A bioassay for measuring the intensities of ant defenses on Macaranga myrmecophytes

In the Southeast Asian tropics, the tree genus Macaranga includes many myrmecophytic species that associate with ‘plant-ants’ nesting in their domatia spaces. Plant-ants on Macaranga myrmecophytes protect their host-plants against herbivores. Because interspecific differences in ant defense intensities among Macaranga myrmecophytes affect the host-plant use by herbivorous insects, they need to be studied to better understand the ecology and evolution of herbivores on Macaranga myrmecophytes. In this study, to examine whether larvae of a lycaenid species, Arhopala major, which potentially feeds on some Macaranga myrmecophytes, can be used for a bioassay that assesses relative ant aggressiveness towards general herbivores on Macaranga plants, we experimentally introduced A. major larvae onto leaves of three Macaranga myrmecophytic species. We measured (1) the time required for the first touch by plant-ants on an introduced larva and (2) the number of plant-ant workers aggregating 3 min after the first touch. The order of three Macaranga species in ant defense intensity, as estimated by the two measurements, corresponded with the results of previous studies investigating the interspecific differences in ant defense intensities using ant-exclusion experiments. This suggests that the bioassay using A. major larvae is valid for the assessment of relative intensities of ant defenses on Macaranga species. As time and labor cost are low and standardization is easier, compared with the other methods, such as ant-exclusion experiments, the bioassay tested in this study is practical for assessing ant defense intensities on Macaranga plants.