| Larval growth and feeding, and adult ovipositional preferences of the cabbage looper, Trichoplusia ni, were investigated on broccoli and eight common agricultural weeds: cow cockle (Saponaria vaccaria ), common groundsel (Senecio vulgaris), lamb's-quarters (Chenopodium album), redroot pigweed (Amaranthus retroflexus), sheep sorrel (Rumex acetosella), shepherd's-purse (Capsella bursa-pastoris), stinkweed (Thlaspi arvense ), and wild mustard (Sinapis arvensis). Plants were grown in a greenhouse in soil-less potting mix for six to eight weeks. In a seven-day growth trial, larvae grew larger on excised broccoli leaves than on cow cockle, lamb's-quarters, redroot pigweed, sheep sorrel, shepherd's-purse, stinkweed, and wild mustard. Only one insect of 64 in the growth experiment survived for seven days on stinkweed, indicating the possible presence of antifeedant or toxic compounds in the weed. In a leaf disc choice bioassay, 4th instar larvae showed a significant feeding preference for common groundsel over broccoli, but a similar preference for oviposition was not observed in adult females. Stinkweed shows promise as a trap species for this insect in broccoli crops since adult females preferred it to broccoli, but larvae could not survive on it. Water and nitrogen stress were applied to plants to determine what effects these plant stresses have on the insects. T. ni larvae grew larger on water-stressed broccoli plants compared with less-stressed plants. However, they showed no preference among water-stress treatments of broccoli as either larvae or adults. They exhibited an ovipositional preference for water-stressed weeds. Nitrogen status of the plant had little effect on T. ni growth or feeding preference, but common groundsel plants receiving high nitrogen levels were oviposited on preferentially compared with low-nitrogen plants. Adult host-plant preference did not always correlate with improved larval performance. This study suggests ways in which these weed species could be managed to reduce pest infestation levels in broccoli crops. It also shows that environmental stresses can have important consequences for both the plants and the insects. In examining performance and preference at different life stages and under different stresses, it shows that this insect does not always make the best choices for optimal growth and survival. |
