Long‐distance insect migration is poorly understood despite its tremendous ecological and economic importance. As a group, Nearctic hover flies (Diptera: Syrphidae: Syrphinae), which are crucial pollinators as adults and biological control agents as larvae, are almost entirely unrecognized as migratory despite examples of highly migratory behavior among several Palearctic species. Here, we examined evidence and mechanisms of migration for four hover fly species (Allograpta obliqua, Eupeodes americanus, Syrphus rectus, and Syrphus ribesii) common throughout eastern North America using stable hydrogen isotope (δ2H) measurements of chitinous tissue, morphological assessments, abundance estimations, and cold‐tolerance assays. Although further studies are needed, nonlocal isotopic values obtained from hover fly specimens collected in central Illinois support the existence of long‐distance fall migratory behavior in Eu. americanus, and to a lesser extent S. ribesii and S. rectus. Elevated abundance of Eu. americanus during the expected autumn migratory period further supports the existence of such behavior. Moreover, high phenotypic plasticity of morphology associated with dispersal coupled with significant differences between local and nonlocal specimens suggest that Eu. americanus exhibits a unique suite of morphological traits that decrease costs associated with long‐distance flight. Finally, compared with the ostensibly nonmigratory A. obliqua, Eu. americanus was less cold tolerant, a factor that may be associated with migratory behavior. Collectively, our findings imply that fall migration occurs in Nearctic hover flies, but we consider the methodological limitations of our study in addition to potential ecological and economic consequences of these novel findings.
This project investigated associational interactions (associational resistance or susceptibility) between native and non-native trees commonly found in urban landscapes in the southeastern United States. Non-native plants offer limited ecological services because few native herbivore species are capable of feeding on them. In a 2-yr field study, abundance and species richness of caterpillars, plant damage, and herbivore natural enemies were evaluated in plots where a native red maple (Acer rubrum L. [Sapindales: Aceraceae]) was planted singly (no neighbors) or interplanted with either non-native non-congeneric crepe myrtles (Lagerstroemia indica L. [Myrtales: Lythraceae]), non-native congeneric Norway maples (Acer platanoides L. [Sapindales: Aceraceae]), or other red maples. Dryocampa rubicunda Fabricius (Lepidoptera: Saturniidae) accounted for most of the damage and caterpillar abundance. There were few significant differences between treatment groups in the establishment year of 2014, but in 2015 there was greater tree defoliation, caterpillar abundance, and caterpillar species richness when red maples were surrounded by crepe myrtles. We describe this as a biological fence effect in which the presence of crepe myrtle causes caterpillars to accumulate on the focal red maples over multiple generations. Red maples interplanted with Norway maple neighbors hosted an intermediate abundance and species richness of caterpillars compared to red maples interplanted with crepe myrtles and those with other red maples, indicating a spillover of herbivores to the related maple. No significant trends in insect natural enemy abundance or diversity between treatment groups were detected. These results highlight the necessity of considering plant associational interactions in context of species origin to alleviate pest outbreaks and develop sustainable landscape designs.
Semi-natural field borders are frequently used in midwestern U.S. sustainable agriculture. These habitats are meant to help diversify otherwise monocultural landscapes and provision them with ecosystem services, including biological control. Predatory and parasitic arthropods (i.e., potential natural enemies) often flourish in these habitats and may move into crops to help control pests. However, detailed information on the capacity of semi-natural field borders for providing overwintering refuge for these arthropods is poorly understood. In this study, we used soil emergence tents to characterize potential natural enemy communities (i.e., predacious beetles, wasps, spiders, and other arthropods) overwintering in cultivated organic crop fields and adjacent field borders. We found a greater abundance, species richness, and unique community composition of predatory and parasitic arthropods in field borders compared to arable crop fields, which were generally poorly suited as overwintering habitat. Furthermore, potential natural enemies tended to be positively associated with forb cover and negatively associated with grass cover, suggesting that grassy field borders with less forb cover are less well-suited as winter refugia. These results demonstrate that semi-natural habitats like field borders may act as a source for many natural enemies on a year-to-year basis and are important for conserving arthropod diversity in agricultural landscapes.
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