This map shows Monarch abundance across North America during July, based on butterfly counts from 1977 through 1999 - starting that year because counts from 1975-76 (the first two years of the program) had more missing data, and ending in 1999 because that's as far as yours truly has analyzed. I totaled the number of Monarchs as well as the number of "party-hours". Party-hours = the sum of hours each group (or party) of counters spent tallying butterflies on a given count.

     The counts are grouped by states or provinces, with special attention to geography relevant to Monarchs. For example, I was curious about the Intermountain West - e.g., Nevada, Utah, and Arizona, which fall between the known breeding grounds for Monarchs that migrate to winter in Mexico and those that winter along the Pacific coast. To allow comparisons among regions, I divided the Monarch total by the total number of party-hours for each region, to produce a "rate" of Monarchs recorded per party-hour of counting effort.

     As expected from other sources of information, Monarchs in mid-summer are most abundant in the Northeast, Midwest, and adjoining Ontario, with a secondary concentration in California. These are the prime breeding areas for the two migratory populations.

     In Florida, Monarchs had a very low density but rather high rate of presence - 50% of July counts recorded the species. Monarchs were similarly low in abundance in British Columbia and Alberta, which are at the northern edge of milkweed range (the caterpillar food for Monarchs). But only 2% of those Canadian counts recorded any Monarchs - Monarchs are a true rarity in that area. But in Florida, at least where the counts are held, Monarchs are a routine but low-density summer butterfly.

     Scientists have found that Monarchs have both minimum and maximum overnight temperatures suitable for successful breeding. The areas of highest abundance fall within that climatic range in mid-summer. But never say always or never. Monarchs somehow persist, or keep re-invading, where nights are hottest. Low mid-summer numbers southward may also correspond to died-back milkweeds, as well as hot nights. More observations on southern counts by watchers in well-watered butterfly gardens would address that question.

     These results underscore the unique contribution of the count program. It visits more places over much longer time periods than we are capable of covering as individuals. We can't cover all regions of North America at once in July, but the count program does.

Copyright © 2001 by the North American Butterfly Association, Inc. All rights reserved.