Monarch Watch Blog

When monarchs are like corn

Tuesday, May 17th, 2022 at 11:48 pm by Chip Taylor
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I received an email this past week (<1May) that referenced an article with an alarming headline. "US corn planting slowest since 2013, yield risks still premature". Why should this be alarming? Well, I’m going to tell you why and when monarchs are like corn. It’s been on my list to do so for some time. This headline gave me the motivation to start this discussion. Let's start with the obvious, corn doesn't fly, lay eggs, visit flowers or migrate, but it does grow and every farmer knows that yields are dependent on numerous factors and that two of the most important are timing of planting and temperatures. There's the similarity – timing and temperature. Do you remember the size of the monarch overwintering population in 2013? It was the lowest ever measured, a mere 0.67 hectares. The number was alarmingly low, a number so shocking that it led to an international agreement between the governments of Canada, the United States and Mexico and a White House invitation to stakeholders to a meeting at the Eisenhower Office Building in D. C. to discuss the issue. This meeting was followed by a Presidential Memorandum on the 20th of June 2014 asking 14 Federal Agencies to actively pursue actions that would benefit monarchs and pollinators. Later, five parties petitioned the Department of Interior to declare monarchs a threatened species. The sharp decline in monarch numbers in 2013 led to an abundance of research directed at trying to understand monarch biology as well as how best to address the loss of habitat which was seen as the major cause for the decline. For my part, I tried to focus on how monarch population growth was affected by seasonal conditions and wrote the following about the decline: Monarch population crash in 2013. Most of this text is still relevant though I know more about monarch population growth now than I did then. Here are the main reasons for the decline.

“The low number of monarchs reported at the overwintering sites in Mexico in the winter of 2013-2014 appears to have been the result of a series of negative weather events that began in the summer of 2011. Excessive temperatures and droughts in 2011 and 2012 followed by low initial colonizing numbers in the spring of 2013 account for the decline. More favorable conditions for population growth allowed the population to increase in 2014 and 2015.” In addition, the timing of the returning monarchs in 2013 was late, in fact, the latest in the record. Which brings me back to corn and timing and temperature.

Both corn and monarchs were affected negatively by these conditions. Corn yields in 2012 were the lowest since the early 90’s – estimated to be -22.2% below the long-term trend. And monarchs were the lowest measured (1.19 hectares) since the start of record keeping in 1994. Corn yields were also below the long-term average in 2013 and monarchs were, as mentioned, were even lower in the winter of 2013.

To get a sense about how the weather this spring compares with that of 2013, I looked up the the deviations from long term mean temperatures for March-May from Texas to Minnesota for both 2013 and 2022.


As you can see, the March temperatures in 2013 were substantially below normal except for Texas. April means were universally low. Though this spring is later than many in recent years, the mean temperatures for March were above average. April temperatures were lower and the first week of May has been colder than predicted from past means. If May temperatures continue to be cooler than normal, the northern movement of first-generation monarchs originating in Texas will be delayed and so will corn planting, resulting in a lower fall population for monarchs and a lower corn yield per acre. At the time of this writing (1 May), it seems likely that the overwintering number next winter will be higher than in 2013. However, the number will certainly be no more than 3 hectares and perhaps much lower.

Monarchs dipped and recovered from 2011-2015 and so did corn. The general pattern is similar. So, in the sense that both responded similarly to the physical conditions, monarchs are like corn and the converse. This response exists in spite of the fact that the areas represented in the measures of monarchs and corn are quite different. Most monarchs that reach the overwintering sites originate from the western part of the Midwest while corn production includes large areas with lower monarch recovery rates in Mexico. Corn recovered from the downturn faster than monarchs and that’s to be expected due to the manner of propagation. Corn starts with similar acreage each year whereas monarchs start with the number and timing of those returning from Mexico which can vary greatly from year to year.

(bushels per acre)

The existence of data on both monarchs and corn that can be related to temperatures and weather extremes allows for these comparisons. Similar relationships surely exist for a large number of co-occurring plants and animals. Soybeans also declined from 2010 through 2012 yet increased again in 2013. However, the overall similarity between crop yields and monarch numbers is compromised in this case by the adoption of new varieties and better production methods through the years. The larger point here is that what happened to monarchs from 2010-2015 was not unique. It’s likely that a large number of species were similarly affected during this interval.

As of 13 May, the warmer weather in the Midwest that began on the 8th has changed the outlook for both the corn growers and monarchs. Planting has intensified and the northward migration of first-generation monarchs has been aided in an extraordinary manner by strong winds from the south and southwest. Again, these improvements demonstrate that corn, monarchs and many other species respond in a similar manner to weather conditions.

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