Contributed by: Sonia Altizer, Karen Oberhauser, Dara Satterfield, Candy Sarikonda
There has been much discussion about a recent scientific study (Satterfield et al., 2015). The press release for the article is available here. This research shows very clearly that monarchs breeding on tropical milkweed throughout the winter have higher levels of protozoan infection (caused by Ophryocystis elektroscirrha, OE) compared to monarchs in the migratory cycle. This result is not debatable. However, the implications of this for monarchs are more complicated. Concerned monarch advocates have raised many important questions. As scientists familiar with this research, we address below several of these questions with our best understanding of the science to date.
1) Will infected butterflies at winter-breeding locations affect the monarch population as a whole?
The truth is that we don't really know. This depends on the abundance of non-migratory monarchs relative to the whole population, and the degree to which migratory and winter-breeding monarchs come into contact. Many of the winter-breeding locations occur along the migratory route in the southern U.S., so winter-breeding monarchs could spread parasites to migratory monarchs if they mate with each other or lay eggs on the same milkweed plants. (OE parasite spores are transmitted from infected adults to milkweed to caterpillars; spores can also be passively transferred from adult to adult during mating). As the overall monarch population in eastern North America becomes smaller, it is possible that winter-breeding monarchs will make up a larger proportion of the population. This could lead to a population-wide increase in infection rates. The potential mechanism for impact is clear, and we are strong believers in precautionary principles when it comes to conservation, so it seems prudent to err on the side of caution and avoid anything that could lead to a decline in monarch health.
The availability of tropical milkweed (Asclepias curassavica) on the landscape in the U.S. is likely increasing as people become more aware of monarchs and their plight and seek to plant milkweeds that are sold in local garden stores. Tropical milkweed itself is not “bad.” (It provides larval food for monarchs in many places where it occurs naturally, such as across the Caribbean, Mexico, and Central America.) Rather, it is winter-breeding that leads to increased parasitism, and tropical milkweed planted in warm areas of the U.S. enables monarchs to winter-breed. It's important to understand the effects that increased planting of this particular milkweed might have for monarch migration. Is it going to drive monarchs to extinction? No. Could its proliferation lead to greater fall and winter-breeding and increased disease? Probably. We think that the risk is real enough and there are enough milkweed species that don't have this effect that it makes more sense to plant natives. If people want to keep planting the non-native tropical milkweed, they should understand and be comfortable with the likely consequences.
2) Isn't it normal for some monarchs to encounter milkweed and lay eggs on milkweed in the fall and winter as they move through Texas (i.e., is the problem really new)?
While native milkweed plants are sometimes green and available during the fall as monarchs are moving through Texas, this generally only happens in years with significant rainfall during the late summer and early fall, as documented in a study by Reba Batalden and Karen Oberhauser, coming out in the upcoming monarch book (Cornell University Press, 2015). And the native milkweed generally does not stick around all winter. Reba also showed that the vast majority of sites that reported monarchs during the winter in Texas (through the Monarch Larva Monitoring Project) had tropical milkweed, and that monarch eggs were much more likely to be found on tropical milkweed. So, it is quite clear, as the authors of the recent study (Satterfield et al. 2015) point out, that tropical milkweed frequently supports monarchs breeding during the winter, whereas native milkweeds do so only in extremely rare circumstances.
3) I read that tropical milkweed is the ‘medicinal milkweed’ that helps monarchs when they are infected with OE. Does that mean tropical milkweed can keep my monarchs from becoming infected?
Many people have cited other recent work from Jaap De Roode's lab at Emory University showing that tropical milkweed can have a medicinal effect on monarchs infected with OE, and that infected female monarchs seek out highly toxic milkweed like tropical milkweed to lay their eggs. This is interesting and important work. However, it cannot be emphasized enough that tropical milkweed does not 'cure' monarchs of infection. If this were true, we would not see such high levels of infection in monarchs sampled in the winter-breeding tropical milkweed patches in the wild. In some of these patches, every single monarch was heavily infected. Tropical milkweed, like other toxic milkweed species, reduces disease severity (spore load) in infected monarchs – sometimes by half – and thus allows infected monarchs to live longer. But living longer can give infected monarchs more time to spread parasites. In other words, feeding on toxic milkweeds is beneficial to individual infected monarchs because they have a better chance of surviving long enough to mate and lay eggs; but if they do reproduce, their offspring will also become infected. In this way, tropical milkweed could lead to high levels of infection in the wild. This is somewhat like parents giving a child Tylenol and sending her to school when she wakes up feeling ill, resulting in the transmission of disease to her classmates.
4) What about tropical milkweed leads to high infections in winter-breeding monarchs?
The problem is not with tropical milkweed per se, but is rather with the winter-breeding behaviors that it enables. Tropical milkweed that grows year-round prolongs monarch breeding. In warm parts of the country, if tropical milkweed persists long enough so that multiple generations of monarchs can lay eggs on the same plants, this results in the build-up of OE spores on the milkweed leaves and the transmission of parasites to caterpillars. OE spores deposited by infected monarchs are known to persist on surfaces for a long time – several months or longer – unless they are exposed to harsh chemicals or extreme temperatures. The situation is different for migratory monarchs: When monarchs leave for Mexico in the fall and milkweed plants die back in the winter, this allows the monarchs to come back to ‘clean’ habitats in the spring, because the parasites die during the monarch’s long absence and the new growth of milkweed is parasite-free.
5) Could the milkweed be disrupting the migration by pulling monarchs out of reproductive diapause (mentioned but not tested directly in the Satterfield et al. 2015 study)? We can’t be sure of this. It is possible that tropical milkweed can disrupt migration for some monarchs, but this has not been tested in the wild. One previous study (Batalden and Oberhauser, 2015, to be published in forthcoming book by Cornell University press) using outdoor cages suggested that a small fraction of fall migratory monarchs will break diapause when they encounter milkweed (of various species) in good condition. There are also a few anecdotal accounts of monarchs arriving at tropical milkweed locations in the fall and remaining there or reproducing. In general, though, we have not gathered enough data to be conclusive about this question. So it remains unknown where the adult monarchs found at winter-breeding sites originate from. Some people think these monarchs simply come from local year-round tropical milkweed sites, and the monarchs remain in these areas generation after generation. Other observers think the monarchs found at winter-breeding sites were once fall migrants (or the offspring of fall migrants) that halted migration once they encountered tropical milkweed.
There is evidence suggesting that there is more winter-breeding in the southern U.S. now than in the past. If this is the case, we don't really know the impacts on the population. Our sense is that the precautionary principle should apply; we should not take the risk of promoting a behavior whose consequences for the population are unknown, especially when we have better alternatives such as planting one of the dozens of native milkweed species.
6) Are gardeners to blame?
No. Gardeners who have planted tropical milkweed are not at fault, despite what recent newspaper headlines proclaim. Rather, this problem stems from the fact that – until very recently – there was little to no scientific knowledge about the effects of year-round milkweed availability (which we now better understand). Another challenge for gardeners is the limited supply of native milkweeds available for purchase in some parts of the country. Gardeners have been helping – not hurting – monarchs by planting milkweeds and nectar flowers. And many gardeners and monarch enthusiasts have collected crucial scientific data on monarchs for this and other studies. Thanks to these citizen science efforts, we now know that native milkweeds support healthier monarchs in the southern U.S. Gardeners have been and continue to be imperative to monarch conservation. We depend on their help.
7) What can gardeners do to support healthy monarchs?
Now that we understand that monarchs are healthier when milkweed is seasonal and not year-round, we can work to improve monarch habitat. Below are recommendations for gardeners interested in reducing monarch disease and non-migratory behaviors:
8) Where can I learn more information?
Xerces’ Milkweed Seed Finder: http://www.xerces.org/milkweed-seed-finder/
Monarch Watch Milkweed Market: http://monarchwatch.org/milkweed/market/
Milkweed: A Conservation Practitioner’s Guide (Xerces): http://www.xerces.org/milkweeds-a-conservation-practitioners-guide/
Tropical milkweed fact sheet (Monarch Joint Venture): https://monarchjointventure.org/images/uploads/documents/Oe_fact_sheet.pdf
Native milkweed fact sheet (Monarch Joint Venture) (*examples of native milkweed by region): https://monarchjointventure.org/images/uploads/documents/MilkweedFactSheetFINAL.pdf