Pollinators in the Landscape I: Importance of Pollinators and Causes of Decline

Photos by Mandy Bayer

The Importance of Pollinators

Concerns about honeybee populations and monarch butterfly migration have brought attention to the need to protect and support pollinators in the landscape. To understand the importance of pollinators, it is necessary to have an understanding of pollination. There are two types of pollination, self-pollination in which a plant is able to pollinate itself without outside help, and cross-pollination which is pollination aided by animals, wind, and water. The majority of pollination is cross-pollination with animal assisted pollination accounting for around 75%. Pollination is necessary for fruit development, seed production, and can result in better quality of some fruits (such as tomato). Almost all fruit and grain crops grown in the US require pollination (over 150 crops!) and crops dependent on pollinators are estimated by the USDA to be worth more than $10 billion per year.

Pollinator Decline

Pollinator decline is an area of controversy and concern, with the growing consensus being that it is not a singular cause, but a combination of multiple stressors leading to population declines. Pesticides, with a focus on neonicotinoids, are one of the most controversial contributors to decline. The concern with pesticides and herbicides is twofold: 1) the direct health implications of pesticides on pollinators and 2) the use of herbicides in cropping systems, which creates monocultures that poorly support pollinators by reducing the variety of flowering plants in the area. The issue with neonicotinoid insecticides is that they travel systemically throughout the plant, including to pollen and nectar, increasing the likelihood of pollinator contact.

Loss of suitable habitats is another cause of concern. Urbanization and land conversion have contributed to loss of habitat. A combination of habitats is needed for food, nesting and mating areas, and migration. Urbanization has led to the fragmentation of habitats and the disruption of some migratory pathways. Land conversion from diverse natural landscapes to intensive cropping areas that are almost monocultures, decreases the variety of pollen and nectar sources for pollinators. Flowers vary in protein content and nutrient composition, so a diet of a singular plant species has the potential to impact pollinator health.

Transportation of bees and an increase of commercial bee colony trade have contributed to the spread of parasites and diseases outside of the normal range. The spread of parasites and pathogens is problematic because the new host species often lacks resistance, increasing the likelihood of death.

The impact of current and future climate change on pollinators is not yet well understood. Historically, changes in climate have caused the native ranges of plants and animals to shift to where conditions are more favorable. Many plants and pollinators have evolved to have a mutualistic dependency, so with potential range shifts there is concern that plant and pollinators will shift in different directions. Range shifts due to climate change have already been observed with some butterflies. Another concern is the alteration of bloom time and pollinator emergence. Climate change has the potential to lead to earlier warmer temperatures in the spring, which could disrupt the synchronization of flower development and pollinator emergence. There may also be increased potential for flower and bud damage due to increased frequency of late frost events if warmer temperatures come early.

The good news is that urban landscapes have great potential to support pollinators, both as nectar and pollen sources and as pollinator habitats.

Pollinator Plant Spotlight: Milkweed

Milkweed (Asclepias spp.) is the larval host plant for the monarch butterfly, meaning that milkweed is the only plant that monarch caterpillars feed on. Many plants and their pollinators have evolved together creating mutualistic dependencies. Milkweed contains toxins that are not harmful to monarch caterpillars, but are poisonous to many animals, providing protection for monarchs both as larval caterpillars and as adult butterflies.

Milkweed species for the Northeast:
Asclepias incarnata	Swamp milkweed
Asclepias tuberosa	Butterflyweed
Asclepias verticillata	Whorled milkweed
Asclepias exaltata	Poke milkweed

Pollinator Highlight: Butterflies and moths

Butterflies have four life stages: egg, caterpillar, pupa, and adult; all of which have differing habitat needs. There are many habitat options for butterfly eggs including leaves or branches of various plants and the soil. Larval, or caterpillar, host plants are much more limited (for example Monarch butterflies and milkweed) and need to provide the caterpillar with both food and shelter. At the pupa stage butterflies need a protected area, such as shrubs, tall grasses, or fallen branches. As adults, butterflies need plants that provide a nectar source. Butterflies are attracted to flowers that are brightly colored, open during the day and provide a good landing platform. Nectar is usually deeply hidden within the flower; however butterflies are able to use nectar guides to find nectar. Nectar guides are markings on flowers that are not visible to humans but can be seen by pollinators and direct them to nectar. While some butterfly species migrate in the winter, others hibernate, requiring leaf litter, bark, or other landscape debris as cover.

Moths can be either nocturnal or day-time pollinators depending on the type, with the majority being nocturnal and very important for the pollination of night blooming flowers. They are generally attracted to white or dull colored flowers that are heavily fragranced.

Plant features for attracting butterflies include:

• Bright colored flowers
• Wide landing pad
• Tubular flowers

Plant features for attracting moths include:

• Pale/dull colored flowers
• Strongly sweet fragrance (especially at night)
• Tubular flowers

Plants for Butterfly Gardens

Plants that attract butterflies (nectar sources):

Annuals
Scientific Name	Common Name	Size	Color	Bloom Time
Cosmos bipinnatus	Cosmos	12-48"	pink, red, white	June to frost
Lantana camara	Lantana	36-48"	yellow, pink, white, orange, red, purple, mixed	July to frost
Tagetes erecta	African marigold	12-48"	yellow, orange, white	June to frost
Tagetes patula	French marigold	6-12"	yellow, orange, red, bicolor	June to frost
Tithonia rotundifolia	Mexican sunflower	48-72"	orange-red	July to September
Tropaeolum	nasturtium	12-36"	red, orange, yellow, or cream	May to September
Zinnia spp.	zinnia	6-48"	red, orange, yellow, pink, white, purple, cream	June to frost

Perennials
Scientific Name	Common Name	Size	Color	Bloom Time	Native
Allium cultivars	ornamental onion	12-60"	purple, lavender	May-June
Asclepias incarnata	swamp milkweed	48-60"	white, pink, mauve	July - August	*
Asclepias purpurascens	purple milkweed	24-36"	pink to purple	May- July	*
Asclepias tuberosa	butterfly weed	12-30"	yellow/orange	June-August	*
Aster spp.	Aster	12-24"	purple, blue, lavender, white	August - September	some
Coreopsis cultivars	tickseed	12-24"	yellows	June - October
Coreopsis rosea	tickseed	12-36'	pink	June - September	*
Dianthus barbatus	sweet William	12-24"	red, pink, white, bicolor	May - frost
Echinacea cultivars	coneflower	6-48"	purple, pink, white, yellow, orange, lime	June - August
Echinacea purpurea	purple coneflower	24-60"	purple/pink	June - August	*
Eutrochium purpureum	Joe Pye weed	60-84"	pink	July - September	*
Gaillardia spp.	blanketflower	8-36"	yellow, orange, red	May - September
Helenium cultivars	sneezeweed	12-48"	red, orange, yellow	August - October
Lavandula angustifolia cultivars	English lavender	8-36"	purple, lavender, pink, blue	June - October
Leucanthemum x superbum	shasta daisy	22-36"	white	July - September
Liatris spicata	blazing star	8-18"	purple	July - August	*
Lobelia cardinalis	cardinal flower	24-48"	red, white, or pink	July - September	*
Lobelia siphilitica	blue cardinal flower	24-36"	blue	July - September	*
Monarda cultivars	bee balm	8-36"	reds, pinks, or purples	June - August
Phlox paniculata cultivars	garden phlox	12-48"	pink, red, white, purple, lavender, bicolor	July - September
Phlox subulata	creeping phlox	4-6"	red, purple, lavender, pink, white	March - May	*
Rudbeckia fulgida	black-eyed Susan	24-36"	yellow/orange	June - October
Scabiosa 'Butterfly Blue'	pincushion flower	12-18"	lavender blue	April - frost
Sedum spp.	stonecrop	2-18"	pink, red, white	July - September
Symphyotrichum novae-angliae	New England aster	36-72"	deep pink purple	August - September	*

Shrubs and Trees
Shrubs
Scientific Name	Common Name	Size	Color	Bloom Time	Native
Aesculus parviflora	bottlebrush buckeye	8-12'	white	June-July
Buddleja alternifolia	alternate-leaved butterfly bush	8-15'	lilac purple	May
Buddleja davidii cultivars	butterfly bush	3-12'	lilac, purple, pink, red, white	June - September
Ceanothus americanus	New Jersey tea	3-4'	white	May - July	*
Cephalanthus occidentalis	buttonbush	5-12'	white	June	*
Clethra alnifolia and cultivars	sweet pepperbush/ summersweet	2-8'	white, pink	July - August	*
Fothergilla gardenii	dwarf fothergilla	1.5-3'	white	April-May
Kalmia latifolia cultivars	mountain laurel	2-15'	pink, white, multicolor	May - June	*
Kolkwitzia amabilis	beautybush	6-10'	pink	April-May
Lindera benzoin	spicebush	6-12'	yellow/green	March	*
Rhododendron spp.	Rhododendron and azalea
Syringa vulgaris cultivars	common lilac	3-16'	lavender, purple, white, pink, blue	May
Vaccinium spp./cultivars	blueberry	3-12'	white	May	*
Viburnum spp.	viburnum	2-20'	white	April - June	some
Trees
Aesculus glabra	Ohio buckeye	20-40'	greenish yellow	April - May
Amelanchier spp.	Serviceberry	6-25’	white	April - May	*
Cornus florida and cultivars	flowering dogwood	15-30'	white, pink	April - May	*
Malus cultivars	flowering crabapple	7-30'	white, pink, coral
Prunus spp.	cherry, flowering almond	10-40'	white, pink	March - April

Host Plants (plants that caterpillars like to feed on):

Larval Host Plants
Scientific Name	Common Name	Size	Native
Alcea rosea	hollyhock	5-8'
Anethum graveolens	dill	3-5'
Asclepias spp.	milkweed	24-60"
Artemisia spp.	wormwood	6-36”
Aster spp.	aster	12-24”
Carex	sedge	0.5-1.5'
Echinacea purpurea	coneflower	18-36”
Panicum	switch grass	2.5-6'
Petroselinum crispum	parsley	0.5-1'
Schizachyrium scoparium	little bluestem	2-4'
Sedum spp.	stonecrop	2-18"
Viola spp.	violet	4-12"	some
Aesculus glabra	Ohio buckeye	20-40'
Amelanchier spp.	serviceberry	15-30'	some
Betula spp.	birch	30-70'	some
Carpinus caroliniana	hornbeam	20-35'	*
Ceanothus americanus	New Jersey tea	3-4'	*
Cephalanthus occidentalis	buttonbush	5-12'	*
Cercis canadensis	redbud	20-30'	*
Clethra alnifolia and cultivars	sweet pepperbush/ summerweet	2-8'	*
Cornus florida	flowering dogwood	15-30'
Cornus alternifolia	pagoda dogwood	15-25'
Fraxinus spp.	ash	35-80'	some
Lindera benzoin	spicebush	6-12'	*
Populus spp.	poplar, aspen	30-80'	some
Quercus spp.	oak	30-80'	some
Rhododendron spp.	rhododendron and azalea	2-12'	some
Rhus spp.	sumac	1.5-30'	some
Sassafras albidum	sassafras	30-60'	*
Salix spp.	willow	20-40'+	some
Syringa vulgaris cultivars	common lilac	3-16'
Vaccinium spp./cultivars	blueberry	3-12'	*
Viburnum spp.	viburnum	2-20'	some
Ulmus spp.	elm	30-80'	some

Sources:

Goulson, D., E. Nicholls, C. Botias, and E.L. Rotheray. 2015. Bee declines drive by combined stress from parasites, pesticides, and lack of flowers. Science 26 Feb 2015.

USDA Forest Service. Pollinators. https://www.fs.fed.us/wildflowers/pollinators/animals/

USDA Forest Service. Plant Milkweed for Monarchs. https://www.fs.fed.us/wildflowers/pollinators/Monarch_Butterfly/documents/MilkweedInfoSheet.pdf

University of Maine Extension. Landscaping for Butterflies in Maine. https://extension.umaine.edu/publications/7151e/

Massachusetts Butterfly Club. http://www.naba.org/chapters/nabambc/downloads/Butterfly%20gardening%20101%20-%20Western%20Massachusetts.pdf

USDA Natural Resources Concervation Service and the Wildlife Habitat Council. Native Pollinators. https://plants.usda.gov/pollinators/Native_Pollinators.pdf

UMass Center for Agriculture Food and the Environment. Protecting Bees and Pollinators from Pesticides in Home Gardens and Landscapes. https://ag.umass.edu/home-lawn-garden/fact-sheets/protecting-bees-pollinators-from-pesticides-in-home-gardens-landscapes

Missouri Botanical Garden Plant Finder http://www.missouribotanicalgarden.org/plantfinder/plantfindersearch.aspx