Phase Of Epidemic Research Articles

Characteristics of an outbreak of West Nile virus encephalomyelitis in a previously uninfected population of horses

Equine West Nile virus (WNV) encephalomyelitis cases – based on clinical signs and ELISA serology test results – reported to Texas disease control authorities during 2002 were analyzed to provide insights into the epidemiology of the disease within a previously disease-free population. The epidemic occurred between June 27 and December 17 (peaking in early October) and 1698 cases were reported. Three distinct epidemic phases were identified, occurring mostly in southeast, northwest and then central Texas. Significant ( P < 0.05) disease clusters were identified in northwest and northern Texas. Most (91.1%) cases had no recent travel history, and most (68.9%) cases had not been vaccinated within the previous 12 months. One-third of cases did not survive, 71.2% of which were euthanatized. The most commonly reported presenting signs included ataxia (69%), abnormal gait (52%), muscle fasciculations (49%), depression (32%) and recumbency (28%). Vaccination status, ataxia, falling down, recumbency and lip droop best explained the risk of not surviving WNV disease. Results suggest that the peak risk period for encephalomyelitis caused by WNV may vary substantially among regions within Texas. Recumbent horses have a poor prognosis for survival. Vaccines, even if not administered sufficiently in advance of WNV infection within a district, may reduce the risk of death by at least 44%.

Landscape analyses of Douglas-fir beetle populations in northern Idaho

Spatial relationships of Douglas-fir beetle (DFB), Dendroctonus pseudotsugae, infestations were examined over a 13-year period on the Priest Lake Ranger District, a USDA Forest Service ranger district in Idaho, USA. In addition, population trends were examined on three other USDA Forest Service ranger districts. Using aerial detection survey maps and GIS to derive nearest neighbor distances, trends in the proximity of DFB infestations within a single year ( t), and between-years ( t + 1) were determined and spatial patterns of infestations were investigated. DFB infestations (i.e., trees killed, area affected) were described with patch metrics. When DFB populations were low, average within-year nearest neighbor distances were typically above 1400 m. As DFB populations increased, average nearest neighbor distances dropped below 500 m. Simulation-based analyses of the spatial pattern of DFB infestations suggested a clustered distribution of infestations when populations were high. Between-year average nearest neighbor distances followed a similar pattern, with half of the infestations close to one another especially during epidemic population phases. During periods with endemic populations, infestations were found at various distances (0–5000 + m) from other infestations. Average patch size of infestations on Priest Lake Ranger District varied from 0.8 to 19.7 ha and was positive exponentially related to number of infestations. Average number of trees killed per infestation ranged from 4.7 to 124.3 and was also positive exponentially related to number of infestations. As numbers of infestations increased, infestations formed larger patches with an increasing number of trees killed.

The role of spatial mixing in the spread of foot-and-mouth disease

A model of epidemic dispersal (based on the assumption that susceptible cattle were homogeneously mixed over space, or non-spatial model) was compared to a partially spatially explicit and discrete model (the spatial model), which was composed of differential equations and used geo-coded data (Euclidean distances between county centroids). While the spatial model accounted for intra- and inter-county epidemic spread, the non-spatial model did not assess regional differences. A geo-coded dataset that resembled conditions favouring homogeneous mixing assumptions (based on the 2001 Uruguayan foot-and-mouth disease epidemic), was used for testing. Significant differences between models were observed in the average transmission rate between farms, both before and after a control policy (animal movement ban) was imposed. They also differed in terms of daily number of infected farms: the non-spatial model revealed a single epidemic peak (at, approximately, 25 epidemic days); while the spatial model revealed two epidemic peaks (at, approximately, 12 and 28 days, respectively). While the spatial model fitted well with the observed cumulative number of infected farms, the non-spatial model did not ( P < 0.01 ). In addition, the spatial model: (a) indicated an early intra-county reproductive number R of ∼ 87 (falling to < 1 within 25 days), and an inter-county R < 1 ; (b) predicted that, if animal movement restrictions had begun 3 days before/after the estimated initiation of such policy, cases would have decreased/increased by 23 or 26%, respectively. Spatial factors (such as inter-farm distance and coverage of vaccination campaigns, absent in non-spatial models) may explain why partially explicit spatial models describe epidemic spread more accurately than non-spatial models even at early epidemic phases. Integration of geo-coded data into mathematical models is recommended.

RESISTANCE TO BIOTIC STRESS IN FRUIT TREES

Resistance to pests and diseases is a highly desirable objective in fruit breeding but is a long-term endeavor. It requires appropriate tests for early screening and an analysis of host-parasite interactions for durability of resistance. Host resistance is considered durable when it remains effective for a long period despite an intensive exposure to the parasite. Durability has to be the key-point of any fruit breeding programme for resistance. Non-durable resistance is usually associated with a hypersensitive reaction (HR) and major gene resistance which often occur in multiple allelic series and/or complex loci. HR resistance or race-specific resistance is often rendered ineffective because the pathogen population shifts genetically to avoid the resistance gene. Resistance of a more durable nature against fungi and bacteria is often polygenic and based upon the additive or interaction effects of a few to several genes not promoting a hypersensitive reaction. The more complex the genetics of the resistance, the more difficult it is for the pathogen to evolve to a form that can overcome the resistance. Polygenic resistance is usually associated with a quantitative expression that is characterized by a continuous variation between susceptibility and resistance. To approach the number of genes involved in polygenic resistance (QTL) and the relative effect of each of these genes, they have to be mapped on the genome. Marker-assisted selection (MAS) gives the possibility to recombine QTL for resistance; major genes could be associated with QTL. The combining of resistance genes is desired to: acquire a higher level of resistance against a particular pathotype, obtain resistance against a broader range of pathotypes, and combine different mechanisms of resistance. The current situation of fruit crops - monocultivar orchards established for several years - enhances the selection pressure on the pathogen population and results in population shifts which could be dramatic. Guidelines have to be proposed to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure; the most important parameters to consider are reproductive/mating systems and gene/genotype flow. Pathogens that have the greatest risk of breaking down resistance genes are those that possess a mixed reproduction system, with one sexual cycle per growing season and asexual reproduction during the epidemic phase, and a high potential for gene flow. Several examples of different strategies used by fruit breeders will be reported mainly on Coffea (coffee rust), Musa (black leaf streak disease), Prunus (root knot nematodes) and Malus (scab, mildew, fire blight). Fruit breeders can use the lessons learned from deploying resistance genes in annual crops and selecting resistance in forest trees and apply these to their strategies for fruit tree improvement

Detecting Rice stripe virus (RSV) in the small brown planthopper ( Laodelphaxstriatellus) with high specificity by RT-PCR

Rice stripe disease, caused by Rice stripe virus (RSV), may lead to severe or even crippling losses in many rice-cultured countries and regions. As the most important vector of RSV, the small brown planthopper (SBPH) ( Laodelphax striatellus) is largely responsible for the epidemic phase of the disease. Therefore, a rapid identification of RSV in the SBPH is of a great need for disease forecasting. A reverse transcription polymerase chain reaction (RT-PCR) assay is described to amplify a RSV gene in individual L. striatellus. By using primers matched to the viral RNA dependent RNA polymerase gene in RNA1, a 445 bp product was detected in viruliferous SBPHs. Meanwhile, the PCR products produced by the SBPH actin primers constructed across the boundary of an intron and an exon were used as RNA specific positive control for each stage of the experiment to ensure the validity of the negative results. Duplex RT-PCR conditions were established for the simultaneous detection of RSV and actin. This approach can be used for the early detection of RSV in L. striatellus and the subsequent rice stripe disease forecasting.

HIV in IDUs: should we bother?

HIV and drug use continue to pose challenges in tandem*/like a bicycle with seats for two, one riding behind the other*/to the practitioners of public health as well as intervention researchers worldwide. For example, while cities in Australia have recorded a low HIV prevalence in injection drug users (IDUs) (Wodak and Lurie, 1997) and New York has reported declines in HIV incidence among IDUs (Des Jarlais et al., 2000), Kathmandu*/the capital city of Nepal*/has experienced initial containment followed by explosive spread of HIV among IDUs in the last 6 years (Pokharel et al., 2000). These challenges are made tougher by a few hurdles, such as the assumption that the HIV epidemic in heterosexuals is independent from that in IDUs and the intermittent switch from non-injection forms of drug use to injecting under various socio-political influences. Many national HIV/AIDS Control Programmes in developing countries also add to these challenges by having most of their resources spent on breaking the chain of transmission in the population groups who are vulnerable to getting HIV through sex and by having abysmally low support for interventions targeting injection drug users. The mainstay of this approach is formed by the belief that HIV epidemics in IDUs do not contribute significantly to the overall HIV epidemic. Evidence, however, suggests the contrary. Lowndes et al. (2003) identify an interplay of forces which contribute towards conditions favourable for the heterosexual spread of HIV in the Russian Federation, which since 1996 has witnessed a series of large and explosive HIV epidemics among IDUs in different cities. Such a rapid spread of HIV among IDUs reminds us of the HIV epidemic among IDUs in Manipur, a northeastern state of India sharing a border with Myanmar. This response looks back to the past when the epidemic happened in Manipur, as well as taking account of current research evidence to examine critically whether one really needs to bother about HIV epidemics in injectors. Twelve years ago, in the winter of 1989, an outbreak of HIV occurred among a large number of injection heroin users of Manipur (Sarkar et al., 1991). The virus not only infected more than 50% of the local IDUs within 9 months, but it spread into other population groups as well. Some 45% of the non-injecting wives of IDUs (Panda et al., 2000) and 1.2% of the antenatal clinic-attending women (representing female general population) in the state (NACO 1997 /1998) contracted the virus within the next 5 years. A small study from Manipur, conducted in 1997, recruited female sex workers operating from the streets of Imphal, the capital city (there is no brothel-based sex industry in Manipur) and revealed that while 20% of those who had never injected drugs were HIV positive, 57% of female injection drug users involved in sex work had HIV (Panda et al., 2001). About one-third of the female IDUs involved in sex work in this study had male IDUs as their regular sex partners. Lowndes et al. (2003) document a similar progression of HIV among different population groups in the Russian Federation, highlighting different links between IDU and non-IDU populations that may be responsible for generating a wider spread of HIV in countries where epidemic outbreaks first emerged among IDUs. While it took some time for policymakers to see how different pieces of information relating to the progression of HIV in Manipur were parts in one jigsaw puzzle, an early attempt to do so in Russia and elsewhere would certainly pay dividends for HIV prevention. Otherwise, a familiar story of the diffusion of injection drug use in a new area followed by rapid HIV spread among IDUs and subsequent spread to non-IDU population groups may repeat itself. The evidence generated through the randomised controlled trials in African countries on the role of improved STD management in reducing HIV incidence in the early phase of HIV epidemics (Grosskurth et al., 2000) emphasises how STIs could play a * Tel.: /91-332-5910-334; fax: /91-332-3376-290. E-mail address: samiran_panda@rediffmail.com (S. Panda). International Journal of Drug Policy 14 (2003) 83 /85

Paleodemography and Historical Demography in the Context of an Epidemic: Plague in Provence in the Eighteenth Century

This article sheds an entirely new light on the study of the plague thanks to contributions from disciplines that are only seldom brought together around the same research topic: anthropology, archaeology, historical demography, history, microbiology and paleodemography. It confronts two types of documents: biological archives (the skeletons) and historical archives, the comparative study of which has provided new and original information.Distribution of the casualties by sex, age and intensity of the epidemic phase has been established and compared with previous results. The age distribution of deaths from the plague differs from “natural” mortality profiles and from deaths resulting from other epidemics or other demographic crises. The plague epidemic may be characterized as “non selective”, given that the bio- or paleodemographic sample may be considered as a reflection of the structure of the living population, something which is rarely observed.The results that we present concern a recent period for which the historical archives are of exceptional richness and provide a great deal of information on modern plague epidemics. The results from anthropological fieldwork and microbiology enable us to envisage similar research on more ancient epidemics, even in the absence of written documents.

Sexual network structure as an indicator of epidemic phase.

Ascertaining epidemic phase for a sexually transmitted disease (STD) has depended on secular trend data which often contain significant artefacts. The usefulness of sexual network structure as an indicator of STD epidemic phase is explored in an analysis of community wide genital chlamydia reports, with network analysis of interviewed cases and linked sexual partners, in Colorado Springs, USA, 1996 to 1999. In this period, the chlamydia case rate per 100,000 increased by 46%. Three quarters of cases (n=4953) were interviewed, nominating 7365 partners; these, combined with index cases, made up the 9114 persons in the network. Epidemiologic analysis of cases suggests that secular trend increases are artefactual. Network analysis supports this view: overall network structure is fragmented and dendritic, notably lacking the cyclic (closed loops) structures associated with network cohesion and thus with efficient STD transmission. Comparison of network structure with that of an intense STD outbreak (characterised by numerous cyclic structures) suggests low level or declining endemic rather than epidemic chlamydia transmission during the study interval. These observations accord with intuitive and stochastic predictions.

The population genetics of plant pathogens and breeding strategies for durable resistance

The durability of disease resistance is affected by the evolutionary potential of the pathogen population. Pathogens with a high evolutionary potential are more likely to overcome genetic resistance than pathogens with a low evolutionary potential. We will propose a set of guidelines to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure. Under our model of pathogen evolution, the two most important parameters to consider are reproduction/mating system and gene/genotype flow. Pathogens that pose the greatest risk of breaking down resistance genes are those that possess a mixed reproduction system, with at least one sexual cycle per growing season and asexual reproduction during the epidemic phase, and a high potential for gene flow. The lowest risk pathogens are those with strict asexual reproduction and low potential for gene flow. We will present examples of high- and low-risk pathogens. Knowledge of the population genetic structure of the pathogen may offer insight into the best breeding strategy for durable resistance. We will present broad guidelines suggesting a rational method for breeding durable resistance according to the population genetics of the pathogen.

Paleodemography and Historical Demography in the Context of an Epidemic

This article sheds an entirely new light on the study of the plague thanks to contributions from disciplines that are only seldom brought together around the same research topic: anthropology, archaeology, historical demography, history, microbiology and paleodemography. It confronts two types of documents: biological archives (the skeletons) and historical archives, the comparative study of which has provided new and original information.Distribution of the casualties by sex, age and intensity of the epidemic phase has been established and compared with previous results. The age distribution of deaths from the plague differs from &#8220;natural&#8221; mortality profiles and from deaths resulting from other epidemics or other demographic crises. The plague epidemic may be characterized as &#8220;non selective&#8221;, given that the bio- or paleodemographic sample may be considered as a reflection of the structure of the living population, something which is rarely observed.The results that we present concern a recent period for which the historical archives are of exceptional richness and provide a great deal of information on modern plague epidemics. The results from anthropological fieldwork and microbiology enable us to envisage similar research on more ancient epidemics, even in the absence of written documents.

Statistical and dynamical study of disease propagation in a small world network.

Statistical properties and dynamical disease propagation have been studied numerically using a percolation model in a one dimensional small world network. The parameters chosen correspond to a realistic network of school age children. It has been found that percolation threshold decreases as a power law as the shortcut fluctuations increase. It has also been found that the number of infected sites grows exponentially with time and its rate depends logarithmically on the density of susceptibles. This behavior provides an interesting way to estimate the serology for a given population from the measurement of the disease growing rate during an epidemic phase. The case in which the infection probability of nearest neighbors is different from that of short cuts has also been examined. A double diffusion behavior with a slower diffusion between the characteristic times has been found.

Balsam fir (Abies balsamea) establishment dynamics during a spruce budworm (Choristoneura fumiferana) outbreak: an evaluation of the impact of aging techniques

The effects of recurrent spruce budworm (Choristoneura fumiferana (Clem.)) outbreaks on balsam fir (Abies balsamea (L.) Mill.) mortality have been extensively studied in Canada. Most studies report substantial seedling recruitment of balsam fir during outbreaks when reproductive trees are dying. According to previous research, this contradiction could be due to inaccuracies in the conventional aging method. Counting the maximum number of growth rings found at the tree base would significantly underestimate tree age. Counting terminal bud scars found on the entire trunk, including buried stem, would give a more accurate tree age. In this study, we compare recruitment dynamics obtained for aging seedlings in two balsam fir populations (about 500 km apart) using (i) the conventional method and (ii) bud scar counts. For both populations, the conventional method shows substantial recruitment during adult mortality, while the second aging technique reveals reduced recruitment during the epidemic phase of the spruce budworm outbreak.

Analysis of the spatiotemporal structure of rice sheath blight epidemics in a farmer's field

The spatiotemporal structures of two rice sheath blight epidemics were studied during two consecutive (rainy and dry) rice cropping seasons in a farmer's field in the Philippines. The amount of primary inoculum in the soil measured at transplanting was higher during the dry than during the rainy season, and showed nonaggregated spatial patterns in both seasons. Disease incidence (percentage tillers infected per hill) was monitored at weekly intervals on a (14 × 17) grid of rice hills 1 m apart, at the centre of adjacent, nonoverlapping elementary quadrats of 5 × 5 hills. The rate of disease increase at the onset of the epidemics was higher in the dry than in the rainy season, but was afterwards higher in the rainy than in the dry season. A C(α) test was used to test for overdispersion, and the beta‐binomial parameter θ was used to assess disease aggregation in quadrats consisting of one, two, four, eight and 16 elementary quadrats. Aggregation was detectable at all quadrat sizes throughout the epidemic, according to the C(α) test. θ was highest at epidemic onset and at the smallest (elementary) quadrat size, indicating a very strong aggregation of disease at the individual hill scale. θ increased until disease incidence reached 40%, then declined over time. θ values were generally smaller in the dry season. Semivariograms for both epidemics showed strong initial discontinuities, that is, large variance of disease incidence at the elementary quadrat scale. Some aggregation in the spatial distribution of disease incidence was indicated at higher scales in the rainy season, coinciding with the highest rate of disease increase. Semivariograms did not suggest any structure in the dry season, except for one date in the early epidemic stage. Spatiotemporal autocorrelation of disease incidence at 45 days after transplanting yielded very few variables that contributed significantly to describing the disease incidence and its increase in a given hill. In the rainy season these variables were disease incidence (or increase) in the nearest sampled neighbours, and disease incidence (or increase) observed 1 week earlier in the same hill. In the dry season only disease incidence (or increase) observed 1 week earlier in the same hill was significantly correlated with the current incidence (or its increase). The development of a spatial structure in distribution of disease is hypothesized to coincide with the spread of the pathogen from one hill to another, and with the shift from monocyclic to predominantly polycyclic processes in sheath blight epidemics. These results concur with previous reports in suggesting that management of rice sheath blight should first be directed against the leafborne, polycyclic phase of epidemics, rather than their initial phase.

Introduction, persistence and fade-out of porcine reproductive and respiratory syndrome virus in a Dutch breeding herd: a mathematical analysis.

The objective of this study was to investigate the dynamics of PRRSV infection and to quantify transmission within a breeding herd, and its impact on herd performance. For this purpose a longitudinal study was performed in a closed breeding herd of 115 sows. Statistical methods and Monte Carlo simulations based on stochastic SIR models were used to analyse the observational data. Moreover, a case-control study was performed to determine whether seroconversion of sows during gestation was associated with aberrant litters. The transmission parameter R was estimated to be 3.0 (95% confidence interval 1.5-6.0) for the model version based on the most plausible assumptions that the infectious period lasts 56 days and no lifelong immunity exists after infection. Based on simulations using a breeding herd of equal size the average time-to-extinction was estimated to be 6 years; using a herd of twice the size, it was 80 years. Furthermore, in contrast to the epidemic phase of the disease, the endemic phase was not detrimental to herd performance.

Temporal duration and event size distribution at the epidemic threshold.

The epidemic event, seen as a nonequilibrium dynamic process, is studied through a simple stochastic system reminiscent of the classical SIR model. The system is described in terms of global and local variables and was mainly treated by means of Monte Carlo simulation; square lattices N×N, with N=23, 51, 100, 151, and 211 were used. Distinct extensive runs were performed and then classified as corresponding to epidemic or non-epidemic phase. They were examined with detail through the analysis of the event duration and event size; illustrations, such as density-like plots in the space of the model's parameters, are provided. The epidemic/non-epidemic phase presents smaller/larger relative fluctuations, whereas closer to the threshold the uncertainty reaches its highest values. Far enough from the threshold, the distribution φ(t) of the events time duration t shows a step-like appearance. However at the threshold line it shows an exponential behavior of the form φ (t) ∼ exp (-ωt); the same behavior is observed for the event size distribution. These results help to explain why the approach to epidemic threshold would be hard to anticipate with standard census data.

Epidemic phase and the site percolation with distant-neighbor interactions

A generalized site percolation model is used to construct an analogy with the epidemic problem, involving spatial coordinates. Epidemic phase and concepts like herd immunity are analyzed in terms of connectivity in a 2D square lattice. The epidemic model used in this work considers a specific interaction topology that includes up to the fifth-nearest neighbors. The results, obtained by Monte Carlo simulation, emphasize the meaning of the spatial coordinates and are illustrated by an epidemic/non-epidemic phase diagram.

Epidemiology of porcine reproductive and respiratory syndrome (PRRS): An overview

PRRS disease was first recognised in the USA in 1987 and in Europe in 1990 and since then the disease has spread widely throughout many pig-producing countries. After a severe epidemic phase, the infection has become endemic. The prevalence of infection is generally high in infected countries. However, in areas with a low density of pigs, infection may spread slowly and if infected animal movements are not significant, farm-to-farm sperad can be controlled and prevalence of infection maintained at a low level. The PRRS virus (PRRSV) was completely unknown before 1986, and the question of its origin remains unanswered. The exact epidemiologic relationship between American and European strains of PRRSV is difficult to establish because different isolates appear to belong to distinct sub-populations which are only distantly antigenically related. In the environment, virus survival is optimal when temperature is cold and when ultra-violet light exposure is low (little sunshine). These conditions are easily attained in winter and that may explain why virus spread increases during this period. Pigs of any age (including wild boars) are the only animals known to be naturally infected with PRRSV. Relatively close contact between pigs is the primary factor in virus transmission. Aerial transmission is a second mechanism of spread, particularly in winter and particularly over distances of less than 3 km. A third route of transmission is via semen. The role of fomites is not clearly documented, however since the virus is excreted in faeces and urine, slurry should be considered as a potential source of contamination. Within herds, the virus spreads rapidly with up to 85 to 95% of pigs in a herd becoming sero-positive within two to three months. Thereafter, virus activity persists for extended periods (several month to years). Nevertheless, some authors have reported spontaneous elimination of PRRSV from infected farms. For the future, there remain questions concerning the possible evolution of the disease (in terms of its sanitary and economic impacts), and the possible influence of vaccines on the epidemiological features of PRRS.

A multidimensional analysis of scientific dynamics. Part I. Case studies of mathematical logic in the 20th century

Sequences of empirical Lotka-like distributions of the publications of scientific areas are mapped into a multidimensional parameter space. On this basis a new definition of the notion of an epidemic phase of a discipline is introduced. A graphic representation of the parameter space along with results of an exponential regression analysis of the Lotka exponent yield an image of the inner state of a discipline and renders possible a prognosis. Examples, primarily from mathematical logic, are described in detail. The notion of a scientific elite is discussed and the hypotheses of Ortega, Merton, and Price are critically assessed.

Colonization trials with Cryptochetum iceryae and Rodolia iceryae for improved biological control of Icerya purchasi in Israel

Colonization of additional natural enemies of Icerya purchasi Maskell (Homoptera: Margarodidae) in Israel was considered because of the frequent outbreaks of the scale during the 1980s and the chronic severe injuries to certain ornamental plants whose toxic effects through the scale reduce the effectiveness of Rodolia cardinalis Mulsant (Coleoptera: Coccinellidae). Colonization trials were conducted with the parasitic fly Cryptochetum iceryae Williston (Diptera: Cryptochetidae) collected in southern California and with Rodolia iceryae Jenson sent from South Africa. Rearing experiments of both natural enemies were conducted using I. purchasi and I. aegyptiaca (Douglas) infesting different plant species. C. iceryae developed on I. purchasi growing on 17 host plant species. I. aegyptiaca was not affected by the fly. R. iceryae could not complete its development on either I. purchasi or I. aegyptiaca. C. iceryae was released at nine sites planted with Erythrina corallodendrum, Spartium junceum, or Retama raetam infested with I. purchasi. In citrus groves, after 2 years and following several releases, it is still uncertain whether C. iceryae has become established. C. iceryae was acclimatized at all sites planted with E. corallodendrum, S. junceum, and R. raetam and suppression of the scale was achieved in 2 years, probably because of the lack of significant competition with Rodolia cardinalis. I. purchasi populations were very sparse in the sampled citrus groves. Only in one site did we observe a rapid switch from the latent epidemic phase. This outbreak resulted in serious damage to the trees and was followed by a rapid breakdown of the scale population due to a sharp increase in the R. cardinalis population.

Endemic and Epidemic Populations of Southern Pine Beetle: Implications of the Two-Phase Model for Forest Managers

Abstract Typical of many Dendroctonus species, the southern pine beetle (D. frontalis Zimmerman) exhibits dramatic fluctuations in population levels. Using field observations of beetle damage, this study empirically demonstrates that a two-phase model described for mountain pine beetle (D. ponderosae Hopkins) is appropriate for southern pine beetle at each of five hierarchical levels: the tree, the local infestation, the county, the state, and the region. The model has three points that represent a stable low-level (endemic) phase maintained by host-tree-defensive capabilities, a transient high-level (epidemic) phase determined by the availability of host material, and a threshold between the two phases. The threshold depends on local environmental and biotic factors and is a rarely observed phenomenon. Implications of these results for southern pine beetle management and control are discussed. The necessity for establishing and maintaining control programs during the endemic phase is stressed. For. Sci. 35(4): 1075-1087.