December 16, 2017

Milkweed Adaptation

Contact: Dr. Emily Mohl, St. Olaf College,

Initiated: 2015

Investigating clines and local adaptation in common milkweed.

Part 1 As climate changes, plants may either shift their ranges or adapt to avoid extinction. In either case, plant responses to climate change may impact their interactions with herbivores. Previous research (Woods et al. 2012) observed that northern milkweed populations emerge earlier, grow more slowly and are more resistant to herbivory, even though herbivore diversity and damage was greatest in the central part of the range. They hypothesized that northern plants have the lowest ability to tolerate herbivory, favoring higher resistance because herbivory has a greater impact on plant fitness. Participating schools will contribute seeds that will allow us to test this hypothesis at St. Olaf College. They may also gather data on the extent and types of milkweed herbivory at their site in spring and fall. Together, these data will contribute to our understanding of the way shifts in climate-related growth traits might also alter plant-herbivore interactions.

Part 2 As the public has become aware of declines in monarch populations, more people are planting milkweed, a critical resource for this specialist butterfly. Participating schools will plant seeds from local and non-local populations and gather data annually in spring and fall to determine whether milkweed populations are locally adapted, meaning it would be beneficial to plant locally sourced milkweed genotypes, or whether there are certain superior milkweed genotypes that perform better in all locations. Note that concerns about introducing non-local genotypes will be addressed in protocols; however, preventing pollen dispersal may be challenging.

I am interested in collaborating with a small group of motivated colleagues to develop protocols and analysis plans. I aim to test the hypotheses below. Tests of the first two hypotheses are related to Part 1 and should lead directly to a publication about gradients in tolerance. Based on initial responses, I intend to investigate the potential for grant funding to support participation in Part 2 of the project, which should ultimately lead to a publication about milkweed local adaptation. I plan to build some optional modularity into Part 2, which will allow for other participants to take the lead on testing hypotheses that lead to additional publications.

This study is designed to test the following hypotheses, using Asclepias syriaca as a focal sepcies.

1. There is a latitudinal cline in tolerance to herbivory that is correlated with growth patterns. Alternative hypotheses predict opposite correlations, which we aim to test. Based on previous research, northern populations are expected to be slower growing. Potentially, northern populations are also less tolerant of herbivory because loss of tissue is more costly, or because herbivory is less common so there has been less selection for tolerance. Alternatively, northern populations could be more tolerant of above-ground herbivory because they store more resources in roots that can be reallocated to shoot growth or because their phenology allows for greater growth prior to herbivory.

2. Alternatively, tolerance to herbivory is correlated with rates of herbivory, especially by specialist herbivores against which defenses may be ineffective. If herbivory is most intense in the range center and declines toward the margins, then there may not be a latitudinal cline in tolerance.

3. Milkweed plants are locally adapted to their region of origin. By measuring performance of milkweed genotypes in sympatric and allopatric habitats, we will be able to assess whether milkweed plants are locally adapted or whether some genotypes regularly outperform the rest (see Blanquart et al. 2013). There is potential to test additional hypotheses through optional manipulations of other variables (water, herbivory, nutrients, etc.). Finally, we can test whether herbivory varies on sympatric and allopatric plant genotypes, and there may be potential to devise tests of the hypothesis that sympatric herbivores outperform allopatric.


Milkweed Seed Collection Protocol

Data Collection:
Milkweed Adaptation Sample Data Sheet

Participating schools can choose to participate in one or both parts of the study by simply providing milkweed seeds or by transplanting milkweeds as well. Being able to locally collect seeds from native Asclepias syriaca plants is a requirement. Collections from the north central region of the United States are particularly desirable. Protocols for measuring plant growth traits and assessing herbivory in spring and fall will be developed to use with students. Plants will be censused for invertebrates like milkweed bugs, weevils, caterpillars, aphids and snails, and herbivore damage will be recorded. Part 2 offers the greatest potential for hands-on education research by students, and may be particularly useful as a way to incorporate evolutionary ecology labs into the curriculum. Tentatively, each participating school will plant a minimum of about 16 plants-a mix of local and randomly assigned non-local genotypes. Participating sites may decide to plant replicate blocks that could be experimentally manipulated (eg. add water, fertilization, pesticide).

Because we are interested in local adaptation, it is preferable to collect seed from naturally occurring milkweed populations rather than planted populations. For Part 2, it will be necessary to transplant milkweed seedlings into a common area. Ideally, seedlings would be transplanted into the original collection site, but any local site with similar conditions will provide some useful information. The original timeline (collecting seeds in fall 2015, transplanting in summer 2017, and data collection thereafter) has been changed.  There will be an expanded pilot study this year: participants will collect seeds this fall (2016), send them to St. Olaf, and receive a group of seeds to plant in the spring, utilizing revised protocols.  Data can be collected at the sites in the fall of 2017, but the entire process will need to be repeated again during 2017-2018.   The official experiment has been delayed to 2017-2018.

View the Milkweed Adaptation Project Participants - [Updated October 2015]