Effect of common wild crucifer species of Kenya on fitness of two exotic diamondback moth parasitoids, Cotesia plutellae and Diadegma semiclausum

Abstract

Plutella xylostella is the most abundant and damaging pest of cruciferous crops in Kenya and has gained economic importance over the years. During a survey conducted in the major crucifer areas of Kenya a number of wild crucifers recorded in the region were found infested with diamondback moth. As a result, research was conducted in the laboratory and greenhouse to investigate whether exotic parasitoids recently introduced would develop and survive on the wild crucifers. Experiments were conducted on development, survival, reproductive potential and parasitism of Cotesia plutellae and Diadegma semiclausum on cultivated Brassica ( Brassica oleracea var. capitata and B. oleracea var. acephala) and wild crucifers ( Erucastrum arabicum, Raphanus raphanistrum, Rorippa micrantha, Rorippa nudiuscula and Brassica juncea). Both diamondback moth and parasitoids survived and developed on the wild crucifers. Egg-larval period of C. plutellae was shortest on E. arabicum (7.4 days) and longest on R. raphanistrum (9.3 days) while that of D. semiclausum was shortest on B. juncea (6.7 days). Ro. micrantha and R. raphanistrum recorded the lowest cocoon weight of 1.67 mg and 7.1 mg from C. plutellae and D. semiclausum, respectively. Longest pupal period was recorded on C. plutellae and D. semiclausum reared on B. oleracea var. acephala and Ro. micrantha, respectively. Egg-adult development time of C. plutellae was significantly longer on R. raphanistrum (15.2 days) and shortest on B. juncea and E. arabicum (12.2 days) while that of D. semiclausum was longest on Ro. micrantha (16.1 days) and shortest on E. arabicum (12.4 days). Mortality was higher on wild crucifers than on the cultivated Brassica species. In free choice tests, parasitism and parasitoid emergence were significantly higher on cultivated cultivars. Relatively higher percentage of larvae exposed to C. plutellae was not re-collected. The results suggest that wild crucifers can support development of the exotic parasitoids and thus provide refugia, thereby lowering the risk of local parasitoid extinction after pesticide application or harvesting. However, they might also be a source for diamondback moth and their presence near cultivated crucifer fields could lead to early population build-up.