Declining Monarchs

Monarch Butterfly

Monarch Butterfly

Brad Plumer has an interview with Lepidopterist, Lincoln Brower about the declining numbers of Monarch Butterflies. The overwintering population in Mexico has shrunk to the lowest on record. Brower cites the following reasons for decline:
1. Habitat destruction and illegal logging in protected areas.
2. Massive drought in Texas two years ago that left few nectar sources for migrating Monarchs. Many Monarchs may not have reached the overwintering site because they ran out of fuel (nectar).
3. Spring 2103 was cold in the Southern states for a lengthy period and that may have delayed the northward migration.
4. The cold weather may have resulted in the loss of one generation. Typically the numbers of Monarchs in the first (migrating) generation are small in number. Subsequent generations increase in number. The generation that migrates south will be the largest. If the last generation is missing the migrating population will be far smaller.
5. Changes in agricultural practices have led to a decline in the population of all milkweed species in many areas of the midwest. One study estimates a 60% reduction in milkweed populations.
6. Agricultural practices are reducing the numbers of nectar producing plants that provide food to adults.

Our own observations at the Tippecanoe County Butterfly Encounter have noted similar declines. The spring weather in 2011 was very cool and we noted lower counts. To be fair, some of the lower numbers may have been due to longer development times and delayed emergence of the population. 2012 was a record low count: 2 Monarchs. It was a drought year with many 100 degree (F) days in the week prior. We saw some Monarchs early and a huge migration of Red Admirals but they appeared to pass through and not stop in the area. In 2012, Monarchs were reported in far northern Canada, unusually far north. We don’t know what limits the northern migration of Monarchs. Perhaps the warmer weather encouraged more movement north? In 2013, we saw more Monarchs, but numbers were far below median numbers.

Will Monarchs recover? We don’t know. We do know that habitat protection, steps to increase or protect milkweed and increased numbers of plants that produce nectar would be a positive step.

About jjneal

Jonathan Neal is an Associate Professor of Entomology at Purdue University and author of the textbook, Living With Insects (2010). This blog is a forum to communicate about the intersection of insects with people and policy. This is a personal blog. The opinions and materials posted here are those of the author and are in no way connected with those of my employer.
This entry was posted in by jjneal, Environment, Policy. Bookmark the permalink.

8 Responses to Declining Monarchs

  1. GarryRogers says:

    Thank you. I pinned, tweeted, and scooped your article (see the scoop at http://scoop.it/t/ecoscifi).

  2. Pingback: Purdue University’s Jonathan Neal Writes about Declining Monarch Population – Entomology Today

  3. Don in VA says:

    Reasons 5 and 6 are not just general “agricultural practices” changing; the reason there are fewer milkweeds and nectar-bearing plants is mostly due to the prevalence of glyphosate-resistant crops accompanied by virtual monoculture in those crops (corn, soybeans, cotton) over the vast acreages of continent.

    • GarryRogers says:

      I haven’t seen any numbers, but I’ll bet use of glyphosate use has skyrocketed. Next I believe we will have 2, 4-D resistant crops just to add some variety.

    • jjneal says:

      Glyphosate resistance is one major reason. It is more effective weed control and some estimates are a 50 % reduction of milkweed in agricultural fields since 2000.
      There are reductions in the conservation acres in part due to high corn prices, converting conservation areas to production to grow feedstock for ethanol production. Management of the conservation areas is important. Many areas are being planted to grass and cover crops that reduce milkweed. Drought is very important because milkweed is not drought tolerant. Urban sprawl and replacement of natural areas containing milkweed with lawns and golf courses can eliminate a lot of milkweed.

      Glyphosate is certainly a factor but is it the most important factor? Atrazine also provides milkweed control but it is used in greater amounts and can contaminate ground and surface water.

      Drought in the Midwest may be an imporatant factor. There is greater decline of Monarchs in the Midwest than the East and both areas use glyphosate.

      • Anonymous says:

        Thanks Jonathan for your thoughtful reply. I agree there are a number of factors likely responsible for the recent decline. Glyphosate (and soon, dicamba and 2,4-D) resistance of corn and soybeans is a key piece, and so influential in EVERY year that it should be singled out in any listing of probable causes. Thanks, Don

  4. Don in VA says:

    Thanks Jonathan for your thoughtful reply. I agree there are a number of factors likely responsible for the recent decline. Glyphosate (and soon, dicamba and 2,4-D) resistance of corn and soybeans is a key piece, and so influential in EVERY year that it should be singled out in any listing of probable causes. Thanks, Don

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