Louie H. Yang, Lizbeth Gonzalez, Sarah Solís & Prabhjot Singh

Abstract

Plants are known to affect their soils in ways that can have positive or negative effects on themselves and other plants, often described as plant-soil feedbacks. However, less is known about the potential for herbivores to change the soil in ways that affect plant growth. Many sap-feeding insect herbivores produce a sugary secretion called honeydew. Here, we ask if seasonal pulses of accumulated aphid (Aphis nerii) honeydew reduce the availability of soil nitrogen below milkweed (Asclepias fascicularis) host plants, and how honeydew-conditioned soil affects the relative seedling growth of host plants and other, potentially competing plant species. We hypothesized that: (1) honeydew would reduce plant-available nitrogen in the soil, reflecting the expectation for increased microbial immobilization in response to high carbon inputs, and (2) reduced nitrogen availability would have a greater negative effect on the growth of cheatgrass (Bromus tectorum) and yellow star-thistle (Centaurea solstitialis) than milkweed, reflecting the expectation of greater nitrogen limitation in disturbance-adapted, non-native, annual plant species compared with native, perennial plants. We found that the addition of honeydew at accumulated levels reduced total inorganic soil nitrogen by 48.8% relative to controls. Unexpectedly, milkweed seedlings were 90% taller and had 4.2 times greater dry biomass in honeydew-conditioned soil than in the control group, but we did not observe any effects on the growth of cheatgrass (Bromus tectorum) and yellow star-thistle (Centaurea solstitialis) seedlings. These results suggest the possibility of a positive plant-herbivore-soil feedback for milkweeds, mediated by aphid honeydew.

Oecologia

https://doi.org/10.1007/s00442-026-05931-4

Ian D. Jett and Louie H. Yang

Abstract

Trapped arthropods have been shown to benefit plants in several ways, but few studies have examined the potential costs of arthropod entanglement. Milkweeds (Asclepias spp.) have an unusual pollination system that requires relatively large packets of pollen (pollinia) to become attached to the appendages of insect pollinators and be pulled through a narrow opening in the flower. Honey bees (Apis mellifera) commonly become trapped and die with their legs still attached to milkweed flowers. In this study, we conducted a field experiment to examine how dead trapped honey bees affect floral visitation. We expected that the presence of a dead trapped bee would reduce floral visitation via two non-mutually exclusive pathways: (1) a direct deterrent effect on floral visitors and (2) an indirect deterrent effect mediated by an increased abundance of scavenging predators such as ants. The presence of a dead bee reduced floral visitation by 37% compared with controls, and this effect was stronger and more robust for honey bee visitors than non-Apis visitors. While ant densities were 51% higher on floral umbels with a dead bee and ants reduced floral visitation by 30%, our path analysis indicated that the direct deterrent pathway explained 91% of the total effect, consistent with an aversion to dead conspecifics among honey bees. Our results suggest that the lethal entanglement of honey bees is likely to incur an ecological cost for milkweed flowers, although the deterrence of honey bees could also shift the pollinator communities on milkweeds with unexpected consequences.

Ecology

https://doi.org/10.1002/ecy.70452

Prabhjot Singh and Louie H. Yang

Abstract

The nonconsumptive effects associated with the predator-avoidance behaviors of potential prey species may commonly be context-dependent. In this study, we examined how temperature differences between the phyllosphere and the ground change the consequences of predator-avoidance dropping behavior in monarch caterpillars. We hypothesized that these consequences change on both diel and seasonal timescales, and that the risk associated with exposure to potentially high ground temperatures is likely to be greater for smaller caterpillars that have less thermal inertia and movement ability. We conducted field experiments to assess the mortality risk associated with dropping to the ground for different-sized caterpillars at a wide range of ground temperatures. We also assess if a caterpillar’s probability of dropping in response to a standardized, simulated attack is consistent with adaptive expectations given variation in observed mortality risk under different conditions. We found that the survivorship consequences of dropping depend on both temperature and caterpillar size and that monarch caterpillars show context-dependent predator-avoidance behaviors consistent with changing adaptive expectations across a wide range of temperatures and body sizes. However, we also observed a potentially nonadaptive willingness to drop at high temperatures, consistent with a biological constraint imposed by increased reactivity. These results contribute to a general knowledge gap regarding the context dependence of nonconsumptive effects in nature.

Oecologia

https://doi.org/10.1007/s00442-025-05837-7

Dylan J. MacArthur-Waltz, Dylan S. Sommer, Prabhjot Singh, Louie H. Yang

Abstract

1. Understanding phenological strategies can be complicated by organisms’ use of multiple cues, changing effects of cues over phenophases and local adaptation. We investigated the importance of spring temperature and rain-year precipitation on phenology in five species of milkweeds (Asclepias spp.) in the western United States, a region with variable temperature and moisture conditions.
2. We categorized participatory science milkweed observations into budding, flowering and post-flowering phenophases and matched observations to year- and site-specific climate conditions. We separated climate effects on phenology into variation across sites versus variation across years to look for evidence of local adaptation in phenology.
3. Warmer mean spring temperatures generally advanced phenology, while wetter rain-years generally delayed phenology. Across all milkweed species × phenophase combinations, spring mean temperature advanced phenology by 2.56 ± 1.71 days × spring temp°C⁻¹ (mean ± SD), and precipitation delayed phenology by 0.12 ± 0.079 days × cm precip⁻¹.
4. We found evidence for local adaptation in phenology with respect to temperature in three of fifteen species × phenophase combinations and for precipitation in four of fifteen species × phenophase combinations. All significant local adaptation with respect to temperature and precipitation reflected counter-gradient local adaptation (increasing phenological synchrony across the range).
5. Synthesis. Combining analytical approaches for studying phenology using observation datasets with the wide availability of observational data from participatory science sources offers an exciting possibility to rapidly generate hypotheses about how organisms’ phenology may react to novel cue combinations under global change, and how these reactions may differ based on species traits.

Journal of Ecology

https://doi.org/10.1111/1365-2745.70185

Mingxuan Ge

Abstract

Parnassius clodius, a Snow Apollo butterfly species native to western North America, overwinters as pharate first-instar larvae enclosed within the chorion. Different populations can experience vastly different overwintering temperatures, which are influenced on a microclimatic scale by both snowpack depth and air temperature. Higher elevation populations are likely to experience more moderate overwintering temperatures than lower elevation populations due to the generally greater snowpack volume and stability at higher elevations. Thus, I hypothesized that higher elevation populations would exhibit reduced cold tolerance compared to lower elevation populations, reflected in a higher average egg supercooling point. The results showed that the high elevation population had a supercooling point 0.61 °C warmer than that of the low elevation population (F test, F1,48 = 10.38, P = 0.0023). This pattern supports the hypothesis that overwintering pharate larvae in high elevation populations are less cold-tolerant than their low elevation counterparts and highlights the potential for increased overwintering mortality at high elevations with reduced snowpack under climate change.

The Journal of the Lepidopterists’ Society

https://doi.org/10.18473/lepi.79i3.a6

Louie H. Yang, Elizabeth G. Postema, Heran Arefaine, Fernanda Y. Cohoon, Emma A. Deen, Yvonne L. Durand, Gwendolyn I. Erdosh, Hailey Ma, Courtney N. Mausling, Sarah Solís, Madeline R. Wilson

Abstract

While both daytime and nighttime temperatures are increasing with climate change, few studies have experimentally investigated their differential effects under field conditions. We conducted a factorial field experiment examining how day- and night-warming impact the growth, survivorship, and behavior of cabbage white caterpillars (Pieris rapae). In this experiment, the night-warming only treatment showed the highest rates of caterpillar growth, but also showed the highest mortality, the shortest maximum caterpillar lengths, the least accumulated herbivory, and reduced pupation. Caterpillars in the treatments that were not warmed during the day showed daytime-shifted growth, and caterpillars in the combined day- and night-warming treatment showed strongly night-shifted herbivory. Both biotic (e.g., predation risk) and abiotic (e.g., thermal) factors could have contributed to these results. Broadly, these results show the importance of temperature-mediated behavioral changes in diel activity for caterpillar development and survival. These results also support the emerging hypotheses that periods of reduced activity may be important for successful development, that warmer nighttime conditions could limit a temporal thermal refuge for caterpillars, and that increasing temperatures could increase the likelihood of metabolic meltdown. This experiment also illustrates the value of field studies to provide insights into how ectotherms might respond to ongoing climate change.

Ecology

https://doi.org/10.1002/ecy.70150

Tracie E. Hayes, Léo Lassérès, Louie H. Yang

Abstract

Shifts in abiotic factors such as temperature and moisture can change the availability of resources, especially under climate change. Both abiotic and biotic drivers can have profound, rapid effects on species distribution, survival, and reproduction. Little is known about how abiotic factors affect the availability of ephemeral resources. Burying beetles Nicrophorus spp.) are specialist users of ephemeral resources, as their reproduction requires locating, defending, and burying a small carcass. Environmental moisture, such as coastal fog, could change how quickly carcasses dry out. We tested the role of carcass moisture and interspecific competition with a generalist scavenger, Heterosilpha spp., on reproduction by placing pairs of Nicrophorus guttula in field chambers with control and experimentally dehydrated mouse carcasses. Pairs that were given control mouse carcasses were more likely to carry out reproductive behaviors and produce viable offspring than pairs that were given a partially dehydrated mouse. For those pairs that reproduced, competition limited the number of offspring. These results indicate that shifts in abiotic factors under climate change, along with biotic factors like competition, can reduce the availability and quality of ephemeral resource patches for consumers.

Ecology

https://doi.org/10.1002/ecy.70127

 

Jolene A. Saldivar will be completing her Ph.D. in Plant Biology at UC Riverside this June before starting as a UC Davis Chancellor’s Postdoctoral Fellow in our lab later this summer. Jolene also received an NSF Postdoctoral Research Fellowship in Biology, which she has deferred for a year. Jolene is interested in documenting plant-butterfly interaction networks across a management gradient, using isotopes to learn about the migratory ecology of painted lady butterflies, and developing species distribution models for butterflies and their host plants.

Melanie is a third-year Environmental Science and Management major, working primarily with Tracie on her Pacific Northwest beetle project. Annalee is a first-year Wildlife, Fish, and Conservation Biology major with experience at the UCD Stable Isotope Facility and an interest in butterflies. Annalee will be helping us with a variety projects while developing her research interests.

Kate Cox will be a full-time field assistant on the “Monitoring Monarch Use of Early-Season Milkweeds in the California Coast Range” project this winter/spring. Kate graduated from UC Davis in December 2022 with a major in Plant Biology, and will start with our lab in the new year.