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Lack of Milkweed Is Not Harming Monarch Butterfly Populations

People who attended the 2013 Annual Meeting of the Entomological Society of America are likely to remember Dr. Anurag Agrawal, who talked about Dame Miriam Rothschild when he presented the Founders’ Memorial Award lecture. The ESA Founders’ Memorial Award is one of the highest honors that can be bestowed upon an entomologist.

Now Dr. Agrawal is talking again, but this time it’s about monarch butterflies.

Some groups have blamed herbicides and genetically modified crops for the decline of monarch populations, but new research by Dr. Agrawal and colleagues suggests that lack of milkweed is unlikely to be driving the monarch’s population decline.

Using years of data collected by the World Wildlife Fund and citizen scientists across North America, the researchers looked at monarch populations throughout North America during different times of the year. They found that monarch numbers begin declining at the end of the summer, when the butterflies begin their long migration to Mexico, and the numbers continue to decline as they travel. During this southern migration, adult monarchs do not feed on milkweed, so lack of milkweed is unlikely to be the problem, according to the researchers.

“Milkweed is probably not the limiting thing for monarch populations,” Dr. Agrawal said. “At the end of the summer in August, when they begin that epic journey down to Mexico, they no longer are feeding on milkweed, they’re not nectaring on their flowers. They are simply making that journey. The only thing they eat then is water and flower nectar from other species like golden rods.”

The research also suggests that planting milkweed is not likely to help increase monarch populations.

“By the time they get to Mexico their numbers are plummeting, but at the end of the summer when they start their migration, their numbers are not down,” Dr. Agrawal said. “That presents an important mystery, but it also presents an important finding — that perhaps planting milkweed will not improve or increase the population as much as we thought in the past. It would be a mistake to think that planting milkweed alone is going to solve this problem. Planting milkweed probably isn’t a bad thing to do, but it’s not going to increase their populations or save them from some demise.”

The research also seems to vindicate genetically modified crops that are herbicide resistant.

“Our findings suggest that monarch populations are not limited by milkweed, which in a way suggests that herbicides are not likely to be the problem, and genetically modified crops that are herbicide resistant are not likely to be the problem for the monarch,” Dr. Agrawal said. “Given the intense interest in monarch conservation, the blame being put on herbicide use, and the national dialog about potentially listing monarchs under the endangered species act, we have to get the science right.”

Read more at:

Linking the continental migratory cycle of the monarch butterfly to understand its population decline


  1. The authors didn’t actually go into the field and determine “monarchs aren’t finding enough nectar to feed on as they travel long distances,” hence their nectar limitation hypothesis is hypothetical and imaginary. Why is this important? Because even during the ultra severe drought in 2011 in west Texas the millions of monarchs moving through in early October were not showing much interest in nectaring:

  2. Actually the disappearing of meadows with large swaths of milkweed in the north does have an impact on the monarchs. Based on some research and time in the field Monarchs return to the same fields every year and the offspring of them go back to around those fields. So hypothetically if people do plant large swaths of milkweed in new locations near the old fields then you have potential to help spread the population and keep them in that location to further the lack of deaths by multiple factors…. Because the more spread out the species is the more it will survive. If there is 50% the fields we have now in the next year, then the population will mix DNA more but the rate of which the caterpillars survive and population survival rate will drop allot. This is because only 2% of caterpillars in a given location survive, the more caterpillars in location means the more parisitic wasps(will stay around 2%).

  3. As I imagine you’re aware, it takes huge energy stores for the (non-feeding, just nectaring and libating) adults to get to Mexico and survive the winter. The simple fact that they ‘make it’ to adulthood does not necessarily make them travel-worthy. A single “common” milkweed plant can support even three larvae to pupation, but the resulting adults are not as able as if the same host fed one. Simple population size, even without accounting for all the other confounding variables in such a data set (there are many), seems to be a naive measure. A large but starved population won’t make the trip and it seems as if that would present the exact results you’re reporting (due to a stressed hostplant population).

      • I’m suggesting there is a difference worth considering between larval “starvation” or, more aptly, underfeeding, and adult need to ‘refuel.’ They don’t get from nectar and water what they get from leaves. If they are inclined near the end of the season to oviposit on “alternative” milkweeds for lack of A. syriaca milkweeds or whatever is being mismanaged by big ag and city managers…. (you know, the milkweed killers), than many variables required for the adults to make the trip could be compromised.

        I do not believe measurements of population sizes – raw numbers, or modified and adjusted numbers of just counts during the migratory cycle provide enough information to be able to dismiss hostplant stress as a potentially significant factor in the overall reduction in population sizes. ‘Anecdotal’ records of Monarchs not ‘showing interest in nectaring’ are not compelling (beyond on the face of it as ‘data’) because that would not necessarily be symptomatic of adults on the migration not having the reserves to begin with. In fact, they may be more stressed and less likely to nectar on the way.

        I think it is a very difficult argument to make that the replacement of large larval hostplant range with monocultured cropland is not a contributing factor to the subsequent reduction in an ecologically sensitive phenomenon dependent on that hostplant. This study does not seem particularly convincing.

      • It’s easy to video document whether or not the butterflies are behaving as though they are hungry for nectar or water or not. Here’s a case in south-central Minnesota when they were hungry for nectar (also shows monarchs are still abundant on the GMO croplands of the upper Midwest)

  4. Not sure where the author gathered his data but it sure wasn’t in Vermont. These days we’re lucky to see a single monarch all summer long, whereas they were common throughout our state 10 years ago. Who funded this study, by the way?

      • Dear Entomology Today,
        Prof. Ron Herring (Cornell) who has helped to promote GMOs in India and fought to defuse the farmer suicide debate in India.
        Monsanto is deep into academia, all over the country, twisting it’s “scientific” studies, falsifying evidence and burying damaging results.
        “”The real scoop was not the perfidy and deceit of a handful of individual professors. Buried in the emails is proof positive of active collusion between the agribusiness and chemical industries, numerous and often prominent academics, PR companies, and key administrators of land grant universities for the purpose of promoting GMOs and pesticides. In particular, nowhere does the Times note that one of the chief colluders was none other than the President of the American Association for the Advancement of Science (AAAS).”
        Paul Cherubini who is posting on here is a shill for the Industry.
        Entomology Today had better wake up and smell the bacon burning.
        The FDA Allows 10,000 Chemicals Untested For Humans Into Your Food and Food Packaging.
        “The GMO Debate: One Student’s Experience of Pro-GMO Propaganda at Cornell University”
        Read this and realize the corruption that has occurred.
        There are volumes and decades of study that are being buried, due to these maniacal seed and chemical companies.

  5. I have never considered the lack of milkweed a problem as a source of nectar but rather the only plant that monarchs can lay their eggs on. It is my understanding that the problem is not finding milkweed for egg laying on the return trip. NJ’s native milkweed blooms long before the monarchs arrive.

    • And you win the Gold Star!!! These attempts at junk science promoting the lack of effect of GMO’s and the toxic pesticides, mowing, and destruction of habitat, deforestation, etc, are increasing because Monsanto is under attack, and rightfully so.
      For the Monarch, Milkweed provides the preferred larval food of the Monarch, nectar is just a plus provided by the Creator. In Tennessee, along Briley Parkway, a group of students planted and acre of Milkweed, and were fighting the state to keep them from mowing it. They want to plant 3 acres in 2017. The Butterfly’s were showing up and were seen feeding all the way up through the Fall, as blossoms declined.

  6. Are there factors that are not observable about the condition of the milkweed, use of pesticides, GMO or other changes that have caused these changes? Why would there be a drastic population reduction in Vermont?
    Mark said A large but starved population won’t make the trip and it seems as if that would present the exact results you’re reporting (due to a stressed hostplant population).

  7. Unfortunately. the scientific community is prone to doom and gloom for all creatures including us and the planet in general. The monarch migratory population is by no means in danger as demonstrated after this years 4x increase over last year and humans did nothing. Drought weather conditions caused the decline in numbers. There was no significant milkweed planted during the 2015 monarch migratory season yet the population overwintering in Mexico quadruppled from the year before and was 7x more than just 24 months ago. When weather conditions throughout the monarchs eastern range are favorable for milkweed, nectar sources, etc those same conditions are also favorable for monarch increase. There’s less “cropland” on the Eastern range than in 1949 according to USDA statistics. look it up. The BO monarch and pollinator initiative is no more than a funding mechanism for conservation and environmental NGO’s as was the Man Made global warming initiative “created” by Al Gore and the 97% liberal scientist. Unfortunately for the monarch Ngo’s we started coming out of drought and hot conditions in the fall of 2013 and the monarchs recovered and humans continued to survive ;) The continuing California drought continues to keep monarch numbers for the western monarch population down proving my point

    • Oh good, Wickedpedia. Your prof. took references from Wiki?
      All negative factors are coming into play in the stressing of all pollinators and beneficials. The science of compilation in regard to these factors holds true.

    • I am sorry that Butterfly Man feels that man has done nothing in the past few years to help the Monarchs. New Jersey has planted thousands of acres of milkweed through use of land on state and national parks and backyard habitat. Raising townships awareness of the damage that roadside mowing causes when it is done at the wrong time has also helped. We have programs in the schools where raising Monarchs is used to teach ecology, biology, and history. Since as you know Monarchs are solar flyers they enjoy the hot dry weather. The only problem they suffer in drought is if it kills the milkweed before their fall migration.
      I had a great year for raising Monarchs 413 raised and released. I raise them to prevent predators such as praying mantis and tachinid flies from getting them. I usually raise 50. All 413 monarchs caterpillars were found on two 4 x 8 ft. raised beds in my yard. I also have a quarter acres of wild common milkweed that I protect. Great year for me but friends in Penna. and Mass. saw none.

      • Marilyn, enthusiasts who plant milkweed will not boost the eastern migratory monarch population in a mathematically significant way because about 4,000,000,000 stems of syriaca milkweed still grow in wildscape settings in the eastern and central USA and support an average population of 150,000,000 fall migrant monarchs. So whatever extra 1,000’s, 10,000’s or 100,000’s more milkweed stems enthusiasts plant will not boost the fall migratory population even 1/10th of 1%. Enthusiasts also cannot save a substantial percentage of the existing 4,000,000,000 milkweed stems because it’s not logistically possible for them to locate, map and annually monitor even 10% of them to determine if the number of stems is expanding or contracting over time.

      • It is better than doing nothing while thousands of miles of roadside milkweed are destroyed by round-up each year. I cannot change what someone on a Midwest farm does but I can educate locals as to the struggles of the Monarch.

  8. I am going to assume the best: assume no collusion, no intellectual compromises, etc.

    If they are correct and the problem is lack of nectar flowers, where do those plants grow? Are GMO fields removing some?

    Even if they are correct, GMO is not off the hook.

  9. I raise Monarchs in San Diego, California. My BigAg government will not test milkweed plants for pesticide/herbicide contamination, but my experiences and those of others on a greater level is that there is substantial contamination of plants sold in nurseries, e.g., Home Depot. I have witnessed the ravage caused my caterpillars. We now know that pesticide contamination of flowers are killing honeybees inexcusably; I suspect the same goes for adult Monarchs. Worse, the Tachinid and O.E. infections here are so bad that adulthood is the exception, not the rule, in the wild. When I grew up as a kid in the early 1960’s, parasitism by a different Tachinid was extremely common. I have not comment on the issue borne in the title.

  10. We can still help by planting milkweed and wildflowers, though, right? We can atleast do something to help, even if it isn’t the main reason that the Monarchs are declining. We can atleast try to help them, even if we can’t control weather patterns, temperature and other factors that effect their migration.

  11. In my area, the primary villain are multiple species of Tachinid flies not habitat limitations. Here, to increase habitat would primarily serve to increase Tachinid populations. I await scientific information on whether any of the Tachinid species was introduced or bolstered in biological warfare. O.E. is also a major problem here.

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