Staying in touch: how highly specialised moth pollinators track host plant phenology in unpredictable climates

Abstract

For pollinating insects that visit just a single flowering species, the co-occurrence of flowers and insects in time is likely to have critical implications for both plant and pollinator. Insects often utilise diapause to persist through periods in which resources are unavailable, timing their re-emergence by responding to the same environmental cues as their host plants. The obligate pollination mutualisms (OPMs) between Epicephala moths (Gracillariidae) and their leaf flower host plants are some of the most specialised interactions between plants and insects. However, to date there have been very few studies of Epicephala moth lifecycles and none of how they synchronise their activity with the flowering of their host plants. Breynia oblongifolia (Phyllanthaceae) is known to be exclusively pollinated by two highly specific species of Epicephala moth (Gracillariidae). We surveyed populations of both the host plant and it’s pollinators over multiple years to determine their annual phenology and then modelled the climatic factors that drive their activity. Using our newly gained knowledge of moth and host plant phenology, we then looked for evidence of diapause at both the egg and pre-pupal stages. Our phenology surveys showed that although female flowers were present throughout the entire year, the abundance of flowers and fruits was highly variable between sites and strongly associated with local rainfall and photoperiod. Fruit abundance, but not flower abundance, was a significant predictor of adult Epicephala activity, suggesting that eggs or early instar larvae diapause within dormant flowers and emerge as fruits mature. Searches of overwintering flowers confirmed this, with many containing evidence of pollen and diapausing pollinators. We also observed the behaviour of adult Epicephala prior to pupation and found that ~10% of the Autumn emerging Epicephala enter diapause, eclosing to adulthood after 38-56 weeks. The remaining 90% of autumn emerging adults pupate directly with no diapause, suggesting a bet hedging strategy for adult emergence. As such, Epicephala moths appear to utilise diapause at multiple stages in their lifecycle, and possibly bet hedging, in order to deal with variable flowering phenology and climatic unpredictability.