Twelfth Australian Weeds Conference A COMPARISON BETWEEN EUROPEAN AND AUSTRALIAN POPULATIONS OF HOREHOUND, (MARRUBIUM VULGARE L.) John Weiss1, Jean-Louis Sagliocco2 and Emma Wills1 1 Keith Turnbull Research Institute, Victorian Department of Natural Resources and Environment, Cooperative Research Centre for Weed Management Systems, PO Box 48, Frankston, Vic. 3199 2 CSIRO Biological Control Unit, Cooperative Research Centre for Weeds Management Systems, 34980 Montferrier-sur-Lez, France Abstract Horehound (Marrubium vulgare L.: Lamiaceae), a noxious weed throughout southern Aus- tralia, was imported from its native southern Europe during the 1800s. Horehound in its native habitats shows weed potential, but plant populations are scat- tered and their density is lower than Australian infes- tations where the plants are larger and more vigorous. Climate modelling based on overseas populations in- dicate that horehound in Australia maybe close to achieving its maximum potential distribution. The seed production and seed bank studies indicate that there is at least a one hundred fold difference between the populations on the two continents. At Victorian infes- tations, either competition or water appear to be the limiting factors affecting survival and recruitment while sustained herbivory pressure restricts Horehound in its Figure 1. Distribution of horehound in Europe (adapted native range. from Anerberg et al., 1996) The insect fauna of horehound in Europe has been examined and four insects have been identified as suit- able agents for biological control. Two of these have been approved for release into Australia. INTRODUCTION Horehound is a perennial weed native to temperate Eurasia, Europe, the Middle East and the Mediterra- nean region including North Africa (Figure 1). It is also a weed in southern states of north America, in- cluding California and Texas, in South America (Ar- gentina, Chile, Peru, Uruguay etc) and in New Zea- land (Weiss, 1999). It is now naturalised over most of Figure 2. Present distribution of horehound in Aus- the pastoral areas in Australia with rainfall over tralia (from Parson and Cuthbertson, 1992) 200mm. It occurs in all Australian states but is of most importance in Victoria and South Australia where an estimated 26 million hectares (Lane et al., 1980, Carter, Horehound was possibly first introduced into Australia 1990) are infested (Figure 2). from Europe via a shipment of botanical specimens Horehound leaves contain marrubin, a bitter alkaloid, sent by Sir Joseph Banks. A record from Sir Joseph which makes it unpalatable for grazing animals. Hore- Bank’s diary states that M. vulgare was sent to NSW hound burrs are not only of nuisance to people, as they on board the ship Porpoise on 11 October 1798 (Frost, catch in clothing and socks, but they also contaminate 1993). The plant appears to have been introduced wool, reducing the value of fleece. for use as a garden herb, medicinal use and for beer 596 Twelfth Australian Weeds Conference brewing purposes. It was first recorded as naturalised Plant Phenology Table 1 presents a summary of the by 1840s. phenology and demography of two populations of hore- hound in France and two in Victoria. The horehound MATERIALS AND METHODS plants in southern France are much smaller with only Potential distribution of horehound in Australia was about one third the number of stems, and produce about determined using CLIMEX for Windows version 1.1 half the seeds that the Victorian populations do. Most (Sutherst, 1999). The distribution was based on the importantly though, the density of plants in Victoria is provided “Mediterranean template” and the parameters about 20 to 50 times more, 200 to 1000 times as many for temperature, moisture and stress were expanding seedlings occur and 70 to 250 times as much seeds are upon utilising Australian and overseas information in the soil seed bank than southern France. about stress limits and the distribution of horehound. Comparative phenological studies of horehound were Table 1. Demography and seed banks of horehound in carried out at two sites in Europe (Cournonterral and Europe and Australia. La Crau in southern France) and two in Australia (Wyperfeld N.P. and Swifts Creek, in Victoria) during France Victoria, the period 1991-1997. Growth, reproductive capacity, Australia size, and soil seed banks were measured seasonally or Demography Cournt. La Crau Swifts C. Wyper. biennially. Individually marked plants were identified Density of 0.4 0.2 6.9 9.9 on each visit and details recorded. Soil cores were col- 2 lected to measure the soil seed bank. Samples were plants/ m washed free of soil and horehound seeds checked for Mean height 14.6 9.6 36.1 27.1 viability. plant (cm) Mean no. of 6.3 6.8 35.2 18.9 Surveys of the insect fauna of Europe (Portugal, Spain, stems France, Italy and Yugoslavia) and Morocco were car- Mean no. seed 878 174 1,746 498 ried out during 1990-1997. Collected insects were iden- prod. / plant tified by the IIE (British National History Museum, Mean no of seed 139.4 25.6 49.6 26.4 London), CNRS (Toulouse) and MNHM (Paris). prod./per stem Mean no. 0.11 0.32 69.5 196.3 RESULTS seedlings/ m2 Potential distribution Mean soil seed 116.9 61.2 15,290 7,880 bank (seeds/m2) Insect Fauna in Europe Twenty seven species of insects were found on M. vulgare in Europe and Mo- rocco. Figures 1 and 2 show the relevant orders and feeding behaviour of the horehound insect fauna. One other insect, a rare weevil, and a leaf-spotting fungus have been reported in the literature (Sagliocco and Weiss, 1996) but were not found during the surveys. Figure 3. Potential distribution of horehound in Aus- tralia (as predicted using CLIMEX Version 1.1) 597 Twelfth Australian Weeds Conference Horehound Phenology Although Australian plants Root borers Foraging are significantly larger and have more stems, the (4) ants (4) amount of seeds produced per plant is not dramati- 15% cally greater than that in Europe. The number of seeds Stem borers 15% Detritivors produced per stem is roughly equivalent in both (1) (1) populations. However as the density of plants is 20 to 4% 4% Sap suckers 50 times more this greatly increases the seed bank, and recruitment potential of seedlings. The extreme low Leaf feeders (4) 15% levels of seeds in the soil in Europe is most likely due (5) to predation and destruction by the foraging ants spe- 18% cies. Seed feeders Horehound in both Europe and at Swifts Creek, Vic- (8) toria, have consistent populations with very little sea- sonal or yearly fluctuations. In comparison, horehound Figure 4. The different groups of insects recorded feed- in the drier more drought prone Mallee at Wyperfeld, ing on M. vulgare in Europe undergoes dramatic “boom – bust” cycles. Mature Thysanoptera plants die off during severe droughts and the infesta- Heteroptera (1) Embioptera tions rely on the very large seed bank to recolonise (1) 4% (1) when sufficient rainfall occurs. This most likely indi- 4% 4% cates that horehound in these drier areas are surviving Diptera (3) Coleoptera in less than optimum conditions and without the large 11% (11) seed bank reservoirs, the infestations would disappear 40% in time. Control strategies in these dry areas should Hemiptera concentrate on removing the seed bank, by fire for (2) example (Weiss, 1996), or by reducing the seed pro- 7% duction of the plants using biological control agents. Hymenoptera (4) Lepidoptera Seed germination Lippai et al. (1996) reported on 15% (4) the effects of temperature and water potential on the 15% germination of Victorian horehound seeds. They indi- cated that Australian horehound is not a drought toler- Figure 5. The insect Orders recorded to feed on M. ant germinator but more opportunist, germinating vulgare in Europe whenever sufficient rains occur. In contrast horehound seeds from America require much more specific envi- DISCUSSION ronmental factors prior to germination including fluc- tuating temperatures of at least 15°C of stratification. Distribution in Australia and Europe The present and potential distribution maps of horehound in Aus- This contrast is possibly due to environmental differ- tralia (Figures 2 and 3) are very similar indicating that ences the seeds are subjected to in North America and horehound maybe close to reaching its maximum po- Australia, which is substantiated by increased germi- tential distribution. Horehound has the potential of nation following moist chilling of North American increasing in Western Australia and parts of NSW, but seeds, but not of Australian seeds. It would be inter- in South Australia, Victoria and most of NSW, it will esting to see what factors influence the germination of more likely increase the density of present infestations European horehound. Anecdotal evidence indicates rather than move into new localities. that horehound seeds survive in the soil seed bank for between 7 to 10 years (Weiss, 1999). In Europe, horehound generally grows on waste lands, on well drained calcareous soils. The persistence of Potential Biological Control agents In Australia prior permanent plant populations depends on low to 1994 only a couple of arthropods (mites, mealy bugs interspecific plant competition, which is favoured by and one bug) fed on horehound. The native brightly sheep grazing, and a sufficient seed dispersal by ani- coloured orange and black horehound bug, Agonoscelis mals. In western Europe, large horehound populations rutila, is often seen on the weed in great numbers, but are very rare, but are found where sheep or horse graz- does not have any significant impact. ing occurs. 598 Twelfth Australian Weeds Conference In comparison twenty seven species have been found species maybe higher in Europe than in Australia. Hore- to feed on M. vulgare in Europe and Morocco. As ar- hound seeds have a limited ability to germinate in con- eas such as Greece, the Middle East and other coun- ditions where moisture availability, or where tempera- tries of northern Africa also rich in Marrubium genera tures are low.