful to humans and grazing animals. Monarch butterflies are partial migrants, meaning not all will migrate. Eastern populations migrate over to high elevation forests in Mexico, and western monarchs’ winter in the trees on the coast of California (Malcolm, 2018).
Migratory behavior in these butterflies is driven by one factor, the abundance of milkweed host plants. With North American agriculture growing and adopting GM crops the fields over which the butterflies migrate have become a toxic warzone. The soil around Genetically modified crops are host to a bacterium called Bacillus thuringiensis which is a natural form of pest control. The bacteria consist of a spore and a protein crystal within the spore that is toxic to insects and other organisms deemed “pests”. It was concluded that the risk this bacteria posed to monarchs was negligible because the exposure probability was too low in the event of pollen specific expression but, this assessment was never put into context of the spatial and temporal variations in the life history of these butterflies (Malcolm, 2018).
There has been no research on the bacterium and mortality of butterflies but, given constant exposure due to milkweed being very common in the Midwest region, there could be consequences in the future. Many other pesticides are also toxic to these organisms. Approved barrier treatments for mosquito control with synthetic pyrethroid permethrin that was sprayed on milkweed in Minnesota showed that 95% of monarch larvae were killed between 50% and 0.1% dilutions (Malcolm, 2018). With the temperature shifts due to global warming, it is expected to see milkweed shifts northward in North America. With this shift northward monarchs will now have to migrate across less suitable habitats in their searches.
JIM HUDGINS / USFWS MIDWEST REGION
Monarch Butterfly on swamp milkweed
In Mexico, forests cover, and the contiguous canopy is necessary to modify the temperature extreme where the monarchs overwinter. The closed canopy is thought to be important to minimize lipid usage during overwintering, so monarchs do not waste these lipids from unnecessary thermoregulation (Malcolm, 2018) The population of butterflies that winter in Mexico is protected within the “Monarch Butterfly Biosphere Reserve” located in Trans – Mexican Volcanic Belt. Despite logging bans in high elevation fir forests in Mexico, between the years 1986 and 2012, 4,300 hectares of protected forests have been altered from human activities such as logging both large and small scale leaving less than 10% of the canopy intact (Malcolm, 2018). In California, Monarchs winter in over 400 locations within the state and are seen to roost on introduced blue gum eucalyptus, Monterey pine, red gum eucalyptus, and Monterey cypress.
Monarch populations in both the east and west have declined and this only seems to be accelerating. The declines in milkweed resources across the Midwest and specifically around fields where herbicides are used are not helping the problem. Human activities in Mexico and California are also directly linked to the decline of these butterflies despite conservation efforts. Conserving their migration is only possible if migratory conditions are fully understood and If something isn’t done these butterflies will disappear forever and other species will soon follow.
Malcolm, S. B. (2018). Anthropogenic Impacts on mortality and population viability of the monarch butterfly. annual review of entomology, 277-302.
Moore, A. (2019, april 9). Fall of the monarchs. Retrieved from wildlife.org: https://wildlife.org/fall-of-the-monarchs/