The concept that moths pollinate some plants is not novel, with examples featuring in the writings of Charles Darwin and Alfred Russel Wallace. Nevertheless, much of the study of nocturnal pollination has focussed on highly-specialised single interactions (e.g. between yuccas and yucca moths), with comparatively little attention paid to the potential role of moths as generalist providers of pollination to entire communities of plants.
In the last decade there has been an increasing focus on understanding plant-moth interactions at the ecosystem level. A review of the global peer-reviewed literature combined the sum of knowledge from many previous studies (mainly of single or limited, defined sets of interactions) to suggest that moth pollination is important to an extremely diverse set of plant species in a wide range of ecosystems and at a global scale1. Additionally, much has been discovered by a change in sampling approaches, from the classical method of flower-visitation (watching a flower and recording insect visitors) to the alternative approach of pollen-transport (collecting and identifying pollen transported on the body of a flower-visiting insect). Early studies using this technique demonstrated that a substantial proportion of moths (both in terms of individuals and species) carried pollen in a range of ecosystems2,3, with pollen detected on some 76% of individual moths collected in one Portuguese meadow3. More recently still, various studies have indicated that nocturnal pollination may be under threat from drivers of environmental change, especially disruption by artificial light at night4–6.
However, a common theme of many of these studies was the reporting of “unknown” pollen types whose identity could not be determined to species, or in some cases even family level. The pollen-transport approach has, until recently, relied upon the morphological identification of pollen grains under a light microscope, which requires considerable expertise. Identifications are often aided by reference to a collection of the pollen of locally-present plants, but this has the potential to be misleading when dealing with pollen transported by moths, some species of which are capable of ranging long distances each night7 or carrying pollen with them as they undertake trans-continental migrations8. These and other factors have led to the development of approaches that make use of novel DNA-sequencing technology to identify pollen grains by their genetic code9.
A new study published in the journal Ecological Entomology has now used nocturnal moths (sampled on farmland in East Yorkshire) as a study system to test the effectiveness of this new approach, called “DNA metabarcoding”, against the existing light microscopy method10. DNA metabarcoding compared favourably to light microscopy, detecting pollen on a greater proportion of moths and demonstrating a greater ability to distinguish morphologically-similar or related pollen types into separate taxonomic groups, although it did not outperform the existing method in all areas. Pollen types could, in many cases, be identified with finer taxonomic resolution than by light microscopy, but still many were not positively identified to species-level. However, perhaps the greatest demonstration of the power of DNA metabarcoding came from the range of moth-plant interactions that were identified by the new method.
Confidence in the accuracy of the DNA metabarcoding approach was reinforced by its detection of several plant species known from earlier studies to interact with moths, including Buddleja davidii (buddleia), Hedera helix (ivy) and Galium aparine (cleavers). Alongside these familiar interactions, though, was pollen from many species not previously associated with nocturnal pollinators, such as species of the genera Trifolium (clovers) and Epipactis (helleborine orchids), both of which are generalist and more typically associated with pollination by bees. Certain groups of identified pollen types were of particular interest. Some pollen was most likely to have come from garden plants, being carried across hundreds of metres to the farmland field site. This supports previous suggestions that moths may transport pollen over considerable distances and therefore might play an important role in dispersing genes between populations of plants at a landscape-scale.
Additionally, several pollen types belonged to groups that include commercial- or allotment-scale crops, including Pisum sativum (pea), Brassica/Raphanus sp. (a complex which includes the highly-valuable crop oil-seed rape), Prunus sp. (more likely to be a species of cherry than wild P. spinosa due to the phenological timing of the field study), and Rubus sp. (a group which includes raspberry Rubus idaeus as well as many wild blackberry species). These preliminary findings emphasise a need to formally investigate the degree to which nocturnal pollination might contribute to yields in a wide range of insect-pollinated crops.
The growing body of evidence that moths may contribute to the reproduction of a wide variety of plants at a global scale, potentially even including some crops, provides a powerful new motivation to conserve them. Nevertheless, each new study has emphasised how little is still known about the scale of nocturnal pollination and its importance to plants.
Dr Callum Macgregor
Postdoctoral Research Associate, Department of Biology, University of York