New research debunked the speculation that migration mortality has caused the significant decline of the butterfly monarch population, despite it having received much attention.

The study was published in the journal Frontiers in Ecology and Evolution on August 7. 

The monarch butterfly has been declining the past twenty years, according to Monarch Watch Director Chip Taylor. The population recorded in the winter of 2013-2014 at the monarch overwintering sites was an all-time low. 

The alarming decline and low population number led to a petition to declare the butterfly monarch as threatened species and the search for answers on its rapid drop. 

READ: Milkweeds and Butterfly Gardens to Save Butterfly Monarchs

Migration Mortality Hypothesis 

The researchers, Monarch Watch Director Chip Taylor, and colleagues revealed that the decrease in the overwintering numbers of the butterfly monarch population is not because of the monarch's death during the migration: a hypothesis known as the migration mortality.  

Taylor's recent study summarized the tagging results of 1.4 million monarchs, which lead to recoveries of 14,000 tagged monarchs in Mexico. The researchers also found that tagging recoveries did not decrease over time. Tagging recoveries are measures of migration success, which contradicts the argument of migration mortality advocates. 

"Monarch Watch has been collecting recovery data for tagged monarchs since 1992, and we knew that those advocating the migration mortality hypothesis were on the wrong track from the outset and told them so," Taylor said.

This particular study also found out that the size of the summer population determined the annual variation in overwintering population size. 

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Milkweed Limitation Hypothesis 

Monarch Watch believes in the milkweed limitation hypothesis: that habitat loss and prevalence of glyphosate herbicide on corn and soybeans used in the Upper Midwest area led to the decline of the milkweed and ultimately monarch butterfly population. 

The hypothesis was challenged by the migration mortality hypothesis advocates who maintained that the marked decrease is due to high mortality levels during migration. 

Despite the lack of data to support this claim, Taylor said that the migration mortality hypothesis earned substantial attention.

Taylor and his team also found out from this study that the number tagged each year correlates with the overwintering population in Mexico. Moreover, most monarch butterflies that made it to the overwintering sites come from the Upper Midwest. These findings, according to Taylor, complement the milkweed limitation hypothesis. 

The findings suggest that curb the decline of the butterfly monarch's population, milkweed must be restored, especially in the Upper Midwest, in the coming years.

Milkweed 

Milkweed is vital in the survival of monarch butterflies. It is the only crop that monarchs lay their eggs on, as butterfly monarch caterpillar's diet consists solely of milkweed. Milkweed is toxic, but the butterfly has evolved to bear it and store the toxins in their body. Butterfly monarchs may be attractive, but they are foul-tasting and poisonous, thanks to their milkweed diet.

The crop used to be abundant but was steadily removed from the fields over the years. 

Monarch butterflies are known for their seasonal migration that covered thousands of miles from the United States and Canada to California and Mexico. These butterflies have highly efficient muscles designed for thousand miles of flights. 

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Check out more news and information on Monarch Butterflies on Nature World News.