by Roger C. Kendrick
This article is intended to be in part a report on a wildlife observation and in part a provocation of thought pertaining to matters ecological.
In April 2015, the opportunity arose to undertake a night time light-trap moth survey at Mai Po Nature Reserve (MPNR), as part of ongoing biological survey work for beetles being recorded at light. The primary objective was to gain a better understanding of the moth diversity at MPNR, for which there is little data from April: data published by Waring et al. (1997), from two sessions in April 1993 and unpublished data from a single April session of the Territory wide Hong Kong S.A.R. Government moth survey in 2004. Three mercury vapour lights were operated from 18:30 hrs to 22:30 hrs on 17 April 2015, (1) at the birder’s hut adjacent to the Education Centre, (2) on a gei wai bund close to mangrove, 130m north of (1) and (3) in gei wai mangrove some 50m north-west of, and out of sight from, (2). The sites were chosen to try and record species from different habitats, as well as logistical practicalities of running electrical equipment from mains electricity. Species were identified (from Kendrick (2002) and voucher material in the moth collection held at Kadoorie Farm and Botanic Garden, Tai Po) and recorded in situ, photographed where possible, and an abundance count taken at the end of the recording session. A small number of voucher specimens were taken for species not identified in situ.
At the end of the evening, some 250 moths, representing just over 50 moth species, were documented (48 species photographed plus a few species with retained voucher specimens). The most abundant species, by quite a distance, was the looper moth Cleora injectaria (Walker, 1860) [Lepidoptera: Geometridae, Ennominae]. Whilst this was not a totally unexpected result, as the species was recorded in “very large numbers” by Waring et al. (1997) – some 250 individuals in two evenings, what was of note was the high degree of variation observed amongst individuals of this species. Nine different forms were photographed (Figure 1), with the black & white banded forms (d, e & f) more abundant than grey, brown and white forms (g, h & j); the plain forms (a) and (b), as well as a black discal spot form (c) were seen only once each.
Documentation of variation in this species has been made before – Holloway (1993) noted “this is a relatively small, variable species with heavy discal markings. The commonest form has the males dull, dark brown and tends to predominate in coastal habitats. Those from inland, particularly from montane habitats, tend to be greyer. Females are generally much paler, often with a strong black medial band. …. The moth is most abundant in mangrove or esturine habitats, but a distinctly greyer form was taken on . . . limestone . . . in lower montane forest.”. Murphy (1990) also noted variability of pattern and colour of adults (and larvae) of the species. Cleora alienaria was found to be common on Kandelia obovata at Mai Po and Ting Kok (Tai Po) (Tong et al., 2006) and elsewhere in its range it has also been recorded (including defoliating) on other mangrove plant genera found in Hong Kong, including Avicennia and Rhizophora (Piyakarnchana, 1981; Murphy, 1990; Holloway, 1993).
So much for the observation; now to pose questions for deduction (and maybe further research). Why is it that this species exhibits so many different forms in a single large population, and what ecological mechanism (or mechanisms) selects for so many forms? The observations from Mai Po in 2015 (and 1993, Waring, pers. comm.) are somewhat contrary to that reported by Holloway, with mangrove forms of Cleora injectaria in Hong Kong including variations found in sub-montane limestone forest from Borneo. Unfortunately, only one adult specimen was illustrated from Singapore’s mangroves by Murphy (1990), so it is not possible to compare further. There are other moth species with high population abundance associated with mangrove at Mai Po; specifically Dysphania militaris (Linnaeus, 1758) [Geometridae, Geometrinae] and Ptyomaxia syntaractis (Turner, 1904) [Pyralidae, Phycitinae] that exhibit two and many forms respectively. Both species’ larvae also are mangrove feeders, Dysphania on Carallia (Bascombe, in Holloway, 1996) and Ptyomaxia syntaractis (as Nephopterix syntaractis) on Avicennia marina (Anderson & Lee, 1995). By way of comparison, abundant Phragmites associated moth species at Mai Po, like Chilo luteellus (Motchulsky, 1866) [Crambidae, Crambinae], show no polymorphism. Here, perhaps, lies part of the answer – Phragmites is a comparatively uniform environment at the microhabitat level with few niches, whereas there are many niches within mangrove, offering the opportunity for variable gene expression through natural selection from multiple, variable, microhabitats where adult Cleora injectaria rest up during the day.
How does this benefit polymorphic species? Well, for Cleora injectaria, one might speculate that polymorphism allows the species to be abundant – maybe adaptation for hiding from predators (through cryptic patterning) in a wide variety of microhabitats provides a high chance of survival for long enough to pass on the genes for adult moths of each form. Maybe I’ll leave speculation here and ask the professors and lecturers to ask students for other reasons….
Is polymorphism important? For Cleora injectaria, it seems so, not just for maintaining a healthy population, but also probably for the health of the mangrove. High predation (defoliation) can cause major stress to the mangrove (Tong et al., 2006), but could it be that such defoliation events actually improve the genetic fitness of the mangrove plant species involved? If so, the moths would benefit, too, as fitter, longer lived mangrove plants should improve the long term viability of the moth populations, assuming that the habitat is allowed to persist.
As always, one observation gives rise to many questions. I trust that some readers will be able to think more deeply than I have on the issue. One might even think about ways to investigate which forms of Cleora injectaria have evolved to utilise a defined microhabitat within the mangroves – or is there a relationship between the larval host species and adult form? How do abiotic factors affect polymorphism in a single population? Are there cryptic species involved? Over to staff and students to mull over….
Lastly, I thank programme coordinator Paul Aston for inviting me to participate in the research at Mai Po; Bena Smith and his staff at MPNR for facilitating field work and the Director, Agriculture, Fisheries & Conservation Department, HKSAR Government for issuing the necessary permit to record by light trapping.
Anderson C. & Lee S.Y. (1995). Defoliation of the mangrove Avicennia marina in Hong Kong: cause and consequence. Biotropica 27: 218-226.
Holloway J.D. (1993 ). The Moths of Borneo: part 11; Geometridae, subfamily Ennominae. Malayan Nature Journal 47: 1-309.
Holloway J.D. (1996). The Moths of Borneo: part 9; family Geometridae: subfamilies Oenochrominae, Desmobathrinae, Geometrinae. Malayan Nature Journal 49: 147-326.
Kendrick R.C. (2002). Moths (Insecta: Lepidoptera) of Hong Kong. Ph.D. thesis, The University of Hong Kong. xvi + 660pp, 47 plates, 40 figs.
Murphy D.H. (1990). The natural history of insect herbivory on mangrove trees in and near Singapore. Raffles Bulletin of Zoology 38: 119-203.
Piyakarnchana J. (1981). Severe defoliation of Avicennia alba BL. by larvae of Cleora injectaria Walker. Journal of the Science Society of Thailand 7: 33–36.
Tong Y.F., Lee S.Y. & Morton B. (2006). The herbivore assemblage, herbivory and leaf chemistry of the mangrove Kandelia obovata in two contrasting forests in Hong Kong. Wetlands Ecology and Management 14: 39–52.
Waring P., Thomas R.C. & Li K.H.K. (1997). Lepidoptera in Hong Kong, April 1993. British Journal of Entomology and Natural History 10: 77-100.