Integrating conservation biological control and chemical control in cole crops through habitat manipulation

Parasitoid wasps attack imported cabbageworm (Pieris rapae) and diamondback moth (Plutella xylostella), two key caterpillar pests of cabbage. In their adult stage, these wasps feed on nectar and are susceptible to many broad-spectrum insecticides. Floral strips of sweet alyssum can provide parasitoids and other natural enemies with nectar. The integration of selective chemical treatments and floral strips was evaluated in 2011 and 2012 for effectiveness in suppressing the caterpillar complex of cabbage. Sweet alyssum was used for the floral strips due to its parasitoid-accessible nectaries and its long flowering stage. Two cabbage plots, one with sweet alyssum floral strips and one without, were established at each of three field sites in Ohio. Subplots within each main plot were treated with B.t., β-cyfluthrin, or were left untreated. Caterpillar populations were assessed weekly. Density ranged from 0.03 to 0.22 caterpillars per plant for P.  xylostella and 0.01 to 0.13 for P. rapae. Diadegma insulare and Cotesia rubecula were the most common parasitoids. The average parasitism rate ranged from 8 to 100%. Parasitism was highest within plots not treated with insecticide. Parasitism tended to increase over time. The presence or absence of floral strips did not have a statistically significant impact on parasitism, although there was a trend of lower caterpillar density within the alyssum plots at two out of three sites in 2011. In a related study, caterpillar parasitoids were surveyed monthly at ten commercial farms; species found were similar to those seen in the field trial but with greater diversity. Percent parasitism in these fields varied from 0% to 56%. The parasitoid complex for cabbage caterpillars has not been recently surveyed within Ohio; data on diversity and abundance will be useful to cole crop growers that wish to enhance biocontrol and develop multi-tactic pest management programs.