Factors that threaten pollinator health

Most researchers agree that a combination of factors is causing declines in bee and pollinator populations, including loss of habitat or flowers that provide pollen and nectar, pesticide exposure, parasites and pathogens.

Pollinator-friendly flowers.
Help pollinators by planting more flowers. Photo by Rebecca Finneran, MSU Extension.

Most researchers agree that a combination of factors is causing declines in bee and pollinator populations, including loss of habitat or flowers that provide pollen and nectar, pesticide exposure, parasites and pathogens. Each of these has been found to negatively affect bees, but there is also evidence the combination of stresses is especially harmful. Bees and other pollinators depend on flowers for food – nectar provides carbohydrates, while pollen is their source of protein and is necessary for them to rear their young. Flowerless landscapes like mowed lawns with strict weed control or heavily paved areas of cities and fields with no plant diversity contain little food for bees, which leads to poor nutrition and compromised immune systems. Nutritionally weakened bees are more susceptible to disease and pesticides.

Many pests and pathogens also affect bees. The Varroa mite, a parasite of honey bees, is one of the most destructive factors causing honey bee decline. Other parasites and pathogens may become a more serious problem in hives weakened by Varroa mite.

In some cases, the flowers that bees forage on have pesticide (not just insecticide) residue on the petals or in the nectar and pollen. These chemicals can kill bees directly or cause a variety of sublethal effects such as impairing their ability to find their hive or provide food for their larvae. The toxicity of pesticides for bees ranges from highly toxic to relatively safe, depending on the specific chemical and the exposure. In some cases the impacts are worse when pollinators are exposed to combinations of pesticides. Since bees forage through a wide range of landscapes, they may be exposed to a complex mixture of many different chemicals.

One group of pesticides, the neonicotinoids, has recently been studied intensively by scientists to determine their impact on bees, primarily because of their widespread agricultural use on field crops. “Widespread” means that over 50% of the 89 million acres of soybeans and over 90% of the 88 million acres of corn planted in the USA in 2018 were planted using seed coated with a systemic neonicotinoid insecticide. However, in urban areas neonicotinoids used by homeowners in yards and gardens may be just as important.

Neonicotinoids are a class of insecticide that acts on the insect’s nervous system. They are more selective, having greater toxicity for insects than mammals, and safer for humans to use than most old classes of insecticides. They are toxic to insects when ingested or through direct contact. The most widely used neonicotinoids – imidacloprid, thiamethoxam, clothianidin and dinotefuran – are all highly toxic to bees. Products containing these active ingredients have bee-warning boxes on the label with important instructions for limiting bee exposure that must be followed. However, corn and soybean seed coated with a neonicotinoid do not have bee-warning labels. Neonicotinoids move upwards in xylem sap internally within plants when applied to the plant’s base (to roots via a soil application, or to the stem via injection or a basal spray), where they can later reach nectar and pollen. Pesticides remain primarily in leaf tissue following a foliar spray.

Neonicotinoids, like most insecticides, will cause significant harm if pollinators come directly into contact with them. This exposure generally occurs when a neonicotinoid is misused and sprayed on a blooming plant or one that will bloom soon, or when bees are exposed to dust of seed-coatings at planting time. However, researchers have found seed-coat treatments to canola seed can be harmful to native bees feeding on canola flowers in fields planted with treated seed.

The pollinator protection section of pesticides with active ingredients requiring a bee-warning.

Bees and other pollinators can also collect contaminated pollen or nectar from treated plants and bring it back to their colony, creating high risk of harm to the colony. Research studies have demonstrated native and honey bees can be harmed by small amounts of pesticides in nectar and pollen. When a neonicotinoid is applied as a soil drench (a dilute solution poured around the plant base), it may persist for a year or more, especially in woody plants, and can also move into weeds or flowers growing over the drenched soil. If some of the insecticide moves into pollen or nectar it may kill bees directly, or act as a stressor to affect larval growth, susceptibility to diseases, navigation or winter survival.

How we manage ornamental landscapes has an impact on two of the most important factors affecting pollinators: habitat quality and pesticide exposure. The following two sections explain the best ways to create and maintain good habitat for pollinators, and how to minimize pollinator exposure to pesticides.

Read the next article in this publication series: Creating and maintaining pollinator friendly habitat

Or return to the beginning of this publication: How to protect and increase pollinators in your landscape

Did you find this article useful?

Other Articles from this Publication