Optimizing field collection and laboratory conservation of the parasitoid Trichomalus perfectus before release against the cabbage seedpod weevil Ceutorhynchus obstrictus

Abstract

In Canada, the cabbage seedpod weevil pest, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), causes damage to canola (Brassica napus L., Brassicaceae) crops. Trichomalus perfectus (Walker) (Hymenoptera: Pteromalidae) is the most effective larval parasitoid for controlling it in Quebec and Ontario, but it is absent from the Canadian Prairies. This study aimed to optimize T. perfectus field collection (using sampling methods and selecting optimal collection sites based on local and landscape scales) and laboratory conservation according to cold temperatures, relative humidity (RH), and diet conditions before field releases. Two sampling methods were compared: 54 sweeping nets were conducted for 30 s each, and 20 pod collections lasted 90 s each. At a local scale, 1000 pods were collected from two edges, distributed along adjacent environments, and at two distances of 58 canola fields. At a landscape scale, landscape predictors were measured within a 500-m radius from 37 canola fields. Regarding laboratory conservation, parasitoid females were divided into five groups containing between 50 and 60 females and subjected to different conservation combinations of temperature, RH, and diet to assess survival rates every 8 to 10 days over 105 days. Results indicated that pod collection was superior to sweeping nets for maximizing T. perfectus collection. To release 2000 parasitoids, 223 emergence boxes, each containing 740 pods and generating about 9 parasitoids, would be required, and all pods could be collected in 5.5 h. At a local scale, pod collection along all canola field edges, without specific targeting of adjacent environments, proved effective, maintaining a 5-m distance. At a landscape scale, the collection of T. perfectus is maximized in a more diversified landscape, correlating with eight different landscape elements resulting in 10 T. perfectus per 1000 collected pods. Regarding laboratory conservation, female parasitoids' survival was highest when fed a protein-free diet (only honey solution) and cold-stored for at least 3 months at 5°C and 30%–50% RH.