Australian Entomologist, 2000, 27 (4): 113-116 113 FIRST RECORD OF WHITE MONARCHS, DANAUS PLEXIPPUS (L.) FORM NIVOSUS (LEPIDOPTERA: NYMPHALIDAE: DANAINAE) FROM VANUATU M.S. MOULDS and R.B. LACHLAN Australian Museum, 6 College St, Sydney, NSW 2000 Abstract The only previously known established population of white monarch butterflies, Danaus plexippus (L.) form nivosus Gunder, is that found on the Hawaian island of Oahu. This paper records a second established population, restricted to the island of Aneityum (= Anatom), Vanuatu. The discovery of this second population provides the first opportunity for hybridization experiments which may shed light on the origin of the 8nivosus9 gene. Introduction The 8white9 form of Danaus plexippus (L.), known as nivosus Gunder, is a conspicuous morph in which the normal bright orange of the wings is replaced by white that is slightly tinted pink or grey. For the most part this is a rare phenotype, although it became increasingly abundant on the Hawaiian island of Oahu before declining (Stimson and Kasuya 2000). Other isolated records listed by Vane-Wright (1993) are from Washington D.C., Missouri, Pennsylvania and the coast of California in continental USA, plus the North Island of New Zealand, Ambon and Seram in the Moluccas, Indonesia and Brisbane, Australia. We are aware of one further record recently published (Minno 1996), which added two records from Florida. The Vanuatu population Material examined. VANUATU: 6 GO", 6 99, Umeg, Aneityum, May, June, 1994, Fr A. Sacco. Around 25 years ago, Fr Albert Sacco first noted the occasional white monarch at Anelghowhat on the SW coast of Aneityum (= Anatom), the southern-most island of the island chain that makes up Vanuatu. Some years later, white monarchs frequently were seen and collected by Fr Sacco at Umeg (= Umetch) on the SE coast of Aneityum, where they flew together with the orange form in a field where milkweed, Asclepias sp., was abundant. Form nivosus is now estimated by Fr Sacco to comprise around 20% of the D. plexippus population on Aneityum. Samson (1983) recorded D. plexippus from Aneityum but made no mention of a white form. No intermediate individuals between the white and orange forms have been noted. Despite extensive observations of butterflies by Fr Sacco on other islands of Vanuatu, extending over more than 40 years of residency, form nivosus has not been sighted on any other island. Nor was form nivosus sighted by one of us (RBL) while collecting during the summer of 1987/88 on Efate, Ambrym and Malekula, and during September/October 1989 on Espititu Santo. 114 Australian Entomologist, 2000, 27 (4) All specimens of nivosus collected or sighted on Aneityum (Figs 1-2) were pinkish-white, unlike those from Hawaii which are nearly all grey-white. Only three specimens of the pinkish-white version are known from Hawaii (J. Stimson, pers. comm.). Discussion The Vanuatu population is particularly significant because it is the only known established population of white monarchs apart from that in Hawaii. All other records involve chance encounters with single specimens rather than discernable populations. The size of the Vanuatu population (around 20% of the total D. plexippus population) is greater than that of Hawaii, which in 1988-89 reached its maximum density of approximately 8% (Stimson and Kasuya 2000), confirming that the Vanuatu population has been established for some years. The white monarchs of Hawaii were first noticed in 1965 (Mitchell 1966), at which time they were estimated at around 1% of the local population. This would suggest that the Vanuatu population may have established somewhere between 30-40 years ago and long after the establishment of monarchs through the Pacific islands in the latter half of the 19th century (Vane-Wright 1993). One wonders why white monarchs have established colonies only on Oahu Island (Hawaii) and Aneityum Island (Vanuatu). It may well be a direct consequence of inbreeding the trait due to isolation. Stimson and Meyers (1985) have shown that form nivosus is the result of a simple autosomal recessive allele. If the populations of D. plexippus on Oahu and Aneityum Islands have few or no immigrant individuals, then inbreeding could easily increase the percentage of form nivosus individuals, providing that natural selection does not operate to eliminate homozygotes. Indeed, Stimson and Meyers (1985) hypothesised that the survival of white monarchs on Oahu is most likely because they are not at a selective disadvantage to the orange morph in the presence of bird predators. However, a more recent change in the behaviour of birds to seeking out larvae may now be a cause for the percentage decline in the number of white monarchs on Oahu (Stimson and Kasuya 2000). The discovery of the Vanuatu population of white monarchs provides an opportunity for hybridization experimentation between the Hawaiian and Vanuatu populations, which for the first time may shed light on the origin of the 8nivosus9 gene. As Vane-Wright (1993) noted, if double heterozygous crosses of different origins produce an F1 population of only typical orange D. plexippus, then independent origins for form nivosus can be rejected. However, if the F1 population segregates into 3:1 then a single American origin is plausible, although this would not reject a multiple-origin possibility. Australian Entomologist, 2000, 27 (4) 115 Figs 1-2. Danaus plexippus, form nivosus. (1) male upperside; (2) male underside. 116 Australian Entomologist, 2000, 27 (4) Acknowledgments RBL thanks Fr Albert Sacco for considerable help and companionship during several collecting trips to Vanuatu and for providing the specimens examined. For the accompanying photographs we wish to thank Paul Overden, Australian Museum. John Stimson and Maiko Kasuya kindly made available to us a copy of their unpublished manuscript. References MINNO, M. 1996. White monarchs found in Florida. News of the Lepidopterists9 Society 38(4): 184, 190. MITCHELL, W.C. 1966. Danaus plexippus (L.). Proceedings of the Hawaiian Entomological Society 19: 129. SAMSON, C. 1983. Butterflies (Lepidoptera: Rhopalocera) of Vanuatu. Naika (Journal of the Vanuatu Natural Science Society) 10: 2-6, 1 pl. STIMSON, J. and KASUYA, M. 2000. Decline in the frequency of the white morph of the monarch butterfly (Danaus plexippus plexippus L., Nymphalidae) on Oahu, Hawaii. Journal of the Lepidopterists9 Society 54: 24-32. STIMSON, J. and MEYERS, L. 1985. Inheritance and frequency of a color polymorphism in Danaus plexippus (Lepidoptera: Danaidae) on Ohahu, Hawaii. Journal of Research on the Lepidoptera 23: 153-160. VANE-WRIGHT, R.I. 1993. The Columbus hypothesis: an explanation for the dramatic 19th Century range expansion of the monarch butterfly. Pp 179-187, in Malcolm, S.B. and Zalucki, M.P. (eds), Biology and conservation of the monarch butterfly. Natural History Museum of Los Angeles County, Los Angeles; xii+419 pp.