18.104.22.168. External factors
The results of the field studies can be affected by several factors outside the intrinsic toxicity of the substance. This includes the attractiveness of the target crop and the other plants surrounding the test field, the weather conditions during the experimental test, and the modality of the treatments. Honey bees’ forage an average of 1.5 km radius around their nest (Crane, 1984). However, this can extend to > 9 km under stressed food conditions (Seeley, 1985). For this reason, it is possible that bees from colonies in treated fields could forage in untreated areas and vice versa, thus underestimating pesticide exposure. In order to reduce this “dilution factor”, the colonies in the test field should be isolated from other important blossoms and the test crop should be very attractive to bees (see section 22.214.171.124.). Ploughed fields, rivers and highways can be used as natural barriers to isolate the test fields. The negative reference field (if present) should be located at least 4 km from the treated field and in an area with similar climatic and landscape conditions.