Marathon (imidacloprid) used on ornamental plants kills bees

I want to discuss a 2021 paper published by two researchers at the University of California, Riverside. It was published in Proceedings of the Royal Society B, the Royal Society’s flagship biological research journal.

The researchers applied Marathon insecticide to a common flowering plant (lacy phacelia) that’s sold in nurseries and grown in gardens. The active ingredient in Marathon product is Bayer’s imidacloprid, the leading neonic (neonicotinoid) insecticide.

In order to determine the impact on wild bees, the researchers reared alfalfa leafcutting bees (ALCBs) in several caged enclosures along with these potted plants. In some enclosures the plants were treated with Marathon (imidacloprid) and in other enclosures the plants were left untreated, as a control.

The following should tell you everything that you need to know: the researchers applied Marathon to the plants at only 30% of the dosage recommended on the label. This is because when they used a full dose, in preliminary work, all of the bees died.

It turns out that, even when less than one third of the recommended dose was applied, the bees were still devastated.

A leafcutter foraging on phacelia, from Rusty Burlew, HoneybeeSuite. Marathon (imidacloprid) used on ornamental plants kills bees.
A leafcutting bee foraging on phacelia, from Rusty Burlew, HoneybeeSuite

Here’s the title of the paper, which you can access on the Royal Society site:

Pollinators and plant nurseries: how irrigation and pesticide treatment of native ornamental plants impact solitary bees  

I’ll go through the findings of the paper with a few quotes and notes

Our study is, to our knowledge, the first to examine the consequences of a nursery neonicotinoid formulation on solitary bee reproduction. Our results have important implications for wild bee conservation in horticulture and other agroecosystems.

We reared alfalfa leafcutting bees (‘ALCB’, Megachile rotundata) on containerized ornamental plants from a nursery grown under different irrigation and neonicotinoid application regimes.

For imidacloprid treatments, we applied Marathon® 1% Granular, a commercial nursery formulation… Formulated for use in greenhouses and nurseries, Marathon® consists of 1% imidacloprid and 99% inert ingredients by mass.

We used approximately 30% the label rate as our ‘high’ dosage to be conservative, as near total ALCB mortality occurred when separate plants were treated at label rate (JM Cecala 2018, unpublished data).

Applying a granular nursery formulation of imidacloprid [Marathon] at only 30% label rate reduced ALCB brood production by 90%

Here’s the key figure that shows the impact of Bayer’s Marathon (imidacloprid) on these bees:

Marathon (imidacloprid) used on ornamental plants kills bees. Figure 3 is shown from the 2021 paper, Pollinators and plant nurseries: how irrigation and pesticide treatment of native ornamental plants impact solitary bees. The charts show how the application of Marathon (imidacloprid) reduces the leafcutter bee population in the study by 90%.   

In the chart on the left, you can see that the number of new nests built by the bees drops to about zero when they are exposed to imidacloprid. The chart on the right shows that the number of brood cells occupied by bees drops by around 90% in the enclosures where the plants were treated with Marathon. The researchers also tested two irrigation regimes:

Most interestingly, irrigation level mediated the effects of imidacloprid application … resulting in higher imidacloprid concentrations in nectar of low irrigation plants.

So, with less irrigation, the imidacloprid becomes more concentrated in the plant and bees receive a higher dose. That’s pretty relevant information, considering that climate change is going to bring more drought conditions to today’s farming regions, like California.

Safety of Marathon and Marathon II insecticides

The two main imidacloprid products sold for horticultural use (nurseries or home) are Marathon and Marathon II, both supplied by OHP Inc, (Olympic Horticultural Products). In several countries, imidacloprid is sold by Bayer under the brand name, Confidor.

The label for the Marathon (the exact same product used in the study above) contains no information about toxicity to bees. While Marathon is a granular product that’s 1% imidacloprid, Marathon II is even more potent – it’s a foliar spray that contains around 21% imidacloprid.

The label for Marathon II directs gardeners to contact a poison control center even if they spill it on clothing. There is a warning that the product is toxic to bees, but take a look at the directions:

Directions from the label of a container of Marathon II insecticide on how to (supposedly) avoid harm to bees. The directions instruct users to avoid application of the product while bees are foraging, but grants several exceptions including the application after sunset or below 55 degrees F. Marathon (imidacloprid) used on ornamental plants kills bees.

So, let’s get this straight: humans who spill the product should remove clothing and call poison control, while for bees (which are massively more sensitive) the precaution is to wait until after sunset until spraying. OK, I guess that’s fine because the insecticide fairy cleans it all away at midnight. I mean, there’s no way it could end up in the pollen or nectar, right?

Sarcasm aside, OHP states that Marathon II protects plants from insect attack for weeks. Imidacloprid, by design, is taken up by the plant and poisons insects that feed on the plant (including pollen and nectar). Applying the product after sunset does not protect bees from harm.

Please see this post on risk analysis of Bayer’s imidacloprid for more detail on toxicity of this insecticide.

Marathon (imidacloprid) insecticide is toxic to bees. A container of Marathon, supplied by OHP Inc, (Olympic Horticultural Products) is shown on the left. On the right is a toxic symbol and below it, a photo of an alfalfa leafcutting bee foraging on phacelia flowers

Advice for gardeners and nurseries regarding the use of Marathon and Marathon II.

Don’t use Marathon or Marathon II, unless you actively want to harm bee populations.

Advice for purchasing plants from nurseries, stores, and garden centers

Ask staff whether the plants were treated with neonics and whether the business has a policy on the use of neonics. If imidacloprid products such as Marathon (II) are used, then let the staff know that you won’t support the business until change is implemented.

Buying plants that attract pollinators may do more harm than good unless they are free of neonics.

6 thoughts on “Marathon (imidacloprid) used on ornamental plants kills bees

  1. Thank you, I no longer buy nursery plants, unless the nursery can certify that neonics are not used on them. It’s tragic, and as a beekeepter who has lost bees, it’s important to spread the word. For bees, the three deadlies are–pesticides (largely neon’s), parasites (mostly varroa and varroa carried viruses) and poor nutrition (because of monoculture.)


    1. Right – and getting away from monocultures by even just planting a small amount of an appropriate sister crop has several benefits. The second crop can confer pest protection by attracting predators or repelling pests, reducing the need for other forms of pest control, and also encourages biodiversity and bee health.


  2. The bee problem is world wide but its effects in the US are masked because hives are transported around the country to help pollenate various crops on the east and west coast. And also because American farmers use so much more pesticides per acre than their European counterparts.


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