Nectar plants used by monarchs during March in Texas

by Chip Taylor, Director of Monarch Watch and Carol Clark, Monarch Conservation Specialist

Introduction

Extreme weather events can have short and long-term impacts on the flora and fauna of a region and can even have an impact on migratory species such as monarch butterflies, birds and bats by limiting the resources available as they arrive in areas affected by such events. So, it was with some concern that we viewed the winter freeze that unfolded in Texas in February. This 11-day cold spell (10-20 February 2021) proved to be a disaster. Freezing temperatures covered the state and extended well into Northern Mexico killing wildlife (TPDW, 2021, Maeckle, 2021) and freezing vegetation across the state. Our post-freeze assessment gave rise to the possibility that monarchs returning from Mexico in mid-March would find little or no milkweeds to lay eggs on or nectar to sustain them. As a result of these concerns, we initiated a project with iNaturalist, with the help of Tania Homayoun of the Texas Parks and Wildlife Department, in which we asked participants to submit sightings (images) of monarchs, nectar sources and milkweeds:

Texas Winter Storm 2021 Plant & Pollinator Survey
https://inaturalist.org/projects/texas-winter-storm-2021-plant-pollinator-survey

Description/justification for the project
https://inaturalist.org/projects/texas-winter-storm-2021-plant-pollinator-survey?tab=about

This project produced an amazing amount of data which will be summarized in several posts to the Monarch Watch Blog. In this text, we will summarize the diversity of nectar plants visited by monarchs in March of 2021 and will compare that list with the records for 2018 and 2020.

The iNaturalist records consist of photos taken of subjects of interest which, due to the time stamp on the photos and the locations submitted by the participant, provide a marker for the time and place of a species. These records can then be assembled to show the spatial and temporal distributions of these sightings. In some cases, the images provide evidence of associations between species and that’s the case with monarchs and nectar sources. The images of monarchs in March 2021 (N=539) showed a large number of them feeding on flowers, most of which could be identified. One of us, (CC), identified these plants for 2021 and then scanned the records for 2018 and 2020 to determine how the plants visited differed among the years.

Results

The results show substantial similarities and differences among years in the plants that were visited by monarchs. The diversity appears to differ, in part, due to the number of records submitted per year and the weather that influences plant growth each season. The records are summarized in Table 1 below. The records for 2021 indicated monarchs visited 69 species per 539 observations, 85 species per 647 observations in 2020 and 41 species per 168 images in 2018. The percentage of photos with nectar plants ranged from 12-13% in 2021 and 2020 to 24% in 2018. In all, monarchs visited 123 species for nectar and of these, 20 were visited all three years. Another 20 species visited in 2021 were recorded as visited in either 2018 or 2020. Interestingly, there were images of 30 species visited in 2021 that were not recorded as visited in the other two years. In contrast, there were 18 species that were not visited in 2021 but were visited in either 2018 or 2020. Ten of these were visited in both 2018 and 2020. Also of interest are the 38 species visited in 2020 but not recorded for either 2018 or 2021.

Discussion

Before getting into to the biological implications of these plant visitation records, we need to acknowledge that the process of obtaining these images has inherent biases such that they may not be fully representative of what monarchs visit. First, the very process of taking pictures involves light with most pictures taken when the photographer determines there is enough light for a photo. Then, there are the hours that both monarchs and people overlap in the field. While both monarch and human activity probably have similar peaks in early to mid-afternoon it’s possible that there is less overlap both early and late in the day that could result in missed opportunities to record monarch activities. It may also be the case that monarchs are more active than the iNaturalist participants when the temperatures are in the 60F range. Further, it’s clear that the records are human centric in that they are concentrated around major cities where access to native vegetation is more limited than in the broader countryside. Visits to trees and vines may be underrepresented as well.

In spite of these limitations, these records show that monarchs visited an amazing diversity of flowering species for nectar (N=123) during March in the three years we examined, and it’s likely the true number is even greater. Included among these plants are a large number of introduced species, including cultivars, ornamentals, weeds and even some invasive species. Natives are widely used as well, but the diversity of nectar plants available to monarchs presently seems likely to be greater than before European settlement.

Comparisons of the species visited from year-to-year lead to a number of interesting questions. First, if we compare 2021 with 2020, we can see that 69 species were visited in 2021 but of these only 38 were visited in 2020. Does this mean that monarchs were forced to visit a range of nectar sources in 2021 that were not visited in 2020 because the richer sources nectar sources available in 2020 were not available in 2021? Similarly, there were 18 species visited in 2018 and 2020, 10 of which were visited both years, that were not visited in 2021. Does this mean that those 18 species, or a least some of them, had not recovered from the freeze and were simply not present or abundant enough to become part of the record? And what about the 38 species visited in 2020 that were not visited in 2018 or 2021. Were they simply not present, or again, too scarce to be included in the 2021 record?

There are basic questions here about how each plant species is affected by either favorable or negative conditions. Several species, such as wild hyacinths, prairie celestials, crow poison, wild onions, and rain lilies, all of which have bulbs well underground, seemed to be unaffected by the freeze and bloomed on time in 2021 while other plants were delayed. Yet, some species such as the Indian hawthorns, so ubiquitous in Dallas landscaping, are just rows of dead bushes at this writing. Statewide, none of them bloomed this year.

While there is much to learn about the calories monarchs need per day to sustain flight, egg production, egg laying and mating, we also need to know what the flowers of each species provide and how monarchs “choose” to visit particular flowers. Though monarchs appear to be opportunistic feeders, monarchs are capable of learning (Rodrigues, et al 2010, Blackiston, et al 2011) and it is possible, perhaps likely, that they prioritize high-quality nectar plants in good years and try nearly anything in years when there is little to offer. 2021 represented a year when many flowers were no-shows at the time monarchs arrived, perhaps causing them to visit a number of species (N=30) not visited in other years.

These records indicate that floral resources available to migratory and reproductive monarchs can, and do, shift dramatically from year to year and this leads to two observations. First, that monarchs are very adaptable and can shift to new flower sources when needed and second, since monarchs can use a variety of nectar sources beyond those that occur naturally, humans can do much to sustain monarch populations by planting non-native as well as native nectar sources in managed landscapes. However, for those introducing non-natives into gardens, care should be taken to avoid the use of species that naturalize easily or are known to be invasive. A list of herbs and forbs considered to be invasive in parts of the United States can be found here https://www.invasiveplantatlas.org/herbs.cfm. There are also lists for shrubs and trees – https://www.invasiveplantatlas.org.

Table 1. Nectar plants used by monarchs during March in Texas. [View as Google Sheet instead]

References

Blackiston, D., A.D. Briscoe, & M.R. Weiss. 2011. Color vision and learning in the monarch butterfly, Danaus plexippus (Nymphalidae) J Exp Biol (2011) 214 (3): 509–520.

Maeckle, M., 2 March, 2021. Wildlife death toll mounts in wake of historic Texas freeze. San Antonio Report. https://sanantonioreport.org/wildlife-death-toll-mounts-in-wake-of-historic-texas-freeze/

Rodrigues, D., Goodner, B.W., & M. R. Weiss. 2010, Reversal Learning and Risk-Averse Foraging Behavior in the Monarch Butterfly, Danaus plexippus (Lepidoptera:Nymphalidae). Ethology 116 (2010) 270–280.

Severson, K., 5 March 2021. Texas Farmers Tally Up the Damage From a Winter Storm ‘Massacre’. N. Y. Times. https://www.nytimes.com/2021/03/04/dining/texas-farms-storm-damage.html

Texas Parks and Wildlife, March 10, 2021. At least 3.8 Million Fish Killed by Winter Weather on Texas Coast. https://tpwd.texas.gov/newsmedia/releases/?req=20210310c