Traffic influences nutritional quality of roadside plants for monarch caterpillars

Mitchell, Timothy S.; Agnew, Lauren; Meyer, Rebecca; Sikkink, Kristin L.; Oberhauser, Karen S.; Borer, Elizabeth T.; Snell‐Rood, Emilie C.

doi:10.1016/j.scitotenv.2020.138045

Cited by 22 publications

(44 citation statements)

References 50 publications

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“…Leaf sodium concentrations were 77.83 mg/kg and 2087.95 mg/kg for the control and elevated sodium treatments, respectively. The elevated zinc treatment exceeds levels of zinc found in milkweed along roadsides, and our elevated sodium treatment is within the upper end of the range of sodium commonly found in roadside milkweed ( Snell-Rood et al , 2014 ; Mitchell et al , 2020 ).…”

Section: Methodsmentioning

confidence: 68%

“…For instance, heavy metals such as zinc, copper and iron are commonly released from vehicle wear-and-tear and accumulate in roadside soils ( Fergusson et al , 1980 ; Ho and Tai, 1988 ; Jaradat and Momani, 1999 ; Councell et al , 2004 ; McKenzie et al , 2009 ). Additionally, sodium-containing salts are applied in large quantities to roads during winter for de-icing purposes ( Novotny et al , 2009 ; Kaspari et al , 2010 ), but sodium remains in roadside soils throughout the growing season ( Mitchell et al , 2020 ). These micronutrients can contaminate leaf tissue of plants that are consumed by leaf-feeding insect pollinators such as larval butterflies and moths.…”

Section: Introductionmentioning

confidence: 99%

“…We focus specifically on sodium and zinc because these are among the most prominent human-released micronutrients in local roadside habitats contaminating common milkweed ( Asclepias syriaca ), the most common roadside larval dietary host plant of the monarch throughout the American Midwest ( Kasten et al , 2016 ). Surveys of common milkweed collected from roadside sites across MN, USA, have shown that leaf zinc and sodium concentrations are higher in plants along busier roads and in plants closer to the road edge ( Mitchell et al , 2020 ; Shephard et al ., in review ). Prior experimental work has investigated the effects of increased dietary sodium ( Snell-Rood et al , 2014 ) and increased dietary zinc ( Shephard et al , 2020 ) on monarch development in isolation, but not yet in combination.…”

Section: Introductionmentioning

confidence: 99%

“…Increased dietary sodium enhances neural and muscle development in monarchs, but can reduce survival at high levels ( Snell-Rood et al , 2014 ). Despite elevated levels of sodium in roadside milkweed ( Mitchell et al , 2020 ) neither caterpillars nor ovipositing females avoid leaves with toxic sodium levels ( Mitchell et al , 2019 ). Roadside milkweed accumulates sodium to levels that are potentially toxic for monarchs, but observed levels of zinc in roadside milkweed are likely not toxic for monarchs ( Mitchell et al , 2020 ; Shephard et al , 2020 ).…”

Section: Introductionmentioning

confidence: 99%

“…Despite elevated levels of sodium in roadside milkweed ( Mitchell et al , 2020 ) neither caterpillars nor ovipositing females avoid leaves with toxic sodium levels ( Mitchell et al , 2019 ). Roadside milkweed accumulates sodium to levels that are potentially toxic for monarchs, but observed levels of zinc in roadside milkweed are likely not toxic for monarchs ( Mitchell et al , 2020 ; Shephard et al , 2020 ). It remains unclear how elevated zinc affects monarch development and performance when experienced in combination with other micronutrients, such as sodium.…”

Section: Introductionmentioning

confidence: 99%

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Monarch caterpillars are robust to combined exposure to the roadside micronutrients sodium and zinc

Shephard

Mitchell

Snell‐Rood

2021

Conservation Physiology

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Human activities are increasing the environmental availability of micronutrients, including sodium and some essential metals. Micronutrients are often limiting in animal diets but may have negative effects when consumed in excess. Though prior research has documented how elevated exposure to individual micronutrients can impact organismal development and fitness, we know less about combined effects of multiple micronutrients. In the wild, monarch butterfly larvae (Danaus plexippus) commonly consume plants in roadside habitats that contain elevated levels of sodium (from road salt) and zinc (from vehicle wear-and-tear). We reared monarch caterpillars to adulthood to test individual and combined effects of dietary sodium and zinc on components of fitness, sodium-linked phenotypes (proxies for neural and flight muscle development) and concentrations of sodium and zinc in adult butterflies. Monarch survival was not impacted by elevated sodium or zinc individually or in combination. Yet, monarchs feeding on sodium-treated milkweed developed relatively larger eyes, consistent with a positive effect of sodium on neural development. Measurements of element concentrations in butterfly and plant tissue indicated that monarchs had higher zinc levels than those present in zinc-treated milkweed but lower sodium levels than those present in sodium-treated milkweed. Monarchs developing on sodium-treated milkweed also had prolonged development time, which might be a cost associated with developing extra neural tissue or investing in mechanisms to excrete excess dietary sodium during the larval stage. Our results indicate that sodium, more than zinc, is likely influencing phenotypic development and performance of insect pollinators in roadside habitats. Yet, in contrast to previous work, our experiment suggests that the highest levels of sodium found along roads are not always harmful for developing monarchs. Future work could consider how potentially stressful effects of micronutrients could be mitigated by increased macronutrient availability or how developmental factors such as migratory status might increase micronutrient requirements.

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Section: Methodsmentioning

confidence: 68%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Monarch caterpillars are robust to combined exposure to the roadside micronutrients sodium and zinc

Shephard

Mitchell

Snell‐Rood

2021

Conservation Physiology

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Traffic patterns, more than adjacent land use, influence element content of roadside forbs for insect pollinators

Shephard

Agnew

Herdtle

et al. 2022

Ecol Sol and Evidence

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Roadsides are targeted for restoration of pollinator‐friendly plants. Yet, roads are sources of macronutrient, micronutrient and heavy metal pollution that may contaminate roadside plants. Adjacent landscape features such as railroads and agriculture provide additional macronutrient and heavy metal pollution that may exacerbate traffic effects. However, we lack perspective on how roads combine with rural landscape features to influence nutrition of roadside plants, which could have implications for pollinator health. We surveyed roadsides across Minnesota, USA and measured foliar levels of dietary macronutrients (nitrogen, phosphorous and potassium), a micronutrient (sodium) and metals (iron, zinc, copper, chromium, nickel, lead, aluminium and cadmium) in six abundant roadside forb species used by insect pollinators: Asclepias syriaca, Dalea purpurea, Monarda fistulosa, Ratibida pinnata, Solidago spp. and Trifolium pratense. We aimed to determine (1) how road variables (traffic volume and distance from road) combine with adjacent land use (railroad and agriculture) to influence element content of roadside forbs and (2) whether some forb species show consistent differences in their accumulation of potentially toxic heavy metals, which could inform selection of species to plant along roadsides. We found that foliar concentrations of nine elements increased with greater traffic volume (nitrogen, phosphorous, iron, zinc, copper, chromium, nickel, lead and aluminium), and concentrations of six elements declined with distance from the road (nitrogen, phosphorous, potassium, iron, zinc and copper). Leaves collected adjacent to railroad had less phosphorous, potassium, iron, nickel and aluminium than leaves collected from sites not adjacent to railroad. Additionally, leaves collected from sites adjacent to agriculture had lower copper levels than leaves from sites without adjacent agriculture. We found no evidence that particular ford species along roadsides consistently rank higher than other species in their accumulation of heavy metals. Our results show that traffic alters more elements in roadside plants than does adjacent landscape context, alleviating concerns that landscape features exacerbate pollutant levels in roadside pollinator habitat. However, nutrient contamination of most roadside plants is unlikely to reach toxic levels for insect pollinators. This work is consistent with the positive conservation potential of low to moderate traffic roadsides for pollinators.

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No escape: The influence of substrate sodium on plant growth and tissue sodium responses

Santiago‐Rosario

Harms

Elderd

et al. 2021

Ecology and Evolution

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Traffic influences nutritional quality of roadside plants for monarch caterpillars

Cited by 22 publications

References 50 publications

Monarch caterpillars are robust to combined exposure to the roadside micronutrients sodium and zinc

Monarch caterpillars are robust to combined exposure to the roadside micronutrients sodium and zinc

Traffic patterns, more than adjacent land use, influence element content of roadside forbs for insect pollinators

No escape: The influence of substrate sodium on plant growth and tissue sodium responses

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