Glyphosate gets off the butterfly hook

An example of a blatant effort to find a pesticide scapegoat

Monarch butterflies hibernating in Mexico on fir trees.

When I lived in Ontario in the early seventies, monarch butterflies were a common sight in the Guelph area, especially in late September. I would be coaching rugby on the University of Guelph rugby field late in the day and I would see hundreds of monarch butterflies heading south over the playing field at a height of around 100 feet (30 metres) on their long 150-kilometre-a-day journey to Mexico. When I moved west to Edmonton, Alta., I saw few, if any, monarch butterflies, and only then in the southern part of the province, again in September.

In 2012, we had an unusually early spring when air currents from the United States swept up massive clouds of leafhoppers onto the Canadian Prairies. It was the year of the destructive aster yellows disease of canola carried by these leafhopper hordes. I was amazed to see in mid-April monarch butterflies in my acreage just west of Edmonton. Normally such butterflies barely reach the Prairies and only then in August to lay eggs that feed as caterpillars on milkweed, the butterfly host plant.

It was in 2012 that American ecologist Karen Oberhauser wrote a paper that blamed glyphosate as greatly damaging the monarch butterfly multiplication.

Monarch butterflies migrate each fall from the northern U.S. states and southern Canada all the way back to a very specific upland site of only around 40 acres of fir trees in Mexico. Their numbers at the turn of the century were around 300 million, but a decade later these numbers fell to only 100 million. Immediately Oberhauser blamed the use of glyphosate for killing off some 70 species of milkweed plants (genus Asclepias) that the monarch butterfly caterpillars fed on (i.e. the butterfly caterpillars were being deprived of their food sources as they returned to the United States and Canada to breed and multiply).

There was immediately a huge hue and cry in the United States to ban glyphosate, since its use was eliminating the milkweed food plants. I read several articles on this and I got confused. This ecologist stated the use of glyphosate on corn and soybean crops was eliminating the milkweed host. Having travelled extensively in the United States and southern Canada, I had never seen milkweed in these crops since the ’70s.

On the other hand, on waste ground and along field headlands and poorly maintained pastures, milkweed species were common plants. No farmers would spray headlands or pastures with glyphosate unless they wanted a massive annual weed infestation since glyphosate killed off all green plant growth.

The monarch butterflies leave their Mexican winter quarters in early spring and travel a few hundred miles north into the southern United States. They feed on plant nectar and lay their eggs on the milkweed hosts. The adult butterfly then dies and the resulting caterpillars develop into new butterflies. This cycle is repeated four or five times each year as the butterfly swarms head north into southern Canada.

A few years later, several well-known researchers in the United States and Canada contradicted the findings of Karen Oberhauser. These other monarch butterfly researchers pointed out that only some 40 per cent of the butterfly population passed north and south through the corn- and soybean-growing areas in the United States. The other 60 per cent of the butterfly population travelled either east or west of the major corn- and soybean-growing areas.

Monarch butterfly counts done in the early fall in both the United States and southern Canada showed that butterfly populations were thriving and normal. These researchers concluded it was during the many-thousand-mile migration to the Mexican winter hibernation area that led to major reductions in butterfly numbers. These other researchers pointed out that the monarch butterflies relied on nectar from wildflowers to fuel their long migration south. They revealed that several droughts in the late fall in the southern United States, in particular, may have deprived the butterflies of the nectar energy source to complete the journey to Mexico and, additionally, they were left with low energy reserves for the long winter and spring migration north into the United States.

These researchers also concluded there were other factors responsible for the butterfly decline, such as pests and diseases as well as disturbances to their small patch of overwintering fir forest. Steps were subsequently taken by Mexico to step up conservation in this area and to preserve suitable trees higher up and cooler in elevation with the looming prospect of global warming.

This article is intended to demonstrate to the farming public and to North Americans in general the blatant effort by so-called environmentalists to look for pesticide scapegoats. Environmental researchers, rather than take the time to rationally and correctly address wildlife and other problems, should not take shortcuts and blame responsible pesticide use.

Before eager environmentalists and the general public “pin the tail on the donkey” they first need to identify the proper cause before taking aim at good and proper farming practices and procedures.

About the author


Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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