In a controversial decision, the world’s most used pesticide, glyphosate, was recently renewed by the European Commission for 10 more years.

This decision is one that has been applauded by many farmers who use it as part of min-till and no-till systems, but has been criticised by those who believe glyphosate to be toxic.

What much of this reporting has highlighted is that when we talk about pesticides being renewed or banned it’s always the main ingredient we discuss (i.e., glyphosate). Main ingredients are what we scientists would call the active ingredient or the ‘active’.

But when farmers reach for a bottle of pesticide to spray it’s not just an active ingredient they’re reaching for, it’s a formulation. A classic example of this would be glyphosate itself, which has been sold under the trade name of Roundup for 50 years.

There’s a disconnect between how we talk about regulating pesticides, and how we talk about using pesticides. Governments regulate active ingredients, but farmers spray formulations. So, what’s the difference between them?

Well, it’s everything else in the bottle apart from the active ingredient. These other ingredients are what we scientists would call co-formulants. Co-formulants are really helpful chemicals that can play all sorts of roles in making pesticides more effective.

They can help spread the spray through the crop properly, give the product a long shelf life and help dilute the active ingredient. Co-formulants are actually the majority of what’s in a pesticide formulation, anything from ~60-99% of the total weight.

Increasing effectiveness

Without the help of co-formulants pesticides wouldn’t be nearly as effective. Without them we’d need to spray at much higher rates to get the same level of pest control. So, by adding in the co-formulants we can spray less active ingredient and still get good pest control. In fact, over the last few decades, discoveries of new active ingredients has slowed down, but co-formulant technology has yielded some real improvements.

The flipside is that co-formulants can actually be just as toxic to humans and the environment as active ingredients. While co-formulants are not designed to be toxic to anything, they are potent chemicals that are sprayed at high levels. Unfortunately, because of how pesticides are regulated, co-formulants aren’t properly safety tested like active ingredients. As a result, some dangerous co-formulants can make their way into pesticide formulations.

One example of this was the glyphosate co-formulant poly-ethoxylate tallow amine (POEA). Its role was to help the pesticide spread out over the weed leaf and allow the glyphosate to get deep into the weed to kill it. POEA was partially banned in 2016, then fully banned in 2021 because it was found to be quite harmful to human health. It interferes with hormone production in humans, which can be very dangerous.

This toxic co-formulant can actually explain why some studies found glyphosate to be toxic. You see, some of those studies weren’t actually testing pure glyphosate (the active ingredient), they were testing a formulation which contained both glyphosate and this toxic POEA co-formulant. The scientists were seeing toxicity, but not from the glyphosate, instead it was from the co-formulant. That nuance can get lost in discussion though, with all toxicity being assumed to be caused by the active ingredient.

Has anything changed?

Now that this co-formulant, POEA, is banned, it has been replaced with a new generation of co-formulants. The trouble is the same system which failed to spot that the POEA was toxic hasn’t worked out a good way of detecting if the new generation of co-formulants are toxic. So, we’re back at square one, not knowing if the ingredients in our pesticides are safe or not.

One example of co-formulants being environmentally harmful is the fungicide Amistar, which contains azoxystrobin as its active ingredient. In testing I ran on bumblebees, I found the active ingredient to be essentially harmless to the bees, but the formulation was killing about 30% of the bees. When I dug into it a bit further, I found that one single co-formulant was responsible for the deaths in the bees. Researchers in the US saw the same thing with honeybees and the fungicide formulation Pristine. The active ingredient on its own didn’t impact the honeybees’ ability to learn, but the whole formulation stunted their learning ability.

Trade secrets

Co-formulants are so overlooked that pesticide companies are still allowed to keep them secret. It’s actually EU law that most of the co-formulants in pesticides are ‘trade secrets’, with only specific co-formulants needing to be listed on the label. This situation makes life very difficult for those of us trying to test if the co-formulants are safe, because quite often we just don’t know what’s in the pesticides we’re working with.

Should co-formulants be tested?

What this all points to is the need for governments to test pesticide formulations better. Rather than focussing on just the active ingredients, both co-formulants and formulations as a whole should be studied in detail. If governments did test co-formulants as rigorously as they test active ingredients it would help us weed out the harmful co-formulants. This would make pesticides safer for farmers who use them and safer for the environment. Further, this kind of pesticide testing wouldn’t impact farmers access to pesticides, it would just slightly change what co-formulants are in the pesticides. All the same products would stay on sale, it’s just that the manufacturers would have to tweak what co-formulants they use.

Dr Edward Straw is a research fellow at Trinity College Dublin. He completed his PhD in 2021, studying how herbicides and parasites impact bumblebees.