Edward Oliver Essig

Aim Many invasive plant species benefit from enemy release resulting from the absence of insect herbivores in their invaded range. However, over time, specialized herbivores may ‘catch up’ with such invasive plants. Black locust is a tree species with a relatively limited native range in North America but has invaded large areas in virtually every temperate continent including North America. We hypothesize that both intra- and intercontinental spread of black locust leads to a parallel, though delayed pattern of intra- and intercontinental spread of insect herbivores. Location Global. Taxon Black locust, Robinia pseudoacacia, and its insect herbivores. Methods We compiled historical records of the occurrence of insect herbivore species associated with R. pseudoacacia from all world regions. Based on this list, we describe taxonomic patterns and investigate associations between environmental features and numbers of non-native specialist herbivores in the portion of North America invaded by R. pseudoacacia. Results A total of 454 herbivorous species are recorded feeding on R. pseudoacacia across the world, with 23 of these being specialized on Robinia. From this group, seven species have successfully expanded their range beyond North America. Within North America, the richness of specialists is explained by a combination of road density, R. pseudoacacia density, distance from the R. pseudoacacia native range, and climate. Main Conclusion Non-native herbivore species have accumulated on invasive R. pseudoacacia in both North America and in other continents. The steady build-up of invasions likely has diminished the enemy release that this invasive tree species has benefited from – a trend that will likely continue in the future. These findings support the hypothesis that invasive plants promote parallel though delayed invasions of specialist insect herbivores.

Species exhibit a range of specialisation in diet and other niche axes, with specialists typically thought to be more efficient in resource use but more vulnerable to extinction than generalists. Among herbivorous insects, dietary specialists seem more likely to lack acceptable host plants during the insect's feeding stage, owing to fluctuations in host‐plant abundance or phenology. Like other herbivores, bee species vary in host breadth from pollen specialisation (oligolecty) to generalisation (polylecty).Several studies have shown greater interannual variation in flowering phenology for earlier‐flowering plants than later‐flowering plants, suggesting that early‐season bees may experience substantial year‐to‐year variation in the floral taxa available to them.It was therefore reasoned that, among bees, early phenology could be a more viable strategy for generalists, which can use resources from multiple floral taxa, than for specialists. Consequently, it was expected that the median dates of collection of adult specimens to be earlier for generalist species than for specialists. To test this, phenology data and pollen diet information on 67 North American species of the bee genus Osmia was obtained.Controlling for latitude and phylogeny, it was found that dietary generalisation is associated with significantly earlier phenology, with generalists active, on average, 11–14 days earlier than specialists.This result is consistent with the generalist strategy being more viable than the specialist strategy for species active in early spring, suggesting that dietary specialisation may constrain the evolution of bee phenology—or vice versa.

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