Just what do the numbers show? While we wait until the end of the season when everyone has entered all of their data to do careful analyses, I pulled up the data from Minnesota, where 33 sites have been monitored this summer. The data from other northern states look pretty much the same. These sites range from the Gunflint Trail way up in northeastern MN, where David MacLean has been monitoring a patch of swamp milkweed since 2010 (David saw no eggs or larvae at all in 2013), to New London-Spicer in west-central MN where Laura Molenaar and many students have been monitoring milkweed patches since 2004 (they saw a few monarchs this year), to many sites in the Minneapolis-St Paul metro area (my daughters and I have monitored our yard since 2002, and on my best day this year I found 20 eggs on about 135 plants). The graph below shows egg densities (as eggs per milkweed plant monitored, so a value of 0.20 means that there were 20 eggs on 100 plants, and value of 0.02 means there were 2 eggs on 100 plants).The blue line represents a fairly typical year for Minnesota, with an early peak in May when the migrants arrive from the south, followed by a second one in July when the next generation is appearing. Last years' (red line) was unusual because of record warmth in the early spring followed by a drought in late summer. This year's data (green line) shows a dearth of Monarch eggs throughout the year. Our family saw the same general pattern here in Wisconsin. (More data and discussion from MIMP here).
The annual Madison (Wisconsin) Butterfly Count has data going back for over 20 years. This year's result (only 2 Monarchs) is, by a wide margin, the fewest ever recorded.
The East Coast of the United States also saw a markedly reduced Monarch population, as reported in Native Plants and Wildlife Gardens:
Normally my wildlife gardens attract Monarchs in the spring... Normally, it is hard NOT to find Monarch eggs when peeking under milkweed leaves. Normally, Monarchs continue to be seen in my garden into the fall when some days the garden glitters with dozens (if not many dozens) of Monarchs.Purely for dramatic effect, I've illustrated this post at the top with a pair of remarkable photographs taken by Dan Sonnenberg, who spotted this road warrior in central Wisconsin in mid-October, presumably trying to migrate south after a rocky summer. He noted that "it still flies." Now I'll emphasize that mechanical trauma from predation is not the cause of nationwide decline in the population; for that we have to look at habitat loss.
As always, the dinner table was set for Monarchs in my garden this year. Offerings included the magic combination of native nectar plants and stands of milkweed. But this year (between June and mid-September), I have seen a total of 11 Monarchs in my garden and found only 1 caterpillar. In over 30 years of gardening for wildlife I have never seen so few Monarchs in my garden. Each day I check my milkweed stands for holes in leaves and each day I am disappointed to find all leaves intact and untouched. The absence of Monarchs unnerves and alarms me. Something is very wrong.
Habitat loss is discussed at Yale Environment 360. Monarchs are evolutionarily adapted to feed only on milkweed, the population of which has markedly diminished after the introduction of Roundup-ready crops that grow in weed-free fields.
Additionally, Monarchs from the Midwest overwinter in the mountains of Mexico, where the trees they assemble on have been harvested for wood. A MonarchWatch survey this past spring showed a record low number of Monarchs in the Mexican mountains:
The percentage of forest occupied by monarch butterflies in Mexico, used as an indicator of the number of butterflies that arrive to that country each winter, reached its lowest level in two decades. According to a survey carried out during the 2012-2013 winter season... the nine hibernating colonies occupy a total area of 2.94 acres (1.19 ha) of forest – representing a 59% decrease from the 2011-2012 survey of 7.14 acres (2.89 ha).I'll close with a photo of a magnificent, healthy Monarch I photographed in our garden during my blogcation -
-nectaring on New England aster, harvesting one of the few locally-available energy sources on his 1500-mile migration to Mexico. It's so sad to see them disappearing.