light

Moths and the night

Spring is here, and the moths are emerging. Overwintering as a pupa, they emerge when the temperatures rise, to mate and lay eggs so their caterpillars can spend summer and autumn feeding and growing. A month ago, a light trap at Gordon Park would attract just one or two moths, but now, after an hour, the white sheet is covered with moths large and small, wasps that parasitise moths, flies, beetles and other nocturnal life. New Zealand has over 1,700 species of native moths, many barely known or still undescribed, and Robert Hoare at Landcare Research is the only full-time researcher working on them – his plate is pretty full.

2. light trap

Light Trap: Using LEDs in the blue and ultraviolet parts of the spectrum shining on a white sheet attracts nocturnal insects, which confuse it with the moon—the only bright light they’ve evolved to deal with. Photo: WRM.

While waiting at a light trap, being savaged by mosquitoes, you’ll occasionally hear a loud rustling and flapping as a huge pūriri moth (Aenetus virescens) or pepetuna, blunders into the sheet. Adult pūriri moths have beautiful mottled green wings, but they have no mouthparts and don’t feed, living on stored fat for the few days it takes them to find a mate and reproduce. In contrast to their brief adulthood their caterpillars, called mokoroa, live for years in holes bored into tree trunks; wētā will often move into the vacated tunnels. Once very common in North Island, pūriri moths are now only seen around native bush. In times past, swarms would emerge and fly into houses, extinguishing lamps and candles.

1. puriri

Pūriri moths emerge on damp nights in September and October to mate, lay thousands of eggs and die. Photo: WRM.

The decline in moth numbers is something that’s been noticeable, even in my lifetime. When I was a child, long drives at night left the windshield plastered with insects, but hardly ever so today. True, cars are more aerodynamic now, but the decline is certainly real, at least in Europe where it’s been carefully measured. Scientists, managing annual light trapping in Germany since 1989, have measured real declines in the numbers of nocturnal insects, down on an average of 45%, or 80% in some areas. There’s been no funding for this sort of long-term monitoring in New Zealand, so we have to rely on anecdotes about windscreens.

3. forest looper

Also known as the conifer flash (Pseudocoremia leucelaea), this moth is common in native bush in spring. Its caterpillars feed on tōtara and miro leaves.  Photo: WRM.

This insect decline has multiple causes: deforestation of course, the switch to intensive agriculture reducing the diversity of habitats in our farmland and the widespread use of pesticides. Systemic pesticides like neonicotinoids are restricted in Europe but widely used in New Zealand, and most discussion of their impact is around their effects on honeybees, not our thousands of native species.

4. cranefly

There are over 600 species of craneflies in New Zealand. Sometimes mistaken for a gigantic mosquito, they are harmless.  Photo: WRM.

Even artificial lighting might be a problem. Everywhere, including Whanganui, old high-pressure sodium street lighting is being replaced with modern LEDs, which are brighter and far more energy efficient, saving councils millions of dollars annually in electricity costs. The problem is that sodium lights were orange, whereas LEDs shine in the blue end of the spectrum, like those in my fancy new German light trap. Blue lights are far more visible and confusing to insects. In the name of energy efficiency, we’re busily lining our streets with thousands of high-powered insect traps, without much thought about the effects on nocturnal pollinators like moths.

Human beings have transformed the night, replacing the moon and stars with so much artificial light that we’ve forgotten what darkness is. When I lead night walks at Bushy Park we stop and do a simple exercise of turning off all our torches. Some of the children in the group are then in complete darkness for the first time in their lives. For many of the creatures of the bush, the night is their habitat, the place they can’t be seen by predators. They evolved in a world that had darkness, and in just a few hundred years we’ve driven that away.

 

Dr Mike Dickison is Curator of Natural History at the Whanganui Regional Museum.

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