FNZ 13 - Encyrtidae (Insecta: Hymenoptera) - Intraspecific variation in New Zealand
Noyes, JS 1988. Encyrtidae (Insecta: Hymenoptera). Fauna of New Zealand 13, 192 pages.
( ISSN 0111-5383 (print), ; no. 13. ISBN 0-477-02517-X (print), ). Published 09 May 1988
Intraspecific variation in New Zealand
A striking feature of the New Zealand encyrtids is the extreme intraspecific variability of most of the endemic or indigenous species. Indeed, fairly detailed preliminary studies indicated that there might be as many as 53 endemic or indigenous species present in the material examined, whereas this revision recognises only 39. Among those species for which sizeable samples have been examined (n > 25), many appear to exhibit a greater degree of variation than is known to exist in any encyrtid species outside New Zealand. This variation is not limited to endemic species; it is greatest in Tetracnemoidea bicolor, which is likely to be Australian in origin, and which has been described four times in three different genera. Furthermore, variation does not appear to be linked to geographical distribution or to altitude, except perhaps in Odiaglyptus biformis, of which completely orange forms are known from more northerly latitudes, and in Adelencyrtoides variabilis, which is generally larger and darker in more southerly latitudes.
This variation is often so extreme that given small, isolated samples of a single species it is very easy to accept that several species may be present. However, with larger samples it becomes impossible to divide the material into discrete morphological segregates. Principal component analysis has been used in an attempt to recognise morphological segregates of some of the more variable species, for which it was difficult to accept that only one species was present, viz, Adelencyrtoides blastothrichus, A. inconstans, A. novaezealandiae, A. otago, A. variabilis, Austrochoreia antipodis, and Tetracnemoidea brounii. The results of these analyses for Adelencyrtoides novaezealandiae are presented in Text-figure 1. An attempted analysis of Tetracnemoidea bicolor proved unsatisfactory because the presence or absence of wings and the segmentation of the antennal club tended to independently distinguish groups. While it is easy enough to remove the effect of segmentation, it is difficult to do the same for the presence or absence of wings because the relative length of wings has an effect on some of the characters used in the analysis, e.g., relative length of the pronotum, mesoscutum, and scutellum.
Intraspecific variation, both geographic and individual, has been noted in other groups of insects in New Zealand, but unfortunately very little information on this has been published to date. Franciscolo (1980) noted extreme individual variation in the mordellid beetle Stenomordellaria neglecta (Broun), and Butcher (1984) noted an unusual degree of variation in some species of the carabid genus Holcaspis, although apparently not to the extent indicated by the present study. J.C. Watt (pers. comm.) maintains that all coleopterists who have worked on the New Zealand fauna have found considerable geographic and individual variation within species. The phenomenon has also been noted in Thysanoptera (L.A. Mound, pers. comm.) and other groups of Hymenoptera Parasitica. e.g., Ichneumonidae (I.D. Gauld, pers. comm.) and Proctotrupoidea (L. Masner. pers. comm.). In the words of one: "It is as though someone has taken the gene pool and given it a good stir". Watt (1983) comments that many beetles with reduced or vestigial wings have limited capability for dispersion and thus they tend to form series of semi-isolated populations with limited gene flow. When such populations come together again they may fail to interbreed, having speciated, or may interbreed producing complex patterns of geographic variation. While this may account for unusual geographic variation in the species of many groups, it is unlikely to be the reason for the extreme individual variation noted in indigenous species of Encyrtidae, for two reasons. First, populations of encyrtids are likely to be isolated for only relatively short periods because, unlike beetles, even flightless individuals can be wind-carried for considerable distances. This would allow periodic genetic exchange between even well separated populations. Second, it is possible that encyrtids are a relativelv recent addition to the New Zealand fauna; many species may even have appeared here in the last 10 000 years or so, i.e., since the last glaciation.
It is probable that many vacant ecological niches have become available only relatively recently. The main hosts in New Zealand are Coccoidea, which are relatively diverse on these islands, there being probably at least 500 endemic or indigenous species and a further 150 recently introduced (J.M. Cox, pers. comm.). About half of the species of Pseudococcidae recognised by Cox (1987) are known from alpine or subalpine habitats, and many are restricted to individual mountains or mountain ranges. Such localised distribution suggests that the species concerned have evolved since the last glaciation, the females being completely flightless, fairly sedentary, and unlikely to be dispersed by wind; a wider distribution could be expected if they had earlier origins. This may also be the case with the other major families of Coccoidea represented in New Zealand, but this is unknown at present. Thus, it is possible that their parasites, which are readily dispersed by wind, are at present adapting to particular hosts and particular habitats, resulting in a high level of genetic variability which in turn gives rise to extensive phenotypic variation. Genetic variability could be maintained by periodic gene exchange between otherwise isolated populations by means of wind-carried individuals.