The assessment of the toxicity of residues on foliage to bees can be managed with several methodologies related to the mode of action and the way of application. From 1998 to 2003 the subject of high bee mortalities during spring when sowing of seeds is common became an important topic. On a review of different hypotheses, it was decided to investigate the ability of seeder machines to leave dust residues in the environment, a suspicion identified because of the use of insecticide coated seeds in southwest France. Consequently, it became necessary to determine if increased bee mortalities were related to the dust from coated seeds or alternative routes of exposure.
Crops of maize and sunflower were suspected to trigger such mortalities because of the numerous surfaces and AIs of the insecticide seed protection. As mortalities were mainly located in apiaries of this area, a major link was established with the sowing time of sunflowers.
The following field-lab protocol was developed after initial tests of dust emission.
1) In indoor conditions, non-moving seeder machines are used to collect dust from different varieties of seeds and dressings. Seeders are equipped with filters that permit one to analyse the source and quantity of dust when working.
2) Coated seeds are classified from a screening with different kinds of varieties as well as different dressings for the same variety.
3) Among all dressing coated seeds, two modalities are selected for comparison of pesticide impact on honey bees. One concerns the low level of dust emission and is expected to have a minor impact when contacting honey bees. The second modality focuses on higher dust emission data and is tested for assessment of an eventual impact to honey bees.
4) The field part of this protocol aims to collect dust from a sowing operation in agricultural conditions. Fields of at least two hectares are separate from one another by about three kilometres in order to avoid a cross-contamination under wind conditions. These fields are bordered by a hedge on the edge of plot so that the wind creates turbulence on site. Dusts are expected to drop to the ground instead of being borne away. Dedicated sentinel plants are arranged on the ground to catch dust. They must have hairy leaves with good hair disposition on the upper leaf surface such as with Tibouchina (Order: Myrtales; Family: Melastomataceae) or other ornamental plants. They are placed in fields before sowing starts and they remain in the field for 2 days post sowing.