Peak Monarch 2014

Monarch sharing a pitcher sage plant with a bumble bee

This week we're hitting "peak monarch" in Fort Worth. According to Monarch Watch estimates, tomorrow is the midpoint of the height of monarch butterfly abundance for Fort Worth's fall migration [1]. I'd actually noticed the up swell in the population over the past week, especially after Thursday's passing storm. Every time I looked up I'd see at least one monarch flapping hard to the south, sometimes as many as three. It makes sense to me that they would follow the fronts. They do, after all, ride the wind. The last of this year's monarchs should finish passing through by November.

Monarch Fall Migration, from USFS pollinator site [2]

Texas is the funnel through which many migrating monarchs pass on the way to their Mexican wintering grounds. While most people are aware that monarchs migrate south to avoid our North American winters, I still find the flip side of this equation shocking: they must recolonize their entire range every year. Instead of overwintering in the litter and duff, brave monarch parents must fly the length of a continent.

Unlike the Methuselah-monarchs bent on the southern horizon in the fall, the spring migration requires multiple lifetimes [2]. They drift northward over 3 or 4 generations.  It appears to be these shorter lived summer monarchs that are the sink of the decreasing monarch brood. For years people blamed declining populations on habitat loss in the monarch wintering roosts in Mexican forests. Recent studies suggest that it is actually the Midwest's ultra-efficient weed control in the summer breeding grounds that is at the root of the decline. Monarchs simply can't find enough milkweed on which to lay their eggs.

Asclepias asperula, antelope horns/spider milkweed

In addition to being the last stop on their way south, Texas is their first stop on the way north to summer pastures. Increasing the abundance of milkweeds in Texas could be hugely beneficial to these amazing creatures. There are over 30 species of milkweed which can be found in Texas [5]. You can find range maps of the milkweeds (Asclepias spp) at the BONAP (The Biota of North America Program North American Vascular Flora) website [3]. In my neighborhood, I tend to see Asclepias asperula, A. viridiflora, and A. viridis. All three species are documented larval hosts to monarchs. 'Documented larval host' means monarch caterpillars have been officially observed and recorded feeding on these plants [4, see appendix for list of milkweeds]. Milkweeds you may find in north central Texas include [3]:

A. amplexicaulis*
A. arenaria
A. asperula*
A. engelmanniana
A.incarnata*
A. latifolia
A. linearis (likely endemic to Texas)
A. longifolia*
A. obovata
A. oenotheroides*
A. perennis*
A. stenophylla
A. subverticillata*
A. texana (endemic to Texas)
A. tomentosa*
A. tuberosa*
A. variegata*
A. verticillata*
A. viridiflora*
A. viridis*

* denotes documented larval host to monarch caterpillars [4]

It is possible that some of the milkweeds that have not been documented as larval host plants have successfully nourished monarch brood. Some, like the Texas endemic Asclepias texana, are relatively uncommon and are less likely to be observed, period. I'm a bit of a plant collector so I'm going to try to get my hands on some native milkweed seed to sow this winter. In the spring, after receiving the cold/moist stratification they require in my fallow garden, these seeds should emerge and start my monarch buffet. Eventually my plants will provide high-quality nectar to monarch butterflies on their way north, and leafy lunches to little monarch caterpillars beefing up for their transformation and long journey.

As an FYI to any North Texas readers: this week is the Fort Worth Botanic Garden's fall plant sale. I have not confirmed that they will be selling milkweed, but considering their focus on native plants and insects I think it is fairly likely. The plant sale starts the evening of Friday, October 9th and goes through October 11th. You can find more information here.

Good bye monarchs, at least until March next year!

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1. http://www.monarchwatch.org/tagmig/peak.html
2. http://www.fs.fed.us/wildflowers/pollinators/Monarch_Butterfly/migration/index.shtml
3. http://bonap.net/NAPA/TaxonMaps/Genus/County/Asclepias
4. http://www.xerces.org/milkweeds-a-conservation-practitioners-guide/
5. http://npsot.org/wp/story/2012/2235/