Powderbark wandoo (Eucalyptus accedens) has a powdery triterpenoid-containing substance on the surface of its smooth bark, which is formed from sloughing peridermal cells. When compared with the similar-appearing wandoo (E. wandoo), which occurs inthe same area and which does not accumulate powder, fewer bark-associated arthropods are found. Exposure to this powder accelerated mortality of the ant, Iridomyrmexchasei, a species that tends scale and other sap-sucking insects on the foliage of eucalypts. Ants of this and two other species were unable to reach baits on the top of vertical wooden dowels that had been coated with powder taken from the bark of E. accedens. The powdermay deter arthropods from living or moving on the bark by chemical or physical means. It is postulated that the function of the powder is to reduce the threat from herbivorous or scale-tending arthropods that may live on, or traverse, the bark of this species

Cocquyt, R.

Majer, Jonathan

Recher, H.

English

Powderbark wandoo (Eucalyptus accedens) has a powdery triterpenoid-containing substance on the surface of its smooth bark, which is formed from sloughing peridermal cells. When compared with the similar-appearing wandoo (E. wandoo), which occurs in the same area and which does not accumulate powder, fewer bark-associated arthropods are found. Exposure to this powder accelerated mortality of the ant, Iridomyrmex chasei, a species that tends scale and other sap-sucking insects on the foliage of eucalypts. Ants of this and two other species were unable to reach baits on the top of vertical wooden dowels that had been coated with powder taken from the bark of E. accedens. The powder may deter arthropods from living or moving on the bark by chemical or physical means. It is postulated that the function of the powder is to reduce the threat from herbivorous or scale-tending arthropods that may live on, or traverse, the bark of this species

Majer, J.D.

Cocquyt, R.D.

Recher, H.F.

Research Repository

81Powdery bark inEucalyptus accedensdeters arthropods? Anevaluation using antsJ D Majer1, R D Cocquyt1 and H F Recher21Department of Environmental Biology, Curtin University ofTechnology, Perth, Western Australia 6845   J.Majer@curtin.edu.au2School of Natural Sciences, Edith Cowan University,Joondalup, Western Australia 6027   hjrecher@pacific.net.au(Manuscript received January 2004; accepted June 2004)Abstract. Powderbark wandoo (Eucalyptus accedens) has apowdery triterpenoid-containing substance on thesurface of its smooth bark, which is formed fromsloughing peridermal cells. When compared with thesimilar-appearing wandoo (E. wandoo), which occurs inthe same area and which does not accumulate powder,fewer bark-associated arthropods are found. Exposure tothis powder accelerated mortality of the ant, Iridomyrmexchasei, a species that tends scale and other sap-suckinginsects on the foliage of eucalypts. Ants of this and twoother species were unable to reach baits on the top ofvertical wooden dowels that had been coated withpowder taken from the bark of E. accedens. The powdermay deter arthropods from living or moving on the barkby chemical or physical means. It is postulated that thefunction of the powder is to reduce the threat fromherbivorous or scale-tending arthropods that may liveon, or traverse, the bark of this species.Keywords: Tree trunk, invertebrate, Eucalyptus forest,powder, Eucalyptus accedens.IntroductionBark provides a wide range of refugia forinvertebrates and represents an important source ofnutrition to such animals, either directly, or through thefungi, lichens, algae and microorganisms that live on andwithin it. It also acts as a resting place for invertebratesthat move within or across the area in which the treesoccur. As part of a larger study of the fauna associatedwith trunks of Australian eucalypts, we recentlyexamined the arthropod fauna associated with wandoo(Eucalyptus wandoo) and powderbark wandoo (E.accedens) in the Western Australian wheatbelt (Majer et al.2002, 2003).Both species possess exfoliating bark and havesmooth, light-coloured trunks that are similar inappearance. Powderbark wandoo differs from wandooin having a bark that is covered in a powdery substance,from which it receives its name. Majer et al. (2002, 2003)quantified the species richness and general abundance ofvarious arthropod orders on the bark of these two treespecies. They found that powderbark wandoo, whencompared with wandoo, yielded generally lowernumbers of arthropods (6301 vs 7754 individuals in barktraps on 20 trees) and fewer species (422 species from 140families vs 497 species from 159 families in bark plusintercept traps on 20 trees). These differences weresignificant for many of the invertebrate orders.Measurements of nutrient content of live and deadbark of the two tree species indicated some significantdifferences (Majer et al. 2003), but these differences werenot consistent across elements, suggesting that nutrientlevels are not a major determinant of the trends inarthropod levels on the two tree species. Ourobservations also indicate that there are no largedifferences in the bark roughness, presence of fissures, orgeneral heterogeneity of bark microhabitats between thetwo species (see photos in Majer et al. 2002, 2003). Ittherefore appears that nutrient levels or gross barktexture is not related to the differences in arthropodassemblages between the two tree species.There are examples in the literature of leaves, fruits orstems of plants being covered with microscopic crystalsof epicuticular wax, which may protect the plant againstherbivorous invertebrates (Stork 1980; Edwards 1982;Eigenbrode 1996; Juniper 1995) or Hemiptera-tendingants (Haberlandt 1909). In view of the similarity inappearance of the trunks of the two eucalypt species inour study, and also the fact that they grow at the samesites, we postulated that the powder on the bark ofpowderbark wandoo contributed to the observeddifferences in arthropod abundance and richness. In thisnote, we report on the origin and nature of the powderof powderbark wandoo and the possible role of thepowder in limiting arthropod use of the trunk of the tree.MethodsThe study was undertaken in wandoo/powderbarkwoodland at Dryandra (32° 56’S, 117° 11’E), southwestAustralia, approximately 160 km south-east of Perth.Trees were distributed throughout a 20 ha section of theadministrative compartment known as Peters Block;powderbark wandoo trees tended to be on the upperhalf, while wandoo trees tended to be on the lower halfof the landscape. Except where indicated, allmeasurements and samples were taken at 1.5 m abovethe ground. It was not possible to measure live barkbecause it merged in with other living tissue of the trunk.However, both species decorticate bark, so the thicknessof 10 samples of freshly-fallen bark beneath each tree wasmeasured with a micrometer.Powder was collected from powderbark wandoo treesusing a clean paint brush by brushing the trunk in adownwards motion into a plastic food container. Thismaterial was stored in a refrigerator at 4°C. Sub-sampleswere plated on malt agar to see if there were anyassociated microorganisms. These were identified by alocal specialist. A Liebermann-Burchard test (Brieskornand Herrig 1959) was performed on the powder to testfor the presence of triterpenoids. Slivers of phelloderm ofboth species were also collected for sectioning andmicroscopic examination.To examine the impact of powder on arthropods, fivefilter paper discs were dipped in bark powder, and theJournal of the Royal Society of Western Australia, 87:81–83, 2004© Royal Society of Western Australia 200482Journal of the Royal Society of Western Australia, 87(2), June 2004surplus was shaken off to leave a film of powder ofsimilar thickness to that on the trunks. These, and fiveclean filter papers, were individually placed in petridishes. A drop of water was added to maintain moisture.Five worker ants (Iridomyrmex chasei) were then placed ineach petri dish and the lids were replaced. Dishes weremaintained in darkness at 20oC, with a drop of wateradded daily, and observed at 12 h intervals to record thenumbers of live and dead animals. This experiment wasconducted at Curtin University using locally-collectedants. However, I. chasei also occurs at Dryandra and isknown to ascend the trunks of various tree species,where it tends sap-sucking insects (Hemiptera).A further experiment was performed in the vicinity ofCurtin University in which 20 cm long, 13 mm diameterwooden dowels were lightly dusted with powder fromthe powderbark wandoo trees and the upper end wascoated with dilute honey bait. Ten powdered and 10powder-free dowels were then ‘planted’ vertically in theground and continuously observed over a 24 h period.Ants that climbed the dowels to reach the honey werescored and collected for subsequent identification. Thisexperiment was performed on 5–6 April 2003 and againon 1–2 May 2003; the second experiment differed fromthe first in that the dowels were placed at an angle of 45ofrom the vertical in order to make them easier to climb.ResultsPowderbark wandoo had a much thinner exfoliatedbark than wandoo (mean ± SD: 0.28 ± 0.20 vs 1.16 ± 0.45mm). Examination of the powderbark wandoo barksurface under a stereo-microscope revealed a coating ofperidermal cells that were sloughing off, either in clumpsor singly. Examination of the powder that had beenbrushed from the trunk indicated that this comprised ofsloughed-off peridermal cells, with powder grainsmeasuring on average 35 µm in length. The cellsappeared to be empty, although some containedstructures that may be fungal hyphae. Plating of the sub-sample of powder revealed the presence of a range ofcommon saprophytic, epiphytic fungi (Scytalidium sp.,Aspergillus sp., Ulocladium sp., Aureobasidium sp.), whosepresence would be expected on decaying organic matter.The chemical analysis of the powder confirmed thepresence of triterpenoids. Wandoo bark was similar inappearance and also exhibited sloughing peridermalcells, which did not accumulate to form a powder.Ant mortality was much greater on the powderedpaper discs than on the control discs (Fig. 1), with allants on the powdered discs having died after 72 h,compared with only 1 death per disc in the controls.Inspection of dead ants indicated that the powder hadadhered to their body surfaces, with the greatestconcentration around the mouthparts and on the tarsi.All ant species that ascended the dowels werecommon species that also occurred at Dryandra. In thefirst baiting experiment, single individuals of I. chasei,Pheidole ampla perthenesis and Paratrechina longicornisascended the clean dowels to reach the bait; althoughseveral ants attempted to climb the powdered dowels,only one ant (I. chasei) ascended a powdered dowel, andthis ant fell off after climbing half the distance. In thesecond experiment, in which dowels were set at a 45ºangle, ants reached the honey bait on seven of the 10clean dowels (mean ± SD: 2.5 ± 2.2 ants), but no antssuccessfully ascended those that had been powdered; allants were P. longicornis.DiscussionOur findings suggest that the powder of powderbarkwandoo trunks may be responsible for the lower usageof trunks by arthropods, when compared with the similarappearing, and co-occurring wandoo tree species.The powder of powderbark wandoo trunks mayinfluence arthropods in a number of ways. Thetriterpenoids or other unknown chemicals in the powdermay have a repellent or toxicity effect against arthropods.Repellency is a possibility that we did not test directly,although it is noteworthy that the powder was odourlessto humans. Our bioassay with I. chasei shows that thepowder is capable of causing direct insect mortality,which could have been caused by the toxic triterpenoidsor other chemicals in the powder. Evidence against thisis the fact that ants attempted to climb the dowels, evenwhen powder was present. Toxic or repellent effects ofchemicals in the powder could be tested in the future byexhaustive solvent extraction of the powder beforeconducting boassays.Secondly, the powder may have blocked the spiraclesof certain arthropods, hence interfering with their normalrespiration. Close inspection revealed that this did nothappen, as the powder grains are at least double thediameter of the spiracles of I. chasei, which are of a sizethat is typical of many bark-associated arthropods.Thirdly, the powder may render the bark difficult towalk on. This is supported by the concentrations ofpowder that had accumulated on the tarsi of ants whichwere exposed to this material, and also the fact that fewor no ants were able to climb the powdered dowels. Asimilar mechanism has been reported in the pitcher plant,Sarracenia purpurea, in which wax inside the pitchersloughs off when insects attempt to climb out, causingthem to fall into the fluid within (Juniper 1986). Waxblooms on the stems of various species of Macaranga havealso been found to prevent ants from walking acrossthem (Federle et al. 1997), and wax layers on the surfaceof juvenile Eucalyptus nitens and E. globulus preventFigure 1. Mean mortality (number of ants per dish ± SD) ofIridomyrmex chasei ants (n = 5 replicates of 5 ants per dish)exposed to discs of filter paper that have, or have not, beencoated with powder from the bark of powderbark wandoo trees.83chrysomelid beetles from adhering to their surface(Edwards 1982).The final possibility is that the powder is simply aphysical nuisance to the normal movement and activityof animals to which it adheres. The accumulation ofpowder around the mouthparts of the I. chasei we studiedsuggests that they tried to remove the powder from theirbodies. Possibly this diverted their attention or energyfor climbing the trunks of E. accedens. Whatever theexplanation, our bioassay with dowels shows that thepowder effectively prevents ants from climbing thedowels, and thus can deter ants and potentially otherarthropods from climbing the trunks.There may, of course, be other reasons why this treespecies accumulates powder on the surface of its bark.Our data in Majer et al. (2002) indicate that theinvertebrates that are apparently deterred from walkingon the powderbark wandoo trunks represent severalfeeding guilds. Importantly, ants and sap-suckinghemipterans are noticeably less diverse on thepowderbark wandoo than the wandoo (43 vs 54, and 9 vs15 species respectively in bark traps) (Majer et al. 2002).The powder does not totally exclude trunk-associatedarthropods but, by reducing the abundance of bark-feeding species on the trunk, and by making it moredifficult for folivores to reach the canopy of the tree(Proctor et al. 2002), the powder on the bark of E. accedensmay reduce the overall levels of herbivory and associatedinjury to the tree.Acknowledgements: This work was funded by a grant from theAustralian Research Council. We thank Geraldine Garrioch for assistingwith the ant baiting experiment, Dr Steve Errington for performing thetriterpenoid analysis and Lydia Kupsky for plating the powder. Dr ElaineDavison identified the microorganisms in the powder, and Prof NealEnright and Prof Byron Lamont and three anonymous refereescommented on an earlier draft of this paper.ReferencesBrieskorn H and Herrig H 1959 The chemical mechanism of theLiebermann-Burchard color reaction in steroids, terpenes,and their esters. Archiv für Pharmacologie 292: 485–496.Edwards PB 1982 Do waxes on juvenile Eucalyptus leavesprovide protection from grazing insects? Australian Journalof Ecology 7: 347–352.Eigenbrode SD 1996 Plant surface waxes and insect behaviour.In: Plant Cuticles – An Integrated Functional Approach (ed GKerstiens) Bios, Oxford, 201–222.Federle W, Maschwitz U, Fiala B, Riederer M and Hölldober B1997 Slippery ant-plants and skilful climbers: selection andprotection of specific ant partners by epicuticular wax bloomsin Macaranga (Euphorbiaceae). Oecologia 112: 217–224.Haberlandt G 1909 Physiologische Pflanzenanatomie. WilhelmEngelmann, Leipzig.Juniper BE 1986 The path to plant carnivory. In: Insects and thePlant Surface (eds BE Juniper, TRE. Southwood) EdwardArnold, London, 195–218.Juniper BE 1995 Waxes on plant surfaces and their interactionswith insects. In: Waxes: Chemistry, Molecular Biology andFunctions (ed RJ Hamilton) Oily, West Ferry, Dundee, 157–174.Majer JD, Recher HF, Heterick BE and Postle AC 2002 Thecanopy, bark, soil and litter invertebrate fauna of the DarlingPlateau and adjacent woodland near Perth, WesternAustralia, with reference to the diversity of forest andwoodland invertebrates. Pacific Conservation Biology 7: 229–239.Majer JD, Recher HF, Graham R and Gupta R 2003 Trunkinvertebrate faunas of Western Australian forests andwoodlands: Influence of tree species and season. AustralEcology 28: 629–641.Proctor HC, Montgomery KM, Rosen KE and Kitching RL 2002Are tree trunks habitats or highways? A comparison oforibatid mite assemblages from hoop-pine bark and litter.Australian Journal of Entomology 41: 294–9.Stork NE 1980 Role of wax blooms in preventing attachment tobrassicas by the mustard beetle, Phaedon cochleariae.Entomologica Experimentalis et Applicata 28: 100–107.Majer, Cocquyt & Recher: Powdery bark in Eucalyptus accedens

Powdery bark in Eucalyptus accedens deters arthropods? An evaluation using ants

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Powdery bark in Eucalyptus accedens deters arthropods? An evaluation using ants

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