Lepidopteran pests Plutella xylostella, Pieris rapae, and Trichoplusia ni are the major species that attack crops in the family Brassicaceae. They are typically controlled by insecticide application, but there are parasitoid species that contribute to mortality. However, parasitoids alone are typically unable to exert sufficient control to reduce pest numbers below economic thresholds. Pest density tends to be lower in more diverse systems, including those that integrate flowering plants that parasitoids use as food resources. These resources are lacking in most agricultural landscapes. Parasitoids tend to be negatively impacted by exposure to broad-spectrum insecticides.
The first goal of this research was to investigate the integration of habitat manipulation and insecticide treatment through the planting of floral resources and use of selective microbial insecticides. The integration of sweet alyssum (Lobularia maritima) insectary strips and the insecticide Bacillus thuringiensis (B.t.) was investigated in a cabbage field trial conducted in 2011 and 2012. The results of this study are discussed in Chapter 2. The objectives of this study were: 1) to determine the effects of insectary strips on parasitism rates and pest density; and 2) to determine the effects of insecticides on parasitism rates and pest density. The main plot treatment was the presence or absence of insectary strips and the subplot treatment was insecticide. Plots were sampled weekly. Insectary strips were found to have no significant effect on pest density in or parasitism in 2011. In 2012, insectary strips were found to have increased pest density but also increased parasitism. Parasitism rate was lower in subplots treated with B.t. than in untreated subplots. Pest density in subplots treated with B.t. was not significantly different from subplots treated with cyfluthrin. These findings indicate that insectary strips can increase parasitism but may also increase pest density in some circumstances. The use of microbial insecticide did not increase parasitism but did keep pest density low. Because parasitism appeared to be density dependent, use of lower rates or longer intervals of B.t. may increase parasitism while maintaining low pest density.
Another goal of the research was to determine what parasitoid species were present in Ohio. The diversity and relative abundance of parasitoid species found in the survey are discussed in Chapter 3. The objectives of this study were: 1) to determine parasitoid diversity and parasitism rate on commercial farms; and 2) to determine whether differences in parasitism rates among farms are associated with differences in insecticide use as measured by the environmental impact quotient (EIQ). Ten commercial fields were used for the surveys in 2011 and 2012. Caterpillars were collected and held until the emergence of parasitoids or pests. Eleven parasitoid species were found. Species abundance varied between years. Farms with a higher EIQ rating tended to have decreased species abundance and percent parasitism, but this correlation was not significant. These findings indicate that many species of parasitoids are present in Ohio and that insecticide usage can impact parasitoid species diversity and abundance. Diadegma insulare and Cotesia rubecula should be the primary target of conservation biocontrol tactics within integrated pest management programs.