Agricultural landscape complexity has mixed effects on patterns of parasitoid abundance and diversity (Pseudaletia unipuncta, Glyptapanteles militaris, Meteorus communis)

The armyworm Pseudaletia unipuncta (Haworth) (Lepidoptera: Noctuidae) is a sporadic pest of maize and cereal grains in North America. Field experiments were conducted to determine how landscape structure and host density affect parasitism and parasitoid diversity. Sentinel host larvae were used to assess P. unipuncta parasitism in maize fields located in a complex and a simple agricultural landscape in Michigan, during 2000 and 2001. Neither landscape structure nor host density significantly influenced overall parasitism (80.2% on average) or parasitoid diversity (2.8 species per field on average). However, the two main parasitoid species reared from P. unipuncta (Hymenoptera: Braconidae) responded differently to host density. Glyptapanteles militaris (Walsh) parasitized proportionally more hosts at low host density, while Meteorus communis (Cresson) parasitized more hosts at high host densities. These results contrast with previous findings of increased parasitism in the complex landscape in 1993 and 1996, when overall parasitism was dominated by M. communis . However, this study corroborates that similar levels of parasitism in the complex and simple landscape are obtained in years when parasitism is dominated by G. militaris, such as 1998, 2000, and 2001. Laboratory experiments showed that carbohydrate resources significantly increase the longevity of both parasitoid species and M. communis fecundity.