In the laboratory at a photoperiod of 16:8 (L:D) and 21°C, larvae ofLobesia botrana(Denis&Schiffermueller) developed faster on grapes and apples infected with the fungusBotrytis cinereaPersoon than on healthy grapes and apples. The duration of each of the five larval stadia on healthy berries was 5.3, 4.3, 4.2, 6.3, and 8.5 d in 1981, and 6.2, 4.3, 4.8, 6.9, and 9.7 in 1982. On infected berries the respective durations of the stadia were 5.8, 5.2, 5.1, 5.2, and 6.3 d in 1981, and 6.6, 4.2, 4.6, 5.3, and 8.0 d in 1982. The duration of the fourth and fifth stadia was significantly shorter on infected berries. When the larvae grew on infected berries during either the third or fourth stadia, the duration of the fourth and fifth stadia on healthy berries was significantly shortened. This faster larval growth was observed when the berries were infected 10–12 d before placing the larvae on them, but not when infected 25 d before placing the larvae on them. The yield in adults, when the larvae were reared on grape berries and apples infected with the fungus, was higher than on healthy fruits. This resulted in a 2.2- to 2.5-fold population increase on grapes and 1.4- to 2.7-fold on apples, as compared with fruits not infected with the fungus.