Towns, 2002. Interactions Between Geckos, Honeydew Scale Insects and Host Plants Revealed on Islands in Northern

Home , Fauna of New Zealand, Honeydew (secretion), New Zealand parrot, Nothofagus solandri

Interactions between geckos, honeydew scale insects and host plants revealed on islands in northern New Zealand, following eradication of introduced rats and rabbits

D. R. Towns Science and Research Unit, Department of Conservation, Private Bag 68 908 Newton, Auckland, New Zealand. E-mail: [email protected]

Abstract Invasive species that reach islands can have effects that ripple through communities. As a corollary, once invasive species are removed, the responses by resident species may also have ripple effects, sometimes with outcomes that are unpredicted. One such unpredicted response is reported on islands off north-eastern New Zealand following the removal of rabbits (Oryctolagus cuniculus) and Pacific rats or kiore (Rattus exulans). As composition of the vegetation changed and geckos became increasingly abundant, a source of energy for the geckos was revealed: honeydew produced by the scale insect Coelostomidia zealandica (Hemiptera: Margarodidae) infesting ngaio (Myoporum laetum) and karo (Pittosporum crassifolium). Honeydew may have significant effects on the carrying capacity of invertebrates and birds in mainland forests of New Zealand. However, its importance for geckos on islands was apparently masked by reduced gecko abundance in the presence of introduced predators, and suppression of host plants by introduced herbivores. Possible mechanisms of spread and new hosts of C. zealandica are described, and the vulnerability of the scale insect on islands with introduced mammals that suppress recruitment of selected host species is emphasised.

Keywords Scale insects, Coelostomidia zealandica; parasites; honeydew; geckos, Hoplodactylus maculatus, Hoplodactylus duvaucelii; hosts; ngaio, Myoporum laetum; karo, Pittosporum crassifolium; habitat modification; restoration.

INTRODUCTION

Island archipelagos show levels of recent extinction com- (Campbell and Atkinson 1999), reduced diversity and parable to the most severe mass extinctions recorded in abundance of large, ground-dwelling flightless inverte- the Earth’s history (Paulay 1994). These losses are largely brates such as weta (Orthoptera), some spiders and darkling the result of direct or indirect human activity, including beetles (Atkinson & Towns 2001), reduced capture fre- the accidental or deliberate introduction of a large number quencies, abundance and diversity of lizards (Towns 1994, of alien organisms. For example, in New Zealand over 1996), impaired recruitment of tuatara (Tyrrell et al. 2000) 1600 species of plants, 1500 species of insects and 90 spe- and reduced productivity of small burrowing seabirds cies of vertebrates are established alien species (Atkinson (Pierce 1998). and Cameron 1993), and some of them are now serious weeds and pests. Among the vertebrates, most extinctions In addition to direct effects, invasive species are likely to of island plants and animals are attributable to predation have other effects that ripple through communities as and browsing by about nine species of mammals, includ- changes in the populations of one species affect others ing humans (Atkinson 1989). Two of the most widespread (Simberloff 1990). For example, goats (Capra hircus) that mammals are rabbits (Oryctolagus cuniculus) and Pacific reduce vegetation cover leading to increased light levels rats or kiore (Rattus exulans). The effects of these two may also make sites prone to the effects of additional in- species on island systems have not been well documented. vasive weeds (Towns et al. 1997). Ripple effects may also In an extreme example, rabbits appeared responsible for lead to further pressure on indigenous species. Atkinson the loss of three species of endemic birds and 22 species (1989) described modification of forest by introduced her- of plants from Laysan Island (Williams et al. 1995 and bivores, leading to increased exposure of New Zealand references therein), but around New Zealand they more land snails to introduced predators. commonly induce low-diversity browse-resistant vegetation (Ogle 1990; Towns et al. 1997). Over the last 15 years, Ripple effects need not operate in one direction as a spiral rabbits and Pacific rats have been removed from at least of degradation. In theory, the removal of a catastrophic 24 islands on the continental shelf of New Zealand (Veitch disturbance event could lead to ripple or interactive ef- 1995 and unpublished data). The islands cleared of these fects in the course of recovery or succession. Because the organisms provide opportunities to determine the direct pre-disturbance history of many sites is unknown, the re- and indirect effects of the introduced species and to responses by resident species may sometimes be either un- store depleted island systems. For example, recent studies predictable or unpredicted. on islands from which Pacific rats were removed indicated pervasive direct effects while the rats were present. These In this account I describe a hitherto unknown plant-scale included suppressed recruitment of selected woody plants insect relationship that was revealed following the removal

Pages 329-335 In Veitch, C. R. and Clout, M. N. (eds.). Turning the tide: the eradication of invasive species. IUCN SSC Invasive Species Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK. Turning the tide: the eradication of invasive species of rabbits and Pacific rats from islands in north-eastern patches on parts of Korapuki Island in 1962 (Hicks et al. New Zealand. The relationship is between a scale insect 1975), but was rare in the understorey by the time rabbits that exudes honeydew, the margarodid Coelostomidia were removed in 1987 (Towns et al. 1997). zealandica, its host plants, and vertebrates that feed on the honeydew. This example illustrates how human-in- Coelostomidia zealandica duced disturbance, and the presence of introduced mammals, can suppress populations of scale insects, their host Ten native species of plant-sucking scale insects in the plants, and the links to vertebrate honeydew feeders. family Margarodidae are associated with native trees and shrubs. Margarodids produce sugary secretions (honey- STUDY AREAS dew) that are used as a food source by forest insects and birds. The secretions also provide a medium for sooty The study was based on Korapuki Island and neighbour- moulds that blacken the trunks of heavily infested host ing Green and Middle Islands in the Mercury Islands, New plants (Morales 1991). The life history of C. zealandica is Zealand (36o40’S, 175o52’E), a group of seven islands and unknown, but that of a related species, C. wairoensis, was associated islets and stacks of volcanic origin. Korapuki summarised by Morales (1991). In brief, the female life Island (18 ha) was inhabited by rabbits and Pacific rats cycle includes a mobile crawler, non-mobile intermediate until the rats were eliminated using rodenticide in 1986, feeding stages, and a fully legged, mobile, non-feeding, and the rabbits by shooting in 1987 (Towns 1988; flightless adult. The male life cycle includes a mobile McFadden and Towns 1991). Vegetation on Korapuki Is- crawler, a non-mobile intermediate feeding stage, non-feed- land was, until 1987, dominated by flax (Phormium tenax), ing pre-pupa, pupa and fully-winged adult male. shrubs including mahoe (Melicytus ramiflorus) and a canopy of pohutukawa (Metrosideros excelsa) consistent Female C. wairoensis may remain and deposit eggs in a with extensive forest clearance (probably by burning early hard ‘test’ formed on its host, but females of C. zealandica in the 20th century) and subsequent browsing of regrowth are free living and oviposit in the soil or under bark (Mo- by rabbits. Since removal of rabbits, there have been spec- rales 1991). Crawlers settle in cracks on branches, insert tacular increases in the recruitment of soft-leaved their mouthparts, and produce a long anal tube to void understorey species as well as increased canopy develop- sugary waste. The other visible sign of infestation of hosts ment of species such as mahoe (Towns et al. 1997). are white cocoons spun by male prepupae on the trunks of trees. Hosts include a wide variety of coastal and forest Green (3 ha) and Middle Islands (13 ha) have never had shrubs, trees, and vines. Amongst these are ngaio and introduced mammals. The islands support coastal Pittosporum tenuifolium (Morales 1991). broadleaf forest with little evidence of previous human occupation (Towns et al. 1990). METHODS

STUDY ORGANISMS Observations of the distribution and abundance of vegetation infected by margarodids were made on Korapuki Is- Myoporum laetum (ngaio) land opportunistically between 1986 and 2000 and on Lady Alice Islands and adjacent islets between 1992 and 2000. Ngaio is a light-demanding shrub or tree that is often low- Infected trees were identified by the presence of sooty growing in coastal areas, but can reach to 10 m with a moulds, anal tubes of margarodid nymphs and, on some trunk diameter of 30 cm (Allan 1961). The species was trees, by the presence of cocoons and adult males and fe- regarded as uncommon on Korapuki Island by Atkinson males. Species identification was confirmed by R.C. in 1962 (cited in Hicks et al. 1975), but in 1974 a stand of Henderson (Landcare Research, New Zealand Arthropod ngaio was present (Hicks et al. 1975). When rabbits were Collection). eradicated in 1987, there were two identifiable stands of ngaio and occasional scattered trees on cliffs. The total On Korapuki Island, counts of geckos on trees were made number of trees was estimated as less than 10 by Towns et by capturing the animals after sunset (between 2200 and al. (1997) and the largest tree had a basal circumference 2300 hrs) and marking them with a dot of correcting fluid of 1.25 m (unpublished data). (TWINK). Estimates of relative density on beaches were obtained from sightings within a fixed period of ‘catch Pittosporum crassifolium (karo) per-unit-effort’ (CPUE) at five stations along a set transect line parallel with the high tide line (Towns 1991). The Karo is a shrub or tree that can reach 9 m and, although CPUE transect line was adjacent to the smaller of the two found in forest margins and stream sides (Allan 1961), stands of ngaio. Comparative data were obtained from also inhabits coastal areas, including small rocky islets. mammal-free Middle Island. The species is able to germinate in very low light conditions. On Green and Middle Islands, karo is a significant Levels of scale infestation were estimated by searching component of the understorey as well as an emergent tree for deformations of the bark of karo and visible anal threads near the coast (Atkinson 1964; Cameron 1990; pers. obs.). with honeydew droplets on karo and ngaio. Thread den- Karo was ranked as frequent, with plants seen singly or in sity was estimated by counting the active threads (those

330 Towns: Geckos, honeydew and host plants with droplets) within a defined 20 cm length of plant stem ducted on Korapuki Island in the presence of rabbits and beginning 20 cm from ground level and continuing at 20 cm Pacific rats (Whitaker 1973; Hicks et al. 1975; Towns un- intervals (five per plant). The start of each counted cylin- published data). CPUE data obtained in 1985, 1990, and der was therefore 40 cm from the start of its predecessor. 2000 indicated a 28-fold increase in sightings of common Thread density was calculated from stem circumference geckos along a coastal transect when comparing data from measured at the beginning of the counted sections and a year before campaigns against introduced mammals be- converted to threads per square metre. gan (1985) against data from 13 years after the campaigns’ completion (2000) (Fig. 1). Similarly, counts of geckos RESULTS on flax flowers between 1986-1990 increased from 0% to 24% occupancy (Towns 1994). Both sets of data indi- Spread of ngaio and karo cated an expanding population of geckos. Margarodids on Korapuki Island and Before removal of Pacific rats and rabbits from Korapuki Island, the remaining ngaio were infected by scale insects, nearby islands and blackened by sooty mould. There was no visible recruitment of young ngaio to these populations. Likewise, By 1994, eight years after the beginning of the campaigns karo was uncommon; there were no plants around the against rats and rabbits on Korapuki Island, young ngaio coastal beaches and apparently complete recruitment fail- trees on the coast adjacent to the original ngaio stands were ure (Towns et al. 1997). infested with margarodid scale insects. In addition, the margarodids had infested karo trees growing beneath the Following the removal of mammals, ngaio proliferated over original ngaio. most of Korapuki Island, but the seedlings and saplings failed to survive unless they occupied a canopy gap. The On karo, infestations were in the form of scattered raised most rapid expansion was around the north-western coast wart-like growths on the bark with the margarodid anal of the island where ngaio formed almost continuous cover tubes projecting from these growths. Occasionally, lower behind rocky beaches. branches were buried in leaf litter, and on these were ex- ternal encrustations of scale. Of 20 karo checked within a Likewise, after removal of mammals, karo proliferated ngaio stand in December 2000, 19 (95%) were infected throughout the island to become abundant on the coast by scale. These included saplings only 50 cm tall. and throughout forested areas (Towns et al. 1997). In coastal sites karo and ngaio formed mixed stands. Unlike the localised and scattered infestations of scale on karo, those on ngaio were present over the entire trunk. In Spread of geckos December 2000, mean infestations on five young trees (range 17-69 cm basal circumference) ranged from The two resident species of geckos, common gecko 70-2200 threads/m2. The most heavily-infected tree had a (Hoplodactylus maculatus) and Duvaucel’s gecko (H. thread density of up to 3300/m2. duvaucelii), were regarded as rare during surveys con- Margarodid infestation of karo and ngaio plants checked on Green and Middle Islands indicated similar levels of infestation to those found on Korapuki Island. On Green Island, ngaio and karo were infected and large karo trees (basal circumference >50 cm) had deformed sections of bark up to 5 cm across as evidence of previous margarodid presence. On some trees the deformed areas had cracked open and were oozing sap. On Middle Island, evidence of scale was present on 24 of 25 (96%) karo trees checked along 400 m on or within 50 m of the coast (mean basal karo circumference ± SE: 45.06 ± 5.48 cm).

Few adult margarodids were seen on Korapuki Island when trees were inspected in late spring (November-December), but males and females were seen frequently during late Fig. 1 Mean numbers (+ SE) of geckos summer (February-March). Adult females were orange- sighted in November at five sites along a 100 m pink in colour and were visible during daylight slowly coastal transect on Middle Island (pale shad- crawling about on the bark of ngaio and on the leaf litter ing) and Korapuki Island (dark shading) one beneath ngaio trees. Females were seen once on karo trees. year before the start of campaigns against introduced mammals on Korapuki Island and in years subsequent to the completion of the campaigns.

331 Turning the tide: the eradication of invasive species

Invertebrates and geckos on honeydew var. solandri), mountain beech (N. solandri var. sources cliffortioides) and red beech (N. fusca) in the northern South Island. The honeydew produced is rich in fructose, Introduced Vespula wasps (probably V. germanica) were sucrose, glucose, and oligosaccharides, but low in protein present on Korapuki Island until about 1990, but have not (Grant and Beggs 1989) and is sufficient to provide the been seen since. Other introduced wasps include the Asian daily energy requirements of kaka (Nestor meridionalis), paper wasp Polistes chinensis and the Australian paper a large native parrot, after only three hours of feeding wasp P. humilis. Neither species has been seen accumu- (Beggs and Wilson 1991). The sooty moulds that grow on lating around honeydew sources on ngaio. Two native the honeydew are also a food for arthropods (Morales et insects were commonly active on ngaio at night: a cock- al. 1988), and there is evidence from northern hemisphere roach (probably Parellipsidon lattipennis) and a byrrhid studies that honeydew washed into soil promotes the beetle (Pedilophorus crysopepsis). During the day, the growth of microorganisms and these in turn affect proc- native ant Monomorium antarcticum was seen on some esses such as carbon throughfall and nitrogen flux (Beggs trees and there were also occasional trails of the introduced 2001 and references therein). ant Technomyrmex albipes. Two species of Margarodidae were probably once wide- Geckos were first seen aggregating on ngaio trees in March spread on the northern offshore islands: Coelostomidia 1992. Both common and Duvaucel’s geckos were observed wairoensis, most commonly on kanuka (Kunzea spp.), and at night on ngaio. Up to 11 Duvaucel’s geckos and up to C. zealandica on a variety of hosts including ngaio and eight common geckos were observed simultaneously on individual ngaio trees. Both species were observed lick- ing at the bark surface. An estimate of numbers of geckos using individual trees was obtained over six nights in De- cember 1996. Of two trees checked, one had 10 common geckos and two Duvaucel’s geckos over a total stem length of 3 m, and the other had 32 common geckos and one Duvaucel’s gecko over a stem length of 7 m. Excluding recaptures, the number of common geckos observed on the two trees over six nights was 0.43 geckos/m/hour and 0.76 geckos/m/hour respectively. DISCUSSION

The ecological role of honeydew

Congregations of geckos feeding on exudates from margarodids on ngaio trees have not previously been reported. There was little chance of observing the phenom- enon before removal of introduced mammals from Korapuki Island because at that time geckos were rarely seen. Indeed, there is little published information on the use of honeydew by geckos anywhere in New Zealand, although the attraction of trees bearing honeydew to geckos has been exploited when surveying for geckos (A.H. Whitaker pers. comm.). Whitaker (1987) found at least 80 geckos (Hoplodactylus pacificus) feeding on honeydew on a 6 m tall karo on rodent-free Little Ohena Island. This is the only report now attributable to honeydew produced by Coelostomidia zealandica. Bishop (1992) included geckos in a diagrammatic representation of the us- ers of honeydew in beech forest of the South Island, but gave no further details. Beggs (2001) included lizards Fig. 2 Conceptual models of interactions among species that consume honeydew in beech forests between honeydew, their host trees and honey- of the South Island, but did not elaborate. dew feeders before (A) and after (B) the removal or disappearance of introduced mam- Honeydew can provide a major energy source in forest mals and wasps (circles) from Korapuki Island. ecosystems (Beggs 2001). The most intensively studied Arrow width denotes likely interaction effects; honeydew-producing system in New Zealand is that of unshaded arrows are for potential rather than Ultracoelostoma spp. on black beech (Nothofagus solandri observed effects.

332 Towns: Geckos, honeydew and host plants karo. Kanuka can form large stands in regenerating for- on ngaio would have eventually lost their original host est, such as those on Whatupuke and Lady Alice Islands trees, as they became overtopped by an expanding canopy in the Marotere Islands (Bellingham 1984). Duvaucel’s of mahoe. geckos were observed on Whatupuke Island by A.H. Whitaker and R. Parrish (pers. comm.) feeding on honey- Regeneration of karo is strongly inhibited by Pacific rats dew exudates on kanuka. The combined output of honey- (Campbell and Atkinson 1999), so it is conceivable that dew from scale insects on kanuka, ngaio, and karo may the presence of these rats alone may be sufficient to initi- therefore once have been considerable on offshore islands. ate the disappearance of C. zealandica from some islands. For example, no evidence of margarodids was found on The most frequently observed vertebrates attracted to hon- regenerating karo eight years after the removal of Pacific eydew produced by C. zealandica were geckos (Fig. 2), rats from Red Mercury Island (I. A. E. Atkinson pers. although the honeydew also appeared attractive to comm.). Similarly, following the eradication of Pacific nectivorous birds. For example, on Korapuki Island and rats from Lady Alice Island in October 1994 (K. Hawkins in the Poor Knights Islands, bellbirds (Anthornis melanura) pers. comm.), surveys for margarodids on extensive ngaio were seen foraging on ngaio and karo trees, presumably on the western coast failed to reveal visible signs of infes- feeding on the exuding honeydew produced by C. tation by margarodids (unpublished data). There were no zealandica (pers. obs.). Similarly, tui (Prosthemadera visible anal tubes and the bark of all trees was clear of novaeseelandiae) were observed on Muriwhenua Island sooty moulds. Only three adult karo trees are known to in the Marotere Islands foraging on ngaio infested with have survived on the island before removal of Pacific rats honeydew scale (Towns and Parrish unpublished data). and these trees also appeared free of margarodids. By comparison, on islets such as Muriwhenua Island adja- Unlike flowering plants, which often only provide seasonal cent to Lady Alice Island, ngaio were heavily infested by nectar sources, honeydew has the advantage of being avail- margarodids. Muriwhenua Island also supports dense able continuously, although whether there is seasonal vari- populations of common and Duvaucel’s geckos (Whitaker ation in the quantity and sugar concentration of honeydew 1978; Towns and Parrish unpublished data). produced by Coelostomidia zealandica is unknown. In beech forest, the composition and production of honey- On the mainland, introduced organisms, especially wasps, dew available varies during the year (Gaze and Clout can have devastating effects on the availability of honey- 1983), but declines in honeydew production during late dew for native species, reducing the standing crop of hon- summer and autumn were probably due to consumption eydew by 90% for five months of the year (Beggs 2001). by introduced wasps (Moller and Tilley 1989). Activity On offshore islands, the threats to these resources appear by geckos is temperature dependent (Angilletta et al. 1999), to be more through the loss of infestation sources as a re- so geckos may only feed on honeydew during the warmer sult of habitat destruction and modification of forest com- months. There are no such constraints on birds such as position by introduced mammals (Fig. 2). Because the bellbirds and tui. female margarodids are flightless, once the source of infestation is lost, there is no way for scale insects to re- Implications for restoration on islands establish, even when abundant host plants are present. This situation appears to prevail for C. zealandica on Red Mer- New Zealand margarodids are either quite host-specific cury and Lady Alice Islands. Because of the implications or polyphagous. The most polyphagous species is C. that the presence of margarodids has for carrying capacity zealandica (Morales 1991). However, only two of its pre- of invertebrates, geckos, and nectar-feeding birds, artifi- viously reported host species commonly grow on the cial spread of scale insects as part of island restoration smaller northern offshore islands (less than 100 ha): the projects now needs to be seriously considered. creeping vine Muehlenbeckia spp. and ngaio. The hitherto unreported capability of C. zealandica to infest karo ACKNOWLEDGMENTS provides a third host on these islands. Karo may also play a vital role in the scale insect’s transmission. Unlike ngaio My thanks to Northland and Waikato Conservancies of and Muehlenbeckia, karo is able to survive in low light the Department of Conservation for permission to visit the conditions under the forest canopy. On Korapuki Island, Marotere and Mercury Islands (respectively), Richard despite the small number of parent trees, regeneration by Parrish and Chris Green who assisted with transport be- karo was rapid and widespread (Towns et al. 1997). Since tween the islands, and to Ian Atkinson, Lynette Clelland, adult female margarodids have limited powers of disper- Rod Hay, Keri Neilson, Tony Whitaker, and two review- sal, progressive infestations of karo have apparently ena- ers, Peter Gaze and Rosa Henderson, for useful comments bled the spread of margarodids through some of the gaps on the manuscript. The study was part of Science and between new stands of ngaio (Fig. 2). As a corollary, the Research Unit Investigation 3236. absence of karo (and perhaps other hosts) from low-light areas could break the infestation pathway. Without removal of rabbits from Korapuki Island, the margarodids

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