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> ARCHIVIO EVENTI INA | |||||||||||||||||||||||||||
VIII SIMPOSIO INTERNAZIONALE ICPBR HAZARDS
OF PESTICIDES TO BEES Metodologie dei test e studi sugli effetti dei pesticidi sulle api Assessment of the synergy and repellency of pyrethroid/fungicide mixtures Selwyn Wilkins and Helen Thompson National Bee Unit, Central Science Laboratory, Sand Hutton, York, YO41 1LZ, UK. E-mail: p.wilkins@csl.gov.uk There has been considerable concern over the last few years about the potential for synergism between pyrethroids and fungicides applied to oilseed rape. Incidents reported through the Wildlife Incident Scheme in the UK have, according to field data, included mixtures of pyrethroids with fungicides which are not synergistic under laboratory conditions. Interpretation and regulation may be difficult if it is unclear whether synergism or a change in repellency is responsible for the incidents. The repellent nature of pyrethroids are important in limiting the exposure of honeybees to this highly toxic group of insecticides. Therefore it is important that this is investigated to ensure increased exposure of honeybees to pyrethroids is not occurring by reducing the repellent nature of the insecticide, i.e. the risk assessment based on repellency is valid. This study aimed to determine if the mixing of fungicides with pyrethroids alters the repellent properties of the pyrethroid and thus increases the risk to honeybees when applied to flowering crops. Repellency The amount of sucrose consumed in a laboratory trial was used as an indirect measure of the repellency of the treated filter paper on which it was placed. Both the pyrethroids, alpha-cypermethrin (Contest) and lambda-cyhalothrin (Hallmark), showed significant repellency although the scale of the repellency differed. For both pyrethroids three fungicides significantly decreased the repellency of each of the pyrethroids, but they were not the same fungicides. The greatest decline in repellency was observed with alpha-cypermethrin and chlorothalonil (Bravo) where repellency was significantly reduced (p<0.001) from a mean of 80% to a mean of 6%. Toxicity The LD50 and 95% confidence limits for the pyrethroids and their combination with fungicides were calculated. The maximum increase in toxicity (decrease in LD50) observed was a 6.7 fold increased in the toxicity of l-cyhalothrin in the presence of prochloraz. Six of the eight fungicides increased the toxicity of l-cyhalothrin and three increased the toxicity of alpha-cypermethrin. The maximum increase observed in combination with alpha-cypermethrin was 2.2 fold with prochloraz. Discussion The increase
in exposure and toxicity for the pesticide combinations are summarised
in Table 1 together with the consequent predicted increase in risk associated
with the mixture over the pyrethroid alone. This predicts that chlorothalonil
significantly increases the risk posed by alphacypermethrin due to a
reduction in the repellency of the pyrethroid. Therefore the repellency
and toxicity of the combination of chlorothalonil and alphacypermethrin
will be tested in the semi-field trial with chlorothalonil alone and
alphacypermethrin alone as the controls. Table 1: Change in risk (exposure x toxicity) in mixtures of fungicide and pyrethroid compared with pyrethroid alone. Values less than 1.0 indicate decrease in risk, values greater than 1.0 indicate increased risk.
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