Survey Methods

Overview

Surveying for moths is most often and easily accomplished by setting lights at night to attract adults. Moths are primarily nocturnal, and most species are attracted to lights, some much more strongly than others. A single bright ultraviolet or incandescent light placed in a rich temperate habitat will reliably attract well over 100 species of moths on on calm, moonless, and warm summer nights. Even in quite degraded and fragmented semi-natural habitats, such as the post-agricultural meadows and shrublands that cover much of Block Island, a light can attract over 100 species of moths on the best nights. While certain moth taxa are diurnal or not attracted to lights, most species present in any given area will come to lights at least occasionally.

Light surveying has been the main method used in the Block Island moth survey, and only a handful of the more than 1,200 species known from Block Island have been recorded on the island only by other survey means. The most conspicuous and diverse group of Lepidoptera not attracted to lights is the butterflies, which comprise a single large diurnal superfamily, Papilionoidea. (Butterflies evolved from moths and from the perspective of cladistics are a group of diurnal moths in the same way birds are therapod [dinosaurs], hominids are apes, and ants are wasps.) Moths are also commonly surveyed by searching flowers on warm sunny days for nectaring diurnal adults, sweeping fields for adults, and searching for larvae on and in plants; I have not utilized these methods as much as I would like so far.

What makes my survey nearly unique among moth surveys is that since 2018, I have made comprehensive records of abundances of all species each survey night, recording more than 40,000 individual moths in four seasons. This dataset allows for quantitative analysis of the fauna, which I have only just begun. For each species, I have generated a flight time phenology chart and a map of relative abundance in each area surveyed. For common species, my observations reliably measure flight times and habitat preferences. My data can also be used to characterize Block Island’s moth fauna in great detail. So far, I have produced measures of the similarity of the fauna between survey locations and, using my observations at my porch lights, its change over the course of the season.

Timeline

The methods, goals, and scope of this survey evolved organically over the course of several years beginning in 2014. This survey began as part of a more general survey of Block Island’s animal fauna I began in August 2014 with no guidance or prior experience in faunal surveying methods or species identification. Over the following few years, moths become nearly the exclusive focus of the project, and the moth component of my survey gradually grew into an effort at a comprehensive survey of Block Island’s moths. I began collecting moths in 2016 but did not intensively collect for identification until 2018. I began blacklighting around the island, collecting smaller microlepidoptera, and counting common species at my porch lights in 2017. In 2018, my survey efforts took close to their definitive form. I regularly set up sheets for blacklighting at sites across the island that summer, recorded all individual moths sighted both at my porch lights at at blacklighting sheets, and intensively collected taxa I could not identify to species by sight. In 2021, I began using no-kill bucket traps to supplement my surveying with sheets, particularly in dunes, where winds slacken enough for surveying with sheets only a few nights each summer. Future plans, aside from additional surveying for moths, include floristic and habitat surveys of each location sampled and a detailed plant inventory of the Hunt property and surrounding area.

I began nightly estimates of species counts at my porch lights in 2017 for a set slate of several dozen common species and species complexes and in 2018 for nearly every species. Certain groups, such as Pero spp. and Eulithis spp., in which the species present on Block Island are difficult to impossible to separate on sight, I have recorded jointly. Some groups I could not personally identify on sight in 2018 or into 2019, notably all Renia spp., and some Acronicta spp., I always photographed and/or collected for later determination. Anything unrecognized I likewise photographed and/or collected. Since 2019, I have always collected each species I have not previously seen. For Coleophora spp. and Blastobasidae, I recorded morphospecies as precisely as I could, though I shortsightedly did not consistently make note of the streaky tan Coleophora spp. in 2019. I have since identified many of these morphospecies in through identification of collected specimens, though probably several of Block Island’s Coleophora species are indistinguishable by sight due their similarity, low contrast patterning, and small size. Aside from the omission of some Coleophora spp. in 2019, I have recorded in some manner all moths seen since 2018.

While the great majority of my survey effort has been with lights, I have searched in daytime for adults of diurnal species on flowers and vegetation and by sweeping meadows and for larvae, especially leafminers. Additionally, Nigel Grindley has regularly photographed and collected moths at lights at his home in the southwest of Block Island for several years, by a remarkable coincidence starting the same year I did. In the last few years, he has also surveyed at sheets with blacklights several years while I have been off the island. By my count, he has found 58 species I have not found, of these, fewer than ten were recorded by previous surveyors, making Nigel’s contribution to the island’s species list quite significant. (He has of course seen several hundred of the species I have seen as well.) His surveying has disproportionately added species that fly only very early or late in the season, when I have rarely been on Block Island.

Recording Sightings

The methodology I have used for counting numbers of each species at my porch lights is roughly as follows: I start by scanning the porch, walls, and eaves for moths and note the species I see on my phone. As I go, I photograph uncommon or otherwise interesting moths and capture specimens I wish to collect, usually for identification. I frequently update estimated numbers of each species based either on spotting specimens that were worn or differently patterned than those previously counted or by scanning the entire area for all examples of a set of species for a more complete count. For especially abundant species, I estimate numbers. While my methodology is highly inexact, especially during the most productive nights, it has been fairly consistent. The time I spend at the porch lights each night varies based on the number of moths present, but during the peak of the season, 45-60 minutes is probably typical.

During blacklighting sessions, I follow a similar procedure to that I employ at my porch lights, except in that I spend most or all of the night observing moths until I take down the sheets. When I use two sheets, as I typically do, I record moths separarately at each sheet. I make multiple visits to each sheet each night, and I often note the moths I see flying past when walking between the sheets. My notes on moth found away from the sheets have informed my understanding of which common species are poorly drawn to lights. My typical procedure for counting individuals of a species at a sheet is to quickly but thoroughly scan each side of the sheet and then any nearby plants as well as the ground, continually scanning the sheet and updating counts as long as the total number of individuals present appears to be increasing. As at the porch lights, I use photography and collection to fill all gaps in the data created by identification uncertainty, excepting certain species complexes.

Data Analysis

At the end of each season, I compile all my sighting records into a single document in chronological order. I manually add my sightings to a spreadsheet with a line for each species and a column for each night or blacklighting sheet (Fig. 1). Records of unidentified species are marked in red and with asterisks and revisited when I identify the species they represent. I add photographic and specimen records, not all of which I include in my nightly notes, to my tables to complete my records.

I use a series of spreadsheets to store all my photographic records from 2017 and earlier, Nigel’s photographic records, and records from older surveys; and I use these along with my 2018–2021 sighting tables to make a single table with presence/absence data for each species in each 1/3-month period from March through December (Fig. 2). I use the sighting records from my porch lights from 2018 to present to calculate flight time phenology in individuals per survey night and as a percent of all moth records for each 1/3-month period (Fig. 3).

Figure 1. A section of the table with sighting records from 2020; columns for blacklighting nights are shaded gray. Photographic sightings are indicated by light gray shading.
Figure 2. A section of the table with species presence (1) or absence (0) over the season.
Figure 3. A section of the table with calculated species abundances over the season. The number of individuals per night of surveying is the total number of individuals recorded at my porch lights in a 1/3-month period from 2018 to 2021 divided by the number of nights I surveyed at my porch lights in that period. The denominator excludes extremely poor nights with numbers far below normal due to high winds. Abundance is also calculated as the number of individuals of a species divided by the number of individuals of all species of moths recorded at my porch lights in each 1/3-month period. Records of a few taxa, such as Coleophora, remain very incomplete, so the true values are slightly lower than currently calculated. Percent representation of each species will fall as records of particularly difficult taxa are identified and added.