Bekæmpelsesmiddelforskning fra Miljøstyrelsen, 98 – Kombinationseffekter af pesticider

Kombinationseffekter af pesticider

Summary

In the present study we examined the joint action of pesticide mixtures using different test systems to determine 1) if it was possible to predict the joint action of pesticide mixtures based on a priori knowledge of the sites and modes of action of the components of a mixture and 2) if it was possible to generalise concerning the joint action of pesticides across test systems.

In previous studies of the joint action of xenobiotics two different reference models have been applied, the Additive Dose Model (ADM) and the Multiplicative Survival Model (MSM). Comparisons of the two reference models have revealed that in most cases ADM predicts the largest effects of mixtures, and it has been suggested to apply this model is risk assessment studies. Hence, in the present study we only adopted ADM, however the model was improved to allow for statistical tests of the hypothesis of additivity.

In the first phase of the project we examined 104 different combinations of binary pesticide mixtures and test systems. In total 22 different pesticides were included in the study. One group of mixtures consisted of pesticides with similar sites of actions, a second group consisted of mixtures of pesticides with different sites of action but similar modes of action while a third group consisted of mixtures of pesticides with different modes of actions. In the study seven different standardised test systems were included of which five are routinely used to assess ecotoxicological effects: terrestrial plants (Tripleurospermum inodorum and Stellaria media), aquatic plants (Lemna minor), algae (Pseudokirchneriella subcapitata), aquatic bacteria (Microtox metoden) and daphnia. The other two test systems were developed to assess the estrogenic and antiandrogenic effect of pesticides and the pesticides we examined using these test systems had previously been found to mimic the effects of endogenous estrogens. In the second phase of the project we examined 20 different combinations of ternary pesticide mixtures and test systems. The compositions of the ternary mixtures were based on the results with the binary mixtures to obtain mixtures where all three binary mixtures followed ADM and where one or more of the binary mixtures deviated from ADM. Ternary mixtures were not studied on algae and daphnia.

Binary mixtures of pesticides with similar sites of action generally followed ADM, while mixtures of pesticides with different sites of action but similar modes of action either followed ADM or performed antagonistically. In contrast, the responses with binary mixtures with different modes of action were more erratic, and in eight cases synergy was observed. In six of the eight cases prochloraz was one the pesticides of the mixture. Prochloraz was only tested on aquatic bacteria and daphnia, hence it was not possible to determine whether synergy was linked to the test system, the site of the action of prochloraz, or both. In other studies on aquatic plants and algae, binary mixtures containing prochloraz did not deviate from ADM, indicating that prochloraz only respond synergistically in some test organisms. Despite the very large number of mixtures included in the present study, it was not possible to draw general conclusions regarding the relationship between the sites and modes of action of the pesticides and their response in joint action studies, except that mixtures of pesticides with similar sites of action tended to perform additively. Overall, additivity was by far the most likely response. Synergy was only observed with eight of 101 combinations of mixtures and test systems, while antagonism was found with 14 of the 101 combinations.

No relationship could be established between the responses of ternary mixtures and the responses of the respective binary mixtures with the exception of the Microtox test system where synergy with one of the three ternary mixtures in five out of six cases resulted in a synergistic response of the ternary mixture. In contrast synergy was not observed in any of the 11 ternary mixtures, if the response of binary mixtures were additive or antagonistic.

Comparing the responses of the different test systems revealed very similar responses on terrestrial and aquatic plants with 11 and 13 of the 17 mixtures responding identically on each of the terrestrial plant species and Lemna minor, respectively. Thus, deviations from ADM found on Lemna minor were also likely to be found on terrestrial plants suggesting that available data on the former can be extrapolated to the latter in future cases where joint action data on terrestrial non-target plants is required for pesticide registration. The correlations between algae and terrestrial and algae and aquatic plants, respectively were less significant that between terrestrial and aquatic plants. A good correlation was also observed between the responses observed using the Microtox test and daphnia with seven out of eight binary mixtures responding similarly.