Distribution and phenology of monarch butterfly larvae and their milkweed hosts in the South Central US

Abstract

South Central US milkweeds (Asclepias) are critical adult nectar and larval food resources for producing the rst spring and last summer/fall generations of declining eastern migratory monarch butter ies (Danaus plexippus). This study addresses multiple gaps in assessment of monarch conservation priorities for the South Central US through analyses of monarch larval host selectivity, phenology, and spatial density, as well as the phenology, niche modeled distribution, and land cover selectivity of important milkweed hosts. Results are synthesized to estimate seasonal milkweed resource areas. About 70% of monarch larval activity occurred from mid-March to mid-July (early season) and 30% from mid-August to late November (late season). Twenty-six wild milkweed (Apocynaceae) hosts were mapped, including four new records for North America. Important hosts included Asclepias a. ssp. capricornu, A. viridis, and A. oenotheroides, that were utilized more frequently during early season, and Asclepias latifolia, utilized more frequently during late season. Landscape host selectivity was positive for A. viridis and A. a. ssp. capricornu in late and early seasons, respectively, and negative for A. oenotheroides in late season. Milkweed land cover selectivity was positive for Developed-Open Space and Grassland Herbaceous, and negative for Cultivated Crops and Shrub/Scrub. Seasonal milkweed resource areas and larval spatial densities resolved interior and coastal corridors providing functional connectivity for monarch spring and fall migrations. A potential gap in milkweed land cover bene t was identi ed in South Texas. The novel merging of milkweed niche models with larval phenology, host selectivity, milkweed phenology, and land cover selectivity informs conservation assessment.

Changes in landscape and climate in Mexico and Texas reveal small effects on migratory habitat of monarch butterflies (Danaus plexippus)

“…This extent generally agrees with the study extent in Texas used by Ref. 7 and their modelled kernel densities of monarch larvae. We combined the municipalities in Mexico and counties in Texas for a final study extent (Fig.…”

“…For example, central Texas counties dominated by urbanization and eastern Texas showed larger declines in nectar and milkweed. These same regions may have the highest spring suitability for adults 3 and the urban counties around San Antonio-Austin and Dallas-Fort Worth were most utilized by caterpillars 7 . More formal analysis of these patterns is needed, but this concordance suggests strategies to enhance habitat in urban areas 44 , 60 and rights of ways in central Texas may help counter the conversion of rangelands and shrublands to urban in these areas.…”

“…During fall migration, monarchs utilize nectar resources in this region to sustain their migration and build fat reserves necessary to survive the winter in central Mexico 5 , 6 . In the spring, monarchs utilize nectar sources to migrate north through Mexico and for mating and laying eggs on milkweed in Texas 7 , and possibly some regions of northern Mexico 4 , 8 .

Figure 1 Study extent in Mexico (with municipalities) and Texas (with counties) created using ArcGIS Pro 3.2.1, Copyright © 1995–2023 Esri ( https://www.esri.com/en-us/arcgis/products/arcgis-pro/overview ).

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“…Land-use land-cover (LULC) studies describe how land cover, and land use by humans, changes through time and the causes/consequences of those changes. LULC analyses have tracked the status of oyamel fir ( Abies religiosa ) forests supporting monarchs in Mexico 38 – 42 , identified restoration potential of monarch habitat in the US 43 – 45 , modelled monarch and milkweed distributions 7 , 46 , 47 , migratory connectivity 48 and threats during migration 49 . Analyses of how LULC may impact monarch habitat in Texas and Mexico, have not, to date, occurred, despite the important role this region plays in the eastern population’s life cycle.…”

Where the toad crosses the road: Multi-method Texas herpetofauna roadkill modeling for conservation planning

“…The AUC bgp is less than (as much as 8%) or equal to AUC psa , since AUC psa has lower false negative error rates as a result of absence points being represented solely by pseudoabsence points that are buffered a certain distance (2 km in this study) from presence points. In contrast, AUC bgp uses background points as absence points and includes presence points, which thus increases the false negative error rate (see Supplement in Tracy et al 2022). We utilized the following general categorization of AUC values to describe the performance of our nal feature selected models: ≤ 0.5 = none; 0.5 < AUC < 0.7 = poor; 0.7 ≤ AUC < 0.8 = acceptable; 0.8 ≤ AUC < 0.9 = excellent; 0.9 ≤ AUC = outstanding (Hosmer et al 2013).…”