Effect of trap background on cabbage root fly landing and capture

Pieces (600 mm × 800 mm) of coloured board, plastic sheeting and woven materials, placed beneath water traps prevented the traps from becoming soiled during rainy weather. Such backgrounds are not recommended for use with traps for monitoring populations of the cabbage root fly ( Delia radicum L. ‐ Diptera:Anthomyiidae), however, as, instead of increasing trap catch they reduced the numbers of female flies caught by 70%–90%. The main effect was that the visually attractive stimuli from the introduced backgrounds competed with those from the trap. A white background competed with a white trap on a direct fly/unit area basis. Green backgrounds stimulated males to land and the vertical stems of both real and artificial grass induced trivial flights that resulted in greater numbers of males entering traps resting on short grass. Most females were caught over bare soil. To minimize the variation in catch between traps used for monitoring cabbage root fly populations, the background beneath each trap should be similar. For maximum capture, the background should be of grass for male flies and of bare soil for female flies. Care is required if data from traps within mulched crops are used to make pest control decisions, as they will underestimate considerably the numbers of flies in such crops.