Which Annuals And Perennials Are Good For Pollinators?

Bees on flowers
Growers are more aware of the need to protect pollinators, particularly on plants that are highly attractive to bees. Photo courtesy of Jason Gibbs/Michigan State University Extension.

When most people hear “pollinators,” they think of honeybees, and in fact, protecting honeybees is an important part of President Obama’s 2015 plan to protect pollinators. The three key objectives in the President’s plan are:

1. Reduce honeybee colony losses during winter to no more than 15% within 10 years

2. Increase population of the monarch butterfly to 225 million butterflies in an area of approximately 15 acres in the overwintering grounds in Mexico

3. Restore or enhance 7 million acres of land for pollinators over the next five years

Each state is working on a pollinator protection plan that contributes to these objectives, and establishes many other ways to protect pollinators from harm.

Troubling Reports And Unanswered Questions Fuel Uncertaintly

Greenhouse and nursery growers have become part of the discussion about protecting pollinators because of the formerly widespread practice of using neonicotinoid insecticides to protect plants from aphids and whiteflies, which are often difficult to control in production. Most of the news coverage of this issue came from the annual release of the “Gardeners Beware” report by Friends of the Earth, which calls for a ban of spraying neonicotinoid insecticides.

In spring 2016, Friends of the Earth collected 60 plant samples from 12 states in the U.S. and sent them for analysis. They found that 24% of the samples tested positive for neonicotinoids, much less than in similar surveys in 2013 and 2014 when 50% of the samples were positive.

These surveys are interesting, but leave many unanswered questions, like:

Q: Are steps being taken to reduce residues of other insecticides used by growers, which could be just as harmful to pollinators?
A: The answer is yes, but there is no sample data to verify this.

Q: Will contract-banning of growers from using neonics result in an increase in the use of other pesticides?
A: Perhaps, but growers are now much more aware of the need to protect pollinators, particularly on plants that are highly attractive to bees.

Q: What is the relationship of the neonicotinoid concentration in leaves and petal tissues to what is actually found in nectar and pollen?
A: The concentration of neonics in pollen and nectar is usually much lower than what is found in leaves and petal tissues.

Because of extensive press coverage, many people came to believe that the heavy colony loss of honeybees during the winter is related to the use of neonicotinoids when greenhouses and nurseries grow plants for sale in garden centers. In fact, this probably has very little to do with the winter colony loss because most of the honeybee colonies in North America are kept by large beekeepers with thousands of colonies, and they rarely forage in heavily populated cities.

But it doesn’t matter, because in order to preserve honeybees and hundreds of species of native bees, along with the natural enemies that provide biological control, we as an industry want to grow flowers that are safe for pollinators. This has also opened some emerging markets because some fruit growers and gardeners want to buy plants that are good food sources for bees and butterflies.

Caterpillar on leaf
Pollinator food plants can be grouped into annuals, herbaceous perennials, and wildflowers. A fourth category would be good food plants for butterfly adults and the caterpillars that develop into them.

Growing Food Plants For Bees And Butterflies

Many greenhouse and nursery growers have adopted new pest management strategies to grow plants without using neonicotinoids, and to avoid leaving harmful residues of other pesticides on plants going to garden centers. For growers who want to learn more about how to manage difficult-to-control greenhouse and nursery pests and still grow pollinator-friendly plants, a good resource is a new Extension bulletin, “Protecting and Enhancing Pollinators in Urban Landscapes.”

The bulletin has excellent information on which types of flowers are the best food plants for bees and butterflies. Public interest in protecting bees and other pollinators has initiated a new market for flowers that are good food plants for pollinators. There are at least eight types of buyers who may be looking for plants that support bees and butterflies: landscapers, home gardeners, small fruit (blueberries, raspberries, etc.) and tree fruit (apples, cherries, etc.) growers, golf courses, arboreta, hobbyist beekeepers, schools, and college campuses.

The biggest single demand will probably be for milkweed, either as seed or seedlings to plant in the spring to provide summer food for monarch caterpillars and butterflies. More public and private gardens are now including pollinator gardens.

Recent research at Michigan State University has shown that the pollination of small fruits and tree fruits can be improved by installing food plants for native bees and honeybees. With an estimated 50% or more of pollination being carried out by native bees, putting in a variety of perennials that are attractive to bees can help increase pollination rates, and may reduce reliance on commercial beekeepers that bring hundreds of colonies into the orchards for spring bloom.

If an orchard decides to adopt this strategy, it may need hundreds of perennial plants. An added advantage to planting flowers for bees and butterflies is that these same flowers are an important resource for other beneficial insects — the predators and parasitoids that keep plant pests under control.

Growers may want to test the market for pollinator food plants. They can be grouped into the following categories for honeybees and native bees: annual flowers, herbaceous perennials, and wildflowers. A fourth category can be added for plants that are good food sources for butterfly adults and the caterpillars that develop into them. Each of these lists can be found in the Extension publication: “Protecting and Enhancing Pollinators in Urban Landscapes for the U.S. North Central Region.”

Safe Production Of Pollinator-Friendly Plants

When growing food plants for pollinators, it is important to make sure that the plants will be safe for bees that consume their nectar and pollen, or for caterpillars that will eat the leaves. Growers are increasingly faced with the difficult task of producing high-quality plants, free of insects and insect damage, without leaving harmful residues of insecticide on the plant surface or in the nectar or pollen. Retail buyers hold greenhouse and nursery growers to very high standards, so that insects or insect damage are not tolerated at any detectable level.

This means that plants cannot be treated with systemic insecticides for a year or perhaps longer before the plants are sold. Also, nearly all of the insecticides used in the greenhouse are toxic to bees, so they cannot be sprayed after flowers begin to develop. If plants are sprayed three weeks or more before the shipping date, most of them do not have flowers present at that time, and therefore the risk to pollinators is reduced because bees are only landing on flowers and not on leaves.
That raises the question for growers: What can we use in the last three weeks if we find aphids, whiteflies, thrips, caterpillars, or mites?

The new “Protecting Pollinators” bulletin provides a list of products that can be used during the last three weeks of production. However, the list requires a knowledge of which products work for which groups of pests, because many of these pollinator-safe products are specific for one or two types of pests.

More work is needed to determine how and when insecticides can be used in the greenhouse in a way that will be safe for pollinators. A new USDA National Institute of Food and Agriculture Specialty Crop Initiative (IFA SCRI) grant has been funded to address these issues for greenhouse and nursery growers. The research will begin this fall and continue for several years. Part of that research will test the persistence of systemic insecticides, and another part will evaluate the most popular annual and perennial flowers to determine how valuable they are as food plants for bees and butterflies.

If growers plan to grow food plants for pollinators, the best way to avoid harmful residues is to use biological control. If you are new to biological control, it means starting clean (new crop with no pesticide residues, not even on the cuttings), making multiple releases of predators, and avoiding all insecticides with the exception of insecticidal soap, horticultural oil, Beauveria bassiana, Bacillus thuringiensis, and few highly selective insecticides like chlorantraniliprole, which are also listed and explained in the new pollinator bulletin.

 

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