Researchers say that the steep decline in the population of the monarch butterfly that reached and returned from their wintering area in Mexico are not completely explained by the decreasing numbers of milkweed.
A new study has been recently published the BioScience journal. The study reviewed decades of monarch butterfly studies and included an analysis of Illinois’ milkweed populations. Illinois is the heart of the monarch butterflies summer range.
The number of monarch butterfly in Mexico have greatly dropped from 682 million back in 1997 to a mere 42 million two years ago. according to researchers, they are struggling to discover what has caused this decline. Some researchers believe the loss is caused by the loss of milkweed, which are the only plant that monarch butterfly larvae can feed on.
The loss of Milkweed has been attributed to the over use of crops that are resistant to herbicide. This practice started in the 1990s and it permitted farmers to use herbicides on their crop fields. Milkweed is extremely susceptible to glyphosate, which is the most commonly used herbicide, reports the University of Illinois research site, Life Sciences.
The new study shows that numbers of milkweed have fallen by 95 percent in Illinois over the past 20 years. The new study was conducted by Greg Spyreas and David Zaya, who are Illinois Natural History Survey plant ecologists. Spyreas and Zaya worked closely with Ian Pearse a research ecologist.
Zaya says that they discovered more milkweed in natural areas than previous studies had found. Zaya believes that milkweed found in natural areas is buffering milkweed loss in agriculture areas. Zaya does add that milkweed in natural areas has also declined over the past two decades. He believes that the decline has been caused by the conversion of pastures into cropland. The study revealed that the loss of milkweed is not as huge as the great decline in numbers of monarch butterflies returning from Mexico
Researchers found evidence to challenge the belief that the decline in milkweed is the sole cause of the decline in monarch butterflies. Researchers did note that despite the decreased numbers of the monarch butterfly returning from Mexico they appear to rebound once they reach Illinois.
The number of monarch butterflies lost in Mexico are rebuilt within a couple of generations of reproduction during the summer. That fact seems to prove that the decline of milkweed plants in the region is not the primary problem.
The fluctuations of the number of the monarch butterfly, year by year suggest that they are not in an overall decline.
By Tammy Marie Rose