Model of termite distribution in Portugal. Follow-up

Transcription

1 IRG/WP THE INTERNATIONAL RESEARCH GROUP ON WOOD PRESERVATION Section 1 Biology Model of termite distribution in Portugal. Follow-up Tânia Nobre and Lina Nunes Timber Structures Division, National Laboratory for Civil Engineering, Av. do Brasil 101, Lisbon, Portugal Paper prepared for the 34th Annual Meeting Brisbane, Australia May 18-23, 2003 IRG Secretariat SE Stockholm Sweden

2 Model of termite distribution in Portugal. Follow-up Nobre, T. & Nunes, L LNEC, Timber Structures Division, Av. Brasil, 101 P Lisboa Abstract: In Portugal, subterranean termites of the genus Reticulitermes are indigenous and a well-established pest of wood in service. Mapping their distribution has therefore become important. A field sampling campaign was established along a North-South transect based on 50x50 km cells (Universal Transverse Mercator grid system, UTM). A variety of typical habitats, ranging from forest sites of Eucalyptus, Pinus, Pseudotsuga and Quercus, to scrubland and low vegetation, was included. The universal presence of the termites underlined the difference between the field distribution of termites and the occurrence predicted by risk modelling. In countries like Portugal, where termites can be found in suitable habitats almost anywhere, the assessment of risk requires more attention to construction type and the immediate surroundings. Key-words: Reticulitermes, distribution, modelling, descriptive variables Introduction The Reticulitermes genus is naturally distributed trough Asia, North America, North Africa and Southern Europe (Eggleton 2000). However, an increase in the demand for wood products for construction may be expected in the face of the growth of human population, which in turn will accelerate the spread of the pest species well beyond their natural distributions. This is already observed in relatively northerly cities such as Paris, Hamburg and Toronto. The growth of urban populations of pest subterranean termite species is now a matter of concern. The importance of termite attack risk assessment in any scheme of monitoring, prevention and control cannot be understated. The determination of potential high-risk areas where termites can establish and multiply themselves is a necessary task to be undertaken in order to prevent further spreading of the urban pest species. An attempt was already made by Lainé (2002) for the U.K. situation. However, different questions are put forward when dealing with countries belonging to the natural distribution area of the pest species. In Portugal, termites are a well-establish pest of wood in service and all known infestations involve termites from the species complex formerly referred to as Reticulitermes lucifugus (Rossi), which is indigenous. The taxonomy of this Genus is now going through many changes (Clément et al. 2000; Marini & Mantovani 2001; Jenkins et al. 2001) and is still in revision. Accordingly, hereafter, the Portuguese inhabiting species will be referred as Reticulitermes spp.. Accurate mapping of termite distribution is now urgent, and a preliminary model of subterranean termite distribution in Portugal has been attempted (Nobre & Nunes 2001; Nobre & Nunes 2002), and aimed to predict the probability of the presence of termites based on ecological variables (Table 1). 1

3 Table 1: Model of termite distribution (Nobre & Nunes 2001; Nobre & Nunes 2002). (DR) Days of Rainfall; (IS) Insolation; (LS) Litosoils; (HP) Human Population. Prob (R. lucifugus presence) = 1 / 1 + e -z ; Z = (HP) (LS) (IS) (DR) The basis for this model were data from past surveys and bibliographic references, and the results presented showed a close agreement between the predicted distribution of infestation sites of termites based on environmental criteria and their reported occurrence. However, the prediction of termites presence/absence was biased towards the way how the absence data were introduced into the model (randomly selected among the cells with absence of records; see Nobre & Nunes (2002) for details). Accordingly, the current knowledge of the distribution and risk associated with the presence of Reticulitermes spp. in Portugal is to be discussed in light of the results obtained trough a field sampling campaign. Methodology A North-South transect was established, spanning the country using the 50x50 km Universal Transverse Mercator grid system (UTM) already established (Nobre & Nunes 2002). Each square was subdivided to allocate one sampling site at each quadrant of 25x25 km. Four different colonies were sampled per cell and a total of forty four colonies in the transect as a whole. In each cell quadrant, a systematic search for termites was conducted for no longer than 30 min. and at a maximum of two locations. If no termites were encountered trough this sampling procedure, the quadrant should then be considered free of termites for modelling proposes. When selecting the sampling transect, efforts were made to maximise the range of the ecological variables previously described by the model as the ones that best explained the distribution encountered at that time (Nobre & Nunes 2002) (Figure 1). The spot sampling sites were intended to cover the most common habitats in Portugal thought to have suitable conditions for termite establishment. The sampling took place at different times of the year, with field campaigns in March, May, September and November

4 Days of rainfall Hours of Sunshine Litosoils Absent Present Figure 1: Chosen transect for termite sampling plotted against average days of rainfall, average hours of sunshine and the presence of litosoils (a type of rocky soil). Images adapted from SNIG National Geographical Information System. Results Subterranean termites belonging to the genus Reticulitermes were found at the first attempt, at every sampling site, with two exceptions where a second attempt had to be made (Figure 2). The field sampling sites ranged across different habitats, including forest stands containing Eucalyptus, Pinus, Pseudotsuga and Quercus, scrubland and low vegetation. As to forest stands, different levels of disturbance were found with samples collected from sites with a high level of human intervention (e.g. recently planted pine forests or recently burned areas) to sites almost untouched in the last 5 to 10 years. Table 2 show the different plant species where termites were collected from. The majority of the subterranean termites were encountered on dead logs although samples were also collected from a live Olea europaea and from a Populus sp. 3

5 Figure 2: Sampling sites along the selected North-South transect Table 2: Plants from where Reticulitermes sp. were sampled Bushes Trees Cytisus sp Eucaliptus sp. Pinus pinaster Pseudotsuga sp. Vitis vinifera Ficus sp. Pinus sp. Quercus rotundifolia Juniperus sp. Populus sp. Quercus suber Olea europaea The average annual temperatures observed within the transect comprised also the minimum and maximum values observed in Portugal, with a spot where this parameter is usually bellow 10ºC, until an area where the average annual temperature ranges from 16ºC to 20ºC. The same could be observed for the average annual air relative humidity, with areas ranging from 65-85% relative humidity. The range of altitude sampled varied from m above sea level. Subterranean termites were always present. The average time between starting to look for them and finding them was recorded to be circa 12 min., though depending much on the type of habitat. In a place where wood was abundant, it took less than 5 min. to find a well populated log. However, in a Quercus suber plantation, a site with almost no dead logs, the most time consuming task was to find a lying wood element and as soon as one was spotted, it was occupied by subterranean termites. 4

6 Discussion The universal presence of the genus underlined the difference between the field distribution of termites and the occurrence in construction, previously predicted by risk modelling (Nobre & Nunes 2002). Reticulitermes spp. seems to be able to establish under the range of environmental conditions found in Portugal, in different types of wood resources and soil types. Therefore, no further attempt was made to update the risk model previously drawn as the variables chosen, in face of the new data obtained, are no longer discriminative and do not describe the field distribution. Under the conditions found in Portugal, the presence of termites in natural environment has no direct relation with the risk of becoming a pest. In urban environments, the climatic conditions no longer play the major role. The man made structures provide the termites with available food and, quite commonly, a warmer environment due to the use of heating systems in winter time. The straight relation of many buildings with gardens allows them a connection to the soil, where they are known to build their nests. However, the nearby presence of a garden is not a pre-requisite for risk of termite infestation as a colony of Reticulitermes was found to be spread over a total area of 2400 m 2 (Verkerk & Bravery 2001) which gives an indication of how far they could be from the central nest. Considering subterranean termites as a building pest, the logistic model has proved to be a good tool to predict a discrete event (presence/ absence; failure/ success) using a set of descriptive variables, requiring fewer assumptions than discriminant analysis (Hosmer & Lemeshow 1989; Tabachnick & Fidell 1983; Kleinbaum 1994). However, both the choice of the descriptive variables and the reliability of the dependent variable data are the keys for drawing a successful model. As already stated, the variables should be scaled down as much as possible. The data are not yet available and efforts are being made to collect a referential data set. This should be accomplished by collecting data on variables specific to the building structure and history as well as from the timber type and conditions. During inspections, data on the location and age of the building, previous interventions made (what and when), wood species, timber moisture content and sanitary conditions should be collected. The required data should not only comprise of situations where a subterranean termite infestation is detected, but also on buildings free of termites, in order to have data on both possible states of the dichotomous dependent variable (presence or absence of termites infestation). These scaled down variables, together with data on the urban area itself (e.g. number of buildings, number of residential houses, and density of human population) and on environmental data should allow for a way to discriminate the variables which best determine the presence of a termite infestation. Understanding the range of termite infestation problems in order to predict (to some extent) the occurrence and severity of potential problematic areas in countries like Portugal, where termites have a wide range, not obviously defined by temperature, humidity, vegetation or soil type, the mapping of risk areas needs to consider variables specific to the construction type and its immediate environment. Acknowledgements The authors acknowledge the financial support of the FCT Fundação para a Ciência e Tecnologia within the project Metodologias para a Mitigação do Risco Associado à Degradação das Construções. 5

7 References Clément, J.-L., Bagnères, A.-G., Uva, P., Wilfert, L., Quintana, A., Reinhard, J. & Dronnet, S. (2000). Biosystematics of Reticulitermes termites in Europe: morphological, chemical and molecular data. Insectes Sociaux 48, Eggleton, P. (2000) Global patterns of termite diversity. Termites: Evolution, Sociality, Symbioses, Ecology (eds T. Abe, D. E. Bignell & M. Higashi), pp Kluwer Academic Publishers. Dordrecht, Netherlands. Hosmer, D. & Lemeshow, S. (1989) Applied logistic regression. John Wiley & Sons. Jenkins, T. M., Dean, R. E., Verkerk, R. & Forschler, B. T. (2001) Phylogenetic Analyses of two mitochondrial genes and one nuclear intron region illuminate european subterranean termite (Isoptera: Rhinotermitidae) gene flow, taxonomy and introduction dynamics. Molecular Phylogenetics and Evolution, 20, Kleinbaum, D. (1994) Logistic regression: a self-learning text. Springer-Verlag. Lainé, L. (2002) Biological studies on two European termite species: establishment risk in the UK. PhD Thesis. Imperial College, University of London. Marini, M. & Mantovani, B. (2001) Molecular Relationships among European Samples of Reticulitermes (Isoptera, Rhinotermitidae). Molcular Phylogenetics and Evolution, 22, Nobre, T. & Nunes, L. (2001) Preliminary assessment of the termite distribution in Portugal. Silva Lusitana, 9, Nobre, T. & Nunes, L. (2002) Subterranean termites in Portugal. Tentative model of distribution. The International Research Group on Wood Preservation Doc. Nº IRG/WP [Stockholm], 8pp. Nunes, L., Nobre, T. & Saporiti, M. (2000). Degradação e Reabilitação de Estruturas de Madeira. Importância da acção de térmitas subterranêas. [Encontro Nacional sobre Conservação e Reabilitação de Estruturas], Lisboa, LNEC. Tabachnick, B. & Fidell, L. (1983) Using Multivariate Statistics. Harper & Row, Publishers. Verkerk, R. & Bravery, A. F. (2001) The UK termite eradication programme: justification and implementation. Sociobiology, 37,