Graduate Thesis Or Dissertation
 

Conservation of arthropod natural enemies in broccoli with relay strip-cropping

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  • Relay strip-cropping combines two vegetation management tactics, under-sowing and strip-management. In this study conducted from 1994 though 1997, a cover crop seed-mixture containing oats (Avena sativa L. var. "Monida"), common vetch (Vicia sativa L.), red clover (Trifolium pratense L.), annual ryegrass (Lolium multflorum Lam.), and buckwheat (Fagopyrum esculentum Moench) was broadcast over established broccoli (Brassica oleracea L.) 32 days after planting. The broccoli was planted, under-sown, and harvested in sections alternated with strips of cover crop. The cover crop relay provided tillage and pesticide refuges for beneficial insects without taking land out of broccoli production. Relay strip-cropping increased the abundance and diversity of ground beetles (Coleoptera: Carabidae), spiders (Araneae), harvestmen (Opiliones: Phalangiidae), lady beetles (Coleoptera: Coccinellidae), and damsel bugs (Heteroptera: Nabidae) in the cropping system; however, this did not always result in increased natural enemies in the broccoli. Arthropod species varied significantly in their propensity to leave the non-crop vegetation and move into the broccoli. Movement of the lady beetle Coccinella septempunctata L. from the cover crop strips into the broccoli may have depressed aphid populations in 1996. The non-crop vegetation may have served as a sink rather than a source for Nabis species and appeared to reduce their density in the broccoli growing nearby. There was no evidence that background vegetation surrounding the broccoli reduced colonization by winged aphids. The density of the cabbage flea beetle Phyllotreta crucferae Goeze was lower in the broccoli relay compared with the clean-tilled broccoli. The number of third instar imported cabbage worm Pieris rapae (L.) and cabbage looper Trichoplusia ni (Hubner) was reduced on broccoli leaves and harvested buds in the relay strip-cropping system compared with the clean-tilled system. The number of diamondback moth Plutella xylostella (L.) pupae was higher in the clean-tilled broccoli; percent parasitism of the pupae was similar in the two cropping systems. Although reduction in broccoli contamination associated with relay strip-cropping was significant, the residual insect contamination was unacceptable for commercial broccoli production. Relay strip-cropping may be useful for preventing soil erosion and protecting water quality. Relay strip-cropping may be a useful component of an integrated pest management program.
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