Impact of pesticide adjuvants on bee olfaction


The adjuvants of pesticides and fungicides contribute to the toxicity of ‘plant protection products’ for bees, which lose their sense of smell because of them. And so is the ability to communicate, essential for the health and survival of the colonies.

A study published in Science Report (Wu et al., 2023) highlights the need to extend risk assessment to these substances hitherto considered ‘harmless’. (1) Which still receive little attention, even in the EFSA guidelines.

1) Honey bees, smell for survival

Honey bees live in dynamic communities where communication is constant and uses chemical substances as carriers of social signals:

– nurse bees constantly monitor the larvae they are responsible for looking after in the dark using pheromones

– the larvae in turn emit brood pheromones to indicate that they need food

– the workers produce alarm pheromones to warn other bees of danger. If these signals are muffled or not perceived correctly, the colony’s survival is at risk.

2) Bees and pesticides, the California crisis

A systematic review – ‘Effects of pesticide-adjuvant combinations used in almond orchards on olfactory responses to social signals in honey bees (Apis mellifera)‘ received primary support from the ‘Almond Board of California’. Two-thirds of the total honey bees raised in the U.S. it is transported every year to the almond groves of the West Coast, during flowering, to encourage the pollination of over 400.000 hectares of crops.

The crisis of recent years has reached its peak with the extermination of over 50 billion bees, in the winter of 2018/2019, right in the Central Valley where 80% of global almond production takes place. Pesticides had been identified as the first probable cause of Colony Collapse Disorder (CCD), as it turns out. (2) However, the role of the different components of the products sprayed on the plants had not been investigated.

3) Pesticide adjuvants, the missing piece

The searches on Colony Collapse Disorder have so far focused on the active substances contained in pesticide mixes. And it was then extended to fungicides because they, ‘although designed for fungal targets, they have a negative effect on bees‘.

The combination of pesticides and herbicides – explained entomology professor May Berenbaum (GEGC/IGOH. See note 3) – ‘can disrupt the functioning of colonies. Nurse bees are particularly vulnerable to these combinations, and the health of the queens is paramount. If healthy queen bees do not develop, the colony may suffer‘.

The missing piece, to understand how ‘Plant Protection Products’ influence nurse bees, is the analysis of the effects of their adjuvants. Apparently inactive chemical substances, used to increase the adhesiveness of the insecticide and retain it on the plants.

4) The study (Wu et al., 2023)

The study under consideration (Wu et al., 2023) tested the effect of pesticide adjuvants on the olfactory system of honey bees by subjecting four groups of bees, for one week, to untreated commercial pollen or to pollen treated – separately and in combination – with the adjuvant Dyne-Amic, the fungicide Tilt and the insecticide Altacor.

Researchers then anesthetized the bees with ice to remove one of each’s two antennae. The antennae were then exposed to chemical mimics of brood and alarm pheromones, to record their levels of response to different signals using electroantennography.

Some broods and alarm pheromones in the antennae of nurse bees that had consumed pollen contaminated by the three chemical substances were found to be altered. This finding suggests that these commonly used pesticides may interfere with honey bee communication.

5) Conclusions of the study

There are many possible explanations to explain how the consumption of these chemicals can influence the sensory responses of bees‘, commented researcher Ling-Hsiu Liao. ‘The antenna detects and activates the response to olfactory signals. In this study we did not examine what other changes are triggered, especially changes in behavior‘.

This approach can now be extended to other commonly used pesticide mixtures, examining the response of various populations of bees and other pollinating insects. As well as identifying the molecular pathways involved. They hope their work can help farmers and beekeepers rethink the way they manage crops and protect bee colonies.

6) EFSA, guide to the risk assessment of ‘plant protection products’ for bees

EFSA (European Food Safety Authority) published in 2023 the revision of its previous guide (2013) on the risk assessment for honey bees, bumblebees and solitary bees associated with exposure to ‘plant protection products’. (4) The guidance document, for the purposes of applying the Pesticides Regulation (EU) No 1107/2009:

  • outlines a step-by-step approach for estimating exposure under different scenarios and levels.;
  • ‘includes hazard characterization and provides a risk assessment method covering food and contact exposure’;
  • ‘provides recommendations for higher level studies, for risk from metabolites and for plant protection products as a mixture‘.

6.1) EFSA guidance, limits and general uncertainties

The limits and uncertainties declared in the EFSA guidance (2023) concern, among others, the following areas:

  • potential contamination of untreated areas by solid pesticide preparations‘;
  • ‘behavior of pesticide residues in different matrices, such as plant tissues’. This behavior is influenced precisely by the adjuvants object of the study in question (Wu et al., 2023);
  • the risk deriving from combined exposure to different pesticides [and other PPPs, ed.] that could be used in an agricultural landscape over the course of a season‘;
  • the biology and ecology of bees of the non-Apis genus‘.

6.2) Active substances vs. formulations

Risk assessment of the mixturesis not routinely included in active substance risk assessment processes, but may be primarily relevant in the context of the authorization of plant protection products at national level‘ (see Chapter 12 of the EFSA guidance).

Chronic toxicity is in fact considered an intrinsic property of the active substances only and only in some cases is it required to also carry out the relevant tests on the ‘plant protection product’ (PPP) formulations. See Section 6.1.1.

The tests of chronic toxicity are therefore required if, for example, on the basis of acute toxicity data, the PPP was found to be more toxic than the single active substance, or contains more active substances (Section 8.3. Active substances vs. formulations).

6.3) Sub-lethal effects, partial risk assessment for bees

Some of the main obstacles to the development of a comprehensive assessment of sub-lethal effects are their almost unlimited diversity, the lack of standardization of measurement and the lack of a proven link with specific protection objectives (SPG)‘.

The EFSA working group, ‘to address these issues, decided to focus primarily on a subset of sublethal effects, particularly those that noticeably alter bee behavior‘ (Chapter 9, Sublethal effects on honey bees in risk assessment. Section 9.1, Overall strategy).

Sub-lethal risks relating to the impact of ‘plant protection products’ (active substances and adjuvants) on the olfactory system of bees have not yet been considered, also because the publication of the study mentioned above (Wu et al., 2023) is a few months after that of the revision of the EFSA guidance.

6.4) ‘Tank mixtures‘ and adjuvants

Pesticides Regulation (EC) No 1107/2009 provides that ‘in the evaluation and authorization of a plant protection product, the interaction between the active substance, safeners, synergists and co-formulants is taken into account‘(Article 29).

It refers therefore explicitly to the PPPs on the market which are, ‘by their very nature, technical mixtures containing from one to more active substances and, in general, different co-formulants‘ (EFSA Guide, Chapter 12, Mixtures).

‘Tank mixtures’ – considered in the study in question (Wu et al., 2023) – are therefore excluded from the provisions of the obsolete European regulation and the EFSA guide which ‘the concepts and the scheme‘ proposed in its Chapter 12 (Mixtures) applies only if information is available on certain combinations of substances to be mixed in the container.

The fate and the behavior of the co-formulants in pollen and nectar are finally the subject of a recommendation for further investigation (EFSA guide, Chapter 15, Recommendations- Mixtures).

7) European Union, toxic lobbies and rules to be rewritten

The great confederations European agricultural companies (e.g. Copa-Cogeca, Farm Europe) and national ones (e.g. Coldiretti) often complain about the decline of bees and honey production in the European Union. But they are precisely the protagonists of the crisis, having ordered MEPs to boycott the proposed reform of the SUR (Sustainable Use Regulation) regulation which could and should have introduced a drastic restriction on the use of pesticides. (5)

Health of bees and of us all are held hostage by the toxic lobbies of Big 4 – the four global seed pesticide monopolists (6) – and of organizations that express only their own economic interests and that of their leaders rather than those of the farmers they are supposed to represent. This explains, among other things, the defeat of the objectives of the Nature Restoration Law already denounced on this site. (7)

The obsolete rules on which the risk assessment of pesticides and other ‘plant protection products’ in the European Union is still based, as we have seen, completely neglect the cocktail effect which is as dangerous for bees and pollinating insects as it is for human health. (8) Furthermore, these rules are systematically disapplied, given that around a third of the ‘plant protection products’ authorized in the EU today lack an updated risk assessment. (9)

Dario Dongo


(1) Wu, WY., Liao, LH., Lin, CH. et al. Effects of pesticide-adjuvant combinations used in almond orchards on olfactory responses to social signals in honey bees (Apis mellifera). Sci rep 13 (15577).

(2) Dario Dongo, Marina De Nobili. Massacre of bees in the USA, fake news on almonds and neonicotinoidsGIFT (Great Italian Food Trade). 29.1.20

(3) GEGC/IGOH (Genomic Ecology of Global Change, Infection Genomics for One Health), University of Illinois Urbana-Champaign

(4) European Food Safety Authority (EFSA), Pauline Adriaanse, Andres Arce, Andreas Focks, Brecht Ingels, Daniela Jölli, Sébastien Lambin, Maj Rundlöf, Dirk Süßenbach, Monica Del Aguila, Valeria Ercolano, Franco Ferilli, Alessio Ippolito, Csaba Szentes , Franco Maria Neri, Laura Padovani, Agnès Rortais, Jacoba Wassenberg, Domenica Auteri. Revised guidance on the risk assessment of plant protection products on bees (Apis mellifera, Bombus spp. and solitary bees). EFSAJournal. Volume 21, Issue 5 e07989 First published: 11 May 2023.

(5) Dario Dongo. No to reducing pesticides, yes to glyphosate. ToxicEuropeGIFT (Great Italian Food Trade). 23.11.23

(6) Dario Dongo. Seeds, the 4 masters of the worldGIFT (Great Italian Food Trade). 15.1.19

(7) Dario Dongo, Alessandra Mei. ‘Nature Restoration Law’, green light with downward agreementGIFT (Great Italian Food Trade)… 14.11.23

(8) Marta Strinati. Not just glyphosate. 33% of pesticides used in the EU are without risk assessmentGIFT (Great Italian Food Trade). 26.11.22

(9) Marta Strinati. Pesticide cocktails cause toxicity, even at the doses allowed in the EU. New study. GIFT (Great Italian Food Trade). 24.10.20

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Dario Dongo, lawyer and journalist, PhD in international food law, founder of WIISE (FARE - GIFT - Food Times) and Égalité.