Incredible Creatures: Monarch Migration Monitoring and Languishing Lady Beetle Levels

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Want more things to be thankful for this Thanksgiving? Monarch butterfly levels seem to be doing well. And the species of lady beetle that seemed to fill many homes last year are lower. At this time of year, insects and many other animals get ready for winter by either migrating or finding suitable locations to wait out the winter. Last year we saw huge levels of multicoloured Asian lady beetles getting into people’s homes. Some were concerned that this may be the new norm. This month’s Incredible Creatures will provide an update on the status of some late-summer insects of interest. Wondering how the monarchs are doing on their migration? Will your house be an overwintering haven for lady beetles this year? Read on and we will explore some of these questions.

Roosting Monarchs

Monarch butterflies only migrate during the day. They come down at night and gather in clusters. A cluster of butterflies is called a roost, or sometimes a bivouac. When seeing a roost of monarch butterflies it would be easy to assume they migrate in flocks, the way some birds do. But this is not the case. Monarchs migrate alone. Yet they still form these roosts. Some roosts have only a handful of butterflies, while other roosts have too many butterflies to count! Most roosts last for only a night or two. In other places, they may last as long as two weeks. There are a couple of factors that seem to be important in determining where monarchs form their roost. Trees that provide shelter from the wind are an important factor. Having a source of nectar nearby also appears to be important. Often migrating monarchs will come down in the afternoon to feed. As the sun sets, they stop feeding and fly to nearby trees where they begin a search for each other to form these transient nighttime roosts.

Why do monarchs roost? Like the migration itself, we do not have all the answers. One hypothesis is that roosting is an anti-predator strategy. Cool temperatures make it hard for monarchs to move, making them vulnerable to predators. A roost provides safety in numbers. When overnight temperatures are warm, monarchs may not aggregate as tightly or roost at all. Perhaps monarchs shift to roosting behaviour when cold overnight temperatures make them more vulnerable.

Each year, roosts suddenly begin to form in the north in mid-August. Maps are prepared that show when and where people reported roosts during fall migration. The data reveals the migration pathway to Mexico.

Sharing more than Free Trade

Canada, the United States and Mexico share more than just a free trade agreement. They share in the migration route of the monarch butterfly. Each year the migration of monarch butterflies, both north in spring and south in late-summer and fall, can be followed on maps posted on the website Journey North (https://maps.journeynorth.org). In early-September the organization Monarch Watch reported “it’s been more than a few years since we have seen a monarch migration as promising as the one that is taking place at this time”. The estimated numbers of monarchs were reported to be higher than seen in several years, with 17 different roosts estimated to contain 1000 or more monarchs.

At the time of writing this article (early-October), most of the monarch roosts are currently in the central and southern United States. Monarchs traditionally reach Mexico’s overwintering region each year by November 1st.

Where are the Asian lady beetles?

Last year we had incredible numbers of a species of lady beetle called the multicoloured Asian lady beetle.

However, this year we did not see the high levels we had last year. Although I can’t guarantee you won’t find the odd one in your house this year, it’s not likely the level of overwintering beetles will be anywhere close to last year.

So why the difference in lady beetle levels? One of the favourite foods of Asian lady beetles most of the year is aphids, some of which can be pests of plants. The average number of aphids consumed by one of these lady beetles can be quite high. In a study at the University of Guelph, where multicoloured Asian lady beetles were feeding on high densities of soybean aphids, the older juveniles each ate on average 112 aphids per day, adult females ate 95 aphids per day, and adult males ate about 54 aphids per day. Lots of aphids are needed for there to be lots of healthy and well-fed lady beetles. The missing part of the equation this year was that the levels of many species of aphids were much lower this year than last year. As an example, last year we had extremely high levels of soybean aphids in Manitoba, and many soybean growers applied insecticides to protect their yields. This year I am not aware of even one soybean field in Manitoba having been sprayed for soybean aphids. Fewer aphids = less lady beetle food = fewer lady beetles.

Now that we are having some cooler October weather, does that mean the Asian lady beetles have found their overwintering sites already? It depends. A study in Ohio found that overwintering migration began the first day the temperatures exceeded 18⁰C after temperatures had dropped to near freezing. They will migrate towards prominent, isolated objects on the horizon. And during fall migration they prefer to choose and land on white or light-coloured objects. They are looking to form mass aggregations in dark, concealed locations. So now that we have had some colder weather, if it gets up around 20°C again we will know just what we are dealing with for overwintering lady beetles. But don’t be surprised or insulted if your overwintering guests don’t show up in large numbers this year.

So hopefully thanksgiving was truly a time of being thankful for the abundance of good things that we are blessed with. You now have a couple more things to add to the list. Hopefully, the monarchs do well on the rest of their journey and overwinter successfully. And for those of you who didn’t appreciate all the unexpected house guests last year, enjoy the relatively lower levels this year. If by chance mother nature proves me wrong expect an apology and eviction tips in the November Incredible Creatures.

Incredible Creatures is a monthly contribution to provide information on some of the common yet often not well-known creatures that we share space within Manitoba and abroad.

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