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2-1 Chapter 2 Pollinator Biology and Roadsides This chapter provides a brief background on general pollinator biology, the conservation status of pollinators, threats pollinators face, and habitat needs for different groups of pollinators. The habitat needs outlined here are the basis of conservation actions in the chapters that follow. This chapter also includes an overview of roadsides as habitat for pollinators and threats to pollinators associated with roads. 2.1 WhyÂ AreÂ PollinatorsÂ Important?Â Â Approximately 85 percent of the worldâs flowering plants depend on animalsâmostly insectsâto move pollen between flowers, a transfer that is essential for flowering-plant reproduction (Ollerton et al. 2011). Pollinators are critically important for the wildlife food webs that depend upon these plants (Kearns et al. 1998; Summerville and Crist 2002). Fruits, seeds, and nutsâthe products of pollinationâare food for many insects, birds, and mammals. Pollinators themselves are an important food source for wildlife that feed on insects for a portion or all of their diet. For example, butterfly and moth caterpillars are an important part of the diet of many young birds (Buehler et al. 2002). Pollinators also play a significant role in agriculture. Of the worldâs most commonly cultivated crops, 70 percent are dependent on animal pollinators (Klein et al. 2007). Many minerals, vitamins, and nutrients such as vitamin C, calcium, and folic acid needed to maintain human health are found in crop plants that rely on pollinators (Eilers et al. 2011). In addition to production of many fruits, vegetables, spices, nuts, and seeds, pollinators are also important to forage plants like alfalfa and clover that provide feed for livestock. Pollinators contribute about $27 billion annually to the United States economy (Calderone 2012). Pollinators are essential to agricultural production and ecosystem health and are fundamental to human wellbeing. 2.2 StatusÂ ofÂ PollinatorsÂ Â The great majority of pollinators in North America are insects, including bees, wasps, flies, beetles, butterflies, and moths (Allen-Wardell et al. 1998; Kearns 2001). Over the last several decades, a number of insect pollinators have steeply declined. Nearly 30 percent of all bumble bee species in North America face some degree of extinction risk (Hatfield et al. 2012), including species that were formerly among the most common (Grixti et al. 2009; Cameron et al. 2011), as do about 20 percent of butterflies (NatureServe 2018). Once common and widespread, even the iconic monarch butterfly (Danaus plexippus), known for its impressive long distance migration, has declined by 80 percent east of the Rocky Mountains and more than 95 percent west of the Rocky Mountains (Semmens et al. 2016; Pelton et al. 2019). As of March, 2022, 47 pollinating insects are listed under the federal Endangered Species Act (ESA)ânine bees, 34 butterflies, two moths, one fly, and one beetleâplus two species of butterflies that are currently candidates or proposed for listing (Table 2-1).
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-2 There are also a number of imperiled species with declining populations, and there is a significant potential for more pollinators species to be listed in the coming years. Chapter 3, Imperiled Pollinator Profiles, includes profiles of the listed and candidate pollinators in this region, as well as other imperiled species in the region that have the potential to become listed. Factors that contribute to insect pollinator declines include: ï· the loss, degradation, and fragmentation of habitat (e.g., Kremen et al. 2002; Potts et al. 2010); ï· introduced species (e.g., Memmott and Wasser 2002; Tallamy and Shropshire 2009); ï· the use of pesticides (e.g., Kevan 1975, 1999; Dover et al. 1990; Baron et al. 2014); ï· diseases and parasites (e.g., Altizer and Oberhauser 1999; Cameron et al. 2011; Koch and Strange 2012); and ï· climate change (Forister et al. 2010; Warren et al. 2008; IPBES 2016; Glenny et al. 2018; Harvey et al. 2022). Threats to pollinator communities affect not only pollinators themselves but also natural ecosystems and agricultural productivity. Table 2-1. List of pollinators protected or proposed for protection by the ESA that occur in the United States as of March 2022. ScientificÂ nameÂ CommonÂ nameÂ StatusÂ RegionsÂ WhereÂ CurrentlyÂ FoundÂ BEESÂ BombusÂ affinisÂ RustyÂ patchedÂ bumbleÂ beeÂ EndangeredÂ GreatÂ Lakes,Â Midâ Atlantic,Â Midwest,Â Northeast,Â NorthernÂ Plains,Â SoutheastÂ BombusÂ frankliniÂ Franklin'sÂ bumbleÂ beeÂ EndangeredÂ California,Â MaritimeÂ NorthwestÂ HylaeusÂ anthracinusÂ AnthricinanÂ yellowâ facedÂ beeÂ EndangeredÂ HawaiiÂ HylaeusÂ assimulansÂ AssimulansÂ yellowâ facedÂ beeÂ EndangeredÂ HawaiiÂ HabitatÂ Loss:Â theÂ eliminationÂ ofÂ aÂ habitatÂ orÂ transformationÂ intoÂ anotherÂ typeÂ ofÂ habitat.Â Â HabitatÂ degradation:Â aÂ declineÂ inÂ habitatÂ conditionsÂ dueÂ toÂ invasiveÂ species,Â pollution,Â development,Â orÂ overutilizationÂ ofÂ naturalÂ resources.Â Â HabitatÂ fragmentation:Â largerÂ habitatsÂ areÂ brokenÂ upÂ intoÂ smallerÂ patches,Â whichÂ mayÂ beÂ tooÂ smallÂ toÂ sustainÂ populationsÂ ofÂ someÂ speciesÂ orÂ speciesÂ areÂ unableÂ toÂ moveÂ betweenÂ patches.Â Â HabitatÂ loss,Â degradation,Â andÂ fragmentationÂ canÂ allÂ leadÂ toÂ reductionsÂ inÂ numbersÂ ofÂ speciesÂ andÂ declinesÂ inÂ populations.Â
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-3 ScientificÂ nameÂ CommonÂ nameÂ StatusÂ RegionsÂ WhereÂ CurrentlyÂ FoundÂ HylaeusÂ facilisÂ EasyÂ yellowâfacedÂ beeÂ EndangeredÂ HawaiiÂ HylaeusÂ hilarisÂ HilarisÂ yellowâfacedÂ beeÂ EndangeredÂ HawaiiÂ HylaeusÂ kuakeaÂ HawaiianÂ yellowâ facedÂ beeÂ EndangeredÂ HawaiiÂ HylaeusÂ longicepsÂ HawaiianÂ yellowâ facedÂ beeÂ EndangeredÂ HawaiiÂ HylaeusÂ manaÂ HawaiianÂ yellowâ facedÂ beeÂ EndangeredÂ HawaiiÂ BUTTERFLIESÂ AnaeaÂ troglodytaÂ floridalisÂ FloridaÂ leafwingÂ butterflyÂ EndangeredÂ FloridaÂ ApodemiaÂ mormoÂ langeiÂ Lange'sÂ metalmarkÂ butterflyÂ EndangeredÂ CaliforniaÂ BoloriaÂ acrocnemaÂ UncompahgreÂ fritillaryÂ butterflyÂ EndangeredÂ RockyÂ MountainÂ CallophrysÂ mossiiÂ bayensisÂ SanÂ BrunoÂ elfinÂ butterflyÂ EndangeredÂ CaliforniaÂ CyclargusÂ thomasiÂ bethunebakeriÂ MiamiÂ blueÂ butterflyÂ EndangeredÂ FloridaÂ CyclargusÂ ammonÂ NickerbeanÂ blueÂ butterflyÂ SimilarityÂ ofÂ appearanceÂ toÂ aÂ threatenedÂ taxonÂ FloridaÂ DanausÂ plexippusÂ MonarchÂ butterflyÂ CandidateÂ LowerÂ 48Â statesÂ EuchloeÂ ausonidesÂ insulanusÂ IslandÂ marbleÂ butterflyÂ EndangeredÂ MaritimeÂ NorthwestÂ EuphilotesÂ battoidesÂ allyniÂ ElÂ SegundoÂ blueÂ butterflyÂ EndangeredÂ CaliforniaÂ EuphilotesÂ enoptesÂ smithiÂ Smith'sÂ blueÂ butterflyÂ EndangeredÂ CaliforniaÂ
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-4 ScientificÂ nameÂ CommonÂ nameÂ StatusÂ RegionsÂ WhereÂ CurrentlyÂ FoundÂ EuphydryasÂ edithaÂ bayensisÂ BayÂ checkerspotÂ butterflyÂ ThreatenedÂ CaliforniaÂ EuphydryasÂ edithaÂ quinoÂ QuinoÂ checkerspotÂ butterflyÂ EndangeredÂ CaliforniaÂ EuphydryasÂ edithaÂ tayloriÂ Taylor'sÂ checkerspotÂ butterflyÂ EndangeredÂ MaritimeÂ NorthwestÂ GlaucopsycheÂ lygdamusÂ palosverdesensisÂ PalosÂ VerdesÂ blueÂ butterflyÂ EndangeredÂ CaliforniaÂ HemiargusÂ ceraunusÂ antibubastusÂ CeraunusÂ blueÂ butterflyÂ SimilarityÂ ofÂ appearanceÂ toÂ aÂ threatenedÂ taxonÂ FloridaÂ HeraclidesÂ aristodemusÂ ponceanusÂ SchausÂ swallowtailÂ butterflyÂ EndangeredÂ FloridaÂ HesperiaÂ dacotaeÂ DakotaÂ skipperÂ ThreatenedÂ NorthernÂ PlainsÂ HesperiaÂ leonardusÂ montanaÂ PawneeÂ montaneÂ skipperÂ ThreatenedÂ RockyÂ MountainsÂ IcariciaÂ (Plebejus)Â shastaÂ charlestonensisÂ MountÂ CharlestonÂ blueÂ butterflyÂ EndangeredÂ SouthwestÂ IcariciaÂ icarioidesÂ fenderiÂ Fender'sÂ blueÂ butterflyÂ EndangeredÂ MaritimeÂ NorthwestÂ IcariciaÂ icarioidesÂ missionensisÂ MissionÂ blueÂ butterflyÂ EndangeredÂ CaliforniaÂ LeptotesÂ cassiusÂ theonusÂ CassiusÂ blueÂ butterflyÂ SimilarityÂ ofÂ appearanceÂ toÂ aÂ threatenedÂ taxonÂ FloridaÂ LycaeidesÂ argyrognomonÂ lotisÂ LotisÂ blueÂ butterflyÂ EndangeredÂ CaliforniaÂ LycaeidesÂ melissaÂ samuelisÂ KarnerÂ blueÂ butterflyÂ EndangeredÂ GreatÂ Lakes,Â Midwest,Â NortheastÂ LycaenaÂ hermesÂ HermesÂ copperÂ butterflyÂ ThreatenedÂ CaliforniaÂ
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-5 ScientificÂ nameÂ CommonÂ nameÂ StatusÂ RegionsÂ WhereÂ CurrentlyÂ FoundÂ NeonymphaÂ mitchelliiÂ francisciÂ SaintÂ Francis'Â satyrÂ butterflyÂ EndangeredÂ MidâAtlanticÂ NeonymphaÂ mitchelliiÂ Mitchell'sÂ satyrÂ butterflyÂ EndangeredÂ GreatÂ Lakes,Â Midwest,Â Midâ Atlantic,Â SoutheastÂ OarismaÂ poweshiekÂ PoweshiekÂ skipperlingÂ EndangeredÂ GreatÂ Lakes,Â NorthernÂ PlainsÂ PseudocopaeodesÂ eunusÂ obscurusÂ CarsonÂ wanderingÂ skipperÂ EndangeredÂ California,Â GreatÂ BasinÂ PyrgusÂ ruralisÂ lagunaeÂ LagunaÂ MountainsÂ skipperÂ EndangeredÂ CaliforniaÂ SpeyeriaÂ callippeÂ CallippeÂ silverspotÂ butterflyÂ EndangeredÂ CaliforniaÂ SpeyeriaÂ nokomisÂ GreatÂ BasinÂ SilverspotÂ ProposedÂ threatenedÂ SouthwestÂ SpeyeriaÂ zereneÂ behrensiiÂ Behren'sÂ silverspotÂ butterflyÂ EndangeredÂ CaliforniaÂ SpeyeriaÂ zereneÂ hippolytaÂ OregonÂ silverspotÂ butterflyÂ ThreatenedÂ California,Â MaritimeÂ NorthwestÂ SpeyeriaÂ zereneÂ myrtleaeÂ Myrtle'sÂ silverspotÂ butterflyÂ EndangeredÂ CaliforniaÂ StrymonÂ acisÂ bartramiÂ Bartram'sÂ hairstreakÂ butterflyÂ EndangeredÂ FloridaÂ MOTHSÂ EuproserpinusÂ euterpeÂ KernÂ primroseÂ sphinxÂ mothÂ ThreatenedÂ CaliforniaÂ ManducaÂ blackburniÂ Blackburn'sÂ sphinxÂ mothÂ ThreatenedÂ HawaiiÂ FLIESÂ RhaphiomidasÂ terminatusÂ abdominalisÂ DelhiÂ SandsÂ flowerâ lovingÂ flyÂ EndangeredÂ CaliforniaÂ
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-6 ScientificÂ nameÂ CommonÂ nameÂ StatusÂ RegionsÂ WhereÂ CurrentlyÂ FoundÂ BEETLESÂ DesmocerusÂ californicusÂ dimorphusÂ ValleyÂ elderberryÂ longhornÂ beetleÂ ThreatenedÂ CaliforniaÂ 2.3 MeetÂ theÂ PollinatorsÂ This guide focuses solely on invertebrate pollinators, due to their widespread importance. North America does have some vertebrate pollinators, including nectar-feeding bat species found in the southwestern United States (Leptonycteris yerbabuenae, Choeronycteris mexicana) and hummingbirds (family Trochilidae) (Grant 1994; Valiente-Banuet et al. 2004); those species are not covered in this guide. The primary groups of insect pollinators are bees, butterflies, and flower-visiting moths, wasps, beetles, and flies (see Figure 2-1). Bees are particularly efficient and effective pollinators because all adult females actively collect pollen to bring back to their nest to provide for their young, and bees are considered the most important group of pollinators for agricultural crops (McGregor 1976; Garibaldi et al. 2013) as well as for many wild plants in temperate climates (Michener 2007).
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-7 BumbleÂ beesÂ Order:Â HymenopteraÂ Family:Â ApidaeÂ Genus:Â BombusÂ BumbleÂ beesÂ formÂ annualÂ socialÂ colonies.Â QueenÂ bumbleÂ bees,Â matedÂ theÂ previousÂ fall,Â startÂ nestsÂ inÂ spring.Â ByÂ midâsummer,Â coloniesÂ canÂ haveÂ dozensÂ orÂ hundredsÂ ofÂ workersÂ (FigureÂ 2â2).Â TheyÂ nestÂ inÂ insulatedÂ cavitiesÂ suchÂ asÂ underÂ clumpsÂ ofÂ bunchÂ grassÂ orÂ inÂ oldÂ rodentÂ nests.Â ThereÂ areÂ speciesÂ ofÂ bumbleÂ beesÂ thatÂ areÂ nestÂ parasitesÂ ofÂ otherÂ bumbleÂ bees.Â TheseÂ cuckooÂ bumbleÂ beesÂ invadeÂ anÂ establishedÂ colony,Â killÂ theÂ queen,Â andÂ layÂ eggsÂ thatÂ theÂ hostÂ colonyÂ thenÂ rears.Â GroundânestingÂ beesÂ Order:Â HymenopteraÂ Families:Â Andrenidae,Â Apidae,Â Colletidae,Â HalictidaeÂ MostÂ nativeÂ beesÂ liveÂ solitaryÂ lives,Â withÂ eachÂ femaleÂ workingÂ aloneÂ toÂ buildÂ herÂ nestsÂ andÂ collectÂ andÂ provideÂ foodÂ forÂ herÂ offspring.Â AboutÂ 70Â percentÂ ofÂ solitaryÂ beeÂ speciesÂ nestÂ underground,Â diggingÂ slenderÂ tunnelsÂ offÂ whichÂ theyÂ buildÂ cellsÂ forÂ eachÂ eggÂ andÂ itsÂ provisions.Â TunnelânestingÂ beesÂ Order:Â HymenopteraÂ Families:Â Apidae,Â Colletidae,Â Halictidae,Â MegachilidaeÂ ApproximatelyÂ 30Â percentÂ ofÂ solitaryÂ beeÂ speciesÂ nestÂ inÂ tunnels,Â insideÂ alreadyÂ hollowÂ stemsÂ orÂ chewingÂ intoÂ theÂ pithyÂ centerÂ ofÂ stems,Â orÂ inÂ existingÂ holes,Â sometimesÂ manâ made.Â MostÂ tunnelânestingÂ beesÂ areÂ solitaryÂ species.Â PhotoÂ Credit:Â JenniferÂ Hopwood,Â XercesÂ SocietyÂ Figure 2-1. Photo gallery of the main groups of insect pollinators.
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-8 ButterfliesÂ Order:Â LepidopteraÂ Families:Â Papilionidae,Â Hesperiidae,Â Pieridae,Â Lycaenidae,Â NymphalidaeÂ WithÂ theirÂ strikingÂ transformationÂ fromÂ aÂ chubbyÂ plantâchewingÂ caterpillarÂ toÂ aÂ delicateÂ pupaÂ toÂ aÂ gracefulÂ nectarâdrinkingÂ wingedÂ adultÂ (FigureÂ 2â3),Â butterfliesÂ areÂ someÂ ofÂ theÂ mostÂ belovedÂ insects.Â SomeÂ speciesÂ haveÂ narrowÂ hostâplantÂ needsÂ forÂ theirÂ caterpillars,Â whileÂ othersÂ feedÂ onÂ aÂ wideÂ varietyÂ ofÂ plants.Â FlowerâvisitingÂ mothsÂ Order:Â LepidopteraÂ Families:Â Sphingidae,Â Noctuidae,Â ArctiidaeÂ Moths,Â whichÂ areÂ oftenÂ subduedÂ inÂ colorÂ andÂ tendÂ toÂ flyÂ atÂ duskÂ orÂ night,Â areÂ lessÂ visibleÂ thanÂ otherÂ groups,Â butÂ severalÂ areÂ importantÂ specialistÂ pollinatorsÂ ofÂ wildÂ plants.Â MothsÂ asÂ aÂ groupÂ formÂ aÂ criticalÂ foodÂ sourceÂ forÂ wildlife.Â FlowerâvisitingÂ fliesÂ Order:Â DipteraÂ Families:Â Syrphidae,Â Tachinidae,Â othersÂ FlowerâvisitingÂ fliesÂ consumeÂ nectarÂ andÂ sometimesÂ pollen.Â ManyÂ hoverÂ fliesÂ (inÂ theÂ familyÂ Syrphidae)Â resembleÂ beesÂ orÂ waspsÂ inÂ coloration.Â LarvaeÂ ofÂ someÂ speciesÂ areÂ voraciousÂ predatorsÂ ofÂ smallÂ insectsÂ (includingÂ cropÂ pests),Â likeÂ aphids.Â FlowerâvisitingÂ waspsÂ Order:Â HymenopteraÂ Families:Â Sphecidae,Â Vespidae,Â Tiphiidae,Â Scoliidae,Â othersÂ PredatoryÂ wasps,Â mostÂ ofÂ whichÂ areÂ solitary,Â huntÂ forÂ preyÂ (includingÂ cropÂ pests)Â toÂ bringÂ toÂ theirÂ nestÂ asÂ foodÂ forÂ theirÂ carnivorousÂ young.Â TheyÂ buildÂ nestsÂ inÂ cavitiesÂ orÂ inÂ theÂ groundÂ andÂ mayÂ useÂ piecesÂ ofÂ grass,Â mud,Â orÂ resinÂ inÂ nestÂ construction.Â AdultsÂ maintainÂ theirÂ energyÂ byÂ consumingÂ nectar,Â andÂ inÂ theÂ processÂ mayÂ alsoÂ transferÂ pollenÂ betweenÂ flowers.Â PhotoÂ Credit:Â JenniferÂ Hopwood,Â XercesÂ SocietyÂ Figure 2-1 (continued)
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-9 FlowerâvisitingÂ beetlesÂ Order:Â ColeopteraÂ Families:Â Cantharidae,Â Coccinellidae,Â Scarabaeidae,Â othersÂ FlowerâvisitingÂ beetlesÂ consumeÂ nectarÂ andÂ pollenÂ andÂ mayÂ alsoÂ chewÂ onÂ flowerÂ parts.Â LarvaeÂ ofÂ someÂ speciesÂ areÂ predatory,Â huntingÂ otherÂ insectsÂ (includingÂ cropÂ pests)Â asÂ food,Â whileÂ othersÂ areÂ herbivorousÂ orÂ areÂ decomposers.Â PhotoÂ Credit:Â JenniferÂ Hopwood,Â XercesÂ SocietyÂ Figure 2-1 (continued) ImageÂ Credit:Â DavidÂ Wysotski/AllureÂ IllustrationÂ Figure 2-2. Although most bee species native to the United States are solitary, with each female building and provisioning her own nest, bumble bees form small social colonies.
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-10 ImageÂ Credit:Â SaraÂ Morris/XercesÂ Society.Â Figure 2-3. The main groups of insect pollinators (bees, beetles, flies, moths, wasps, and butterflies, including the monarch butterfly, the life cycle shown here) all have four distinct life stages: egg, larva, pupa, and adult. These life stages, particularly the larvae and adult, may occur in different habitats. 2.3.1 Honey Bees The western honey bee (Apis mellifera) is the most widely known bee species worldwide, due to its long relationship with humans. Native to Europe, Africa, and Asia, the western honey bee was first domesticated early in human history, thousands of years ago, for the harvesting of honey. Western honey bees are not native to North America; hives were first introduced by European settlers in the 1600s (Engel et al. 2009). The western honey bee lives in large social colonies (hives) of up to 20,000 individuals, with division of labor within the colony. Only the queen bee reproduces, while several generations of her daughters gather nectar and pollen to feed and rear brood and store food (honey) for the winter. Honey bees have increasingly been managed for commercial crop pollination since the 1950s, with beekeepers bringing hives temporarily to agricultural fields or orchards to increase the density of pollinators. Though western honey bees are the most important managed crop pollinator species in the United States. (Morse and Calderone 2000), the species is not the
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-11 only pollinator involved in crop pollination. Many species of native, wild, and unmanaged bees, as well as some other insects like flower flies, play a critical role in crop pollination as well (Garibaldi et al. 2013). Because honey bees are such a familiar species, they can be a way to introduce new audiences to pollination and the role pollinators play in our lives and in our world. Honey bees have frequently been in the news in recent years as colony losses are occurring at a rate that is unsustainable for beekeepers due to pathogens, pesticides, poor nutrition, and other problems associated with industrial agriculture (Shanahan 2022). However, honey bee losses are often viewed as an environmental concern, rather than an agricultural issue, in part because honey bees are not often viewed as the widely distributed livestock animal that they are. Although beekeepers are losing colonies, they are able to replace them every year and treat some of their health problems through management (though at a cost). Honey bees are still extremely numerous, with millions of hives managed by beekeepers in the United States. (National Agricultural Statistic Service, United States Department of Agriculture 2019); they are not in any danger of disappearing from the planet. Honey bees can be a âgateway speciesâ in some instances, but concern will not always spill over to the native species that are actually in need of conservation (Geldmann and Gonzalez-Varo 2018). For this reason, it is important to recognize and emphasize that there are many pollinators other than honey bees. Honey bees also pose potential risks to native pollinators and their associated landscapes. The foraging habits of honey bees, the amount of pollen and nectar that they consume, their interactions with native bees, and their high level of pathogen loads can all negatively affect native pollinators and plant communities. One estimate of honey bee resource use found that a standard apiary with 40 hives removes pollen that would otherwise support four million wild bees (Cane and Tepedino 2017). Honey bees put considerable competition pressure on native pollinators; for example, when honey bee densities increased, competition for floral resources forced a decline in two species of bumble bees (Thomson 2016). Competition from honey bees can also reduce wild pollinator visitation to native plants, thus reducing pollination and reproduction, while increasing the spread of some invasive flowering species that honey bees preferentially visit (Goulson 2003; Magrach et al. 2017). Honey bees can also transmit diseases to native bees through interactions on shared flowers (Singh et al. 2010; Furst et al. 2014). Pathogens are one of the several forces behind the declines of some imperiled species (e.g., rusty patched bumble bee). Because honey bees have the potential to negatively affect native species, caution should be taken when considering the placement of honey bee hives in or near habitat that supports imperiled or listed pollinator species. 2.4 PollinatorsÂ andÂ RoadsidesÂ As pollinators have declined in the United States, there is increased interest in managing existing habitat or creating additional habitat to support pollinators (National Research Council 2007). Roadsides are some of the most extensive networks of linear habitats in the HoneyÂ bees,Â thoughÂ beloved,Â familiar,Â andÂ importantÂ inÂ commercialÂ cropÂ pollination,Â canÂ transmitÂ diseaseÂ andÂ putÂ considerableÂ competitiveÂ pressureÂ onÂ nativeÂ pollinators.Â Â PhotoÂ Credit:Â DavidÂ CappaertÂ
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-12 United States, and they extend across urban as well as rural landscapes. In some highly altered landscapes, roadsides are the only natural vegetation that remains (e.g., New et al. 2021). Pollinator diversity can be high in roadsides, with communities that include a significant portion of the species found in the region (Ries et al. 2001; Hopwood 2008; Noordijk et al. 2009). Roadsides can be home to rare species as well as common species (Munguira and Thomas 1992; Ries et al. 2001). They can support pollinators through a portion of their life cycle, or species may live their entire lives on roadsides. Specifically, roadsides provide pollinators with a place to find food, reproduce, and take shelter or overwinter, and they can increase habitat connectivity. In addition to supporting pollinators and flowering plants, roadsides also provide a number of other services, including support carbon sequestration; regulate air, water, and soil; enhance aesthetics and safety for drivers; and showcase regional beauty (Phillips et al. 2020). 2.4.1 Roadsides Provide Food for Pollinators Nectar and Pollen Sources Adults of bees, butterflies, wasps, and many species of flies, moths, and beetles feed on nectar to maintain their energy levels. Some adult beetles and flies require the protein that pollen provides in order to reproduce. Flowering plants in roadsides are important sources of nectar and pollen for pollinators that reside within the roadside habitat (e.g., Munguira and Thomas 1992) as well as those that use the roadside as a partial habitat for foraging but reproduce or overwinter elsewhere (e.g., Ouin et al. 2004). Native plants are particularly important to pollinators. Native plants are more attractive as sources of pollen and nectar for pollinators than nonnative plants, and they support more species and more individuals, even when both plant types are present at sites (Williams et al. 2011; Morandin and Kremen 2013) because the native species of plants are those that pollinators have evolved to depend on. Some species of native plants are particularly attractive to a wide range of pollinators, offering large quantities of nectar, high-quality nectar, or pollen with high protein content. Native plants are particularly important for imperiled pollinators, which may need specific sources of pollen and nectar. Roadsides with native wildflowers support a greater number of individuals and species of butterflies and bees compared with those dominated by nonnative grass and flowers (Ries et al. 2001; Hopwood 2008). InÂ additionÂ toÂ providingÂ pollenÂ andÂ nectarÂ forÂ adults,Â floweringÂ plantsÂ thatÂ growÂ onÂ roadsidesÂ canÂ alsoÂ serveÂ asÂ hostÂ plantsÂ forÂ caterpillarsÂ ofÂ butterfliesÂ andÂ moths.Â TheÂ commonÂ milkweedÂ thatÂ isÂ growingÂ onÂ thisÂ MarylandÂ roadsideÂ supportsÂ monarchÂ butterflyÂ caterpillars.Â Â PhotoÂ Credit:Â LisaÂ KuderÂ
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-13 Host Plants Butterflies and moths lay their eggs on plants on which their caterpillars (larvae) will feed upon after hatching; these plants are known as host plants. Some butterflies and moths have specific host- plant needs, relying on plants of a single genus or even a single species of host plant. For example, caterpillars of the Karner blue butterfly (Lycaeides melissa samuelis) will only survive feeding on sundial lupine (Lupinus perennis). Other species may exploit a wide range of plants, feeding on trees, shrubs, grasses, or wildflowers. Establishing caterpillar host plants is recognized as a way to sustain butterfly and moth populations (Croxton et al. 2005; Feber et al. 1996). Roadsides with host plants can support habitat generalist butterflies as well as habitat specialists and migrant species such as the monarch butterfly (e.g., Ries et al. 2001). Native plants are important host plants, often preferred by butterflies and moths over nonnative species. For example, native woody plants used as ornamentals in the eastern United States support fifteen times more native butterflies and moths than do introduced species of ornamental plants (Tallamy and Shropshire 2009). Research has shown that adding nonnative plants to landscapes does not increase butterfly and moth diversity or abundance (Burghardt et al. 2010), and nonnative plants can reduce bird populations as a consequence of reduced insect availability as food (Narango et al. 2018). 2.4.2 Roadsides Provide Shelter and Overwintering Habitat Woody vegetation outside of the recovery zone, such as trees and shrubs, can provide cover during the growing season that can serve as shelter for pollinators and can provide niches for overwintering. Some pollinators will overwinter under bark or in the soil just under shallow roots, or in piles of brush. Grasses can provide shelter for a variety of pollinators, most notably for butterflies on roadsides (Saarinen et al. 2005), and the root systems and grass thatch serve as overwintering habitat. Bees and predatory wasps provide for their young by constructing and provisioning nests in which their offspring develop. Some species of bees nest underground, while others nest in tunnels or within insulated cavities (Table 2-2). Nesting is a critical factor affecting the ability of bees to persist within a site (Winfree 2010; Menz et al. 2011; Morandin and Kremen 2013). AboutÂ 70Â percentÂ ofÂ beeÂ speciesÂ inÂ theÂ UnitedÂ StatesÂ nestÂ underground.Â NestsÂ mayÂ beÂ asÂ shallowÂ asÂ aÂ fewÂ centimetersÂ fromÂ theÂ surfaceÂ orÂ mayÂ beÂ asÂ deepÂ asÂ threeÂ feetÂ orÂ more.Â Â PhotoÂ Credit:Â JenniferÂ Hopwood,Â XercesÂ Society TunnelânestingÂ beesÂ willÂ buildÂ theirÂ nestsÂ inÂ existingÂ tunnelsÂ inÂ hollowÂ plantÂ stemsÂ orÂ oldÂ beetleÂ borerÂ tunnels,Â whileÂ othersÂ willÂ excavateÂ pithyÂ stems,Â asÂ seenÂ here,Â toÂ createÂ theirÂ tunnelÂ nest.Â PhotoÂ credit:Â SaraÂ Morris/XercesÂ SocietyÂ NonnativeÂ plant:Â AÂ plantÂ introducedÂ byÂ humans,Â whetherÂ onÂ purposeÂ orÂ accidentally,Â toÂ aÂ newÂ placeÂ orÂ newÂ habitatÂ whereÂ itÂ didÂ notÂ previouslyÂ grow.Â InvasiveÂ plant:Â AÂ nonnativeÂ plantÂ thatÂ establishesÂ andÂ growsÂ quicklyÂ onÂ manyÂ sites,Â spreadingÂ wildlyÂ andÂ disruptingÂ plantÂ communitiesÂ orÂ ecosystems.Â InvasiveÂ plantsÂ canÂ causeÂ economicÂ orÂ environmentalÂ harmÂ orÂ harmÂ toÂ humanÂ health.Â NoxiousÂ plant:Â AÂ plantÂ thatÂ canÂ directlyÂ orÂ indirectlyÂ injureÂ orÂ causeÂ damageÂ toÂ crops,Â livestock,Â poultryÂ orÂ otherÂ interestsÂ ofÂ agriculture,Â irrigation,Â navigation,Â theÂ naturalÂ resourcesÂ ofÂ theÂ UnitedÂ States,Â theÂ publicÂ health,Â orÂ theÂ environment.Â Note:Â TheÂ U.S.Â DepartmentÂ ofÂ AgricultureÂ AnimalÂ andÂ PlantÂ HealthÂ InspectionÂ ServiceÂ maintainsÂ aÂ listÂ ofÂ federallyÂ recognizedÂ noxiousÂ weeds,Â andÂ eachÂ stateÂ alsoÂ hasÂ itsÂ li
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-14 Table 2-2. Nesting habitat for bees. BeeÂ GroupsÂ byÂ NestingÂ HabitÂ ExampleÂ GroupsÂ NestingÂ LocationsÂ HabitatÂ NeedsÂ Groundâ nestingÂ beesÂ SweatÂ bees,Â minerÂ bees,Â longhornÂ beesÂ NestsÂ areÂ excavatedÂ inÂ theÂ soil,Â soÂ theseÂ beesÂ needÂ accessÂ toÂ theÂ soilÂ surface.Â SomeÂ speciesÂ willÂ nestÂ inÂ aÂ varietyÂ ofÂ soils,Â whileÂ othersÂ haveÂ veryÂ specificÂ requirementsÂ forÂ theÂ soilÂ type,Â moisture,Â alkalinity,Â slope,Â andÂ aspectÂ (CaneÂ 1991).Â OpeningsÂ inÂ scrubÂ orÂ forestÂ habitatÂ canÂ promoteÂ groundânestingÂ bees.Â BunchÂ grassesÂ tendÂ toÂ offerÂ betterÂ nestingÂ habitatÂ thanÂ sodâformingÂ grassÂ species.Â RoadsidesÂ withÂ nativeÂ bunchÂ grassesÂ haveÂ moreÂ nestingÂ opportunitiesÂ forÂ groundâ nestingÂ beesÂ and,Â consequently,Â aÂ greaterÂ abundanceÂ ofÂ theseÂ beesÂ (HopwoodÂ 2008).Â Tunnelâ nestingÂ beesÂ LeafcutterÂ bees,Â masonÂ beesÂ NestÂ inÂ hollowÂ stemsÂ orÂ excavateÂ pithyÂ stemsÂ (e.g.,Â elderberryÂ orÂ caneÂ fruits)Â inÂ plantÂ orÂ inÂ tunnelsÂ inÂ wood,Â suchÂ asÂ abandonedÂ beetleÂ tunnelsÂ inÂ logs,Â stumps,Â andÂ snagsÂ (MichenerÂ 2007).Â WhereÂ siteâappropriate,Â plantingÂ nativeÂ wildflowersÂ withÂ pithyÂ stemsâsuchÂ asÂ cupplantÂ (SilphiumÂ perfoliatum),Â ironweedsÂ (VernoniaÂ spp.),Â andÂ sunflowersÂ (HelianthusÂ spp.),Â alongÂ withÂ shrubsÂ suchÂ asÂ wildÂ roseÂ (RosaÂ spp.),Â elderberryÂ (SambucusÂ spp.),Â sumacÂ (RhusÂ spp.),Â yuccaÂ (YuccaÂ spp.),Â orÂ agaveÂ (AgaveÂ spp.)âwillÂ provideÂ resourcesÂ forÂ stemânestingÂ bees.Â Cavityâ nestingÂ beesÂ BumbleÂ beesÂ NestÂ inÂ socialÂ coloniesÂ inÂ insulatedÂ cavities,Â suchÂ asÂ underneathÂ grassÂ clumpsÂ (SvennsonÂ etÂ al.Â 2000),Â underÂ theÂ thatchÂ ofÂ bunchÂ grasses,Â inÂ woodÂ piles,Â orÂ underÂ rocksÂ (HatfieldÂ etÂ al.Â 2012).Â MowingÂ andÂ grazingÂ canÂ haveÂ negativeÂ impactsÂ onÂ bumbleÂ beeÂ coloniesÂ (HatfieldÂ etÂ al.Â 2012).Â IfÂ aÂ colonyÂ isÂ destroyedÂ viaÂ mowingÂ midâgrowingÂ season,Â noÂ queensÂ willÂ beÂ produced,Â whichÂ canÂ lowerÂ populationÂ levelsÂ theÂ followingÂ year.Â Pollinators also need a place in which to overwinter during the dormant season. Syrphid fly species and soldier beetles overwinter in roadside soil or litter (Schaffers et al. 2012). Butterflies and moths also use roadsides as overwintering habitat (Schaffers et al. 2012), though a few species travel long distances to overwinter as adults in warmer locations (e.g., monarch butterflies). 2.4.3 Roadsides Provide Landscape Connectivity As habitat fragmentation increases, landscape connectivity is becoming more important to the conservation of many species, including pollinators (Saunders et al. 1991; Haddad 1999; Haddad and Baum 1999). Roadsides have the potential to act as corridorsâstrips or patches of habitat that serve as stepping stones to connect larger patches of habitatâ that facilitate movement between habitat fragments, aid in establishing or maintaining populations, and increase species diversity within isolated areas (Tewksbury et al. 2002; Ottewell et al. 2009).
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-15 Roadsides extend across a variety of landscapes and often contain greater plant diversity than adjacent lands. The linear shape and connectivity of roadsides can help pollinators move through the landscape (Soderstrom and Hedblom 2007; Daniel-Ferreira et al. 2022a), either in search of food or in pursuit of new habitat (LÃ¶vei et al. 1998; Ries et al. 2001; Valtonen and Saarinen 2005; Hopwood et al. 2010). Additionally, several pollinator species have expanded their ranges along roadsides (Dirig and Cryan 1991; Brunzel et al. 2004). Corridors like roadsides and other linear strips of vegetation such as utility rights-of-way may enhance climate resiliency for pollinators as climate change causes shifts in species ranges. (See Chapter 8 for more information.) Due to their mobility, pollinators have the ability to use partial habitats, habitats that may have one or more components for their survival but not all. For example, bumble bees may nest in one location but forage for food in another. Similarly, an imperiled butterfly might lay eggs on its host plant on a roadside but seek nectar elsewhere. 2.4.4 Risks to Pollinators from Roads Roadsides present many conservation opportunities for pollinators, though roads can also pose certain risks. Roads can be a source of mortality for pollinators due to collisions with vehicles, and roadside vegetation is exposed to vehicle pollution. On roadsides, dense stands of invasive plants reduce pollinator abundance and diversity, and certain types of vegetation management can cause mortality or influence the resources available for pollinators. Vehicle Mortality Collisions with vehicles are a source of mortality for pollinators that use resources in roadsides as well as pollinators traveling through the landscape (McKenna et al. 2001; Baxter-Gilbert et al. 2015; Keilsohn et al. 2018). Mortality rates of butterflies using roadside vegetation range from 0.6 to 7 percent of the population of the butterflies found on the roadside (Munguira and Thomas 1992; Ries et al. 2001; SkÃ³rka et al. 2013), which is lower than the rates of butterflies killed by natural enemies like predators and parasites (Munguira and Thomas 1992). A review of studies on first instar larval Lepidopterans found high rates of mortality from natural enemies and other factors that varied widely from 40â95% (Zalucki et al. 2002). Some species of pollinators are more vulnerable to collisions than others due to their behavior or biology. Butterflies that mud- puddle on rural gravel roads to obtain minerals and water may have higher mortality rates (Campioni et al. 2022), while butterflies that are strong fliers appear to have lower rates of mortality than those that are not (Munguira and Thomas 1992; Ries et al. 2001; SkÃ³rka et al. 2013). However, migratory species may have increased risks due to vehicle collisions. More monarch butterflies are killed due to vehicle collisions during fall migration compared to other parts of their migratory cycle, for example (Kantola et al. 2019). As monarchs funnel through Texas on their way to overwintering grounds in Mexico, there are hotspots of mortality due to vehicle collisions. There are relatively few hotspots but those may kill approximately 2 million VehiclesÂ canÂ beÂ aÂ sourceÂ ofÂ mortalityÂ forÂ pollinators;Â however,Â mortalityÂ ratesÂ tendÂ toÂ beÂ lowerÂ whenÂ roadsideÂ plantÂ diversityÂ isÂ high.Â PhotoÂ Credit:Â JeffÂ NorciniÂ
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-16 monarchs per year; hotspots occur in places that are important migratory crossing locations that are constrained to cross roads in Texas and Mexico (Kantola et al. 2019; Tracy et al. 2019; Mora Alvarez et al. 2019). Temporarily reducing traffic speed, closing outer lanes, and using netting alongside the road in mortality hotspots during key times in butterfly migration are techniques that have significantly reduced mortality rates of another migratory butterfly in Taiwan (Yang Ping-shih, personal communication.) Traffic volume may also influence rates of mortality (SkÃ³rka et al. 2013; Daniel-Ferreira et al. 2022b). For example, queen bumble bees were more often killed on roads with higher traffic intensity, although mortality was slightly lower along roadsides with higher-quality vegetation (Daniel-Ferreira et al. 2022b). On the other hand, several studies have found that traffic volume does not consistently influence observed butterfly mortality (McKenna et al. 2001; Saarinen et al. 2005). Road width may also influence butterfly response to roads; wider roads may increase mortality rates (SkÃ³rka et al. 2013). Additionally, vegetation quality can influence pollinator mortality; roadsides with more species of plants had fewer butterflies killed by traffic (SkÃ³rka et al. 2013; SkÃ³rka et al. 2015). Improving vegetation quality can reduce road crossings (Polic et al. 2014; Ries et al. 2011). The frequency of mowing is also linked to a higher proportion of butterflies killed on roads. Butterflies that had to disperse to find new habitat after roadsides were mowed may have had a greater likelihood of collisions with vehicles (SkÃ³rka et al. 2013). Reducing mowing during peak seasonal butterfly activity can increase butterfly numbers (Halbritter et al. 2015). Steps to ameliorate the impacts of vehicle collisions on pollinators can include: ï· increasing roadside plant diversity and ï· reducing mowing beyond the mown strip in the recovery area (also known as the clear or safety zone, the band of low growing or routinely mowed vegetation directly adjacent to the pavement or shoulder where vehicles that have left the roadway can recover; Figure 2-4). The available science indicates that the benefits of supporting pollinators on roadsides outweigh the costs (Philips et al. 2020), but, when possible, road impacts on imperiled pollinators should be mitigated to reduce threats to local populations. Development of a landscape connectivity map may also be helpful to prioritize locations for high-quality revegetation projects with goals of supporting pollinators (see Chapter 9 for a pollinator habitat assessment guide that can be used to help prioritize locations). It is also worth considering focusing first on sites that are not in areas with a dense network of roads where habitat is fragmented, as some butterflies have decreased movement between sections of roadsides that are not connected to other roadsides (Valtonen and Saarinen 2005).
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-17 ImageÂ Credit:Â ArizonaÂ DepartmentÂ ofÂ TransportationÂ Figure 2-4. Recovery areas on a rural interstate road. A recovery area is free of obstruction and the width is determined by the type of road and traffic volume, as well as the slope of the embankment. Roadside Contamination and Pollutants Routine vehicle use and maintenance of roads contribute to roadside vegetation contamination by depositing pollutants, including vehicle exhaust and de-icing materials. Roadside soils and vegetation can be contaminated with heavy metalsâsuch as lead, iron, zinc, copper, cadmium, and nickelâdeposited from tire rubber, brake dust, and gasoline and diesel combustion products (Gjessing et al. 1984; Oberts 1986; Araratyan and Zakharyan 1988). Vehicle-derived contamination is proportional to traffic levels (Leharne et al. 1992; Mitchell et al. 2020). In general, plant and soil contamination is greatest adjacent to the road and decreases with distance from the road, usually declining significantly within 20 meters (Quarles et al. 1974; Dale and Freedman 1982; Jablonski et al. 1995; Swaileh et al. 2004). Pollen and nectar contamination is also greatest nearest to the road (Jablonski et al. 1995). Few studies have examined the impacts of heavy metal exposure in roadsides on pollinators. One study in Minnesota found elevated levels of lead in roadside soils, but not in leaves of roadside plants, meaning pollinators would not be exposed to the elevated levels by consuming plant parts (Mitchell et al. 2020). Plants that grew closer to the road edge along busier roadsides had elevated levels of zinc in their leaves, but higher concentrations of zinc in plants did not appear to increase higher concentrations of zinc in the caterpillars that consumed the plants (Mitchell et al. 2020). In Britain, heavy metal pollution was highest and pollinator activity was reduced within 2 meters of the road (Phillips et al. 2021). Different plant species uptake heavy metals at different rates, with some taking up more (e.g., Ratibida pinnata) and others taking up PlantÂ diversityÂ canÂ reduceÂ pollinatorÂ exposureÂ toÂ heavyÂ metals.Â PlantsÂ suchÂ asÂ theÂ purpleÂ prairieÂ cloverÂ (DaleaÂ purpurea)Â andÂ theÂ yellowÂ coneflowerÂ (RatibidaÂ pinnata)Â growingÂ onÂ thisÂ ColoradoÂ roadsideÂ uptakeÂ heavyÂ metalsÂ atÂ differentÂ amounts.Â Â PhotoÂ Credit:Â JasonÂ Roth/CDOTÂ
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-18 smaller amounts (e.g., Dalea purpurea) (Emilie Snell-Rood, personal communication). Plant diversity can therefore be a buffer against heavy metal exposure. Dry deposition of particles with nitrogen derived from fuel combustion can create a strip of âfertilizedâ soil along roadsides, particularly in more arid regions (Gade 2013). Busier roads are linked to increased nitrogen in roadside plants (Cape et al. 2004). Higher nitrogen content in leaves can attract insect herbivores, but it is unknown whether caterpillars are more abundant within higher nitrogen areas in roadsides. De-icing salts used on roads alter roadside soil chemistry by increasing sodium levels in soil and plant tissues significantly (Snell-Rood et al. 2014; Mitchell et al. 2020). At high levels, salts can damage some plants (Bogemans et al. 1989), which may indirectly affect pollinators by altering or removing their food sources. Sodium levels in plants can directly affect caterpillars consuming vegetation. Varying levels of sodium in butterfly host plants can affect development of caterpillars in both positive and negative ways. Sodium is an important micronutrient for butterflies, and moderate levels of sodium can increase flight muscle and brain size of adults. However, too much sodium can be toxic (Snell-Rood et al. 2014). Soil and leaf sodium levels are higher close to the road edge and on roads with more traffic (Mitchell et al. 2020). However, sodium levels only reach lethal levels in 1 percent of roadside plants in Minnesota (Mitchell et al. 2020). Invasive Species Many invasive and noxious plants can be present in roadsides (Tyser and Worley 1992; Gelbard and Belnap 2003) due to favorable conditions for plant introductions and invasions (Hansen and Clevenger 2005; Von der Lippe and Kowarik 2007). Noxious and invasive plants can decrease the quality of roadside habitat for pollinators (Hopwood 2008; Valtonen et al. 2006), compete with native plants for resources, and alter habitat composition. Some cause significant reductions in the abundance and diversity of pollinators and other herbivorous insects (Samways et al. 1996; Kearns et al. 1998; Spira 2001; Memmott and Wasser 2002; Zuefle et al. 2008; Burghardt et al. 2009; Tallamy and Shropshire 2009; Wu et al. 2009; Hanula and Horn 2011; Fiedler et al. 2012). Noxious and invasive plants can spread to nearby properties and affect working lands. They can also alter wildfire cycles, increasing the frequency of burns over native vegetation (e.g., Balch et al. 2012). A number of native insects will feed on noxious and invasive plants when few natives are available (Zuefle et al. 2008; Burghardt et al. 2009; Wu et al. 2009; Williams et al. 2011). Monarch butterflies, for example, will nectar on Canada thistle (Cirsium arvense). Despite potential use by pollinators, the threat that noxious and many invasive weeds pose to landowners adjacent to roadsides, to pollinators themselves, and to biodiversity overall is much greater than the benefits of using invasive species. Canada thistle, for example, only blooms for a short period of time, providing a temporary pulse of nectar, as compared to the season-long resources that would be available through a diverse plant community. It is not realistic to remove all nonnative plant species from roadsides, nor is it always possible to remove some invasive species due to limited resources and time. State departments of transportation (DOTs) may prioritize controlling certain invasive species that are more problematic than others. However, noxious and invasive species do not need to be retained solely as a method to support pollinators.
ChapterÂ 2.Â PollinatorÂ BiologyÂ andÂ RoadsidesÂ 2-19 Vegetation Management The management of roadside vegetation can have a significant impact on pollinators. Mowing vegetation beyond the mown strip in the recovery area multiple times a growing season, for example, can cause direct mortality to pollinators in the egg or larval stages (Humbert et al. 2010; Steidle et al. 2022); can deprive pollinators of sources of pollen, nectar, and host plants for larvae (Johst et al. 2006); and can destroy bumble bee colonies (Hatfield et al. 2012). However, the timing and frequency of mowing can be adjusted to substantially reduce the impacts on pollinators (e.g., Halbritter et al. 2015), as can adjustments to other management techniques. For additional information about vegetation management and strategies that can support pollinators, see Chapter 6, Roadside Maintenance and Vegetation Management for Pollinators. Balancing Risks with Conservation Opportunities There are risks to pollinators from roads, such as mortality from traffic. However, available evidence suggests that the benefits of roadsides to pollinators far outweigh the costs (Phillips et al. 2020). Roadsides often have vegetation not found in the surrounding landscape, and roadsides can be enhanced through management or revegetation to provide quality habitat for pollinators. Adjusting management of roadside vegetation can be effective, and DOTs can strategically enhance roadside vegetation across the road network to promote connectivity and prioritize sites with the greatest capacity to benefit pollinators (Phillips et al. 2020).