Invasive paper wasps have strong cascading effects on the host plant of monarch butterflies.
The direct and indirect impacts that invasive predators have on communities within their invaded range are poorly understood, particularly in the early stages of invasion. Through top-down control of their prey, predators have the capacity to trigger cascading effects on lower trophic levels. We found the recent arrival of the invasive paper wasp Polistes dominula Christ has been associated with substantial declines in local butterfly abundance in New Zealand. One of the butterfly species we observed to be affected is the monarch, Danaus plexippus Linnaeus with densities reduced by 66% at the study site. Field experiments were conducted to examine the strength of the predation pressure exerted by P. dominula on monarch caterpillars and the cascading effects on milkweed (Gomphocarpus physocarpus E. Mey.) fitness. A survival study of monarch caterpillars was conducted across three habitat types (coastal, forest, and suburban). Caterpillar survival in suburban areas was lowest, with only 45% of caterpillars remaining after just 6 h of exposure to wasp foraging. Predation by P. dominula explained 85% of caterpillar deaths within the trial period. The cascading effects of P. dominula presence were quantified through changes in the height, foliage, and reproductive output of milkweed plants. Monarch caterpillar predation by P. dominula was found to have a positive effect on milkweed fitness. This study demonstrates a strong trophic cascade initiated by an invasive predator. These findings highlight the impacts an invasive species can have on local communities beyond their direct predatory effects.