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Moths in winter?

During winter, on Saturday nights, some people prefer to dance in a night club; others prefer to watch a movie in a cinema or to go to a restaurant. Personally, I like to go to the forest just after dusk. Indeed, some moths species, a bit strange, fly only in winter, during the coldest season of the year. Even more, they share some ecological traits that make them really interesting. They can be so abundant and they are so unique that meeting them leave for sure unforgettable memories.

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Figure 1: Mating of winter moths (Operophtera brumata). The male is fully winged and the female is nearly wingless, so she cannot fly.

In France, there are more than 1600 moth species. Based on flight timings in the book of Robineau et al. (2011), there are about 20 times more moth species leaving as adults in July (~1100 species) than in December (~50 species) in France. However, among the about fifty species that spend December as adults, only a minority of them are really active and fly in winter.

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Figure 2: Moth abundance in terms of species number, along the year in France. The red, black and blue curves show the means of highest, averaged and lowest temperatures of each month, respectively.

Wintering species at rest, sometimes active.

Indeed, most of the wintry species (mostly Noctuids Hadeninae, Catocalinae, Hypeninae and Nolinae) hibernate and stay idle during the cold season. This is the case of several Nycteola, nearly all the Conistra, several Cucullia, several Hypena, nearly all the Lithophane, Scoliopteryx libatrix, Dasypolia ferdinandi Dasypolia templi, Eupsilia transversa and many others. Nevertheless, it is not impossible to watch one of them flying during a mild night. Thus, I could find a Conistra vaccini on December 12th, 2011, by a temperature of 7°C, in the vicinity of Ypres in Belgium. Quick researches on www.observations.be show that other species are observed in December or January, by attracting them with light (UV light or mercury-vapor light) or sugar. As an example, the observations of January 2nd, 2012. To find hibernating moths is not very easy and frequent. Generally, they hibernate in hollow tree trunks, in caves, in cellars or old building with a sufficiently stable temperature. The two major factors that regulate entry and breaking of dormancy are temperature and daylength.

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Figure 3: The chestnut moth, Conistra vaccini, on December 12th, 2011 at the Gasthuisbossen (Bois de l'Hôtel-Dieu) in the vicinity of Ypres (Belgium).

Very few species can be found adults all around the year. This is the case of the humming-bird hawk-moth (Macroglossum stellatarum) that I could find in December about 15 years ago (unfortunately I was too young to think about writing notes about that at that time). However, in Northern half of France, most of the adults do not survive during winter and new immigrants repopulate northern regions at every spring.

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Figure 4: Monthly abundance (number of individuals) of eight moth species that spend winter as adults in Belgium. Blue : individuals attracted by using light or sugar. Brown : hibernating individuals. Data source: www.observations.be. The orange curves show the average temperatures in Brussels (no scale).

Tardy and early individuals

Some autumnal moth species remain active until December. As an example, Alsophila aceraria can be found from November to late December and the December moth (Poecilocampa populi) can be found from October to December (and even January according to http://www.papillon-poitou-charentes.org/). However, it seems very rarely observed as adult in January in Belgium (figure 6) and has a very distinct abundance maximum in November. The Lasiocampidae Poecilocampa coluchei, the Geometridae Eupithecia rosmarinata, the Notodontidae Ptilophora plumigera and the Noctuidae Conistra daubei, Dryobotodes tenebrosa and Ulochlaena hirta are among the other species that we can find in December.

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Figure 5: The December moth, Poecilocampa populi can sometimes be observed until December or January.

Other species are rather spring moths as they usually fly in March-April. However, some of these species are sometimes observed in small number from November. That is the case of several moths of the genus Orthosia: O. cerasi, O. incerta and O. cruda (Figure 6). This is also the case of the Geometridae Alsophila aescularia that we can find from February with an abundance peak in March. As in the "true" winter moth species, the females of A. aescularia are also wingless and as a consequence, they do not fly.

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Figure 6: Monthly abundance (number of individuals) of eight moth species that can be found on early or late winter in Belgium. Data source: www.observations.be. The orange curves show the average temperatures in Brussels (no scale).

The "true" winter species

Other species, of greatest interest here, are "true" winter species. It is very rare to observe them out of the November-March period. Most of them are Geometridae of the sufamilies Larentinae and Ennominae.

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Figure 7: The mottled umber, Erannis defoliaria, 7 males and 2 females. Here, we can see the extreme color variability of males. All of these 7 males were photographed the same day, at the same place (Ploegsteert wood, Belgium).

Erannis defoliaria, the mottled umber, is a widely distributed and usually abundant moth. The males are brownish, ochre or grayish with blurred to well marked drawings. The drawing variability is huge, even within a single population. As an example, the Figure 7 shows 7 males, all distinct and all observed at the same time and the same place (December 7th, 2011, Ploegsteert wood, Belgium). They have a wingspan of 3.6 to 5 cm. The females have atrophied wings and do not fly. They are yellowish white with black spots (figure 7). According to Waring and Towsend (2003), there is a black form. This species can be found in many types of habitats, but they mostly live in forested area, where the caterpillars can develop on several tree species: oaks, beeches, birches, poplars, etc. Adults are found from October to January, with an abundance peak in November-December.

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Figure 8: winter moth, Operophtera brumata, male on left, female on right.

Follow two species of the genus Operophtera: the winter moth, O. brumata, and the Northern winter moth, O. fagata. Both are grayish. In all the Operophtera species, females have atrophied wings and do not fly. O. fagata fly a bit earlier (abundance peak in November) and is far less common than O. brumata (abundance peak in December). Telling these species apart is easy based on females. Indeed, the atrophied wings of females are shorter than half the length of the abdomen in O. brumata and as long as in O. fagata. The identification of males is more difficult. The O. brumata males are a bit smaller than the O. fagata males and their coloration is slightly distinct. Both species develop on a huge number of broad-leaved tree species, and even on conifer species in the case of O. brumata. Both species live mostly in forest habitats but they can also be found in other kinds of habitats. Operophtera are closely related to the genus Epirrita, that comprises autumnal species (from September to November, depending on species). To some extent, the Operophtera males look like the Epirrita species, although the Epirrita females have normally developed wings and are able to fly.

From late December - early January, and until March (abundance peak in February), another winter species can be found: the pale brindled beauty, Phigalia pilosaria. This is a widely distributed forest species. The male has grey wings with dark drawings. Caterpillars feed on various species of broad-leaved trees. Females have very short wings, very hard to see and their body color is grey (the upper body face is darker).

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Figure 9: The pale brindled beauty, Phigalia pilosaria (here is a male), flies in January and February.

Three other species, found in winter, were previously assigned to the genus Agriopis, closely related to the genus Erannis: the dotted border, Larerannis marginaria, and the spring usher, Agriopis leucophaeria, both mostly observed in February-March and the scarce umber, Larerannis aurantiaria, mostly observed in November. All three species live in forests and their caterpillars eat on broad-leaved trees (mostly oaks in the case of A. leucophaeria). Females of these species are easily distinguished from E. defoliaria by their grey color (instead of whitish) and the presence of more or less reduced wings, depending on species. The species with the longest wings in females is L. marginaria the one with the shortest wings is A. leucophaeria. Males of the three species can be told apart from E. defoliaria the lack of the central black spot on hindwings. Nevertheless, be careful as some E. defoliaria males do not have this spot. L. marginaria can be distinguished by a brown, usually slightly contrasted, coloration and, above all, by a row of small black dots along the outer edge of both hindwings and forewings. L. aurantiaria males are yellowish with slight drawings. A. leucophaeria males are usually grey with slightly to well marked drawings. They are smaller than males of other species. We should probably add Agriopis bajaria to this group of species. It is far less frequent and fly in October-November.

The two species of the genus Theria are also "true" winter species: the early moth, T. primaria, and T. rupicapraria. Females of both species also have atrophied wings. Both species appear from January to March although T. primaria seems to flight a bit earlier than T. rupicapraria. Their wings are crossed by a dark grey band. Caterpillars of both species mainly feed on rosaceae shrubs (hawthorn, blackthorn, etc.). They live on forest edges and waste lands.

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Figure 10: Monthly abundance (number of individuals) of eight "true" winter moth species in Belgium. Data source: www.observations.be. The orange curves show the average temperatures in Brussels (no scale).

Other Geometridae species are also known to fly in winter. However, I will not enlarge a lot on them because I have very few information about them and I never observed them. They are Chemerina caliginearia (December to March in Southern France), Eupithecia rosmarinata (November-December in Southern France), Eupithecia lentiscata (January-February in Corsica) and Colostygia multistrigaria ssp. olbiaria (December-January in Southern France). The females of these species have fully-developed wings and are able to fly.

Life cycle

The life cycle is nearly the same in all "true" winter species (Erannis, Operophtera, Agriopis, Larerannis, Phigalia, Theria). Adults emerge in winter. Males start to fly at dusk in search of the females. Females climb on tree trunks and produce pheromones to attract the males. Mating occurs on tree trunk, usually at the beginning of the night. After it, the females continue to climb up the trunks, then the branches and the buds. They laid their eggs there and die. At Spring, at the time of bud break, the eggs hatch and the young caterpillars immediately feed on young tender leaves. They pupate a few weeks later in the ground litter. These moths spend the whole summer and autumn as pupas. The adults emerge in winter and the cycle starts again. Adult moths do not feed.

How and where to observe winter moths ?

Based on their biology, watching "true" winter moths is very simple and does not require specific equipment. The use of a white sheet and an attractive light (UV tube of vapor-mercury light) is not essential, although it is a plus to observe males. The butterfly net is nearly useless. Indeed, you just have to go to a forest at dusk and to look on large tree trunks using a flashlight or, better, a headlamp. You will see many males and times to times females. The later ones are usually less frequent than males (10 to 100 times less). Spotting matings of Erannis and Operophtera is very simple. Usually, males stay at rest the head up. However, when mating, they keep their head down, because the females pull them upwards.


Figure 11: Males winter moth, Operophtera brumata, resting on a tree trunk.

Thus, using a simple headlamp, I was able to watch between 1000 and 1500 winter moths, Operophtera brumata, one female Erannis defoliaria and one Conistra vaccini in one hour and a half of prospecting (about 150 trunks examined on a ~700m walk). As always with moths, the temperature strongly affects the activity levels of moths. The higher the temperature, the more active the moths. The best nights have a temperature above 5°C, although I was able to watch many males at a temperature of 1°C. They were nearly petrified by coldness on the tree trunks and they did not fly. Another solution is to look for the caterpillars at Spring, particularly O. brumata and E. defoliaria can be so abundant that they are considered as forest pests.

Watching species that fly mostly at Spring or Fall, or species hibernating as adults, is more difficult. First, because they are less abundant during winter and, second, because you need a specific methods for that : an attractive light source (e.g. UV light or mercury-vapor light) or sugaring. Both are complementary and seem to give good results.

Conclusion

As shown above, winter is not an inactive time for moth enthusiasts. We can watch a certain number of species, many of which can be observed only during the cold season. Some of them can be found by hundreds or even thousands without great effort. They have several ecological traits and a very unusual life cycle. Their observation always leaves an amazing experience and an unforgettable memory. A future paper will talk in more details about the biology of the winter moth Operophtera brumata that was widely studied.

More pictures of winter species of moths here.

References :
Robineau et al. 2011 - Guide des papillons nocturnes de France. Ed. Delachaux et Niestlé. 287 p.
Orhant 2011 - Atlas des papillons de nuit du Nord-Pas de Calais- Lépidoptères Macrohétérocères. 473 p.
Waring, Townsend et Lewington 2003 - Field guide to the moths of Great Britain and Ireland. British Wildlife Publishing. 432 p.
Young 1997 - The natural history of moths. Poyser Natural History. 271 p.
http://www.lepinet.fr/"
http://www.papillon-poitou-charentes.org/