Spring Showers of… Pesticides

The other day, as I drove down my street, I saw several of our neighbors out doing yard work. Spring is finally and truly here. For many, that means it’s time to start cleaning up the yard from winter, before things really start growing and your yard gets out of control.

Spring brings longer days, rain, and warmer weather. These are prime conditions for growth. Not just growth of the plants you want in your yard and garden, but also growth of insects, weeds, and fungus. For many of you, insects, weeds, and fungus means adding treatments to the plants and soil in your yard and garden.


Before you break out the treatments, please take a few moments to consider what you are applying, and how it can affect the bees. Even if you think you only use bee-friendly treatments, take a moment to read this post. You might be surprised by what I learn.

The Buzz Behind the Pesticides

To start with, remember that even if you don’t have bee hives, bees still forage in your yard. Bees routinely search for food (nectar, and pollen) and water up to 2 miles away from their hive. That means that even if you don’t have bees, the treatments you use on your yard can affect bees.

A lot of you have heard about pesticides called neonicotinoids. They’re harmful to the bees, so many of you steer clear of these. That’s a good thing, since at higher concentrations, neonicotinoids kill bees outright. With repeated exposure to low levels of neonicotinoids, bees exhibit a whole host of problems. The problems are both physical and cognitive. Cognitively, the problems include difficulties with disorientation, communication, memory, developmental delays, and learning. Physically, bees experience problems with flight and movement, reduced levels of foraging for food, and a compromised immune system. Chronic exposure to low levels of neonicotinoids weakens the bees, and in this way neonicotinoids can contribute to the eventual death of a hive.

What Science Has Shown

Studies have shown that even when neonicotinoids are applied in a small restricted area, these compounds soon show up in the greater environment around there areas of use. These pesticides are added to the soil, and taken up by the plant you wish to protect. If this is a flowering plant, the nectar and pollen produced will contain low levels of neonicotinoids. But it is even worse than that. It has been discovered that these compounds do not stay in the soil to which they were applied. Runoff carries them out of the soil and allows them to enter the waterways. This water can then go on to contaminate other plants, and animals, including bees.


Because bees have been struggling, and because neonicotinoids have been found to be widespread in the greater environments of the areas where they are applied, many countries have passed bans on neonicotinoids. Other countries are working to ban them. It is important to realize that neonicotinoids are only one class of pesticides known to be highly toxic to bees. Other classes of pesticides known to be highly toxic to bees include organophosphates and pyrethroids (both natural and synthetic). You can find more complete lists from OSU, and from Wikipedia.

So, Pesticide or No?

Unfortunately, only a few of the pesticides known to be toxic to bees are banned in the United States. If you feel you must apply pesticides, please try to pick pesticides from the list that are relatively nontoxic to the bees (see Wikipedia and OSU links). Always apply as directed at the concentration specified. Also consider applying the pesticide in the evening or at night. Bees don’t fly or forage at these times. Finally, try not spray any open blossoms or contaminate the water supply used by the bees.


Pesticides have gotten a lot of attention lately, but they aren’t the only thing you need to worry about when treating your lawn and garden.


A growing body of research is beginning to show that some herbicides and fungicides once believed to be harmless to bees may actually pose serious risks. While not directly fatal to the bees, exposure seem to weaken honeybee to other threats. For example, one group of researchers found that bees exposed to fungicide sprays are 3 times more likely to die, even when the fungicides tested were generally believed to be safe to use around bees.

Can We Combine Low Level Products

Another group of researchers has found that glyphosate (the active ingredient in Roundup weed killer) damages the gut microbiomes of honeybees. It causes them to lose as many as half of the beneficial bacterial species in their guts. This makes it harder for the bees to digest their food, leading to malnourishment. It also makes bees more vulnerable to infection and death from harmful bacteria and viruses, like those carried by varroa and tracheal mites.

To make matters worse, another group of researchers has found that bees not only like, they preferentially seek out food tainted by certain herbicides and fungicides. They discovered this by offering bees the choice two ‘nectar’ drinks. One was sugar-water one of 20 naturally occurring chemicals commonly encountered by bees in plants and/or in their hives. The other was sugar water tainted with one of either 2 herbicides, or three fungicides. The experiment showed that out of all the sugar-water mixtures tested, bees repeatedly showed the most preference for sugar-water tainted with glyphosate (an herbicide) or chlorothalonil (fungicide). And they told the other bees in their hives which drink to collect ‘nectar’ from. Bees are able to communicate with each other this way.


By preferentially consuming sugar-water containing these chemicals, bees are getting much higher doses than previously anticipated. This calls into question whether the bees are actually ingesting levels of these compounds that might be toxic, instead of negligible levels as previously assumed.

What is All This?

What do all of these findings mean? It means that we are likely poisoning the bees, contributing to their decline. Fungicides can reduce a bees abilities to metabolize pesticides, like the most common pesticide used to treat varroa mites inside a hive. Or some of the pesticides used to treat agricultural or yard and garden pests. That means that even if you started with a pesticide that was considered safe for use around bees, the fungicides may make the pesticide toxic to bees at much lower than anticipated doses. Adding glyphosate (and possibly other herbicides) into the mix leads to malnourished bees with weakened immune systems. It becomes easy to see how we are contributing to the precipitous and perplexing loss of bees. For these reasons, we ask that you use caution and choose wisely when applying any treatments on your lawn or garden.


Thank you for caring enough about the bees to read this post. Next week, We’ll explore some of the green bee-friendly methods being used by many to combat pests, weeds, and fungal infections in the yard and garden.











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