The relationship between plant disease management and pollinator conservation

What does disease management have to do with pollinators?

European honey bee on flower
This European honey bee (Apis mellifera) is just one of the hundreds of bee species that pollinate the plants in our orchards, fields and gardens in Michigan. Photo by Emily Pochubay, MSU Extension.

You may not think that plant disease management would have much to do with bee conservation. However, nature is a highly interconnected biological and physical system. Whether in an agricultural field or a residential yard or garden, pollinators, like all living things, require food, water and shelter to survive. Unfortunately, some of the important practices that we implement for plant disease management can negatively impact those resources for pollinators as well as affect pollinators directly. Two relevant plant disease management practices that we will discuss here are fungicide use and the sanitation practice of plant residue removal.

Bees and fungicides

Fungicides are formulated to inhibit the growth of fungi, not to affect insects, but that does not mean that all fungicide products are completely benign for bees. The risk of harm to an organism from a substance, such as a pesticide, is determined by the immediate or acute toxicity of that substance and the amount of substance that the organism is exposed to. Scientists measure the acute toxicity of pesticides to honey bees using a standardized index called the LD50. The LD50 level is the amount of fungicide that would be predicted to kill 50% of the group or population of the organism.

High acute toxicity of fungicides is less common for bees, but there are a few fungicides with a moderate or high toxicity to honey bees, based on laboratory tested LD50 levels. You can find the latest data for a fungicide’s LD50 for bees by looking up its Safety Data Sheet (SDS), which can be downloaded from the manufacturer’s website. You can also consult the table of Bee LD50s in the Michigan State University Extension Bulletin E3245, “Minimizing Pesticide Risk to Bees in Fruit Crops.”

The Bee LD50 level is determined by laboratory testing on adult honey bees only, so it tells us little about risks to immature bees or adult bees under field conditions. Indeed, recent research is telling us that there may be other impacts to bee health to consider. For instance, there is evidence that some fungicides can have different effects at different stages of bee development, such as the larval stage. Other examples of impacts of certain fungicide ingredients on bees demonstrated in studies include synergistic effects with other pesticides, reducing bees’ ability to detoxify pesticides, reducing the nutritional value of the larval food source (bee bread), and increasing bees’ susceptibility to parasites and diseases.

Fungicide effects on bees is an active area of research and recent studies have indicated that some fungicidal chemicals may be detrimental to bee health under certain circumstances. However, fungicides play an essential role in protecting our economically important plants from disease. To learn more so that you can make informed choices about how to reduce the risk to pollinators while using pesticides, visit these MSU Extension resources:

Pollinator habitat and sanitation for disease management

One of the cornerstones of good disease management with an integrated pest management (IPM) approach is the practice known as sanitation. Sanitizing an area by removing plant material that is dead, damaged or diseased prevents disease organisms from propagating themselves. Sanitation includes careful pruning of fruit trees and woody ornamentals, which increases air flow and light penetration, both of which are key enemies of plant disease. Leaf cleanup at the bases of our multi-stemmed perennial shrubs keeps humidity down and reduces the chances that disease organisms will infect the plants’ crowns.

Our gardens, landscapes and farms are habitat for pollinators whether we intend for them to be or not. There is almost no activity that you can undertake that will not have some impact (good or bad) on pollinators. With that said, there are some things you can keep in mind during your farm and garden sanitation practices.

The overwintering stage is one of the more vulnerable times in a pollinator’s life cycle. In the case of bumble bees, for example, the entire colony is reduced to a single queen bee whose job it is to establish a new colony composed of 60 to 80 worker bees in the following season. Many bee species also spend the winter in sheltered locations that often bring them into proximity with our buildings and managed landscapes. These locations include under mulch or layers of dead leaves, the stems of old plants or in the soil beneath our lawns. You can choose to take extra care when working in those sensitive areas if you would like to protect bees and other pollinators.

The necessity of plant debris removal depends on annual conditions. If you do not see signs of serious plant disease this year, you can choose to practice minimal garden sanitation. On the other hand, if you are dealing with high plant disease pressure, then sanitation is essential to keeping your plants healthy and reducing the need for pesticides.

Finally, let’s not forget that diseased plants will produce fewer flowers, and bees need flowers, too. Protecting both plant and pollinator health will sometimes be a balancing act. You can improve your performance with knowledge!

Learn more about creating and maintaining pollinator habitat from Bee Habitat Tips by Extension.

This article was originally featured in the Michigan Farmer Magazine.

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