El Niño oscillations impact anti-predator defenses to alter survival of an herbivorous beetle in a neotropical wet forest

Authors: VENCL, FREDRIC - Stoneybrook University; Srygley, Robert

Submitted to: Journal of Tropical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/2023
Publication Date: 9/1/2023
Citation: Vencl, F.V., Srygley, R.B. 2023. El Niño oscillations impact anti-predator defenses to alter survival of an herbivorous beetle in a neotropical wet forest. Journal of Tropical Ecology. 39. Article e34. https://doi.org/10.1017/S0266467423000226.
DOI: https://doi.org/10.1017/S0266467423000226

Interpretive Summary: The El Niño-La Niña climate cycle (ENSO) is a primary driver of outbreaks and migrations of herbivorous insects globally. A major gap in our understanding of herbivore outbreaks driven by ENSO is the response of enemies that control herbivore populations. To fill this gap, we investigated a beetle that feeds on a tropical vine and measured mortality resulting from five enemy guilds (bugs, wasps, ants, egg parasitoids, and larval parasitoids) over a five year period characterized by one El Niño and three La Niña events. We also experimentally manipulated the beetles’ defenses against its enemies to assess whether defense efficacies change with ENSO. We found that egg parasitism increased in El Niño and this decline in hatching then increased apparent competition between egg parasitoids and ants. Maternal guarding was most effective against social wasps in La Niña, whereas guarding and shields proved most effective in El Niño when ants were the primary attackers. Larval parasitism was more prevalent in La Niña years, when carnivorous wasps were also most prevalent, in a second case of apparent competition. Asynchronous fluctuations in enemies with ENSO stabilize the prey-enemy network effectively controlling the herbivore. ENSO is forecast to increase in frequency and amplitude with global warming. Coupled with forest fragmentation, this projected change threatens stability of prey-enemy networks controlling insects that feed on crops and forest resources.

Technical Abstract: Little is known about the effects of El Niño-Southern Oscillation (ENSO) on tropical insect communities even though they are suffering rapid declines in complexity and stability due to climate change. We explore the impact of fluctuations in local climate imposed by ENSO on the performance of herbivore defenses mediating enemy interactions. In a widespread rainforest edge community, we quantified the mortality caused by five enemy guilds on the immature stages of the herbivorous beetle, Acromis sparsa. ENSO was a significant determinant of beetle mortality. During warmer, drier El Nino years, the survival of beetles decreased. This was due to increased egg parasitism by wasps, which reduced hatching. Additionally, ant predation on beetle larvae increased. Flies and wasps were in competition for larval prey in wetter, cooler La Niña years. Experimental removal of maternal guards or chemical shields revealed which ENSO-related parameters predicted larval mortality. Guarding was most effective against social wasps in La Niña, whereas shields proved most effective in El Niño. Two ENSO-related defense-enemy breakdowns occurred: (1) decoupling whereby the efficacy of a narrow defensive adaptation was reduced to increase mortality, and; (2) mismatching whereby the resistance of a narrow defensive adaptation against non-targeted enemies was further reduced to increased mortality. These results highlight that defense efficacy against natural enemies can vary predictably with biotic and abiotic environmental conditions. ENSO events will increase breakdowns in defense-mediated interactions, shifts in competition among enemies, and species loss.