Northern Prairie Wildlife Research Center
While many of the moths illustrated here are common, only a small percentage of the species in the Northwest are represented. You may collect a specimen that does not match any of the photographs or fit any of the descriptions of similar species in this publication. If you cannot determine the identity of a macromoth from the western United States, then look in Covell (1984). Covell (1984) provides an extensive assortment of photographs for species that occur in the eastern United States, so the probability of a match to a western species is limited to those species that are widespread across the North American continent. Also, serious students of moths should look in the references cited at the end of the discussion of each family in the section on Macromoth Family Notes.
For each of the 251 species presented with a photograph of the adult, we provide a narrative that includes (1) the wingspan of a typical adult, (2) a description of diagnostic morphological traits, (3) comments on relative abundance, (4) biogeographical range and general habitat, (5) flight period, (6) caterpillar host plants, and (7) similar species.
Size of the moths in the pictures is difficult to compare among species because the images were taken to maximize the size of the moth to the dimensions of the photograph. Thus, small moths appear to be the same size as large moths. Note that the image of Eurois astricta, a relatively large moth, is similar in size to the image of Xestia oblata which in reality are much smaller moths. Because maximizing the size of each image was a high priority we offer a measure of a typical wingspan for each species. Using the example of Eurois astricta with a wingspan of 5.3 cm the size difference becomes apparent when compared with the 3.8 cm wingspan of Xestia oblata.
The measurements for wingspans were taken from properly spread specimens of typical size for the species. The distance from the tip of the left forewing to the tip of the right forewing was used to represent the wingspan and is presented with a resolution to within 1 mm. We did not attempt to assess the statistical distribution of wingspan values for each species but in general most species will exhibit a size range of 15 - 20 percent above and below the average value. Thus, it should not be surprising to collect a moth slightly smaller or slightly larger than the dimensions presented in the species diagnostic traits.
The field identification of species of macromoths may be conducted by comparing combinations of features involving colors, patterns, or shape of the wings, head, thorax, and abdomen.
Wings. We use certain terms to describe the appearance of forewing patterns according to their utility in distinguishing species (Figure 1). The most important of these features are dashes, lines, bands, patches, special spots, and special areas. Dashes are narrow, short marks extending less than half the distance along the width or length of the wing. Lines are narrow marks extending more than half the distance along the width or length of the wing and are associated with a particular area on the wing. Bands are a wide area typically extending more than half the distance along the width or length of the wing and often demarked by lines and are associated with a particular area on the wing. Patches are a small, restricted area of the wing demarked by a distinct color but not delimited by lines. Basal lines occur in the basal area. The postmedian line may be broken or continuous and occurs distal to the reniform spot and demarks the proximal edge of the postmedian band. The submarginal line may be broken or continuous and is proximal to the outer margin. The orbicular spot is a single irregularly shaped (typically near-round shaped) spot that occurs just short of half way along the front edge of the forewing. The reniform spot is a single irregularly shaped (often kidney shaped) spot that occurs just past half way along the front edge of the forewing. The discal spots are the combination of the reniform and orbicular spots. The basal area is the area of the wing nearest the thorax. The midcostal area is in the center of the front edge of the wing below the costal margin. The distal area is between the orbicular and reniform spots. The median area is the central area of the wing. The postmedian area is distal to the reniform spot and proximal to the subapical area. The subapical area is proximal to the apical area. The apical area is immediately below the apex of the wing. The submarginal area is proximal to the outer margin. The anal area is between the outer margin and the inner margin, proximal to the anal angle. The subanal area is proximal to the anal area.
Head. The most obvious features on the head involve the eyes, mouthparts, and antennae. In general, these body parts are not the most useful features for field identification of macromoth species. However, the presence of pectinate antennae can be useful in identifying similar looking species. For instance, the male of Pero occidentalis has pectinate antennae while the male of a similar appearing species, Pero behrensaria, has filiform antennae.
Thorax. The thorax has three segments: prothorax, nearest the head; mesothorax, in the middle; and metathorax, connecting to the abdomen. The forewings attach to the mesothorax, and the hindwings attach to the metathorax. The thorax may have hairs arranged by colors and lengths to form the presence of collars and tufts that may be used to differentiate species. For instance, the pale yellow thoracic collar of Cerastis enigmatica is diagnostic relative to other similar looking moths. Also, identification maybe aided by features of the legs, one pair of which occurs on each thoracic segment, namely colors of hairs [see Spilosoma vestalis]. The forewing and hindwings provide very useful traits for field identification of macromoth species.
Abdomen. The general size and shape of the abdomen is more characteristic of the macromoth family than it is useful in identification of species. For instance, the abdomen of geometrids is typically thin and appears small relative to the wing area. On the other hand, the abdomen of sphingids is robust and distinctly tapered. Because the abdomen is the body segment that contains the genitalia, which are used in describing and differentiating species, the abdomen is of great importance to expert taxonomists. Therefore, it may not be possible to identify certain specimens if the abdomen is missing.
Our comments on abundance are based on how prevalent individuals of a given species are at peak flight periods, in suitable habitat, and within their normal range. We use general terms: abundant, common, uncommon, and rare. Abundant indicates that hundreds of individual moths may be collected in one night of operating a single light trap or that day flying moths may be seen by the dozens within an hour of observation. A designation of common indicates that moths are typically collected in numbers of less than a hundred per night to as few as zero or one on any given night but frequently seen throughout the flight period. Uncommon moths are those species that occur in low numbers, usually no individuals or 1 per trap per night and less than 10-15 per trap throughout the flight period. A rare moth may not be seen in any given year or multiple years, and when observed only a few individuals are noted. As an example, we have observed only a few individuals of Saturnia mendocino during the last decade.
We have used a scaling of geography in our descriptions of each species as follows: coastal refers to the west side of the coast mountains, Pacific West refers to west of the Cascade Mountains and Sierra Nevada, Pacific Northwest as described in the introduction, and western North America refers to west of the Rocky Mountains.
The woodlands and forests of the Pacific Northwest possess many types of habitats based on tree species, geographical location, and climatic conditions. We have placed these woodland and forest types into five categories: (1) subalpine forest, (2) wet forest, (3) dry forest, (4) dry woodland, and (5) riparian forest and woodland.
Subalpine forest. This forest type occurs at high elevation, above 1,500 m, in the Cascade Mountains, Rocky Mountains, and Sierra Nevada, and as isolated montane islands in the Great Basin and the southwest states. Dominant tree species are Engelmann spruce, subalpine fir, lodgepole pine, and quaking aspen.
Wet forest. This forest type is dominated by conifers, particularly Douglas-fir, western hemlock, redwoods, and Sitka spruce. The major hardwood trees are red alder, and big-leaf maple.
Dry forest. This forest type is dominated by ponderosa pine. The associated hardwood trees are quaking aspen in high elevation sites and cherry and serviceberry at lower elevation sites.
Dry woodland. This habitat is characterized by oak woodlands west of the Cascade and Sierra Nevada Mountains and juniper woodlands to the east of the Cascade Mountains. The dry woodland habitat in the Southwest and Great Basin are characterized by pinyon pine.
Riparian forest and riparian woodland. This habitat occurs in dry regions along rivers, creeks, and gullies. The dominant trees are poplars, willow, alder, cherry, and elderberry.
The time of day or night, and the season when a moth flies can be very specific and limited or cover a wide range. Moths may switch between active and inactive periods based on the time of day. Diurnal moths are active during the day and nocturnal moths are active during the night. We have included statements only with reference to day flying activity; therefore, if not stated, the species is a night flying moth.
Seasonal activity may be generally categorized by the standard periods of spring, summer, fall, and winter. Further detail may be added by stating whether the flight period extends into an early or late part of any of the seasons. The seasonal period for flight is mentioned only in general terms because different geographical locations will exhibit different seasonal conditions based on latitude, longitude, elevation, and slope. In general we note the relativity of the seasons based on the western valleys and foothills: spring is February through June, summer is July and August, fall is September through November, and winter is December and January.
Caterpillars, with rare exception, are herbivorous meaning they feed on plants. Most typically caterpillars feed on foliage, but also on roots, within branches and woody stems, in seeds, and on flowers. The caterpillars of many macromoth species may have restricted ranges of suitable host plants upon which they can feed. Such species are termed monophagous or host plant specialists. In such cases the caterpillar may feed on only one species, on only a few related species, or on many species within one genus of plant. For instance, any of the species of Drepanulatrix will feed only on species of Ceanothus.
On the other hand, many caterpillars are generalist feeders and are termed polyphagous. That is, the caterpillar can feed upon many plant species among a wide range of plant species and still develop into an adult in the usual period of time and achieve normal size. Generalist feeders often are able to live on plant species belonging to a wide array of families. For instance, the caterpillars of Neoalcis californiaria, Hesperumia sulphuraria, and Aseptis binotata can develop on 15 to 23 plant species belonging to 10-12 plant families. Although caterpillars may be polyphagous they exhibit preferences for certain host plant species. For instance, caterpillars of Neoalcis californiaria feed on the foliage of conifers and the foliage of flowering plants but each host plant is not equally suitable for growth and more caterpillars can be found on Douglas-fir than any other of the known host plants.
When other species exhibit a similar appearance to that of the featured species we note diagnostic traits for differentiating the species from one another. Sometimes the wing patterns can be used, in other cases the caterpillar host plant or the flight period and geographical range may provide diagnostic traits. Similar species are identified to family only if they belong in a different genus and not one of the numbered species accompanied by a photograph. In discussing some species we did not list any similar species because in our judgment no other species look enough like the featured species to warrant additional diagnostic traits. If a moth in hand doesn't match the photograph of a species, then perhaps the identity of the moth is one of the similar species or a species not mentioned in this publication. We do not cover many of the rare species and mention only a few of the species placed in the larger genera, such as the noctuid genus Euxoa and the geometrid genus Eupithecia.