Once in a Blue Moon

Forest and Bird, Issue 291, 1 February 1999, Page 32

Once in a Blue Moon

PETER MADDISON

PETER MADDISON

Strange butterflies in Net Zealand gardens may be visitors from Australia:

inttod@eessmigrating butterflies and their trans-Tasman flights¢

he headline ‘Painted Ladies Invade Zealand’ might well have been in northern newspapers this spring, but it could have been misinterpreted. For these painted ladies are Australian butterflies which graced various parts of the country in October and November. The painted lady or Cynthia kershawi is one of several kinds of butterfly which migrate from Australia to New Zealand on an irregular basis. Some years many are seen, but in others there are few or none. The painted ladies, looking at a distance like small monarchs, fly differently; quickly, darting close to the ground. When they rest, their wings briefly flick open then close tight so they effectively vanish in a second. Other Australian butterflies which appear in our gardens some summers include the blue moon, the lesser wanderer, some yellow admirals and two blue butterflies which now occur naturally here. How they get to New Zealand has been the subject of much conjecture, but the regularity with which Australian butterflies (and other insects) arrive here has convinced entomologists that many have migrated — that is flown across the Tasman. Whether they have had any assistance, perhaps hitching a ride on a ship or jumbo jet, or whether they have flown or been blown across the sea, has been widely debated. The presence of smoke from Australian bush fires in the New Zealand atmosphere has, however, confirmed air movement across the Tasman sufficient to help migration. Some of the debate revolves around the use of the word ‘migration’ itself. When applied to birds, fish and some mammals, migration generally implies a two-way or cyclical movement as exemplified by the Arctic tern which flies from the Antarctic to the Arctic and back again each year; also to the large movements of wading birds between New Zealand and eastern Asia and North America. Among fish there are the examples of inanga (whitebait) and salmon which migrate between fresh and salt water. Whether such a return journey happens with Australian butterflies visiting New Zealand is highly doubtful. A related phenomenon, however, is ‘dispersal’ — that is the spreading out of a population, generally following increases in numbers. Some such dispersal movements are spectacular — as in the lemmings which disperse and move to the coast where many jump off the cliffs and perish. In insects many examples of dispersal occur — from

the ongoing treks of army ants, to swarms of locusts. Perhaps the best definition of dispersal when applied to butterflies is that of ‘sustained directional flight. Apart from some species which migrate over short distances, only one butterfly makes reverse directional flights (or true migrations) over long distances. That butterfly is the monarch of North America which has become acclimatised in New Zealand. Approximately 30 of the 364 species of Australian butterflies, (principally in the families Danaidae, Nymphalidae and Pieridae), migrate within Australia. To make a trans-Tasman crossing they must journey at least 2000-2500 kilometres at the narrowest points between Australia and New Zealand Generally, butterflies start dispersal flights with large reserves of fat. Flights are generally within a few metres of the ground or sea, but flights up to 60 metres high have been observed in the monarch. In North America, the monarch is known to migrate to the same overwintering sites — usually trees in subtropical California and the Gulf Coast. tions of butterflies build up and start to migrate within Australia, these migrations are occasionally blown off course and become transported across the sea on the prevailing western and northwestern winds. It has been observed that migratory species may persist in their flight, in adverse or high winds, when non-migratory species settle and shelter. Butterflies have been seen flying at sea, but not in the Tasman. Some research on possible flight trajectories for a migration of blue moon butterflies which arrived in western New Zealand in April 1971 has been undertaken by Dr. Graeme Ramsay, formerly of DSIR Nelson, I is suggested that when large popula-

and A.J. Tomlinson of the New Zealand Meteorological Service. They studied the occurrence of 108 butterflies and concluded the weather conditions would have allowed a three-day flight from southern New South Wales or Victoria. Further research indicates that in winds of 15 knots the flight would have taken three days; in 30-knot winds, 1.5-2 days; and in 50-knot winds just one day! Additional evidence of wind-assisted flight across the ocean is that many specimens are in near perfect condition when found in New Zealand, suggesting lack of contact with damaging objects. On land, wings become torn in contact with trees, bushes, and rocks etc. There are of course many questions still to be answered. For example, do butterflies land on the sea surface? — If they do, can they take off again? — Is the flight one of lazy wing beats, or drifting? — Is this "passive displacement or ‘oriented direct flight’? Since several occurrences of Australian butterflies are in western areas of New Zealand it has been suggested that the butterflies migrate on high level winds and are brought down to land by downdraught eddies. Meteorologists confirm the presence of such eddies in Tasman Bay, the Marlborough Sounds, to the north of Hamilton, in the Manawatu, and near Mount Taranaki. One final issue has yet to be determined — what are the chances of return migration? Easterlies blowing for a sustained period between New Zealand and Australia are extremely unusual. On the following pages are some observations and information about migrating butterflies and their trans-Tasman flight.

is an Auckland ento-

mologist and former executive councillor of Forest and Bird.

The migratory Monarch Danaus plexippus

New Zealand’s largest butterfly, with large orange, black-marked wings (span 80100mm) is not a native. Much of the information on butterfly migration comes from studies of this butterfly in North America by Dr.F.A.Urquhart. He instigated a massive ‘tagging’ programme involving hundreds of thousands of butterflies over many years. Small adhesive labels were used to mark them. In North America, some populations of these butterflies spend summer in the central and eastern parts of Canada and the United States, migrating southwards in the autumn to Mexico or California and returning northwards in the spring. Some of the results from the tagging programme showed that individual butterflies flew 1967 kilometres (straight line) from Ontario to Florida and 2257 kilometres from Ontario to Cuba. Observations at points along the flight paths have included the record from Massachusetts of the passage of 100,000-200,000 butterflies in 50 minutes. One tagged individual travelled 128 kilometres per day. In one overwintering colony in trees in Mexico, Urquhart estimated 14.25 million butterflies in 1.5 hectares. One issue that challenges researchers is how the butterflies returning south each winter know how to locate the overwintering trees, since the individuals returning are effectively the grandchildren of those that left! In New Zealand, small over-wintering swarms have been found in coastal areas, for example, Church Hill, Nelson, where they spend from April-May to September. Other wintering sites include Kaeo (Tauranga Bay), Manaia and Hastings. The butterflies are not totally dormant — they will start to fly if the weather warms up, and feed at flowers. No New Zealand records have been found from before the 1860s-70s, though it is notable that there is a Maori name — ‘kakahu’ — for this butterfly. Records suggest this species migrated quickly across the Pacific Ocean,

reaching Hawaii in 1840, Pohnpei 1857, Tonga 1863, Samoa 1867, Rarotonga 1869, Brisbane 1870, Melbourne 1872, Tahiti 1972, with the first confirmed New Zealand record in 1873. Monarch butterflies breed here, depending on the introduced swan plant as a host for its caterpillars.

Lesser Wanderer Danaus chrysippus

This looks like a smaller, paler version of the monarch butterfly and lacks the black colouration of the veins. It is an irregular visitor to New Zealand, first reported in 1904. Most occurrences are from February to April in western districts from Woodhill Forest to Nelson. Caterpillars of this butterfly have been found feeding on swan plant in several areas, including New Plymouth, Wanganui, Glenfield, Henderson and Woodhill in Auckland. The larva is like that of the monarch but has a broader black band on each segment. Of widespread distribution in Australia, this species is also found in Africa, the Middle East, Asia and some Pacific Islands, notably Fiji Larval food plants include Asclepias fruticosa and Asclepias curassavica (both swan plants which are also favoured by monarchs.)

Yellow Admiral Bassaris itea

This large (wingspan 45-55mm) butterfly is easily recognised by the yellow patches on the

mainly black forewings. This is a native New Zealand butterfly, but is also found in Australia, Lord Howe and Norfolk Islands. In Australia it is a well known migrant and may take part in mixed migrations with the painted lady. It is probable that on occasion yellow admirals found here will have migrated from Australia, but these could not be distinguished from their New Zealand forms. In New Zealand, the yellow admiral feeds on the herb nettles, Urtica incisa and Urtica urens. The butterfly flies strongly and may be seen particularly gathering on open tops of hills. Auckland’s volcanic cones (One Tree Hill, Mangere Mountain) are regularly visited.

Australian Painted Lady Cynthia kershawi

This butterfly is patterned with orange and black and has a wingspan of 46-60mm. The Australian painted lady was first reported in New Zealand in 1885 by the Reverend Richard Taylor in his book Te Ika a Maui. The painted lady is a regular visitor, sometimes in large numbers These migrations are often matched by large numbers arriving on Lord Howe and Norfolk islands. Being a strong migrant the painted lady has been found in many parts of the country. Butterflies of the spring generation may lay eggs, and a number of New Zealandbred individuals(with more pink shading)

may be found in January or February. It has been suggested that the butterflies may overwinter but this is not yet proven.. The painted lady is widespread in Australia — the butterflies migrate from mid-August to early October in a southerly or south-west-erly direction, extending from Brisbane to Tasmania. The migration flights are rapid, about 2 metres above the ground and go ina constant direction. A peak of 100 per minute over a 50 metres front has been recorded. Their larval food plants include plants in the family Asteraceae (=Compositae) such as Arctotis, Helichrysum, Arctotheca calendula (capeweed) and Artemisia. v

Blue Moon Butterfly Hypolimnas bolina nerina

This is a large black butterfly (wingspan 85-110mm) with a large white spot, dusted with iridescent blue-purple on each wing. Sometimes the spots are white and there are also orange markings. This is a regular migrant from Australia with most specimens being found from late March until May. In some years large invasions have been recorded: 720 in 1956, 96 in 1971 and 142 in April-June 1995. Most specimens are found in the western part of New Zealand, with particular clusters of

sightings in Nelson, the Kapiti Coast, Taranaki and Auckland. John Early of Auckland War Memorial Museum and others described the 1995 invasion and analysed possible air movements that would have supported this influx. This butterfly is widespread in the Asia Pacific region. The subspecies nerina is common from Australia and the Melanesian Archipelago to the Pacific Islands. In Australia, where it is known as the common egefly, the blue moon is found from the northwest to eastern Victoria and on Lord Howe Island. Its larval food plants include Alternanthera, Portulaca and Sida.

Long-tailed Blue Lampides boeticus

This small blue butterfly (wingspan 20-36 mm) is similar to the common grass blue butterfly, but has two eyespots on its wings and a short hair-like tail at the outer corner of the hindwings. This butterfly was first recorded on Waiheke Island in November 1965 but since then has become successfully established in New Zealand, being now found throughout the North Island. This is an almost worldwide species, being found in Europe, Africa, Asia, Australia and many Pacific Islands including Hawaii. It is a known migratory

species with a successful record in colonising new territory. Perhaps this is linked with the widespread occurrence of its food plants — it feeds on flowers and seedpods of leguminous plants. In New Zealand the main hosts are gorse, broom and garden and sweet peas.

Common Blue Zizina otis labradus

This is the small (wingspan 17-27mm) blue butterfly so common on lawns. It is the most common butterfly in Australia, and in many parts of New Zealand. A separate subspecies, the southern blue (subspecies oxleyi) is found in the South Island. So why is it included here? There are suggestions that this butterfly colonised New Zealand from specimens blown from Australia in the past. There is some controversy amongst the biogeography experts as to whether the species represents an original inhabitant, tracing back to ancestors in the Cretaceous era 60 million years ago, or is a geologically more recent arrival. In both hypotheses a South Island subspecies evolved.