Preference of the cabbage root fly, Delia radicum (L), for coloured traps: Influence of sex and physiological status of the flies, trap background and experimental design

Catches of Delia radicum (L.) (Diptera: Anthomyiidae) were compared in water traps that reflected predominantly wavelengths shorter (violet and blue traps) and longer (green and yellow traps) than 500 nm. Traps were positioned in choice and no-choice situations against backgrounds of bare soil and weeds in the field and against backgrounds of brown and green paper in the laboratory. The physiological status of the flies was modified in the laboratory by denying them access to food sources and oviposition sites. Males discriminated significantly more clearly than females between yellow and blue traps. The discrimination between yellow and blue traps was significantly more pronounced when the traps were presented in the choice than in the no-choice situation in both sexes. Green background (weeds and green paper) was highly preferred for landing and thus competed with the traps to such an extent that few flies were caught when nonpreferred violet and blue traps were sited on green backgrounds. Flies seldom landed on the brown background (soil and brown paper) which resulted in the relative increase of catches in the non-preferred violet and blue traps. The preference for yellow traps was innate even in young flies with immature egg-follicles. Females that were ready to lay eggs, even those deprived of an oviposition site till the age of 8 days, also preferred yellow traps. In the no-choice situation, flies deprived of food landed with the same frequency in yellow and blue traps. Food deprivation, however, did not affect preference for yellow traps over the blue traps presented in a choice situation.