Geology topics

Emma Pelton

Publications and source records attributed to Emma Pelton.

2 recordsLinked to original sources

USGS2026

Density dependence and habitat selection affect overwintering abundance of monarch butterflies at regional and site scales in California

The monarch butterfly ( Danaus plexippus ) is a species of iconic cultural interest. Thanks to annual overwintering monarch counts at hundreds of locations in coastal California, we are able to track fluctuations with high temporal and spatial resolution. Between 1997 and 2024, monarch populations at overwintering sites in the western United States experienced severe dips, at times (2018–2020, 2023–2024) giving the appearance of a population collapse. From 2018 to present, the Pismo State Beach Overwintering Monarch Grove has conducted multiple counts during overwintering and geolocated counts of individual monarch clusters to specific trees within the site. This study determined how annual monarch population variability is influenced by both climate and prior year population density at the state, region, and overwintering-site scale. Furthermore, through a machine-learning process, we investigated how overwintering site configuration and structure drive monarch winter space-use dynamics within the Pismo Beach site. Our approach found monarchs exhibit a preference for specific overwintering sites in California, and that 64% of annual variability of counts across sites can be explained by climate and density dependence, with density dependence explaining 50% of total variability. Within the site we found very little regional climate effect, but individual trees, tree size, distance to boundary, and the amount of shade were all strong indicators of monarch presence. Additionally, only 11 out of 320 trees at the Pismo Beach site accounted for 83.6% of all counts over 6 years, highlighting how monarchs use specific trees and how tree structure may create preferred microclimates for clustering.

California↗

USGS2025

Pyrethroid insecticides implicated in mass mortality of monarch butterflies at an overwintering site in California

Since the 1980s, monarch butterfly ( Danaus plexippus plexippus ) populations across North America have declined by 80–95%. Although several studies have implicated pesticides as a contributing factor to their population declines, our understanding of monarch exposure levels in nature remains limited. In January 2024, a mass mortality event near an overwintering site in Pacific Grove, California, USA, provided an opportunity to analyze dead overwintering monarch butterflies for pesticide residues. Ten recently deceased butterflies were collected and analyzed using liquid and gas chromatography with tandem mass spectrometry (LC-MS/MS and GC-MS/MS). We identified a total of 15 pesticides and associated metabolites in the butterflies, including 8 insecticides (plus 1 associated metabolite), 2 herbicides (plus 2 associated metabolites), and 2 fungicides. On average, each monarch butterfly contained 7 pesticides, excluding transformation products if the parent compound was also detected. Notably, three pyrethroid insecticides—bifenthrin, cypermethrin, and permethrin—were consistently detected at or near each chemical’s lethal dose (LD 50 ). Bifenthrin and cypermethrin were found in every sample, while permethrin was present in all but two samples. The average concentrations of these insecticides were 451.9 ng/g dry weight (dw) for bifenthrin, 646.9 ng/g dw for cypermethrin, and 337.1 ng/g dw for permethrin. These findings demonstrate pesticide contamination in monarch butterflies, including within urban areas, and highlight the risks pesticides, especially insecticides, pose to monarch populations. Additional measures may be required to safeguard this species from pesticide exposure, particularly near aggregation locations, such as overwintering sites in coastal California.

California↗