Conserving parasitoid assemblages of North American pest Lepidoptera: Does biological control by native parasitoids depend on landscape complexity?

The alternate host food–plant relationships of North American pest Lepidoptera were examined to better understand how the vegetational structure of agricultural landscapes may influence parasitoid populations and diversity and thus their potential in conservation biological control. Examined were whether the common pest Lepidoptera of the USA and Canada are generalists or specialists, the successional stage occupied by plants upon which alternative hosts of their hymenopteran parasitoids are found (i.e., what extra-field habitats might support a greater number and diversity of alternate hosts). Forty-two species of native North American pest Lepidoptera in eight families were examined. Lepidoptera were mainly polyphagous feeders. Most of the hymenopteran (53%) and tachinid (72%) parasitoids associated with the Lepidoptera were generalists. Food plant range was determined for 90% of the 649 alternate hosts identified. A majority fed either exclusively on trees (33.3%) or exclusively on ruderal species (29.9%). Half the alternate hosts (50.7%) feed at least sometimes on late successional species. Hymenopteran generalist parasitoids were associated strongly with lepidopteran alternate hosts (47%) that feed on trees and shrubs, whereas hymenopteran oligophagous and specialist parasitoids were associated strongly with lepidopteran alternate hosts (68 and 100% respectively) that feed on ruderals and shrubs. Tachinid generalist and oligophagous parasitoids were associated strongly with lepidopteran alternate hosts (48 and 71%, respectively) that feed exclusively on trees and shrubs. Effective conservation of native parasitoids will probably require the preservation and/or restoration of late successional habitats within the agricultural landscape.