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Monarch butterflies depend heavily on milkweed as one of their most important sources of food, but milkweed losses alone do not explain the massive population decline of monarch butterflies, scientists have said.

Researchers at Illinois Natural History Survey and colleagues published a research in journal BioScience base their findings on in-depth analysis of milkweed populations in Illinois, a state at the heart of the butterflies’ summer range.

Population of monarch butterfly in Mexico have declined massively over the course of two decades from 682 million in 1997 to a recent low of 42 million in 2015 and scientists are still unable to determine what is driving the decline. A popular hypothesis is that the loss of milkweeds – the only plants on which monarch larvae can feed – is to blame.

Milkweed losses are tied to the use of herbicide-resistant crops, a practice that began in the late 1990s and allows farmers to apply nonselective herbicides to their fields. Milkweeds are susceptible to glyphosate, the most common herbicide used to protect those crops from weeds.

While the new study does confirm a decline in milkweed numbers by a whopping 95 per cent in cropland in Illinois over the last two decades, authors of the latest study point out that milkweed in natural areas are increasing than previous levels. Milkweeds in natural areas also have declined in the past two decades, primarily as a result of the conversion of pastures and other marginal sites to cropland scientists point out. But the overall drop in the number of milkweeds in Illinois – roughly 50 percent, the researchers found – is not as large as the huge decline in monarch butterflies making it back to Mexico.

Scientists note that when monarch butterflies leave Mexico in small numbers to migrate they’re able to rebuild their populations within a couple of generations of reproduction in the summer in the Midwest and this suggests that supply of milkweed plants isn’t the primary problem behind the population decline. Scientists have urged conservationists to look beyond boosting milkweed numbers and to look for other reasons that could be causing population decline.

Because the population crashes tend to occur on the way to Mexico, the researchers think a lack of late-flowering nectar sources along the route also may be a significant contributor.

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