Abstract
Interactions of insect pests and their natural enemies increasingly are being considered from a metapopulation perspective, with focus on movements of individuals among habitat patches (e.g., individual crop fields). Biological control may be undercut in short-lived crops as natural enemies lag behind the pests in colonizing newly created habitat. This hypothesis was tested by assessing parasitism of cereal leaf beetle (Oulema melanopus) and alfalfa weevil (Hypera postica) larvae at varying distances along transects into newly planted fields of small grains and alfalfa in northern Utah. The rate of parasitism of cereal leaf beetles and alfalfa weevils by their host-specific parasitoids (Tetrastichus julis (Eulophidae) and Bathyplectes curculionis (Ichneumonidae), respectively) was determined for earliest maturing first generation host larvae. Rates of parasitism did not vary significantly with increasing distance into a newly planted field (up to 250–700 m in individual experiments) from the nearest source field from which pest and parasitoid adults may have immigrated. These results indicate strong, rapid dispersal of the parasitoids in pursuing their prey into new habitat. Thus, across the fragmented agricultural landscape of northern Utah, neither the cereal leaf beetle nor the alfalfa weevil initially gained substantial spatial refuge from parasitism by more strongly dispersing than their natural enemies into newly created habitat. Additional studies, including those of colonization of newly planted crops by generalist pests and natural enemies, are called for in assessing these results with a broader perspective.
Highlights
Study of insect population dynamics across space and time has shifted increasingly in recent decades from a local focus, to a larger, global perspective that stresses population subdivision and exchange among subpopulations collectively comprising metapopulations
The theoretical basis for successful biological control was most often posited to lie in enemy–pest interactions that result in low numbers of pests that are well-regulated locally at stable equilibria [11,12]
Alfalfa weevil larvae were abundant throughout the newly planted alfalfa field sampled in spring 2017
Summary
Study of insect population dynamics across space and time has shifted increasingly in recent decades from a local focus, to a larger, global perspective that stresses population subdivision and exchange among subpopulations collectively comprising metapopulations. This shift can be seen in the conceptual foundations underlying the biological control of insect pests. There has been increasing emphasis on landscape perspective to understand the dynamics of insect predator–prey interactions, including those of parasitoids and their pest hosts [1,2,3,4,5,6,7,8,9,10]. An absence of convincing evidence for similar population dynamics in most cases, has rekindled interest in the early leads of Nicholson [14] and Huffaker [15]
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