Experimental Evaluation of Arthropod Predation on Pieris rapae (Lepidoptera: Pieridae) Eggs and Larvae in Cabbage

Knowledge of factors causing mortality in herbivorous insects is essential to developing a better understanding of their population dynamics and more effective strategies to manage their abundance in crops. In this study we used 2 methods of predator exclusion to evaluate the effects of arthropod predators on Pieris rapae L. eggs and larvae on cabbages ( Brassica oleracea variety capitata L.) in New York State. Survivorship of P. rapae on cabbage plants caged to exclude predators was compared with survivorship on plants in cages that were opened at the bottom to allow access by arthropod predators but not larger predators such as birds. Two cohorts were followed in each of 2 unsprayed cabbage plots in each of 2 yr for a total of 8 cohorts. Estimated mortality of eggs and larvae from arthropod predators ranged from 23 to 80%, averaged 53% for all 8 cohorts, and affected mainly the eggs and 1st instars. Exclusion experiments were also conducted comparing mortality of individual P. rapae eggs protected from predators by rings of Tanglefoot with that of eggs that were left exposed to predators. Mortality attributed to arthropod predators for the entire egg stage among 6 cohorts placed in each of 2 fields ranged from 0 to 44%. Our experiments demonstrate that P. rapae eggs and 1st instars suffer variable, but often quite high mortality from arthropod predators in cabbage fields. Recognizing the important role of these predators is a 1st step toward developing ways to maximize their activity in commercial fields.

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