Pesticide mixtures, fungicides and how bee risk is assessed

Why mixtures are the real question

A bee on a sprayed or seed-treated crop meets whatever that crop and the neighbouring ones have received: insecticides, fungicides, herbicides, adjuvants, and their breakdown products, in its nectar, pollen and water. Regulatory testing has historically assessed products one at a time. Whether that under- or overstates harm depends on how the substances interact — whether their effects add up, cancel out, or multiply.

1. 1AdditiveThe combined effect is what you would predict by adding the separate effects. The assumption most risk assessment has worked on.
2. 2AntagonisticThe combination does less harm than the sum. One substance blunts the other.
3. 3SynergisticThe combination does more harm than the sum. The case that matters most, because a test of each product alone would miss it.

Fungicides that make insecticides deadlier

The best-characterised case involves fungicides that inhibit ergosterol biosynthesis — the azoles, used on many crops. The explanation usually proposed is that they interfere with the enzymes bees use to break down other chemicals; the study below measured deaths, not that mechanism. In a laboratory study, bees given a non-lethal dose of the azole propiconazole together with the dose of the neonicotinoid clothianidin that kills one in ten on its own showed significant synergistic mortality in all three species tested: the honey bee, the buff-tailed bumblebee and the red mason bee.

The species did not respond alike. In honey bees and bumblebees the synergy appeared only in the first hours, but in the solitary mason bee it persisted to the end of the four-day test, and the mason bee was also the most sensitive of the three to clothianidin alone. The authors' conclusion was a regulatory one: test the combinations bees actually meet, and test more than one species.

What 90 studies found

A 2021 meta-analysis brought together 356 interaction effect sizes from 90 studies in which bees were exposed to combinations of agrochemicals, nutritional stress and parasites. Across all of them, multiple stressors interacted synergistically on bee mortality. Broken down, the synergy was strongly supported for combinations of agrochemicals at field-realistic levels, but combinations involving parasites or poor nutrition were no worse than additive. Effects on behaviour, fitness proxies, parasite loads and immune responses were additive or antagonistic, so the mechanism behind the mortality synergy remains unclear.

What the EU now accepts

In 2023 the European Food Safety Authority replaced its 2013 guidance on assessing pesticide risk to bees, covering honey bees, bumble bees and solitary bees and adding methods for metabolites and for products used as mixtures. Underneath it sits a political decision: what size of effect is acceptable. EU risk managers agreed that, for honey bees, a pesticide should cause no more than a 10 per cent reduction in colony size at the edge of the treated field. For bumble bees and solitary bees they could not agree any threshold, because the data to base one on did not exist, and left it undefined.