POTENTIAL SIGNIFICANCE OF FALL BREEDING OF THE MONARCH BUTTERFLY (DANAUS PLEXIPPUS) IN THE ROLLING PLAINS ECOREGION OF WEST TEXAS

The monarch butterfly (Danaus plexippus) is one of the most recognizable North American insects, well known for an annual migration that spans multiple generations and ranges from Mexico to southern Canada (Urquhart & Urquhart 1978; Brower & Malcolm 1991). Unfortunately, this migration is imperiled, as monarchs have been experiencing a substantial population decline due to habitat loss throughout their range (Brower et al. 2012; Flockhart et al. 2015). In particular, the loss of breeding habitat within the United States has been cited as a primary contributor to monarch declines (Pleasants & Oberhauser 2013). This has led to efforts to protect and restore the milkweed (Asclepias spp.) that are necessary for monarch reproduction (Luna & Dumroese 2013). While such initiatives are essential for safeguarding monarch populations, the wide spatial and temporal scales that encompass the monarch migration present obstacles to identifying conservation priorities and developing effective management practices (Flockhart et al. 2013; Flockhart et al. 2015; Oberhauser et al. 2017). As such, conservation planning for monarchs requires a comprehensive understanding of the seasonal population dynamics and migratory patterns of the species (Flockhart et al. 2013; Flockhart et al. 2015; Oberhauser et al. 2017).

To date, much of the focus of monarch conservation in the United States has centered on summer breeding grounds in the Midwestern states. Milkweed colonies in the Midwest have been emphasized as vital repopulation areas for monarchs (Wassenaar & Hobson 1998), with researchers calling for extensive habitat restoration in this region (Pleasants 2017; Thogmartin et al. 2017). However, some studies have advocated the importance of milkweed and nectar plant colonies in the southern portion of the monarch migratory range. Texas has been cited as a strategic bottleneck location along the migratory route, with both the spring and fall migrations of monarchs passing through the Texas corridor (Calvert & Wagner 1999). Studies have shown that many first-generation monarchs hatch in southern states, such as Texas, during the spring (Malcolm et al. 1993; Miller et al. 2012; Flockhart et al. 2013), and these individuals are largely responsible for the recolonization of summer breeding grounds (Flockhart et al. 2013). Texas is also a crucial stopover area for monarchs during the fall migration, as nectar plants in this region are a key source of energy for overwintering butterflies (Brower et al. 2006). Because of this, restoration of habitat in the Texas corridor is likely to be a strong contributor to monarch conservation efforts.

In order to bolster monarch habitat within Texas, the Wildlife Toxicology Laboratory at Texas Tech University has been evaluating habitat restoration within the Rolling Plains ecoregion of West Texas. During this time, monarchs have been observed across the lab's study areas in Stonewall and Fisher counties. In the fall of 2017, monarch reproduction was noted in both counties, with monarch eggs, larvae, and breeding adults observed throughout September and October. On 13 September 2017, a fourth instar monarch caterpillar was photographed on indigenous zizotes milkweed (Asclepias oenotheroides) in Stonewall County (Fig. 1A). Then on 4 October 2017, fifth instar monarch caterpillars were photographed on green milkweed (Ascepias viridis) that were planted by lab personnel in Fisher County during the previous year (Fig. 1B). Although green milkweed is typically found in the eastern portion of Texas, the species was included in preliminary restoration efforts due to the limited availability of plants, particularly those sourced from West Texas. In additional surveys of the sites, monarch caterpillars, ranging from first to fifth instars, were documented on indigenous broadleaf milkweed (Asclepias latifolia) and zizotes milkweed on 12 and 20 October 2017.

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While monarchs are known to enter a period of reproductive diapause on their return to Mexico (Goehring & Oberhauser 2002), there are several documented cases of fall reproduction in Texas that complement these observations. Female monarchs have been shown to be reproductively active during their fall migration through northcentral and southern Texas (Borland et al. 2004), and there are records of monarch eggs and larvae in northern Texas during the fall (Prysby & Oberhauser 2004). Additionally, reports from Journey North, a citizen scientist program that engages volunteers in tracking wildlife migrations, continue to provide evidence of monarchs breeding in Texas during a “pre-migration”, as well as later into the fall (Journey North 2018). The current study's observations of fourth instars in early September and larvae in late October are consistent with these reports, and expanding monitoring efforts may be warranted in order to better assess the significance of these breeding periods. As diapause in monarchs is induced by several factors, such as photoperiod and temperature (Goehring & Oberhauser 2002), it is likely that some monarchs will continue to reproduce in southern areas with higher fall temperatures and longer days. Consequently, fall reproduction of monarchs in areas of Texas, such as the Rolling Plains, may be significant and should be considered when developing conservation practices in the region.

Although the observations presented here are not the first documentation of fall reproduction in Texas, they expand the known range of fall reproduction in Texas, emphasizing the importance of and need to expand research and conservation efforts to the western edge of the monarch migratory corridor. This documentation of fall reproduction of monarchs in the Rolling Plains ecoregion of Texas adds to the body of evidence supporting Texas as a priority for monarch conservation. Additionally, these observations of milkweed, monarch eggs, larvae, and adults in Stonewall and Fisher counties highlight the potential of West Texas being significant to the monarch migration.

Currently, restoration initiatives in Texas focus on the central and eastern portions of the state, with less attention given to West Texas, a trend noted during attempts to find milkweed sourced from western areas of the state. These efforts seem appropriate because central and eastern Texas appear to exhibit higher concentrations of monarch sightings when compared to West Texas (Howard & Davis 2009). However, many of these estimates are based on citizen scientist reports which may over-represent monarch abundance near anthropogenic population centers (Flockhart et al. 2013), such as those concentrated in central and eastern Texas (Fig. 2A). Furthermore, during some years, citizen scientist reports from these same sources show much higher concentrations of monarch sightings in West Texas (Fig. 2B), indicating the potential of variable distribution from year to year (Journey North 2018). This may be a result of inconsistent weather patterns, a factor known to have a strong effect on monarch movement and population dynamics (Zalucki & Rochester 2004; Zipkin et al. 2012; Flockhart et al. 2013).

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As such, research investigating monarch migration and utilization throughout the entire migratory corridor is warranted. This would contribute to a more comprehensive understanding of the migratory patterns and seasonal population fluctuations of monarchs throughout Texas and would aid in the development of more effective monarch conservation and management practices in the state.