Moths complement bumblebee pollination of red clover: a case for day-and-night insect surveillance

Abstract

Recent decades have seen a surge in awareness about insect pollinator declines. Social bees receive the most attention, but most flower-visiting species are lesser known, non-bee insects. Nocturnal flower visitors, e.g. moths, are especially difficult to observe and largely ignored in pollination studies. Clearly, achieving balanced monitoring of all pollinator taxa represents a major scientific challenge. Here, we use time-lapse cameras for season-wide, day-and-night pollinator surveillance of Trifolium pratense (L.; red clover) in an alpine grassland. We reveal the first evidence to suggest that moths, mainly Noctua pronuba (L.; large yellow underwing), pollinate this important wildflower and forage crop, providing 34% of visits (bumblebees: 61%). This is a remarkable finding; moths have received no recognition throughout a century of T. pratense pollinator research. We conclude that despite a non-negligible frequency and duration of nocturnal flower visits, nocturnal pollinators of T. pratense have been systematically overlooked. We further show how the relationship between visitation and seed set may only become clear after accounting for moth visits. As such, population trends in moths, as well as bees, could profoundly affect T. pratense seed yield. Ultimately, camera surveillance gives fair representation to non-bee pollinators and lays a foundation for automated monitoring of species interactions in future.

Keywords: Lepidoptera; biodiversity; computer vision; conservation; entomology; phenology.

Figure 1.

(a) Study site in the Calanda region of the Swiss Alps (photo credit: E.I.). (b) Image from a time-lapse camera, midday, 20 July 2021. (c) Probable Bombus lapidarius visit. (d) Noctua pronuba visit. (e) Frequency of Trifolium pratense visits from moths (dark blue), bumblebees (blue) and other visitors (light blue), as well as T. pratense cover (pink line) recorded by cameras throughout summer 2021.

Figure 2.

The relationship between visitation and seed set is (a) clear when moth visits are accounted for, but (b) veiled when only bumblebees are considered. Box colours indicate how visitation categories in (a) are merged in (b). Letters represent statistical significance within each panel; in (a), seed set differs significantly between groups A and B at p = 0.040. (c) Visit lateness predicts seed lateness in T. pratense inflorescences visited by bumblebees (white), moths (black) or both (grey). The VLI represents mean visitation date relative to peak flowering date of an inflorescence. The SLI indicates whether seeds were more frequent in early-opening florets (low SLI) or late-opening florets (high SLI). The dashed line represents a weighted regression (larger points have larger weights), highlighting how inflorescences with early visits had more seeds in early-opening florets (p = 0 0.0048).