Partnering to conserve the monarch butterfly migration

Assessment of Exotic Milkweeds and the Spread of Disease in Monarchs

Monarch Joint Venture has partnered with Monarch Alert  and Monarch Health in a collaborative effort to examine how the presence of tropical milkweeds (Asclepias curassavica) and year-round monarch breeding activity affect the prevalence and transmission of OE (Ophryocystis elektroscirrha), and to evaluate the potential effectiveness of cutting back tropical milkweeds during the winter months for reducing monarch infections.

Successes of the previous years of research include expanding and maintaining the Monarch Health: Southern Initiative project through recruiting new volunteers to non-destructively test adult monarchs for OE in the southern Atlantic and Gulf areas during the fall, winter and spring months at sites with and without tropical milkweed. They have also launched a new Monarch Health: Western Initiative program recruiting volunteers to test adult monarchs for OE at sites in coastal California with and without tropical milkweed and in close proximity to overwintering colonies. The program performed intensive monthly field monitoring in tropical milkweed patches at 5 sites within each region (FL-TX and CA) and conducted a field experiment to test whether cutting back milkweeds at periodic intervals across a subset of monitoring sites will help lower OE transmission and prevalence at those sites and in wild monarchs.

The proposed management strategy to reduce the accumulation of OE parasite has been to cut back the tropical milkweed, but this has not been previously tested. In October 2015, all tropical milkweed plants at the Wormsloe gardens in Savannah, GA were cut back to examine whether this simple management technique would decrease the number of infected adults over time. A nearby tropical milkweed garden (Pollinator Berm at the Landings community, ~3 miles away) hosting a large monarch population with 75% of adults harboring the OE parasite, acted as a control and the 50 plants at this site were left uncut. During the following months, only two monarchs remained at the Wormsloe gardens, while more than 50 monarchs at Pollinator Berm continued to reproduce and lay eggs. In December, researchers found and collected caterpillars from the regrowth at the Wormsloe gardens. None of these caterpillars (n=7) were infected with OE, suggesting that cutting back tropical milkweed has the potential to reduce disease risk for monarchs, however additional sampling during regrowth events is necessary to make definitive conclusions. Unfortunately, an unexpected freeze in mid-January 2016 killed nearly all monarchs and caused damage to foliage of tropical milkweed plants at the Pollinator Berm, ending the experiment. Photos (right) of defoliated tropical milkweed plants and remaining caterpillar after January 2016 storm by citizen scientist Shirley Brown. 

To examine effects of tropical milkweed on monarchs, researchers followed a monarch population in six experimental butterfly gardens at the Wormsloe Historic Site in Savannah, Georgia, USA. The gardens contained a total of 90 tropical milkweed plants. From May to October 2015, the number of adults, eggs, and larvae on each plant were recorded and tested monarchs for infection with the OE parasite. Extremely high numbers of all life stages were found. For example, one week over 1200 eggs were observed in the gardens! Tropical milkweed plants in the gardens had significantly more monarch eggs per plant than MLMP citizen scientists are reporting in other parts of the country (eg. Stenoien et al. 2015). While numbers were high, survival (from egg to fifth instar) was on average ~9%. In addition, OE parasite infection quickly increased from 0 % in May to 100% in October for both adults and caterpillars. This is a strong contrast to OE levels in natural settings, where no more than 15% of monarchs are infected (Satterfield et al. 2015). An experiment showed that because monarchs continually lay so many eggs on the tropical milkweed plants, OE spores accumulated on the plants which then resulted in very high percentages of monarchs infected with OE. Graph (right): Adult Abundance and proportion of infected adults over time. Spores accumulated on plants over time as shown by proportion of plants that tested positive for presence of OE spores. 

Monarch Health and Monarch Alert will continue this work, directly examining the role of tropical milkweed on monarch migratory behavior and parasite dynamics to better inform management strategies. The research will produce scientifically-informed recommendations for the gardening community on how to better manage tropical milkweed to minimize parasite transmission. The project will create best gardening practices for conserving this iconic species through a short educational video aimed at the community of nature enthusiasts, gardeners and plant growers seeking information about how to effectively and safely manage tropical milkweed for monarchs. 

Please see the Monarch Joint Venture Fact Sheet, our Q&A with monarch and tropical milkweed experts and a recent scientific study (Satterfield et al., 2015) for more information about the potential risks of growing exotic milkweed for monarchs.

References

Satterfield DA, Maerz JC, Altizer S. 2015 Loss of migratory behaviour increases infection risk for a butterfly host. Proc. R. Soc. B 282: 20141734. http://dx.doi.org/10.1098/rspb.2014.1734

Stenoien, C., Nail, K.R., Oberhauser, K.S. (2015). Habitat Productivity and Temporal Patterns of Monarch Butterfly Egg Densities in the Eastern United States. Annals of the Entomological Society of America, 108 (5) 670-679; DOI: http://dx.doi.org/10.1093/aesa/sav054

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