Taro Planthoppers (Tarophagus Spp.) in Australia and the Origins of Taro (Colocasia Esculenta) in Oceania

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Abstract

Taro planthoppers (Tarophagus spp.) may be associated exclusively or primarily with taro (Colocasia esculenta), and the geographical distribution of T. proserpina provides circumstantial evidence that taro is native to the Sahul continental region (as well as being native to Sunda). T. colocasiae (Matsumura) (Asche and Wilson 1989a,b) is reported here for the first time in Australia, and the genus Tarophagus is reported for the first time on the wildtype form of taro (C. esculenta). Three species of taro plant hopper are present in Asia and the Pacific. T. proserpina has a relatively narrow distribution extending from eastern New Guinea to Polynesia (Remote Oceania). This distribution adds support to the suggestion that Polynesian taros are derived from a Melanesian taro gene pool. It is tentatively suggested that different Tarophagus species evolved in association with different taro gene pools, before the domestication of taro in multiple regions within Asia and the Pacific. Plant viruses associated with taro and Tarophagus are also discussed. ********** Insect associations with cultivated plants are important for many reasons, most notably because insects are often major pests and can also transmit various diseases. The economic impacts of insects on crops in the past are rarely known or knowable, although archaeologists have found storage pests (weevils) among archaeological remains of cereal crops (Vartavan 1990). Spriggs (1982) noted the possibility of archaeological preservation of insects associated specifically with taro, giving Papuana spp. (taro beetles) and Tarophagus proserpina (taro planthopper) as examples. Both genera are well known as agricultural pests on cultivated taro (Gagne 1982). Another way to approach agricultural history is to investigate the biogeography of living insects associated with living plants. This is the approach that I will introduce here with respect to taro, Colocasia esculenta (L.) Schott.

In July 1985, near Lae, in Papua New Guinea, I observed an insect pollinator, Drosophilella pistilicola, on wildtype taro in an area where these drosophilid flies had only recently been discovered and described by Carson and Okada (1980, 1982). Although sparse, the existing literature on Drosophilella strongly suggested that wild taro populations in Melanesia could be native (indigenous) to the region (Matthews 1990, 1995). Subsequently, in Australia, I found large numbers of another insect, Tarophagus (literally, 'taro-eating'), on wild and apparently wildtype taros in the coastal region near Cairns, Northeast Queensland, in wet tropical rainforest. In this paper, I introduce the genus Tarophagus and its species in Asia and the Pacific, report my own findings of Tarophagus in Australia, and then interpret the distributions of Tarophagus species in relation to the origins of taro in Oceania. I also discuss plant viruses associated with taro (the host) and Tarophagus (the vector), and recent studies suggesting that there are multiple taro gene pools in Asia and the Pacific.

In this approach to the history of taro I am crossing disciplinary boundaries, just as I did during 1985 to 1990 as a student of Doug Yen, in the Department of Prehistory headed by Jack Golson, at the Australian National University (ANU). Both scholars encouraged me in this dangerous activity, while making clear the perils of treating any one discipline in too shallow a fashion. Another teacher at ANU was David Shaw, an entomologist who encouraged a healthy respect for the difficulties of biological observation and interpretation. Although digging deeply as an archaeologist continues to elude me as an occupation, I am still digging, as best I can, in other fields that can contribute to our understanding of the past.

Tarophagus ecology, taxonomy, and geographical range

The taro planthopper genus, Tarophagus, has three recognised species, and these are considered important insect pests on taro. The entire life history of a taro planthopper occurs on taro leaves, above ground. Taro planthoppers feed on sap and heavy infestations can cause plants to wilt and become stunted. Feeding and egg-laying punctures cause sap exudation which forms red encrustations on the plant. The planthoppers are also vectors of taro bobone virus (a rhabdovirus), and this is of concern for taro growers because the resulting disease can stunt or kill plants. Tarophagus species are widely distributed from East Asia (including Taiwan and the Ryukyu Islands of southern Japan), through Southeast Asia to Australia (Northern Territory and Queensland), Papua New Guinea, New Caledonia, and many Pacific island countries (Gagne 1982, Zettler et al. 1989, Asche and Wilson 1989a, b). Eggs are laid in the bases of petioles and the midribs of leaves. Young nymphs are creamy white, and later stages develop predominantly black with white markings. From egg to adult takes about 18 days. These details may vary according to species, and have not been studied with reference to the taxonomic descriptions provided by Asche and Wilson (1989a, b).

Tarophagus belongs to the family Delphacidae, a large insect group in which most species feed on grasses. Many of them are pests on crops such as rice, sugarcane, wheat, rye, corn, and forage crops (Kisimoto 1994, Fletcher and Lariviere 2001). Delphacid planthoppers are characterised by two wing forms (short and long) that appear either in both sexes or in one of the two. The long-winged forms can fly long distances. For example, in 1967, huge numbers of migrating rice planthoppers were discovered on a weather observation boat floating on the Pacific Ocean, at least 500 km distant from the Japanese mainland (Ichikawa 1994). The possibility of transoceanic migration by the taro planthopper has not been investigated. If taro is the only host for Tarophagus (or in regions where no other hosts are present), effective transoceanic migration by flight would require (or might be made possible) by the prior establishment of natural or introduced populations of taro, at every destination.

The known range of the genus Tarophagus is tropical to subtropical, with no reports from northern Japan or from New Zealand, at the northern and southern limits of cultivated taro in Asia and the Pacific. In Japan and New Zealand, cold temperatures and the winter loss of leaves by temperate-adapted forms of taro might prevent over-wintering and survival of Tarophagus. The genus has not been reported in Subsaharan Africa (see Smithsonian 2001), although taro is an ancient and widespread crop in Africa.

Asche and Wilson (1989a) noted that taro planthoppers were identified in most previous reports as T. proserpina (Kirkaldy). After looking at specimens from throughout Southeast Asia and Oceania, these authors recognised three morphologically distinct species, T. colocasiae, T. persephone, and T. proserpina. The known distributions of these species (see later) overlap and together span most of the possible natural range of taro …