pupa

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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The Regal Monarch

It’s that time of year. Garden centres will make sure they’ve stocked up on swan plants. Harassed parents will be visiting to placate weeping offspring, whose monarch caterpillars have stripped their host plants bare. Nurseries will happily sell a few tiny seedlings, enough to feed voracious caterpillars for a few more days. The cycle will continue until the butterflies finally disappear for winter. Where do they come from, and where do they go?

Monarch butterflies (Danaus plexippus) are a recent immigrant to New Zealand, and unlike most of our introduced butterflies they don’t come from Australia, but from North America. Because they can travel long distances, they’ve dispersed around the Pacific, South-East Asia, and Australia, and in the late 19th century found their way to New Zealand. Because monarchs introduced themselves, they’re technically classed as a native animal, like the Welcome Swallow and Silvereye, and are our largest native butterfly.

The swan-shaped seed pods: There are many species of milkweeds in the monarch’s home, North America, but the swan plant is the only one that’s widely cultivated here. The seed pods can be saved and used to grow a new crop of butterfly hosts next year. photo: Creative Commons BY, Sid Mosdell / Flickr

The swan-shaped seed pods: There are many species of milkweeds in the monarch’s home, North America, but the swan plant is the only one that’s widely cultivated here. The seed pods can be saved and used to grow a new crop of butterfly hosts next year.
photo: Creative Commons BY, Sid Mosdell / Flickr

The biggest impediment to monarchs becoming established here was a lack of host plants. In North America, their caterpillars feed on many different plants in the milkweed family, but none of our New Zealand native plants will do. The introduced milkweed they eat here is swan plant (Gomphocarpus fruticosus), although they can also stomach the noxious weed moth vine (Araujia sericifera), and in emergencies can be fed squash or pumpkin. But the number of monarchs is determined by the number of swan plants we cultivate for them.

Stripey and inedible: The swan plants that monarch caterpillars eat are poisonous, full of noxious chemicals called cardiac glycosides that can actually cause heart attacks if eaten in large quantities. Monarchs are able to store the noxious tasting chemicals and make themselves inedible to birds; the bright colours make caterpillars memorable and ensure a bird will only try eating one once. photo: Creative Commons BY-NC-SA, Vicki DeLoach / Flickr

Stripey and inedible: The swan plants that monarch caterpillars eat are poisonous, full of noxious chemicals called cardiac glycosides that can actually cause heart attacks if eaten in large quantities. Monarchs are able to store the noxious tasting chemicals and make themselves inedible to birds; the bright colours make caterpillars memorable and ensure a bird will only try eating one once.
photo: Creative Commons BY-NC-SA, Vicki DeLoach / Flickr

A female monarch butterfly lays hundreds of eggs, then dies. The eggs take about a week to hatch. The caterpillar can take as little as two weeks to reach full size, eating voraciously, and then it forms a chrysalis in which it stays for about a fortnight. The whole life cycle from egg to adult butterfly can take only a month, although it’s much slower in cooler weather. In the right conditions, one female could produce hundreds of offspring, although almost all of them die from disease, starvation, or being killed by wasps.

Hanging around for a bit: When the caterpillar reaches full size, it attaches itself to a leaf, forms a J shape, and sheds its skin one last time, revealing the pupa. The pupa has to do a little dance to rid itself of the shed skin, and then reshapes itself and hardens to form a chrysalis. photo: Creative Commons BY, Sid Mosdell / Flickr

Hanging around for a bit: When the caterpillar reaches full size, it attaches itself to a leaf, forms a J shape, and sheds its skin one last time, revealing the pupa. The pupa has to do a little dance to rid itself of the shed skin, and then reshapes itself and hardens to form a chrysalis.
photo: Creative Commons BY, Sid Mosdell / Flickr

In their home, North America, monarchs make a extraordinary journey each winter. From as far north as Canada they migrate thousands of kilometres to Mexico, where they find their way to a few sheltered valleys and overwinter in their millions, covering the trees. The weight of massed butterflies bows the branches to the ground, and the fluttering of their wings sounds like a constant roar. In spring they head north again, laying eggs as they go. Because each butterfly lives a few months at most, several generations pass before next winter; the butterflies heading south have never been to Mexico before, and yet find their way to the same trees in the same valleys. Nobody knows how.

Into the chrysalis: When the caterpillar gets big enough, it makes a hard waxy case about itself called a chrysalis, from the Greek chrysos or gold. The jade green chrysalis has golden spots, and is attached to the leaf by a hook called a cremaster. photo: Creative Commons BY, Sid Mosdell / Flickr

Into the chrysalis: When the caterpillar gets big enough, it makes a hard waxy case about itself called a chrysalis, from the Greek chrysos or gold. The jade green chrysalis has golden spots, and is attached to the leaf by a hook called a cremaster.
photo: Creative Commons BY, Sid Mosdell / Flickr

In New Zealand, monarch butterflies don’t migrate in this way. They can’t survive the winter in most of the country, so congregate on a few trees in overwintering spots in Northland, Hawke’s Bay, Nelson, and Christchurch. (There may be an overwintering spot near Whanganui, so let the Museum know if you come across a tree full of monarchs this winter.) When the weather warms up they become active and start laying eggs again.

Nearly ready to hatch: After two weeks in the chrysalis, the caterpillar has rebuilt its body into a butterfly, and is about to emerge. photo: Creative Commons BY, Sid Mosdell / Flickr

Nearly ready to hatch: After two weeks in the chrysalis, the caterpillar has rebuilt its body into a butterfly, and is about to emerge.
photo: Creative Commons BY, Sid Mosdell / Flickr

To ensure monarchs have enough to eat, we can plant swan plants in weedy corners or empty sections, using seed saved from the previous year’s plants. If the seedlings can be kept frost-free over winter, they’ll have a chance to get large enough to host a good crop of caterpillars. But regardless of how many swan plants you grow, or how big they get, there will always be hordes of caterpillars waiting to strip them bare. The best strategy is to pluck off eggs or caterpillars and dispose of them while they’re still small, so that just two or three survive to adulthood per plant. If you’re too soft-hearted to do this, well, there’s a garden centre owner rubbing his hands together with glee, looking forward to a profitable caterpillar famine.

Attractive perfume: Male monarch butterflies have scent pockets on their hindwings, into which they deposit tiny drops of the nectar-scented perfume that attracts females. When they’re about to mate, they brush the scent from these pockets with their hind legs, to ensure the female butterfly stays attracted to the honey-like odour. photo: Creative Commons BY-NC, TexasFlower / Flickr

Attractive perfume: Male monarch butterflies have scent pockets on their hindwings, into which they deposit tiny drops of the nectar-scented perfume that attracts females. When they’re about to mate, they brush the scent from these pockets with their hind legs, to ensure the female butterfly stays attracted to the honey-like odour.
photo: Creative Commons BY-NC, TexasFlower / Flickr

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