Trees are Important for Pollinators too!

By Denise Ellsworth

Important pollinators such as honey bees, bumble bees and monarch butterflies have gained attention in recent years due to concerns about declining populations. Pollinators are currently facing many threats, such as lack of forage (food), pests, pathogens, pesticides, invasive plants, climate change and lack of suitable nesting sites. Fortunately, woodland stewards can take steps to support these and other pollinators through plant selection and woodland management practices.

Pollination is the movement of pollen from the male part (stamen) of one flower to the female part (pistil) of another flower. Without pollination, most plants can’t make seeds and fruits. Many plants are wind pollinated, including most grains such as corn and wheat, but other plants rely on animals to carry pollen from flower to flower.

Animal pollinators are essential to the food we eat. Some scientists estimate that one in three bites of food we take can be traced back to the role of animal pollinators. A 2012 study by Cornell University estimates that bees and other insect pollinators contribute $29 billion annually to U.S. farm income by pollinating 58 crops, including almonds, apples, berries and squash.

In addition to their role on farms and gardens, pollinators are essential to the survival of native plants. Approximately 75% of all plants depend on animal pollinators to move pollen from plant to plant. Without the work of pollinators, many native plants couldn’t produce seeds to ensure the plant’s next generation. These seeds and the fruit that often accompany them also provide important food sources for approximately 25% of birds and many mammal species.Figure 1. Red maple and many other tree species provide an important source of food for pollinators.

Many people are concerned about the health and survival of bees, including honey bees, native bumble bees and the hundreds of lesser-known native and wild bees that call Indiana home. Bees are considered the most important group of pollinators because they are uniquely adapted to gather and transport pollen. Bees’ fuzzy bodies and branched hairs help female bees collect pollen into special structures, such as pollen baskets on the hind legs or long hairs on the legs or abdomen. Bees rely on flowers for food to feed their young, so they actively seek out and visit flowers.

Bees and flowering plants have a critical relationship. Flowering plants provide nectar and pollen for a bee’s diet. Pollen is an essential source of protein for developing bee larvae, and nectar provides a carbohydrate source. Honey bees convert nectar into honey by adding an enzyme which breaks down the complex sugars into simple sugars. Bees, in turn, transport pollen from flower to flower as they forage, allowing for plant fertilization and the production of seeds and fruit.

Flowers can also provide shelter and gathering places for pollinating insects. Flowers depend on repeat visits by pollinators, so they may offer small rewards repeated at regular intervals to encourage return trips. To attract visitors, flowers use a variety of strategies, including petal color, scent, UV light patterns and nectar guides (lines and marks on petals to direct pollinators to a reward). Bees in particular use floral qualities such as polarized light patterns, petal texture, temperature, humidity, and static charge to help them locate flowers.

Bees typically visit one or only a few flowering species during each foraging trip, even when other flowers are available, a behavior called flower fidelity or flower constancy. Bees also forage for food close to their nesting sites, a practice called central place foraging. These practices make bees especially reliable couriers to move pollen to receptive flowers.

While trees provide many well-known ecological benefits, the importance of trees as a source of food for bees is sometimes overlooked. When in bloom, a large tree can provide hundreds to thousands of nectar- and pollen-filled flowers. Because a tree’s flowers are often high up in the canopy out of view, the thousands of insects visiting these flowers are rarely noticed or appreciated.

Early-blooming trees such as maples, willows and redbuds provide food at an especially critical time. In March and April, queen bumble bees are establishing new colonies at a time when few other flowers are in bloom. Foraging honey bees take advantage of tree flowers in early spring to bring food back to the growing hive, which may be short on stored food after a long winter. Later in the spring, black locust and tulip tree provide a rich source of nectar, just the hive’s demand for food increases. For honey bees, honey stored during this period of “honey flow” is crucial to build the hive and ensure the health of the colony into the following winter.

For a sequence of native and non-native woody flowering plants for Ohio, consult the Ohio State Phenology Calendar http://www.oardc.ohio-state.edu/gdd/. This calendar will help Indiana residents predict the sequence of plant bloom from late winter to late summer.

Besides providing an essential source of food for pollinators, trees also provide important nesting and overwintering habitat. Cavity-nesting bees make their nests in the pith of twigs like elderberry or sumac, or in abandoned beetle burrows in dead trees. Brush piles, dead standing trees and fallen wood provide important nesting and overwintering habitat for bees and butterflies. Bare soil can provide nesting habitat for ground-nesting solitary bees such as mining bees or sweat bees that nest in sandy, well-drained soils, commonly on south-facing slopes.

By using an integrated pest management approach with multiple strategies to reduce pest damage, woodland managers can limit negative effects of pesticides on pollinators. Pesticides can kill bees and other insects outright or affect behavior or longevity. Insecticides and fungicides can act together to weaken bees’ immune system. Contact your county Extension office for pest management assistance and best management practices when using pesticides.

Woodland stewards can play an important role in pollinator conservation by providing plants and nesting sites for pollinators and by limiting pesticide impact. To learn more about pollinators and pollinator plants, consult these resources:

•  The Xerces Society for Invertebrate Conservation:
www.xerces.org

•  Pollinator Partnership: www.pollinator.org

•  The Ohio State University Bee Lab: beelab.osu.edu

Denise Ellsworth is the director for the Honey Bee and Native Pollinator Education program at the Ohio State University.