Of milkweed and monarchs

 Bob Bowles

A monarch butterfly gathers nectar on the flowers of swamp milkweed on Pelee Island during the recent Pelee Island butterfly count. (BOB BOWLES/SPECIAL TO THE PACKET & TIMES)

A monarch butterfly gathers nectar on the flowers of swamp milkweed on Pelee Island during the recent Pelee Island butterfly count. (BOB BOWLES/SPECIAL TO THE PACKET & TIMES)

The Civic Holiday weekend in early August marks the end of the summer butterfly and dragonfly surveys that start in early June, peak in early July and wane by mid-July. The last of these surveys every year are the Pelee Island butterfly and dragonfly counts.

When I started these counts more than 20 years ago, we held them on the August long weekend, but with the new Island Music Festival on that weekend now, it became a problem finding accommodations and booking ferry passage. This year, the counts were moved to mid-week before the long weekend.

A female monarch butterfly was observed flying around the Kingsville ferry dock on the afternoon of Aug. 1 while waiting for the MV Jiimaan, which would take us to Pelee Island for the counts over the next two days. Observing this newly hatched monarch flying gave us hope we would see several during the counts. We were not disappointed, and the four teams the following day recorded a total of 122 monarchs, most of them newly hatched with vivid orange and black markings.

The high count of monarchs was not surprising since the counts from mid-July onward had high monarch numbers. The Carden Alvar butterfly count July 28 recorded 100 monarchs. I did a survey of monarchs on Bowles Alvar in Simcoe County July 31 and recorded 20 in just more than an hour, plus almost as many Halloween pennant dragonflies in and around the wild bergamot flowers now in full bloom and the upland white asters just coming into bloom. Even in mid-July, there were at least half a dozen monarchs observed every day in Thunder Bay, which is the northern range of the species in Ontario. Late July and early August this year yielded many newly hatched monarchs. This is the third generation of the monarchs that returned north from Mexico last March. They will now mate and females will lay eggs on milkweed plants. The eggs will hatch and complete the cycle back to adults during the next 28 to 32 days. Adults of the fourth-generation hatching the last week of August or first week of September will not develop sexually to mate but will begin their journey to their wintering grounds in Mexico, arriving there around the end of October. Their arrival in Mexico is celebrated during the Day of the Dead, or All Souls’ Day, Nov. 2.

Records of the total wintering areas in Mexico have been recorded by Monarch Watch since the winter of 1994-95, when monarchs covered 7.81 hectares. This increased to an all-time high of 18.19 hectares during the winter of 1996-97. Monarch numbers dropped sharply over the next few years, then fluctuated up and down between 11.12 to 1.92 hectares in the winter of 2009-10, the lowest since 1994. Numbers have been low over the past seven years, with just more than four hectares for the winters of 2010-11 and 2015-16, but dropping to an alarming low of .67 hectares during the winter of 2013-14. Last winter (2016-17), numbers dropped slightly to 2.91 hectares. Numbers are healthy this summer and there is good growth of milkweed plants in the south, so experts are predicting a strong migration this fall. Some are even predicting the overwintering population this winter will increase from the 2.91 hectares last winter to more than four hectares this winter.

But what has caused these swings with peaks and valleys over the past two decades, with lower coverage in the past decade? There is a combination of factors, like weather during migration and at the wintering sites. Storms can wipe out large portions of the population in any given year. Summer temperatures, with flooding and droughts, have been more extreme in recent years. Good early growth of milkweed is an important factor in early spring during migration.

Researchers of the University of Guelph have been doing studies using chemical isotope signatures to identify the birthplaces of these North American monarch butterflies. One researcher is proposing the significant drop in the monarch population is due to the eradication of milkweed, which began in the mid-1990s. His theory is if we plant more milkweed plants, we will have more monarchs. I don’t agree, since I have observed excellent large stands of healthy milkweed plants in recent years but with no monarch eggs or caterpillars feeding on them. Common milkweed in the past has been considered an unwanted weed in Ontario, and the Ministry of Agriculture listed it on the noxious-weed list until 2014, when it was removed from the list to help monarch populations. When they removed milkweed from the list in 2014, they added a new plant to the list, which also has caused a decline in monarch butterflies. Dog-strangling vine is an invasive, introduced weed that has a white latex sap much like milkweed. Some female monarchs have laid their eggs on this vine, but researchers have found monarch caterpillars that feed on the vine do not survive; they die after a few days.

I feel the biggest contributing factor to the decline of our monarchs is the use of insecticides in the Ohio corn-belt area, which monarchs need to cross during migration. Neonicotinoids, including clothianidin and thiamethoxam, are a class of insecticide commonly applied as a coating to corn and soybean seeds to protect them from early-season pests. A graphite powder is added to the seeds to keep them from clumping when being planted by vacuum systems in the planters. The exhaust from these planters, which contain neonicotinoids, floats in the air and covers the plants, including milkweed, and nearby puddles visited by monarchs and other insects. Researchers have proven it has caused a decline in the honey bee population and probably causes decline in monarch and other butterfly populations as well.

This year, I have observed healthy stands of milkweeds but with no signs of monarchs. Planting milkweed gardens is a great idea. We have several of these in Orillia, but the monarch population fluctuations are much more complex than the theory of more milkweed plants meaning more monarch butterflies.

Bob Bowles is a local naturalist. He can be contacted at

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