Monarch Over-Wintering Count
Monarchs of St Thomas Population Study

Last Updated: June 26, 2021

This study's initial goal was to gather data to ultimately determine if the Puerto Rican monarch butterfly population and range is expanding since Hurricanes Irma and Maria ravaged our islands.

Data collection started the first of January 2019 and due to the amount of positive data that had been collected, along with the negative effects COVID-19 restrictions were having on the study, we at Black Pearl Ecological decided to continue the study as part of our Regenerative Agroforestry research project.

This will allow us to record what effect, if any, our agroforestry project has upon the number of monarch butterflies utilizing our enhanced mini-ecosystem. The data will also be combined with the data we are collecting about the other butterflies found on our site to aid in determining the future direction of our butterfly gardens and agroforestry project.

We are also planning to study in more detail another phenomenon regarding the monarch population trends. Based on the initial observations in 2019, it appears that during the dry months when the milkweed plants die back the time spent in the chrysalis increases dramatically. After observing high volume of mating activity in April, the next generation of monarchs only appeared to start emerging during September when the wet season is beginning to peak (average rainfall > 5").

You can see how our study is progressing by viewing our 2019, 2020 and 2021 Summary Documents.

The project is directed by staff members who have participated in the

  • Capture, tagging and release of monarch butterflies
  • "Northern Gulf Coast Monarch Over-Wintering Count,” and
  • The herpetofaunal research project in Bald Point State Park, Florida.

Common Caribbean Milkweed

tropical milkweed giant milkweed caribbean milkweed


The Puerto Rican monarch butterfly (Danaus plexippus portorricensis) was identified in 1941 as a separate subspecies. It has also been found in Cuba, the Cayman Islands, St. Lucia and Jamaica.

The adult Puerto Rican monarch grows up to 40 millimeters in size, and there are few differences to distinguish between the sexes. Like the monarch, the Puerto Rican monarch is particular about its host plants.

Milkweed plants are crucial to the monarchs’ life cycle. It is the only plant on which they lay their eggs. It provides food for monarch larvae and adds a touch of poison which makes their colors bright as a warning to birds, that they are dangerous to eat. During the first two weeks of their life cycle, they will consume about 30 leaves before they transform into jade green chrysalises, eventually emerging as butterflies.

The Puerto Rican monarch’s primary host plants Asclepias curassavica (also known as tropical or red milkweed), which is native to North and South America, and giant milkweed (Calotropis procera), which is native to Europe. Caterpillars need to feed on milkweed to complete their life cycle, and adult butterflies need the right nectar-producing plants in bloom for needed energy.

Monarchs of St. Thomas population study being conducted by Black Pearl Ecological founder and Master Wildlife Conservationist Roy DuVerger

References, United States Department of Agriculture, Not All Monarchs Migrate! The Puerto Rican Subspecies Stays Put, Julie Wright, Caribbean Area Acting Public Affairs Specialist on June 23, 2016,

Population Study Summary

2022:   0 sightings    Condition: n/a    Location: St. Thomas

As we monitored the effects the road side work to upgrade the power grid is having on the butterfly population, we are sad to say that although the flowers of various types have returned in the disturbed areas, we have no sightings of any type of butterfly or moth in those areas or in our gardens thus far.

2021:   5 sightings    Condition: Good    Location: St. Thomas

This has been a very poor year with no monarch sightings recorded since February at our site. Our travels during this time period across the island did not result in any sightings either.

What we have seen is that the work being done to upgrade the power system here on the island has had dramatic effects on the roadside plant population where most of the tropical milkweed can be found.

True to form, come April the butterfly sightings start reducing to no monarchs and very few of anything else. This lack of activity seems to back up the theory that the butterflies that live here have evolved to a point where they stay in the pupal state longer, only starting to emerge in late August to early September when weather conditions improve and hurricane season is ending.

At our research site we had two females visit and lay eggs in our limited milkweed garden this spring. A great sign for the future of the species in our area.

2020:   8 sightings    Condition: Good    Location: St. Thomas

The effort we have been putting into our butterfly garden paid off as during the last quarter of the year we had several monarch butterfles here feeding for an average of three days, sleeping at night in a dense tree close the the garden. The fact they spent three days here feeding may also indicate that milkweed is getting harder to find due to negative changes in the local habitat.

Due to the COVID-19 outbreak and the restrictions that have been put in place for our protection, Black Pearl Ecological is proud to have assumed responsibility for the Monarch butterfly study being conducted in the Virgin Islands.

This study fits in with our Agroforestry project perfectly as part of our ecological landscape design includes a butterfly garden.

Ferret Fred Productions video team will be producing all of the educational video and photography for this and all of our other projects.

2019:   187 sightings    Condition: Good    Location: St. Thomas

February was the month with the highest count of 97 monarchs, with the number dropping off quickly through April. Our next sightings came in September which supported the theory that the butterflies here in the isands have evolved to spending more time in their chrysalis during the dry season.

At our research site we had one female visit and lay eggs in our limited milkweed garden. Three of the eggs managed to avoid being eaten allowing us to raise and release two male and one female monarchs, all in excellent condition.

Study Area Conservation Strategies

66% of the forests on St. Thomas are classified as sub-tropical dry forest. It is on the northside of the island within these sub-tropical dry forests that are designated Rural Forest that a majority of the monarch butterfly and tropical milkweed plant sightings occur. While areas of sub-tropical dry forest designated as either Urban or Urban Forest and those that are south facing have virtually no sightings.

Projected climate change effects for our island show the east end converting to an arid steppe, while the sub-tropical dry forest will be converting into a tropical savanna. How these changes are going to effect our monarch population and the milkweed they depend on is unknown at this time.

We will be monitoring the changes and effects closely throughout this study and report our findings on this web page.

 References, U. S. Virgin Islands Forest Resources Assessment and Strategies, June 2010

Dry Season

The driest months in St. Thomas are February and March, with less than 1.5" average. This lack of rain was felt starting in April, as the number of monarch sightings dropped off in direct relationship to the die back of the milkweed plants. It wasn't until August, the month after the start of the rainy season (August, 3.7"average rainfall) that monarch sightings began to rebound. This led us to theorize that these monarchs had evolved to stay in the pupal state for longer periods and only emerge when the weather conditions improved.

In researching our theory we have found references that some of the butterflies that are local to the Virgin Islands have indeed developed the ability to wait until the heavy rains return before emerging. We will be continuing to research our theory over the course of this study.

 St. John Tradewinds News, 2015, Butterfly Bloom,

Predators monarch_predator

Our observations have shown that Geckos make up the biggest predator problem for the monarch larva. Their ability to climb about the milkweed leaves the larvae vulnerable and the milkweed toxin appears to have no effect on them.

Jack Spaniard wasps (Hymenoptera: Vespidae) are seen regularly walking about the milkweed plants. Since quite a few of the monarch eggs go missing almost within a day of being laid we are assuming they are the culprit.

However at this time we have noticed that the milkweed plants found in the middle of a field amongst tall grasses appears to have a higher larvae survival rate. We are assuming it is because it is harder to find and the geckos appear to like more open areas rather than pushing their way through the dense grass.

We witnessed what seems to be a never before documented rivalry between the monarch and gulf fritillary butterflies. As seen in the video below, no sooner had the monarch arrived it was attacked by two gulf fritillaries intent on driving it out of the area, which they eventually succeed doing.

We are developing a plan to study the predator issue more closely as the project continues.

Parasite Control

So far in our study we have found no deformed monarch butterflies and have observed no chrysalis with brown spots indicating the parasite has not infected the local population at this time. That is not to say it does not exist here, we just have not observed any sign of it.

There are a couple of factors that could account for this. Most of the milkweed plants we have located are growing along the sides of the roads and in other small open areas that are mowed once or twice a year. The other factor is that we have a dry season and the tropical milkweed dies back in much the same manner as native milkweed dies back during the winter on the US mainland.

Recent research by Jaap de Roode’s lab at Emory University, confirmed that butterflies on the mainland that are infected by OE tend to prefer the tropical milkweed even more than usual. However the tropical milkweed has been found to be “medicinal” in terms of improving parasite resistance suggesting that they are self medicating their offspring. In the final analysis they determined the effect is certainly real.

We are theorizing that since tropical milkweed is the most common form of milkweed available for the larve to eat here on the island, their resistance to OE must be higher than on the mainland, that is why we have not detected it here.

We will continue with our plan to collect data related to parasite sightings, if they exist.

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