R. W. Sutherst

Global Change and Human Vulnerability to Vector-Borne Diseases

SUMMARY Global change includes climate change and climate variability, land use, water storage and irrigation, human population growth and urbanization, trade and travel, and chemical pollution. Impacts on vector-borne diseases, including malaria, dengue fever, infections by other arboviruses, schistosomiasis, trypanosomiasis, onchocerciasis, and leishmaniasis are reviewed. While climate change is global in nature and poses unknown future risks to humans and natural ecosystems, other local changes are occurring more rapidly on a global scale and are having significant effects on vector-borne diseases. History is invaluable as a pointer to future risks, but direct extrapolation is no longer possible because the climate is changing. Researchers are therefore embracing computer simulation models and global change scenarios to explore the risks. Credible ranking of the extent to which different vector-borne diseases will be affected awaits a rigorous analysis. Adaptation to the changes is threatened by the ongoing loss of drugs and pesticides due to the selection of resistant strains of pathogens and vectors. The vulnerability of communities to the changes in impacts depends on their adaptive capacity, which requires both appropriate technology and responsive public health systems. The availability of resources in turn depends on social stability, economic wealth, and priority allocation of resources to public health.

Potential impact of climate change on plant diseases of economic significance to Australia

Burning of fossil fuel, large scale clearing of forests and other human activities have changed global climate. Atmospheric concentration of radiatively active CO2, methane, nitrous oxide and chlorofluorocarbons has increased to cause global warming. In Australia temperature is projected to rise between 1 and 3°C by 2100. This review is the result of a recent workshop on the potential impact of climate change on plant diseases of economic significance to Australia. It gives an overview of projected changes in Australian climate and the current state of knowledge on the effect of climate change on plant diseases. Based on an assessment of important diseases of wheat and other cereals, sugarcane, deciduous fruits, grapevine, vegetables and forestry species, climate change in Australia may reduce, increase or have no effect on some diseases. Impacts will be felt in altered geographical distribution and crop loss due to changes in the physiology of host-pathogen interaction. Changes will occur in the type, amount and relative importance of pathogens and diseases. Host resistance may be overcome more rapidly due to accelerated pathogen evolution from increased fecundity at high CO2, and/or enhanced UV-B radiation. However, uncertainties about climate change predictions and the paucity of knowledge limit our ability to predict potential impacts on plant diseases. Both experimental and modelling approaches are available for impact assessment research. As the development and implementation of mitigation strategies take a long time, more research is urgently needed and we hope this review will stimulate interest.

A Climate Model of the Red Imported Fire Ant, Solenopsis invicta Buren (Hymenoptera: Formicidae): Implications for Invasion of New Regions, Particularly Oceania

Abstract The paucity of empirical data on processes in species life cycles demands tools to extract insight from field observations. Such insights help inform policy on invasive species and on impacts of climate change at regional and local scales. We used the CLIMEX model to infer the response of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), to temperature and moisture from its range in the United States. We tested hypotheses on the mechanisms that limit the distribution of the ant and estimated the potential global area at risk from invasion. The ant can spread further in the United States, including north along the west coast, where patterns of infestation will differ from those in the east. We analyzed the risk of colonization in Australia and New Zealand, where the ant was recently discovered. The patterns of infestation of the ant in Oceania will differ from those in the eastern United States, with slower growth and less winter mortality. This study adds to earlier temperature-based models by incorporating a moisture response; by replacing arbitrary categories of colony size to predict overwintering success with a site-specific model based on the balance between annual growth and survival; and by comparing different hypotheses on low temperature-related mechanisms that limit the geographical distribution. It shows how the response of a species to climate can be synthesized from field observations to provide useful insights into its population dynamics. Such analyses provide a basis for making decisions on regional management of invasive species and an informative context for local studies.

Potential Geographical Distribution of the Mediterranean Fruit Fly, Ceratitis capitata (Diptera: Tephritidae), with Emphasis on Argentina and Australia

Abstract The CLIMEX model was used to infer the climatic requirements of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), from the fruit fly’s observed geographical distribution in the Mediterranean region. The model indicated that the potential distribution was limited by cold to the north in Europe and by dryness in northern Africa and in the south of Spain and Portugal. The model was then used to estimate the potential geographical distribution of the species in Argentina and Australia. The results agreed with the observed distribution in Argentina and much of the historical distribution in Australia, but they did not agree with the present distribution in eastern Australia. In the latter region, another species of fruit fly, Bactrocera (Dacus) tryoni (Froggatt) has been credited with displacing C. capitata. Seasonal and year-to-year variation in climatic suitability was explored at three selected locations in Argentina. The results indicated that some detrimental effects of summer temperatures, or of extremes of precipitation, occurred in particular areas. Some of these limiting factors, especially dry stress, were prolonged enough to restrict the geographical distribution of medfly. However, when irrigation was included in the simulations, the detrimental effect of dryness was removed. Finally, a global risk map for medfly was produced, which highlights the areas at risk from this major quarantine pest.

Modelling non-equilibrium distributions of invasive species: a tale of two modelling paradigms

Invasive species, biological control and climate change are driving demand for tools to estimate species’ potential ranges in new environments. Flawed results from some tools are being used to inform policy and management in these fields. Independent validation of models is urgently needed so we compare the performance of the ubiquitous, logistic regression and the CLIMEX model in predicting recent range extensions of the livestock tick, Rhipicephalus (Boophilus) microplus, in Africa. Both models have been applied to the tick so new, independent data can be used to test their ability to model non-equilibrium distributions. Logistical regression described the spatial data well but failed to predict the range extensions. CLIMEX correctly predicted the extensions without fitting the non-equilibrium data accurately. Our results question the validity of using descriptive, statistical models to predict changes in species ranges with translocation and climate change. More test cases that include independent validation are needed.

IMPLICATIONS OF GLOBAL CHANGE AND CLIMATE VARIABILITY FOR VECTOR-BORNE DISEASES : GENERIC APPROACHES TO IMPACT ASSESSMENTS

Global change is pervasive and occurring at a dramatic rate. It involves changes in land use, vegetation cover, species translocations and even the climate of the planet. The consequences for the biosphere are uncertain. Past research emphasis has been on the science of climate change as the major driver of policy. The present priority in the global-change community is to define the likely nature and extent of those impacts on biodiversity and the functioning of ecosystems. In addition, increasing consideration is now being given to adaptation measures. The way in which that is being initiated is to develop adaptation measures to respond to medium-term climate variability in the form of altered El Nino and similar cycles, and changes in the frequency of extreme events. Given the large number of stakeholders in agriculture, human health and environment, there is a need for great efficiencies if the scientific community is going to be able to respond in a meaningful way with foreseeable resources. The plethora of problems means that generic approaches are needed. The present situation, with parasitologists each doing their own thing in terms of developing and using software tools, is like the tower of Babel. Parasitologists need common tools and languages to facilitate communication and collaboration. Advances in computing, with object-oriented programming languages and seamless exchange of information between different packages and platforms, are providing some exciting opportunities to overcome these problems.

Climate change and biotic invasions: a case history of a tropical woody vine

The impacts of climate change in the potential distribution and relative abundance of a C3 shrubby vine, Cryptostegia grandiflora, were investigated using the CLIMEX modelling package. Based upon its current naturalised distribution, C. grandiflora appears to occupy only a small fraction of its potential distribution in Australia under current climatic conditions; mostly in apparently sub-optimal habitat. The potential distribution of C. grandiflora is sensitive towards changes in climate and atmospheric chemistry in the expected range of this century, particularly those that result in increased temperature and water use efficiency. Climate change is likely to increase the potential distribution and abundance of the plant, further increasing the area at risk of invasion, and threatening the viability of current control strategies markedly. By identifying areas at risk of invasion, and vulnerabilities of control strategies, this analysis demonstrates the utility of climate models for providing information suitable to help formulate large-scale, long-term strategic plans for controlling biotic invasions. The effects of climate change upon the potential distribution of C. grandiflora are sufficiently great that strategic control plans for biotic invasions should routinely include their consideration. Whilst the effect of climate change upon the efficacy of introduced biological control agents remain unknown, their possible effect in the potential distribution of C. grandiflora will likely depend not only upon their effects on the population dynamics of C. grandiflora, but also on the gradient of climatic suitability adjacent to each segment of the range boundary.

The Vulnerability of the Australian Beef Industry to Impacts of the Cattle Tick (Boophilus microplus) under Climate Change

An integrated assessment is presented of the potential impacts of the cattle tick (Boophilus microplus Canestrini) on the Australian beefindustry under climate change. The project was carried out as a case study to test an impact assessment approach that was designed to integrate biological, production and socio-economic impacts on managed and natural systems. A climate-driven, tick population model was run for European, zebu and crossbred cattle breeds having different levels of resistance to cattle ticks. A geographical information system (GIS) was used to organise spatial data on climate scenarios and industry statistics and to undertake regional analyses.A comparison was made of the two available approaches to conducting impact assessments, namely a bottom-up approach using sensitivity analysis and a top-down approach using climate change scenarios from a global circulation model (GCM) (CSIRO, 1996). The output, in terms of the abundance of tick populations and reductions in cattle productivity for each breed showed significant expansions in potential geographical impacts. In the absence of any adaptation measures, the results indicated changes in the losses in live weight gain of cattle tick ranging from 7780 tonnes per year by 2030 to 21637 tonnes per year by 2100, in comparison with estimates for current losses of 6594 tonnes per year.The principal adaptation options available to the beef industry are to switch to breeds that are more resistant to cattle ticks, or to increase the frequency of treatments with various tick control products. In this paper we focus on switching breeds as an adaptive measure when appropriate damage thresholds are triggered under the climate change scenarios. When adaptation measures were put in place, the losses ranged from 4962 tonnes in 2030 to 5619 tonnes in 2100 compared with 2636 tonnes at present if all producers adopted the optimal breed structure. Optimal breed structure was defined as one that would prevent tick numbers per animal exceeding 100 ticks per animal for European and 700 ticks per animal for crossbred breeds of cattle in any week of the year under a tick control strategy that was suitable for present climatic conditions. The lower threshold for European breeds reflects their vulnerability to explosive increases in numbers because of their low resistance to ticks. The results of the analyses using the GCM scenarios were used in an economic model to calculate costs of lost live-weight gain for 2030, 2070 and 2100. The greatest increases in costs were incurred in the southern parts of the current distribution in Queensland and potentially in northern New South Wales if the present quarantine barrier failed.Given the great uncertainty of the nature of possible regional changes in climate, analyses of the sensitivity of losses in live weight gain to changes in climatic variables were also undertaken. The analyses included a measure of likely impacts of cattle tick on the beef cattle industry, in the absence of adaptation measures, as a baseline measure of sensitivity. The likely impacts on crossbred cattle were insensitive to the climatic variables.When adaptive breed changes were allowed, the economic impacts on the industry were insensitive to the GCM scenarios. This suggests that, at least in this instance, reducing the uncertainties in climate change scenarios is not a priority if the adaptation strategies can be implemented in a cost-effective manner. Finally we made a qualitative assessment of the sustainability and robustness of alternative approaches to adaptation and assessed regional vulnerability to cattle tick under climate change. The conclusions were so strongly dependent on assumptions about the future of other global changes, in particular the ability to maintain quarantine barriers and to retain effective acaricides at comparable costs to the present, that we strongly recommend that risk assessments of climate change extend to all relevant variables in involved in global change where possible.

Modelling the population dynamics of the Queensland fruit fly, Bactrocera (Dacus) tryoni: a cohort-based approach incorporating the effects of weather

Queensland fruit fly, Bactrocera (Dacus) tryoni (QFF) is arguably the most costly horticultural insect pest in Australia. Despite this, no model is available to describe its population dynamics and aid in its management. This paper describes a cohort-based model of the population dynamics of the Queensland fruit fly. The model is primarily driven by weather variables, and so can be used at any location where appropriate meteorological data are available. In the model, the life cycle is divided into a number of discreet stages to allow physiological processes to be defined as accurately as possible. Eggs develop and hatch into larvae, which develop into pupae, which emerge as either teneral females or males. Both females and males can enter reproductive and over-wintering life stages, and there is a trapped male life stage to allow model predictions to be compared with trap catch data. All development rates are temperature-dependent. Daily mortality rates are temperature-dependent, but may also be influenced by moisture, density of larvae in fruit, fruit suitability, and age. Eggs, larvae and pupae all have constant establishment mortalities, causing a defined proportion of individuals to die upon entering that life stage. Transfer from one immature stage to the next is based on physiological age. In the adult life stages, transfer between stages may require additional and/or alternative functions. Maximum fecundity is 1400 eggs per female per day, and maximum daily oviposition rate is 80 eggs/female per day. The actual number of eggs laid by a female on any given day is restricted by temperature, density of larva in fruit, suitability of fruit for oviposition, and female activity. Activity of reproductive females and males, which affects reproduction and trapping, decreases with rainfall. Trapping of reproductive males is determined by activity, temperature and the proportion of males in the active population. Limitations of the model are discussed. Despite these, the model provides a useful agreement with trap catch data, and allows key areas for future research to be identified. These critical gaps in the current state of knowledge exist despite over 50 years of research on this key pest. By explicitly attempting to model the population dynamics of this pest we have clearly identified the research areas that must be addressed before progress can be made in developing the model into an operational tool for the management of Queensland fruit fly

Population dynamics of ticks on Ankole cattle in five ecological zones in burundi and strategies for their control

Abstract Tick populations were observed on Ankole (Sanga type Bos indicus x Bos taurus ) cattle at monthly intervals over periods of 2–3 years in 4 ecological zones (Gatumba (830-m alt., 789-mm rain); Gitega (1671-m alt., 1122-mm rain); Kirundo (1420-m alt., 1076-mm rain) and Gihofi (1260-m alt., 1225-mm rain)) of Burundi, Central Africa. Concurrently, observations were made on the development and survival of Rhipicephalus appendiculatus in pastures at each location. Ticks were also counted on cattle in a fifth, high altitude zone (Ijenda, 2191-m alt., 1549-mm rain) for a period of 2 years. Rainfall was monomodal at all sites and fell between September and May. The species composition and total numbers of ticks infesting cattle in the 5 ecological zones varied. The most common species recorded were R. appendiculatus, Boophilus decoloratus and Amblyomma variegatum . The highest average number of adult females of the above species completing engorgement daily was 29 R. appendiculatus at Kirundo, 13 B. decoloratus at Ijenda and A. variegatum at Gitega. The regions differed slightly in their overall level of tick infestation, as measured by the average numbers of adult female ticks of all species engorging daily, i.e. Kirundo (34), Gatumba (27), Gitega (26), Ijenda (21) and Gihofi (11). Adults of A. variegatum exhibited the strongest seasonal pattern of feeding, with peaks early in the wet season. Non-climatic factors were apparently important in determining the relative numbers of A. variegatum in different regions. Development times of the free-living stages of R., appendiculatus in pastures varied between locations, taking 3 weeks for engorged larvae, 4–7 weeks for nymphs and 6–9 weeks for oviposition and egg development. Engorged larvae and nymphs suffered the least mortality while egg hatching was reduced in the dry season. Fifty percent of unfed larvae survived 1.5–3 months, nymphs 3–5 months and adults 16 months. Parasitic ticks completed engorgement more successfully when attached in preferred sites on the hosts body. Computer simulations were used to estimate the proportional reduction in numbers of R. appendiculatus from dipping strategies with different timing, duration and efficiency. Strategies of 3-months duration, with the start timed to coincide with the period of adult feeding activity, were economically the most efficient. Computer simulations suggested that the annual cattle migration from the highlands to the lowlands in the dry season was important in limiting the size of tick populations. These results were taken into account when designing a policy to make the best use of the countrys resources.