Urban tulip poplars (Liriodendron tulipifera) often host localized outbreaks of tuliptree aphids (Illinoia liriodendri), which typically augment local predator populations. We hypothesized that aphid-infested trees would support greater predator abundance and diversity, leading to increased biological control of non‐pest herbivores on the same trees compared to aphid-free trees. To test this, we experimentally infested tulip poplars with aphids and kept others aphid-free. Natural enemy communities were surveyed using a variety of methods to capture the full community of natural enemies. Simultaneously, live Actias luna caterpillars and clay caterpillar models were deployed as sentinel prey to quantify biological control rates associated with aphid presence and absence. We also measured foliar nutrients and defense compounds, alongside tree health metrics, to evaluate resource quality and host vigor. We used these data to quantify predator abundance and diversity responses and apparent-competition effects of aphid outbreaks on non-pest caterpillars. Results shed light on how urban sap-feeding pest outbreaks may interact with conservations efforts in urban forests.
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