Parasitism of diamondback moth (Lepidoptera : Plutellidae) larvae by Cotesia plutellae (Hymenoptera : Braconidae) and Diadegma insulare (Hymenoptera : Ichneumonidae) in cabbage fields after inundative releases of C-plutellae

Cocoons of Cotesia plutellae (Kurdjumov) were released for nine consecutive wk along the margins of two commercial cabbage (Brassica oleracea var. bra ve L.) fields near Bunnell, Flagler Co., FL, in spring 1996. The larval par asitism of diamondback moth, Plutella xylostella (L.), by C. plutellae and by the native parasitoid Diadegma insulare (Cresson) was evaluated in relea se fields and in nearby cabbage fields using two methods-sentinel collard ( Brassica oleracea var. acephala L.) or sentinel cabbage plants and non-sent inel plants. Total parasitism of diamondback moth larvae on sentinel plants in the release and adjacent fields was 35.7%. There were no significant di fferences in the level of parasitism by C. plutellae among sentinel plant l ocations within the release fields. In non-release fields, parasitoids spre ad as far as 1,500 m from the nearest release site during the release perio d, but parasitism of larvae on sentinel plants decreased as the distance fr om the release area increased. Parasitism of diamondback moth larvae by D. insulare was 8.3% in C. plutellae release and adjacent fields, but 14.6% in the nearby fields. Sampling of non-sentinel cabbage plants for diamondback moth larvae demonstrated a total of 37.4% larval parasitism by C. plutella e in the release and adjacent fields, similar to that recorded on sentinel plants. However, C. plutellae were detected only as far as 800 m from the r elease site on non-sentinel cabbage plants, and total parasitism in the dis persal fields also was very low. Diadegma insulare contributed only 1.1% pa rasitism of larvae sampled from non-sentinel plants in all cabbage fields. Cotesia plutellae was more effective than D. insulare in attacking diamondb ack moth larvae in this study where field populations of diamondback moth w ere low (<0.1 larva per cabbage plant).