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

Conservation of North America\u27s 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 x 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

Assessing the risk of insecticide exposures on monarch butterflies

The North American monarch butterfly (Danaus plexippus), an iconic butterfly species, has significantly declined over the last two decades. In December 2020, it was designated as a candidate species under the Endangered Species Act. Conservation of North America’s eastern monarch butterfly population requires establishment of 1.3 to 1.6 billion milkweed (Asclepias spp.) stems in the agricultural landscapes of North Central United States (U.S.). As insecticides are commonly used in this landscape, it is important to assess the risk of insecticide exposure on monarch butterflies. Both foliar and seed treatment insecticide use in North Central U.S. could result in topical and/or dietary exposure to different monarch life stages. Topical and dietary acute and chronic toxicity of six insecticides, encompassing four different modes of action, were studied. Chlorantraniliprole (anthranilic diamide) and beta-cyfluthrin (pyrethroid) were the most toxic while thiamethoxam (neonicotinoid) and chlorpyrifos (organophosphate) were the least toxic. Generally, the larvae and eggs were more sensitive than the adults and pupae. Comparison of toxicity data with modelled and measured environmental insecticide concentrations indicated that foliar applications pose significant risks to monarchs downwind of treated maize or soybean fields. Conversely, seed treatment applications pose little or no risk. The field-scale risk estimates were incorporated into a landscape-scale population model to determine the conservation risks and benefits of establishing milkweed in agricultural landscapes. Toxicity data also were generated for double-stranded RNA (dsRNA) molecules, an emerging class of insecticide products. Monarch larvae were recalcitrant to the dsRNA molecules tested. A review of the literature, including development of species sensitivity distribution models, indicated that while monarch and other lepidopteran larvae were similarly susceptible to the chemical modes of action tested, monarchs were comparatively less susceptible to RNA interference. Finally, a novel mode of action for neonicotinoid insecticides was evaluated. Five of the seven final-instar lepidopteran larvae treated with neonicotinoids failed to expand their pupal appendages and complete pupal ecdysis. Detailed analyses of arrested ecdysis symptomology suggest that neonicotinoids interfere with the function of crustacean cardioactive peptide neurons; adverse outcome pathways for this effect were proposed. Future avenues of research in the field of insect toxicology and risk assessment include the development and use of in vitro and in silico techniques

Assessing the risk of insecticide exposures on monarch butterflies

The North American monarch butterfly (Danaus plexippus), an iconic butterfly species, has significantly declined over the last two decades. In December 2020, it was designated as a candidate species under the Endangered Species Act. Conservation of North America’s eastern monarch butterfly population requires establishment of 1.3 to 1.6 billion milkweed (Asclepias spp.) stems in the agricultural landscapes of North Central United States (U.S.). As insecticides are commonly used in this landscape, it is important to assess the risk of insecticide exposure on monarch butterflies. Both foliar and seed treatment insecticide use in North Central U.S. could result in topical and/or dietary exposure to different monarch life stages. Topical and dietary acute and chronic toxicity of six insecticides, encompassing four different modes of action, were studied. Chlorantraniliprole (anthranilic diamide) and beta-cyfluthrin (pyrethroid) were the most toxic while thiamethoxam (neonicotinoid) and chlorpyrifos (organophosphate) were the least toxic. Generally, the larvae and eggs were more sensitive than the adults and pupae. Comparison of toxicity data with modelled and measured environmental insecticide concentrations indicated that foliar applications pose significant risks to monarchs downwind of treated maize or soybean fields. Conversely, seed treatment applications pose little or no risk. The field-scale risk estimates were incorporated into a landscape-scale population model to determine the conservation risks and benefits of establishing milkweed in agricultural landscapes. Toxicity data also were generated for double-stranded RNA (dsRNA) molecules, an emerging class of insecticide products. Monarch larvae were recalcitrant to the dsRNA molecules tested. A review of the literature, including development of species sensitivity distribution models, indicated that while monarch and other lepidopteran larvae were similarly susceptible to the chemical modes of action tested, monarchs were comparatively less susceptible to RNA interference. Finally, a novel mode of action for neonicotinoid insecticides was evaluated. Five of the seven final-instar lepidopteran larvae treated with neonicotinoids failed to expand their pupal appendages and complete pupal ecdysis. Detailed analyses of arrested ecdysis symptomology suggest that neonicotinoids interfere with the function of crustacean cardioactive peptide neurons; adverse outcome pathways for this effect were proposed. Future avenues of research in the field of insect toxicology and risk assessment include the development and use of in vitro and in silico techniques.</p

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

Iowa monarch conservation, pest management and crop production

The monarch butterfly population has experienced an 80% decline in North America over the past two decades (Brower et al., 2012; Pleasants and Oberhauser, 2013; Jepsen et al., 2015). The three to four hectares of occupied overwintering forest in 2016 and 2015 was well below a target of six hectares needed to support a resilient population and reduce the risk of quasi-extinction (loss of the North American migration) in the next 10 to 20 years (Semmens et al., 2016). In response to a petition to evaluate the status of the species, the U.S. Fish and Wildlife Service (USFWS) is evaluating listing the monarch as a threatened species under the Endangered Species Act (USFWS, 2014 a, b). Under a court-supervised schedule, the USFWS must propose a listing decision in June 2019, which underscores the urgency of establishing viable, voluntary, state-based monarch conservation programs to provide USFWS a credible rationale to not list the species. If the monarch butterfly is listed, it could lead to significant regulatory andmanagement burdens for farmers and livestock producers.</p

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.This article is published as Krishnan, N., Jurenka, R.A. & Bradbury, S.P. Neonicotinoids can cause arrested pupal ecdysis in Lepidoptera. Sci Rep 11, 15787 (2021). doi:10.1038/s41598-021-95284-0. </p

Conservation risks and benefits of establishing monarch butterfly (Danaus plexippus) breeding habitats close to maize and soybean fields in the north central United States: A landscape‐scale analysis of the impact of foliar insecticide on nonmigratory monarch butterfly populations

Establishing habitat in agricultural landscapes of the north central United States is critical to reversing the decline of North America's eastern monarch butterfly (Danaus plexippus) population. Insecticide use could create population sinks and threaten recovery. Discouraging habitat establishment within a 38‐m zone around crop fields is a suggested risk mitigation measure. In Story County, Iowa, United States, this mitigation would discourage habitat establishment in 84% of roadsides and 38% of noncrop land. It is unclear if the conservation benefits from establishing habitat close to crop fields outweigh suppression of population growth owing to insecticide exposure. Consequently, monarch conservation plans require spatially and temporally explicit landscape‐scale assessments. Using an agent‐based model that incorporates female monarch movement and egg laying, the number and location of eggs laid in Story County were simulated for four habitat scenarios: current condition, maximum new establishment, moderate establishment, and moderate establishment only outside a 38‐m no‐plant zone around crop fields. A demographic model incorporated mortality from natural causes and insecticide exposure to simulate adult monarch production over 10 years. Assuming no insecticide exposure, simulated adult production increased 24.7% and 9.3%, respectively, with maximum and moderate habitat establishment and no planting restrictions. A 3.5% increase was simulated assuming moderate habitat establishment with a 38‐m planting restriction. Impacts on adult production were simulated for six representative insecticides registered for soybean aphid (Aphis glycines) management. Depending on the frequency of insecticide applications over a 10‐year period, simulated production increased 8.2%–9.3%, assuming moderate habitat establishment with no planting restrictions. Results suggest that the benefits of establishing habitat close to crop fields outweigh the adverse effects of insecticide spray drift; that is, metapopulation extirpation is not a concern for monarchs. These findings are only applicable to species that move at spatial scales greater than the scale of potential spray‐drift impacts.This article is published as Grant, Tyler J., Niranjana Krishnan, and Steven P. Bradbury. "Conservation risks and benefits of establishing monarch butterfly (Danaus plexippus) breeding habitats close to maize and soybean fields in the north central United States: A landscape-scale analysis of the impact of foliar insecticide on nonmigratory monarch butterfly populations." Integrated environmental assessment and management (2021). doi:10.1002/ieam.4402.</p

Estimating Screening-Level Risks of Insecticide Exposure to Lepidopteran Species of Conservation Concern in Agroecosystems

The U.S. Fish and Wildlife Service defines ‘at risk’ species as those that have either been petitioned for listing, proposed for listing, or assigned a candidate species status under the Endangered Species Act. There are over 30 ‘at risk’ lepidopteran species in the U.S., several of which are found in the north central states. For these species, loss of habitat and exposure to pesticides, particularly insecticides, is often considered a threat to population recovery. Given their range, re-establishment of habitat in agricultural landscapes is typically identified as a primary conservation practice to support species recovery. To evaluate risks associated with habitat established in close proximity to crop fields, estimates of insecticide exposure and toxicity are needed for these species. Based on an evaluation and integration of environmental monitoring and toxicity studies reported in the peer-reviewed literature, we explore an approach to develop screening-level risk analyses for lepidopteran species of conservation concern using data obtained for pyrethroid, organophosphate, neonicotinoid, and diamide insecticides. More specifically, we interpret the utility of existing insecticide residue data to estimate host plant-mediated exposure for lepidopteran species. Based on available lepidopteran topical and dietary toxicity data, we generate Species Sensitivity Distribution models for topical exposures to pyrethroid and organophosphate insecticides. We compare the toxicity results with modeled or available exposure data to explore the potential insecticide risks associated with establishing non-target lepidopteran habitat in agricultural landscapes. Finally, we identify data gaps and needs for future monitoring and toxicity studies

Evaluating toxicity of Varroa mite (Varroa destructor)-active dsRNA to monarch butterfly (Danaus plexippus) larvae.

Varroa mites (Varroa destructor) are parasitic mites that, combined with other factors, are contributing to high levels of honey bee (Apis mellifera) colony losses. A Varroa-active dsRNA was recently developed to control Varroa mites within honey bee brood cells. This dsRNA has 372 base pairs that are homologous to a sequence region within the Varroa mite calmodulin gene (cam). The Varroa-active dsRNA also shares a 21-base pair match with monarch butterfly (Danaus plexippus) calmodulin mRNA, raising the possibility of non-target effects if there is environmental exposure. We chronically exposed the entire monarch larval stage to common (Asclepias syriaca) and tropical (Asclepias curassavica) milkweed leaves treated with concentrations of Varroa-active dsRNA that are one- and ten-fold higher than those used to treat honey bee hives. This corresponded to concentrations of 0.025-0.041 and 0.211-0.282 mg/g leaf, respectively. Potassium arsenate and a previously designed monarch-active dsRNA with a 100% base pair match to the monarch v-ATPase A mRNA (leaf concentration was 0.020-0.034 mg/g) were used as positive controls. The Varroa mite and monarch-active dsRNA's did not cause significant differences in larval mortality, larval or pupal development, pupal weights, or adult eclosion rates when compared to negative controls. Irrespective of control or dsRNA treatment, larvae that consumed approximately 7500 to 10,500-mg milkweed leaf within 10 to 12 days had the highest pupal weights. The lack of mortality and sublethal effects following dietary exposure to dsRNA with 21-base pair and 100% base pair match to mRNAs that correspond to regulatory genes suggest monarch mRNA may be refractory to silencing by dsRNA or monarch dsRNase may degrade dsRNA to a concentration that is insufficient to silence mRNA signaling

Evaluating Toxicity of Varroa Mite (Varroa Destructor)-Active dsRNA to Monarch Butterfly (Danaus Plexippus) Larvae

Varroa mites (Varroa destructor) are parasitic mites that, combined with other factors, are contributing to high levels of honey bee (Apis mellifera) colony losses. A Varroa-active dsRNA was recently developed to control Varroa mites within honey bee brood cells. This dsRNA has 372 base pairs that are homologous to a sequence region within the Varroa mite calmodulin gene (cam). The Varroa-active dsRNA also shares a 21-base pair match with monarch butterfly (Danaus plexippus) calmodulin mRNA, raising the possibility of non-target effects if there is environmental exposure. We chronically exposed the entire monarch larval stage to common (Asclepias syriaca) and tropical (Asclepias curassavica) milkweed leaves treated with concentrations of Varroa-active dsRNA that are one- and ten-fold higher than those used to treat honey bee hives. This corresponded to concentrations of 0.025-0.041 and 0.211-0.282 mg/g leaf, respectively. Potassium arsenate and a previously designed monarch-active dsRNA with a 100% base pair match to the monarch v-ATPase A mRNA (leaf concentration was 0.020-0.034 mg/g) were used as positive controls. The Varroa mite and monarch-active dsRNA\u27s did not cause significant differences in larval mortality, larval or pupal development, pupal weights, or adult eclosion rates when compared to negative controls. Irrespective of control or dsRNA treatment, larvae that consumed approximately 7500 to 10,500-mg milkweed leaf within 10 to 12 days had the highest pupal weights. The lack of mortality and sublethal effects following dietary exposure to dsRNA with 21-base pair and 100% base pair match to mRNAs that correspond to regulatory genes suggest monarch mRNA may be refractory to silencing by dsRNA or monarch dsRNase may degrade dsRNA to a concentration that is insufficient to silence mRNA signaling