Moths in the shadowlands of winter

Winter is a time of meagre fare for butterfly and moth lovers. Sometimes we give up on our passion altogether for this season at least and yearn for warmth’s return, for Emperor moths, Orange-tips and Holly Blues to fly from March onwards.

Yet stirring in the darkness unbeknown to most exist a few select hardy winter moths. These occasionally come to outdoor lights. All three pictured here came to my outdoor light. On milder nights these seek out mates and breeding sites. There are some benefits to flying in winter. Bats, the scourge of nocturnal moths are safely out of the way, in deep sleep. There are far fewer insect predators to trouble moths.  There are fewer insectivorous birds foraging in hedges and woods in daylight hours for resting moths.

These moths do not need to place their eggs on growing leaves. All breed on native trees, placing their eggs on or close to leaf buds or on the bark. The three moths, the December Moth, Winter Moth and Mottled Umber are flying now. The females of the latter two are flightless and simply wait on our beside the cocoon from which they emerged to be located by a male. Soon she lays her batch of eggs and expires, her job done.

Their eggs will hatch in April when the buds open. These will be a great feast for breeding birds who time egg-laying and hatching in synchrony with the availability of the soft, juicy, protein-rich larvae. Enough survives to pupate in June where they will await the onset of cold weather to hatch as adults.

Although winter is a vast empty space for many butterfly lovers especially after the excitement of Christmas has passed and we feel ready for spring only for weeks more winter to cast its colour-drained pall, winter moths offer just a little consolation. Sombre as December weather, they are a fitting complement to the shadowland that is winter.

December moth, male on a limestone wall, December 2019.
This male Mottled Umber fluttered into the hall, initially attracted by the outdoor light.
A male Winter moth found beneath the outdoor light above my front door. Many individuals are paler than this specimen. This moth is frequently attracted to windows when the light is on.

Winter Moths
Sharp winter light slants morning and afternoon
Dank vapour turns all cold blue
Short daylight and long night lonely under the moon
Little to see, less to do, fast time’s slow winter passage to rue.

But huddled close by the outdoor light
A furred, maned December moth
Braves a shivering winter night
Unmoved while light glows, a Sloth.

Fluttering before relaxing on his brightened masonry bed
A Mottled Umber, charcoal or brown on rust or cream
Blends on bark or stone, wings reflect an arrowhead
Vague and defined the impressionists’ dream.

Windows lit from inside casts the outside in darker night
Winter moth mirage, pale and plain,
A sylph of the dark yet a lover of glassy light
Like a wet birch leaf he clings to the pane.

His female patient on cocoon waits for mate and doom.



Modern nitrogen fertilisers shown to destroy butterfly populations

A study published in 2018 (cited as Kurze, S., Heinken, T. & Fartmann, T. Oecologia (2018) 188: 1227. has found that nitrogen enrichment in caterpillar host plants increases the mortality of common Lepidoptera species.

The study involved testing the response of larvae of five common butterfly and moth species to host-plant fertilization using fertilizer quantities usually applied in agriculture. The species involved are Small Heath,  Speckled Wood, Small Copper, Sooty Copper (not found in Ireland or Britain) and two moths, Straw Dot and Blood Vein.

Nitrogen fertiliser was applied to two host plant species, Annual Meadow Grass used by Small Heath, Speckled Wood and Straw Dot and Sheep’s Sorrell used by the Small Copper, Sooty Copper and Blood Vein.

Nitrogen fertiliser kills caterpillars

The researchers found that the addition of nitrogen decreased the survival of all six species by at least one-third.  Richard Fox of Butterfly Conservation UK commented that increases in larval mortality range from 33%-80%. This study presents the first evidence that current fertilization quantities in agriculture exceed the physiological tolerance of common Lepidoptera species. The results suggest that (1) the negative effect of plant fertilization on Lepidoptera has previously been underestimated and (2) that it contributes to the range-wide decline of Lepidoptera.

There are further issues that arise from the study.  The situation may be considerably worse than the study found. Applying 90 kg of nitrogen per hectare per year as was done for the study would be likely to remove Sheep’s Sorrel and Annual Meadow Grass from agricultural grassland altogether. The reason for this is that these caterpillar host plants will be out-competed by coarser, aggressive plants that respond to increased nitrogen by growing at a fast rate.  Even if this is not the case the increased fertility produces a greener, lusher sward where temperatures are lower, delaying or even preventing larval development. In addition,  nectar resources for adults are usually lower than required as grasses out-compete nectar-rich flora.

Furthermore, while application rates of nitrogen on grazing land in Ireland varies, 120 kg of nitrogen per hectare is often applied. It is highly unlikely that the Small Heath or Small Copper can survive on these grasslands. Teagasc, the state agency providing research, advisory and education in agriculture, horticulture, food and rural development in Ireland advises farmers on nitrogen application rates. For dairy grazing, these can rise to 210 kg of nitrogen per hectare per year.

Under such an application regime it is no surprise that butterfly and moth species are being eliminated from the landscape, or why such formerly widespread and abundant species like the Small Heath and Wall Brown are now in serious trouble.

Research needed to save farmland butterflies

Research should be carried out to determine the rate of nitrogen application that grassland Lepidoptera species can tolerate. The Sooty Copper was able to cope with 30 kg of nitrogen but the other species studied were not assessed for nitrogen tolerance.  A study of biodiversity richness under the full range of nitrogen inputs it advises (including nitrogen added by slurry applications) should also be undertaken by Teagasc. Nitrogen application should be reduced or prohibited on farmland adjoining land designated as Special Areas of Conservation and National Heritage Areas to ensure these areas are not contaminated.

This issue can be addressed. Forcing land to produce more food by pouring chemicals into the soil cannot continue at the expense of biodiversity, water quality and possibly animal and human health and our world in general. Butterfly Conservation Ireland will take up the issue with Teagasc and urge the organisation to undertake the research needed to assess the impacts of nitrogen application on biodiversity on Irish farmland and alleviation and amelioration strategies. Good farming and good wildlife conservation must be the outcome if wildlife is to survive in today’s farmed landscape.

This female Small Copper was photographed basking near Common Sorrel plants on which it laid some eggs. This butterfly was breeding on grazing land in County Meath that has never been re-seeded and has not had fertiliser applied for many years. Photo © J. Harding.