The perils of being a Monarch

Monarch butterflies have been getting a lot of publicity in the last few years, and rightfully so. They are undoubtedly the best known butterfly in North America and beyond. They are large, easily identified (except for the Viceroy being an excellent mimic) and are known for their incredible journey to the highlands of Mexico to overwinter.

Unfortunately for Monarchs, they rely almost exclusively on milkweeds for laying their eggs and therefore the survival of subsequent generations. For decades, the agricultural community despised milkweeds, as Common Milkweed is an aggressive invader of fields. There are lots of less common species, none of which are particularly problematic to the farming community but all species were painted with the same brush. Between the use of harsh chemicals and being officially on the Noxious Weed List milkweeds, and therefore Monarchs, did not have a promising future.

Fortunately as more and more people realized the serious decline of Monarchs there was a move afoot to remove milkweed from the Noxious Weed List, and more and more people started planting milkweeds in yards and gardens to provide some assistance to the survival of this magnificent butterfly.

The overall results have been fairly positive, with larger and seemingly more stable populations of Monarchs being observed. But every year is different, for different reasons. The limited overwintering habitat in Mexico is not all that secure from impacts. The weather is one of several factors there. Also the long journey by the survivors as they make their way northward in the spring, is not easy. Those that do survive overwinter begin moving north, laying eggs as they go and then dying, counting on the next generation to make the next leg of the trip north. It may take several generations for the species to get all the way back to the northern limits of their usual range.

In some years it seems there are relatively few that arrive here in southwestern Ontario at the beginning of the season, but gradually their numbers increase. In other years, such as 2018, it seemed there was a decent number present in May, leading one to hope for a good population throughout the summer. But it was not to be, at least in this area. The ones that did arrive in good time had some severe weather to contend with once they got here. Not as in stormy, hurricane type weather or cold, frosty weather. No, it was the shift from a cool damp spring to a very hot and very dry period for several weeks in the early part of the summer.

We regularly saw Monarchs in our yard, both male and female, for the early part of the summer. Eggs were laid.

But there were very few caterpillars that matured to adulthood. It was just too hot and dry for them. In fact many butterfly species experienced lower numbers than usual, which was reflected in the results of the annual butterfly counts in mid to late July. Small species of butterfly in particular were affected, but even the large Monarch was affected.

And then by late July and throughout August, we got more rain than usual. Lawns that were brown returned to luxuriant green. Milkweeds were less stressed and provided a better source for caterpillars. The tough, dry leaves of milkweeds responding to the hot dry weather earlier were not suitable for the caterpillars, so many of them did not develop until the moisture levels improved and there were more and younger, fresh leaves to feed on.

Between the milkweeds in and around our yard as well as those in a pollinator patch in a community garden close by, there are well over 125 milkweed stems. We started seeing more and more caterpillars. They developed and formed their trade mark chrysalis.

On one occasion, Marie noted a chrysalis behind a small cage in the garden. The cage was to protect the vegetables from marauding wildlife, such as cottontails. We didn't expect that a Monarch caterpillar would venture here. The nearest milkweed plant was at least 4 metres (12') away. It had to crawl through a lot of grass and other thick vegetation to get to this spot. But there it was.

 The butterfly eventually emerged, and we were concerned that it could not get out without damaging its fragile wings.

 Even though the 'foot' of the chrysalis was attached to the underside of the leaf, it was also attached to the wire of the cage. I decided to carefully tease the foot of the chrysalis away from the wire, and gently lift it outside the cage.

The Monarch managed to continue with its emergence and fluttered away not knowing how close it might have come to a much less desirable end.

As the season continued, we saw more and more caterpillars. On one occasion I could stand at the edge of the pollinator garden and count a dozen caterpillars of various sizes. But there was something concerning about this. None of them were under the leaf, where they would normally be feeding. Instead all of them were quite visible, including some which were just sitting in full view on the upper side of the leaf.

A closer look showed that some of these caterpillars were dead. Almost full size, but quite dead.

We also noticed that at least a couple of recently emerged adults had deformed wings.

One could fly a bit, the other could hardly even remain upright. Clearly something was not going well as both caterpillars and adults were suffering.

Was it something in the air? Were there pesticides that were affecting them? Were they trapped in a place and couldn't emerge properly?

We are not close to any agricultural field, so pesticide drift surely was not a factor. We had no way of testing air quality. We could only surmise that perhaps some caterpillars decided to place their chrysalis in a tight spot and the adults damaged their wings upon emergence.

But that didn't explain the death of so many caterpillars before ever getting to the chrysalis stage.

Marie took it upon herself to see if Google had any answers. And there were some.

Not surprisingly, there are various things that affect Monarch caterpillars, from fungus to bacteria to flies. Yes, flies, and specifically, Tachinid Flies, also known as parasitic flies.

There are more than 1000 species of Tachinid fly in North America, and at a glance they generally look like the flies we are most familiar with. However they do have some differences. These next few photos are all of Tachinid flies, and probably are all different species. A couple of the key features are that they have somewhat hairy bodies and large, reddish-brown eyes.

As adults, the flies feed on nectar of various flowering plants. But it is their reproductive behaviour that is of concern to Monarchs. The flies glue their eggs on to a caterpillar or, depending on the species of fly, sometimes inject the eggs directly into the body of the caterpillar. When the eggs hatch, they begin to feast on the caterpillar and it is doomed. In some cases the eggs are deposited on the surface of the leaf and it is ingested by the caterpillar. In other cases, the Monarch eggs themselves may have a fly egg attached or injected. Regardless, the result is the same. The caterpillar will not develop to a mature adult.

But before we castigate these flies for what we might consider their nefarious deeds, it is important to look at the bigger picture. The term 'fly' is not nearly as endearing as the term 'monarch'. Even the name of the butterfly is regal and something humans may admire. But these flies are not focusing just on Monarch caterpillars. In fact they are a huge benefit to agricultural and garden crops as they parasitize all sorts of other critters, some of which are very harmful. Think of all the damage that pests such as gypsy moths, squash bugs, cabbage loopers, army worms, Japanese beetles, cutworms, sawflies, codling moths, peach twig borers and tent caterpillars do to fruit and vegetables.

Any and every species of wildlife has natural enemies. It is what keeps things in check....the balance of nature, so to speak. If Monarch butterflies weren't so impacted by human activities, such as loss of habitat and the extensive use of harmful chemicals I suspect the effects of Tachinid flies wouldn't be much of a concern. If things were in a truly healthy state, and Tachinid flies weren't affecting Monarch caterpillars, we may be over run with Monarchs! I don't think anyone would complain if there were a lot more than there are, that is for sure. However Tachinid flies are just doing what they were meant to do, and have a place in the wild.

If you have noticed a similar situation in any milkweed patch/pollinator garden that you are involved with, it might be worthwhile to consider this: an emphasis on helping Monarchs is to plant milkweeds and generally speaking, that is a good thing. But if all you have is a big patch of milkweeds, it makes it easier for the flies to find a lot of Monarch caterpillars and eggs. In a truly natural setting, milkweeds are just one of a multitude of grasses and wildflowers, and are seldom in large patches. Instead they are widely scattered, likely giving any caterpillars on them a better chance at being missed by the flies. So add other pollinator plants to the garden patch, and spread out the milkweeds. You just might help a few more caterpillars survive to become a beautiful adult flitting about your yard.