Comparison of oilseed Brassica crops with high or low levels of glucosinolates and alfalfa as hosts for three species of Lygus (Hemiptera: Heteroptera: Miridae)

The suitability of oilseed rapes, Brassica spp., and alfalfa, Medicago sativa L., as host plants for Lygus spp. is compared. Lygus bugs were monitored in field plots of five cultivars of oilseed rape, Brassica napus L. and Brassica campestris L., containing high or low levels of glucosinolates. The number of lygus collected did not differ among cultivars of the same species. Nymphs reared in the laboratory on the five cultivars had the same survival and developmental rate regardless of the glucosinolate status of the cultivar. Survival, development, and growth were compared for nymphs reared on excised stem tips of oilseed rape and alfalfa that had flower buds or flowers. Nymphs developed faster and had higher survival when reared on oilseed rape than when reared on alfalfa. However, the dry weights of adults collected from an alfalfa field were significantly higher than those reared on alfalfa in the laboratory, indicating that excised alfalfa may not be as good a source of food for conducting laboratory tests as excised oilseed rape. Adult weights of field-collected lygus bugs from alfalfa and oilseed rape did not differ significantly. The results demonstrate that oilseed rapes with high or low levels of glucosinolates are both suitable hosts for the three species of lygus, and as suitable as alfalfa. Furthermore, the change from the older high-glucosinolate rape cultivars to the low-glucosinolate cultivars that are now widely grown probably had no effect on the suitability of the crop for lygus bugs