Successful selection for antixenosis resistance to cabbage root fly (Delia radicum L.) of individual cauliflowers based on egg counts around the stems occurred when the mean egg-laying was between 25 and 50 eggs per plant. Incorrect selection for low egg-laying could still not be excluded due to the random behaviour of the insect.

The performance of selected partially-resistant lines was assessed by:

1. Counting the numbers of larvae and pupae in the soil around plants which had been exposed to variable, naturally-occurring pest populations. In cases, where the mean numbers of larvea and pupae per plant in the standard was below 10, a reduction of egg-laying of 42.1% - 66.7% was followed by a reduction in larvae and pupae of 25.3% - 68.4%. No definitive answer could be given as to whether resistance breaks at higher infestation levels. Selection for resistance to cabbage root fly also had an effect on bean seed fly, D. platura Meigen attack.

2. Offering diminishing choice situations to the cabbage root fly. The experiments showed that the partially-resistant line tested might retain its resistance in a no-choice situation.

3. Reducing the insecticide dose by 40% on the partially-resistant line (0.6n or 0.6g Dyfonate 5G (5% fonofos) per plant instead of 1g). In low pest population situations the reduction in numbers of larvae and pupae around the selected line sprayed with a reduced insecticide dose was comparable to the numbers around the standard sprayed with a normal dose.