Leaf heteroblasty explains unexpected spillover non-target attack on Passiflora edulis by Heliconius erato cyrbia, a biological control agent for Passiflora rubra, in Rarotonga

摘要

Larvae of Heliconius erato cyrbia, a biocontrol agent for invasive Passiflora rubra in the Cook Islands, were discovered feeding on Passiflora edulis seedlings in Rarotonga in April 2018. This represents only the fifth known example worldwide where non-target attack has occurred on a plant species that had been included in host-range tests and deemed not at risk. Field surveys on Rarotonga revealed that non-target attack is rare and trivial, indicating that a major host-range expansion has not occurred. No-choice host-range testing performed on both seedlings and mature P. edulis investigated the hypothesis that heteroblasty (dramatic differences in the leaf morphology and chemistry of seedlings and mature P. edulis plants) explains the unanticipated non-target attack. These tests confirmed that mature P. edulis foliage is toxic to H. e. cyrbia larvae but demonstrated that P. edulis seedlings were highly palatable, resulting in survival rates not significantly different to those on P. rubra. Care should be taken when conducing host-range testing on plant species that exhibit heteroblasty due to the risk of potentially overestimating or underestimating the probability and potential impacts of non-target attack, depending on the growth stage tested.