Triadica sebifera (Euphorbiaceae), a centuries-old invader of southern ecosystems, was virtually free from natural enemies until the specific, hypermetamorphic moth Caloptilia triadicae (Lepidoptera: Gracillariidae) appeared in 2004. Simultaneously, populations of the destructive tawny crazy ant, Nylanderia fulva (Hymenoptera: Formicidae), spread cospatially across the southeast. The potential for positive interactions between these alien species was unknown; therefore, we sought answers to these questions: 1) does hypermetamorphic Caloptilia induce extrafloral nectar production in Triadica; 2) does Nylanderia consume nectar, Caloptilia larvae, or both; and 3) does Nylanderia provide indirect defense to the tree, or to its attacker? To examine potential interactions, and their implications for southern ecosystems, we conducted a series of lab, greenhouse, and field experiments in coastal Texas.
Results/Conclusions
In response to herbivory by Caloptilia, potted Triadica saplings strongly induced extrafloral nectar production. Percent leaf damage explained 57% of the variation in cumulative nectar volume (p<.0001), and nectar volumes were significantly greater on saplings with >20% of leaves damaged compared to controls (p=0.0005). Additionally, nectar production was strongly tied to the life stage of Caloptilia, increasing rapidly with the onset of leaf-rolling and decreasing sharply after adult emergence. Large Caloptilia infestations reduced stem elongation in potted Triadica (R2=0.32, p<.0001), but had no effect on leaf production, biomass, or survivorship. In all trials, Nylanderia exploited available nectar resources without attacking Caloptilia larvae. On potted saplings, nectar volumes were significantly lower when ants were present (p=0.008) while no differences in Caloptilia survivorship were observed (p=0.56). Likewise, no differences in Caloptilia abundance or parasitism rates were observed when ants were excluded from naturally recruited Triadica saplings (p=0.97; p=0.8). These results suggest that Triadica supplies Nylanderia workers with a carbohydrate resource while sustaining heavy herbivore damage, and neither Triadica nor Caloptilia benefit indirectly from Nylanderia foraging. Therefore, Nylanderia appears to be the only beneficiary in these alien, tritrophic interactions.