Uptake and toxicity of clothianidin to monarch butterflies from milkweed consumption

Laboratory Determination of Particulate-Matter-Bound Agrochemical Toxicity among Honeybees, Mason Bees, and Painted Lady Butterflies

Pollinator population declines are global phenomena with severe consequences for native flora and agriculture. Many factors have contributed to pollinator declines including habitat loss, climate change, disease and parasitism, reductions in abundance and diversity of foraging resources, and agrochemical exposure. Particulate matter (PM) serves as a carrier of toxic agrochemicals, and pollinator mortality can occur following exposure to agrochemical-contaminated PM. Therefore, laboratory-controlled experiments were conducted to evaluate impacts of individual PM-bound agrochemicals. Honeybees (Apis mellifera), blue orchard mason bees (Osmia lignaria), and painted lady butterfly (Vanessa cardui) larvae were exposed to bifenthrin, permethrin, clothianidin, imidacloprid, abamectin, and ivermectin via suspended, airborne PM. Agrochemical concentrations in PM to which pollinators were exposed were based on concentrations observed in fugitive beef cattle feedyard PM including a "mean" treatment and a "max" treatment reflective of reported mean and maximum PM-bound agrochemical concentrations, respectively. In general, pollinators in the mean and max treatments experienced significantly higher mortality compared with controls. Honeybees were most sensitive to pyrethroids, mason bees were most sensitive to neonicotinoids, and painted lady butterfly larvae were most sensitive to macrocyclic lactones. Overall, pollinator mortality was quite low relative to established toxic effect levels derived from traditional pollinator contact toxicity tests. Furthermore, pollinator mortality resulting from exposure to individual agrochemicals via PM was less than that reported to occur at beef cattle feedyards, highlighting the importance of mixture toxicity to native and managed pollinator survival and conservation. Environ Toxicol Chem 2023;42:2642-2650. © 2023 SETAC.

A novel laboratory method for simulating pollinator exposure to agrochemical-laden particulate matter

Environmental transport and deposition of particulate matter (PM) associated with toxic chemicals has begun to receive attention as a source of risk to pollinators. For example, dust arising from manipulations of insecticide-treated seed has potential to exert toxic effects among non-target insects. Similarly, synthetic steroid growth promoters, antibiotics and multiple insecticides and parasiticides detected in fugitive beef cattle feedyard PM may also negatively impact pollinators since many of these chemicals have been detected on wildflowers and pollinators collected near beef cattle feedyards. Therefore, there is a need to assess risk to pollinators posed by deposition of agrochemical-laden PM, both in the field and the laboratory. Unfortunately, established laboratory methods for simulating PM exposure or toxicity associated with contaminated PM are few and highly situation-specific. Herein we describe development and use of a PM circulation system that can be employed to evaluate toxicity of agrochemical-contaminated PM in the laboratory under controlled conditions. Two model organisms (honeybees (Apis mellifera) and mason bees (Osmia lignaria)) were exposed to agrochemical-free PM in the circulator system, and post-exposure mortality was compared with controls. No significant differences in mortality between exposed and control bees were observed. Next, honeybees and mason bees were exposed to PM spiked with an insecticide known to exert toxic effects to pollinators (thiamethoxam). Bees experienced significantly higher mortality when exposed to thiamethoxam-laden PM at environmentally relevant concentrations as compared to bees exposed to agrochemical-free PM. These results confirm the validity of these methods for use in controlled laboratory PM toxicity tests and offer a source of positive and negative control groups for laboratory and field experiments examining exposure of pollinators to potentially toxic agrochemical-laden PM. This method facilitates generation of more realistic toxicity data than standard contact toxicity tests when pollinator exposure scenarios involve particulate-based agrochemicals or other toxic chemicals.

Monarch Butterfly Ecology, Behavior, and Vulnerabilities in North Central United States Agricultural Landscapes

The North American monarch butterfly (Danaus plexippus) is a candidate species for listing under the Endangered Species Act. Multiple factors are associated with the decline in the eastern population, including the loss of breeding and foraging habitat and pesticide use. Establishing habitat in agricultural landscapes of the North Central region of the United States is critical to increasing reproduction during the summer. We integrated spatially explicit modeling with empirical movement ecology and pesticide toxicology studies to simulate population outcomes for different habitat establishment scenarios. Because of their mobility, we conclude that breeding monarchs in the North Central states should be resilient to pesticide use and habitat fragmentation. Consequently, we predict that adult monarch recruitment can be enhanced even if new habitat is established near pesticide-treated crop fields. Our research has improved the understanding of monarch population dynamics at the landscape scale by examining the interactions among monarch movement ecology, habitat fragmentation, and pesticide use.

Neonicotinoid pesticide and nitrate mixture removal and persistence in floating treatment wetlands

Mesocosm and microcosm experiments were conducted to explore the applicability of floating treatment wetlands (FTWs), an ecologically based management technology, to remove neonicotinoid insecticides and nitrate from surface water. The mesocosm experiment evaluated three treatments in triplicate over a 21-d period. Floating treatment wetland mesocosms completely removed nitrate-N over the course of the experiment even when neonicotinoid insecticides were present. At the completion of the experiment, 79.6% of imidacloprid and degradation byproducts and 68.3% of thiamethoxam and degradation byproducts were accounted for in the water column. Approximately 3% of imidacloprid and degradation byproducts and 5.0% of thiamethoxam and degradation byproducts were observed in above-surface biomass, while ∼24% of imidacloprid and degradation byproducts, particularly desnitro imidacloprid, and <0.1% of thiamethoxam and degradation byproducts were found in the below surface biomass. Further, 1 yr after the experiments, imidacloprid, thiamethoxam, and degradation byproducts persisted in biomass but at lower concentrations in both the above- and below-surface biomass. Comparing the microbial communities of mature FTWs grown in the presence and absence of neonicotinoids, water column samples had similar low abundances of nitrifying Archaeal and bacterial amoA genes (below detection to 104  ml-1 ) and denitrifying bacterial nirK, nirS, and nosZ genes (below detection to 105  ml-1 ). Follow-up laboratory incubations found the highest denitrification potential activities in FTW plant roots compared with water column samples, and there was no effect of neonicotinoid addition (100 ng L-1 ) on potential denitrification activity. Based on these findings, (a) FTWs remove neonicotinoids from surface water through biomass incorporation, (b) neonicotinoids persist in biomass long-term (>1 yr after exposure), and (c) neonicotinoids do not adversely affect nitrate-N removal via microbial denitrification.

Pollinator biological traits and ecological interactions mediate the impacts of mosquito-targeting malathion application

Mosquito adulticides are perceived by the public as detrimental to nontarget arthropods, contributing to declines of native and beneficial insects. However, the actual impact of adulticides on nontarget arthropods in nature needs to incorporate biological and ecological elements. Here, we investigated the effect of biological/behavioral traits (butterfly roosting at different heights, roosting in sites underneath foliage, bumblebee hive usage) and interactions (parasitism, predation) of pollinators (butterflies and bumblebees) that could mediate the impacts of malathion application in manipulative semi-field experiments in Florida, USA. Roosting height from the spray route had a significant negative relationship with mortality of butterflies treated with ULV malathion, with high survival at the highest roosting height (7 m), but butterflies roosting among vegetation did not have higher survival. Bumblebees with hive access had significantly higher survival than bumblebees without hive access. Host plants treated with ULV malathion significantly reduced parasitism of monarch eggs by Trichogramma platneri, but increased predation of monarch caterpillars by Polistes paper wasps. These data provide insight into the realistic impacts of adulticide applications on pollinators in nature which will enable mosquito control districts to better limit nontarget effects of adulticide treatments and may help to address concerns related to potential nontarget effects.

Host Plant Species Mediates Impact of Neonicotinoid Exposure to Monarch Butterflies

Simple Summary

Neonicotinoids are the most widely used insecticides in North America and many studies document the negative effects of neonicotinoids on bees. Monarch butterflies are famous for their long-distance migrations, and for their ability to sequester toxins from their milkweed host plants. The neonicotinoids imidacloprid and clothianidin were suggested to correlate with declines in North American monarchs. We examined how monarch development, survival, and flight were affected by exposure to neonicotinoids, and how these effects depend on milkweed host plant species that differ in their cardenolide toxins. Monarch survival and flight were unaffected by low and intermediate neonicotinoid doses. At the highest dose, neonicotinoids negatively affected monarch pupation and survival, for caterpillars that fed on the least toxic milkweed. Monarchs fed milkweed of intermediate toxicity experienced moderate negative effects of high insecticide doses. Monarchs fed the most toxic milkweed species had no negative consequences associated with neonicotinoid treatment. Our work shows that monarchs tolerate low neonicotinoid doses, but experience detrimental effects at higher doses, depending on milkweed species. To our knowledge, this is the first study to show that host plant species potentially reduce the residue of neonicotinoid insecticides on the leaf surface, and this phenomenon warrants further investigation.

Abstract

Neonicotinoids are the most widely used insecticides in North America. Numerous studies document the negative effects of neonicotinoids on bees, and it remains crucial to demonstrate if neonicotinoids affect other non-target insects, such as butterflies. Here we examine how two neonicotinoids (imidacloprid and clothianidin) affect the development, survival, and flight of monarch butterflies, and how these chemicals interact with the monarch’s milkweed host plant. We first fed caterpillars field-relevant low doses (0.075 and 0.225 ng/g) of neonicotinoids applied to milkweed leaves (Asclepias incarnata), and found no significant reductions in larval development rate, pre-adult survival, or adult flight performance. We next fed larvae higher neonicotinoid doses (4–70 ng/g) and reared them on milkweed species known to produce low, moderate, or high levels of secondary toxins (cardenolides). Monarchs exposed to the highest dose of clothianidin (51–70 ng/g) experienced pupal deformity, low survival to eclosion, smaller body size, and weaker adult grip strength. This effect was most evident for monarchs reared on the lowest cardenolide milkweed (A. incarnata), whereas monarchs reared on the high-cardenolide A. curassavica showed no significant reductions in any variable measured. Our results indicate that monarchs are tolerant to low doses of neonicotinoid, and that negative impacts of neonicotinoids depend on host plant type. Plant toxins may confer protective effects or leaf physical properties may affect chemical retention. Although neonicotinoid residues are ubiquitous on milkweeds in agricultural and ornamental settings, commonly encountered doses below 50 ng/g are unlikely to cause substantial declines in monarch survival or migratory performance.

Neonicotinoids can cause arrested pupal ecdysis in Lepidoptera

Recently, we reported a novel mode of action in monarch butterfly (Danaus plexippus) larvae exposed to neonicotinoid insecticides: arrest in pupal ecdysis following successful larval ecdysis. In this paper, we explore arrested pupal ecdysis in greater detail and propose adverse outcome pathways to explain how neonicotinoids cause this effect. Using imidacloprid as a model compound, we determined that final-instar monarchs, corn earworms (Helicoverpa zea), and wax moths (Galleria mellonella) showed high susceptibility to arrested pupal ecdysis while painted ladies (Vanessa cardui) and red admirals (Vanessa atalanta) showed low susceptibility. Fall armyworms (Spodoptera frugiperda) and European corn borers (Ostrinia nubilalis) were recalcitrant. All larvae with arrested ecdysis developed pupal cuticle, but with incomplete shedding of larval cuticle and unexpanded pupal appendages; corn earworm larvae successfully developed into adults with unexpanded appendages. Delayed initiation of pupal ecdysis was also observed with treated larvae. Imidacloprid exposure was required at least 26 h prior to pupal ecdysis to disrupt the molt. These observations suggest neonicotinoids may disrupt the function of crustacean cardioactive peptide (CCAP) neurons, either by directly acting on their nicotinic acetylcholine receptors or by acting on receptors of inhibitory neurons that regulate CCAP activity.

Oviposition Response of Monarch Butterfly (Lepidoptera: Nymphalidae) to Imidacloprid-Treated Milkweed

Monarch butterfly (Danaus plexippus) populations have declined over the last two decades, attributable in part to declines in its larval host plant, milkweed (Asclepias spp.), across its breeding range. Conservation efforts in the United States call for restoration of 1.3 billion milkweed stems into the Midwestern landscape. Reaching this goal will require habitat establishment in marginal croplands, where there is a high potential for exposure to agrochemicals. Corn and soybean crops may be treated with neonicotinoid insecticides systemically or through foliar applications to provide protection against insect pests. Here, we investigate whether ovipositing monarchs discriminate against milkweed plants exposed to the neonicotinoid insecticide imidacloprid, either systemically or through foliar application. In our first experiment, we placed gravid females in enclosures containing a choice of two cut stems for oviposition: one in 15 ml of a 0.5 mg/ml aqueous solution of imidacloprid and one in 15 ml water. In a second experiment, females were given a choice of milkweed plants whose leaves were treated with 30 µl of a 0.825 mg/ml imidacloprid-surfactant solution or plants treated with surfactant alone. To evaluate oviposition preference, we counted and removed eggs from all plants daily for 3 d. We also collected video data on a subset of butterflies to evaluate landing behavior. Results indicate that neither systemic nor foliar treatment with imidacloprid influenced oviposition behavior in female monarchs. The implications of these findings for monarch conservation practices will be informed by the results of ongoing egg and larval toxicity studies.

Monarch Butterfly (Danaus plexippus) Life-Stage Risks from Foliar and Seed-Treatment Insecticides

Conservation of North America's eastern monarch butterfly (Danaus plexippus) population would require establishment of milkweed (Asclepias spp.) and nectar plants in the agricultural landscapes of the north central United States. A variety of seed-treatment and foliar insecticides are used to manage early- and late-season pests in these landscapes. Thus, there is a need to assess risks of these insecticides to monarch butterfly life stages to inform habitat conservation practices. Chronic and acute dietary toxicity studies were undertaken with larvae and adults, and acute topical bioassays were conducted with eggs, pupae, and adults using 6 representative insecticides: beta-cyfluthrin (pyrethroid), chlorantraniliprole (anthranilic diamide), chlorpyrifos (organophosphate), imidacloprid, clothianidin, and thiamethoxam (neonicotinoids). Chronic dietary median lethal concentration values for monarch larvae ranged from 1.6 × 10^-3 (chlorantraniliprole) to 5.3 (chlorpyrifos) μg/g milkweed leaf, with the neonicotinoids producing high rates of arrested pupal ecdysis. Chlorantraniliprole and beta-cyfluthrin were generally the most toxic insecticides to all life stages, and thiamethoxam and chlorpyrifos were generally the least toxic. The toxicity results were compared to insecticide exposure estimates derived from a spray drift model and/or milkweed residue data reported in the literature. Aerial applications of foliar insecticides are expected to cause high downwind mortality in larvae and eggs, with lower mortality predicted for adults and pupae. Neonicotinoid seed treatments are expected to cause little to no downslope mortality and/or sublethal effects in larvae and adults. Given the vagile behavior of nonmigratory monarchs, considering these results within a landscape-scale context suggests that adult recruitment will not be negatively impacted if new habitat is established in close proximity of maize and soybean fields in the agricultural landscapes of the north central United States. Environ Toxicol Chem 2021;40:1761-1777. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Effects of early-life exposure to sublethal levels of a common neonicotinoid insecticide on the orientation and migration of monarch butterflies (Danaus plexippus)

Migratory insects use a variety of innate mechanisms to determine their orientation and maintain correct bearing. For long-distance migrants, such as the monarch butterfly (Danaus plexippus), these journeys could be affected by exposure to environmental contaminants. Neonicotinoids are synthetic insecticides that work by affecting the nervous system of insects, resulting in impairment of their mobility, cognitive performance, and other physiological and behavioural functions. To examine how neonicotinoids might affect the ability of monarch butterflies to maintain a proper directional orientation on their ∼4000 km migration, we grew swamp milkweed (Asclepias incarnata) in soil that was either untreated (0 ng g-1: control) or mixed with low (15 ng g-1 of soil) or high (25 ng g-1 of soil) levels of the neonicotinoid clothianidin. Monarch caterpillars were raised on control or clothianidin-treated milkweed and, after pupation, either tested for orientation in a static flight simulator or radio-tracked in the wild during the autumn migration period. Despite clothianidin being detectable in milkweed tissue consumed by caterpillars, there was no evidence that clothianidin influenced the orientation, vector strength (i.e. concentration of direction data around the mean) or rate of travel of adult butterflies, nor was there evidence that morphological traits (i.e. mass and forewing length), testing time, wind speed or temperature impacted directionality. Although sample sizes for both flight simulator and radio-tracking tests were limited, our preliminary results suggest that clothianidin exposure during early caterpillar development does not affect the directed flight of adult migratory monarch butterflies or influence their orientation at the beginning of migration.

Native and Edible Ornamental Plant Congeners Enhance Ecosystem Services Through Key Pest Avoidance and Multifunctionality in Residential Landscapes

Tea scale, Fiorinia theae Green (Hemiptera: Diaspididae), has long been one of the most important pests of Ilex and Camellia plants, particularly in the southeastern United States. This exotic armored scale insect reduces host plant health and function, and often requires insecticide use, which poses risks to nontarget organisms. While the use of Ilex (Aquifoliales: Aquifoliaceae) and Camellia (Ericales: Theaceae) spp. as landscape ornamentals for aesthetic function is firmly established, we have a poor understanding of species-level susceptibility to F. theae. Additionally, two species, Ilex vomitoria Ait. and Camellia sinensis (L.) O. Kuntze are emerging tisane- and tea-producing commodities in the region, respectively. We propose that these consumable plants may be well-suited alternatives to their traditionally used ornamental congeners in residential landscapes where they may provide enhanced ecosystem services. However, the potential impact of key pests, like F. theae, on these species should be evaluated to anticipate pest pressure that may undermine or offset benefits. In this study, we examine six species within the known host range of tea scale, comparing nonnative I. cornuta Lindl. 'Dwarf Burford,' C. japonica L., C. sasanqua Thunb., and C. sinensis, along with native I. opaca Ait. and I. vomitoria. We found that plant species show a wide range of susceptibility to F. theae and associated damage, with the two native Ilex species and tea-producing C. sinensis displaying the least susceptibility. By reducing the impact of a key pest and considering other ecosystem service traits, these results may help guide more sustainable plant selection decisions where the goal is to integrate native and edible plants into residential landscapes.

Larval pesticide exposure impacts monarch butterfly performance

The long-term decline of monarch butterflies has been attributed to loss of their milkweed (Asclepias sp.) host-plants after the introduction of herbicide-tolerant crops. However, recent studies report pesticide residues on milkweed leaves that could act as a contributing factor when ingested as part of their larval diet. In this study, we exposed monarch larvae to six pesticides (insecticide: clothianidin; herbicides: atrazine, S-metolachlor; fungicides: azoxystrobin, pyraclostrobin, trifloxystrobin) on their primary host-plant, A. syriaca. Each was tested at mean and maximum levels reported from published analyses of milkweeds bordering cropland and thus represent field-relevant concentrations. Monarch lethal and sub-lethal responses were tracked over their complete development, from early instar larvae to adult death. Overall, we found no impact of any pesticide on immature development time and relatively weak effects on larval herbivory or survival to adulthood. Comparatively stronger effects were detected for adult performance; namely, a 12.5% reduction in wing length in response to the fungicides azoxystrobin and trifloxystrobin. These data collectively suggest that monarch responses to host-plant pesticides are largely sublethal and more pronounced in the adult stage, despite exposure only as larvae. This outcome has important implications for risk assessment and the migratory success of monarchs in North America.

Optimization of QuEChERS Method for Simultaneous Determination of Neonicotinoid Residues in Pollinator Forage

Consistent with the large-scale use of pesticide seed treatments in U.S. field crop production, there has been an increased use of neonicotinoid-treated corn and soybean seed over the past decade. Neonicotinoids can move downwind to adjacent off-field pollinator habitats in dust from planting and/or move downslope to habitats in surface water. The extent of potential neonicotinoid exposure to pollinators from neonicotinoid movement into these adjacent pollinator habitats is unclear. Pollen and leaf tissue extractions were completed using a quick, easy, cheap, effective, rugged, and safe (QuEChERS) extraction procedure. Samples were subjected to a clean-up step using dispersive solid-phase extraction (dSPE) techniques prior to analysis. The compounds in the extracts were separated on a reversed-phase column with gradient elution and confirmed with tandem mass spectrometry. The extraction method showed acceptable recoveries of analytes ranging from 78.4 to 93.6% and 89.4 to 101% for leaf tissue and pollen, respectively. The method’s detection limits ranged from 0.04 to 0.3 ng/g in milkweed leaf tissue and 0.04 to 1.0 ng/g in pollen. The method is currently being employed in ongoing studies surveying pollen from a diversity of forbs and milkweed leaves obtained from habitat patches established within fields with a history of using neonicotinoid-treated seeds.