Southern Pine Beetle Populations Research Articles

Variation in RNA interference sensitivity in the southern pine beetle, Dendroctonus frontalis (Coleoptera: Curculionidae)

Abstract Intraspecific genetic variation underpins many evolutionary processes and phenomena. For species considered pests, the dynamics of how genetic variation is distributed geographically take on a greater importance, because they can impact pest-management strategies directly. This is particularly true when management strategies are genetic in nature, such as population manipulation via RNA interference (RNAi; a naturally occurring antiviral response that disrupts translation of messenger RNAs into proteins to silence genes). Using RNAi for gene silencing is highly species specific; however, the efficacy of this approach in geographically distinct populations of widespread pest species has not been evaluated comprehensively. Here, we present a preliminary study evaluating differential susceptibility of geographically disparate populations of southern pine beetle, Dendroctonus frontalis, to RNAi. Although the efficacy of RNAi has been demonstrated in localized populations, D. frontalis has a broad distribution across North and Central America and is expanding its range owing to climate change. We observe differences in response between and within geographically distinct populations from its core range and recently expanded range. Our findings have broad implications for the use of RNAi as a management tool in widely distributed species and highlight the need for a deeper understanding of intraspecific genetic variation in this species and others.

Density‐dependent variability in an eruptive bark beetle and its value in predicting outbreaks

AbstractSeveral species of aggressive bark beetle in the genus Dendroctonus (Coleoptera: Curculionidae) undergo large fluctuations in population density with distinct outbreak and non‐outbreak phases. We investigated attributes we hypothesized as subject to density‐dependent variation (in particular, those likely to express phenotypic plasticity) in the southern pine beetle (SPB), Dendroctonus frontalis, as possible indicators of population fluctuations. These traits were morphology (body size and hindwing shape) and the sex ratio of trap‐captured, dispersing SPB populations. We compared attributes of beetles from locations that ranged from having zero to high numbers (>1500) of SPB infestations (spots) at the county level for two summers. Southern pine beetle were obtained from six states in the southeastern USA and had been collected during a springtime, region‐wide trapping survey used for forecasting outbreaks annually. Although we detected an expected but weak sexual dimorphism in both size and shape‐related traits, no morphological differences were found between SPB collected from traps in counties with low or high densities of spots (≤10 or >10 per county, respectively). We found no relationship between numbers of SPB spots per county and trapped sex ratios in 2016, but we observed a strong trend in 2017, with about three times higher proportions of females trapped in counties with low compared with high numbers of spots. This implies that one or more known or possible factors influencing trapped sex ratios (e.g., disparities between the sexes in their responses to semiochemicals or in their propensity to disperse) can be density‐dependent. Including trap‐captured sex ratios in prediction models may improve current forecasting of SPB outbreaks in the southeastern USA, informing more timely and effective management of one of the most economically and ecologically important beetle species of this region.

Contrasting Competitor and Predator Responses to a Short-Lived Southern Pine Beetle Outbreak: A Case Study

Abstract The southern pine beetle (SPB) Dendroctonus frontalis Zimmermann, is the most important insect pest of pines in the southeastern United States, with outbreaks often resulting in thousands of hectares of pine mortality. Natural enemies and competitors have been cited as significant regulators of SPB populations and, therefore, outbreaks. A recent outbreak on the Homochitto National Forest (NF) in Mississippi provided an opportunity to undertake a case study comparing population fluctuations of SPB, its major predator Thanasimus dubius, and its competitors, Ips bark beetles. Trap catches of all three were tracked through the course of the outbreak on the Homochitto NF as well as in two other forests with low or no SPB activity. The number of predators collected initially increased on the Homochitto NF in response to the SPB outbreak, but their impact on reducing infestation numbers was unclear. Numbers of Ips trapped were similar across all three forests, indicating that other factors were regulating SPB populations. The outbreak only lasted a single year, and its brevity likely limited the availability of host resources for natural enemy and competitor populations. Additional studies are warranted to explore the mechanisms affecting the extent and duration of SPB outbreaks, such as active forest management.

Anatomical and Chemical Responses of Eastern White Pine (Pinus strobus L.) to Blue-Stain (Ophiostoma minus) Inoculation

The increases in temperature have recently allowed the southern pine beetle (Dendroctonus frontalis Zimm.; SPB) and its associated fungi to expand its natural range to northern pine forests. In this study, vigorous eastern white pine mature trees were used to evaluate constitutive and induced response to the southern pine beetle, using O. minus as a proxy. We evaluated histological and chemical changes in P. strobus in response to the fungus at 28- and 65-days post inoculation (dpi). Inoculation with O. minus resulted in an induced defense response as evidenced by the increased production of traumatic resin duct, and lesion development surrounding the site of infection. Starch granules accumulated in the epithelial cells surrounding the resin ducts of inoculated trees. Chemical analyses showed that among phloem phenolics, epi/catechin and three unknown compounds were significantly upregulated at 28 dpi due to fungal inoculation. Several phloem terpenoids (α-pinene, β-myrcene, limonene, terpinolene and β-pinene) were significantly increased in inoculated trees compared to controls at both, 28- and 65-dpi. Continuous production of these terpenoids (up to 65 dpi) can be energetically costly for P. strobus as carbohydrate reserves fund monoterpene synthesis, reducing carbon availability necessary for tree development. Induced phenolics along with monoterpenes production and traumatic resin ducts observed in these trees, suggests that vigorous white pine may sustain endemic populations of southern pine beetle and vectored fungi.

Temperature Extremes, Density Dependence, and Southern Pine Beetle (Coleoptera: Curculionidae) Population Dynamics in East Texas

Previous studies of the southern pine beetle, Dendroctonus frontalis Zimm., established that its population in east Texas responds to a delayed density-dependent process, whereas no clear role of climate has been determined. We tested two biological hypotheses for the influence of extreme temperatures on annual southern pine beetle population growth in the context of four alternative hypotheses for density-dependent population regulation. The significance of climate variables and their interaction with population regulation depended on the model of density dependence. The best model included both direct and delayed density dependence of a cubic rather than linear form. Population growth declined with the number of days exceeding 32 degrees C, temperatures previously reported to reduce brood survival. Density dependence also changed with the number of hot days. Growth was highest in years with average minimum winter temperatures. Severely cold winters may reduce survival, whereas warm winters may reduce the efficiency of spring infestation formation. Whereas most previous studies have incorporated climate as an additive effect on growth, we found that the form of delayed density dependence changed with the number of days >32 degrees C. The interaction between temperature and regulation, a potentially common phenomenon in ecology, may explain why southern pine beetle outbreaks do not occur at perfectly regular intervals. Factors other than climate, such as forest management and direct suppression, may have contributed significantly to the timing, severity, and eventual cessation of outbreaks since the mid-1950s.

TARSONEMID MITE ASSOCIATES OF DENDROCTONUS FRONTALIS (COLEOPTERA: SCOLYTIDAE): IMPLICATIONS FOR THE HISTORICAL BIOGEOGRAPHY OF D. FRONTALIS

AbstractSeven species of mites (Acari: Tarsenomidae) were associated with two local outbreaks of the southern pine beetle, Dendroctonus frontalis Zimmerman, in Chiapas, Mexico; three of these species were new records for Mexico and Central America. The morphology and phoretic behavior of these mites differed little between the western and southern populations from the United States. One major difference was that the hyperphoretic ascospores of the southern pine beetle mycangial fungus, Ceratocystiopsis sp. (Ophiostomataceae), were common in sporothecae of Tarsonemus krantzi Smiley and Moser (Acari: Tarsonemidae) and Tarsonemus ips Lindquist in Chiapas, Mexico, whereas the ascospores of the blue stain fungus, Ophiostoma minus (Hedgcock) H. and P. Sydow (Ophiostomataceae), were rare; this situation in the southern United States is reversed. The paucity of behavioral and morphological differences between the two southern pine beetle populations and the relevant historical climatology suggest that the appearance of D. frontalis in the southern United States may be a recent event.

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.

Potential Impact of the Southern Pine Beetle on Red-Cockaded Woodpecker Colonies in the Georgia Piedmont

Abstract Thirty-four red-cockaded woodpecker colonies were studied in the Georgia Piedmont to (1) determine the stand, site, and tree characteristics associated with active colonies, (2) rate the susceptibility of colony stands to southern pine beetle attack, and (3) estimate the probability of beetle attack (risk) and potential for spot spread (hazard) over a projected 30-year period. The colony stands contain mature and overmature trees of predominately loblolly pine sawtimber on soils and sites favorable for tree growth. The susceptibility of these stands to beetle attack is generally low. Estimated timber losses resulting from the probability of attack and potential spot spread are also low. Predicted number of trees killed is strongly influenced by combinations of risk, hazard, and size of southern pine beetle populations. Mature stands can be managed to reduce southern pine beetle damage and establish forest conditions favorable to the red-cockaded woodpecker. South. J. Appl. For. 12(3): 194-199.

Genetic Structure of Southern Pine Beetle Populations

Abstract The pattern of geographic variation among southern pine beetle (SPB) populations of the enzootic infestations of 1978 was similar to that found among populations in the declining epizootic of 1977. Results for two Texas collections made in 1977 and 1978 indicated that these populations were genetically differentiated from each other. Allele frequencies estimated for the Arkansas and Mississippi populations provided evidence that they belonged to the more genetically uniform group of populations found outside of Texas. Two different types of population structures were found in the infestations studied in Arkansas and North Carolina. Infestations within Arkansas were slightly differentiated genetically but displayed little variation in allele frequencies among single-tree subpopulations within infestations. In the North Carolina populations, allele frequencies did not differ among infestations or among single-tree subpopulations within infestations; however, homozygosity in some single-tree subpopulations was greater than expected for random mating. Evidence for considerable two-locus linkage disequilibria was found in subpopulations of unexpanded infestations studied in North Carolina. Within subpopulations of the expanded infestations however, linkage disequilibria were less extensive. For. Sci. 33(1):52-69.

Technique for Rearing Mite-Free Southern Pine Beetle, Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae), Adults1

Southern pine beetles can be reared free of phoretic mites from naturally infested bark if the bark is removed from the tree and air dried. Bark removal does not reduce the number of beetles that emerge. On the average fewer than 1 % of the beetles emerging from removed bark carried one or fewer mites, and 85% of the beetles emerging from attacked bark carried one or more mites. Seven species of mites were found on beetles emerging from attached bark. About 15 species of mites are commonly transported by the southern pine beetle (SPB), Dendroctonus .fron- talis Zimmermann 1868 (Moser and Roton 197 1, Kinn 19761, with the proportion of phoretized flying adults conservatively estimated at 40% (Moser 1976). The trophic habits of these phoretic mites range from predaceous, mycophagous, nematophagous, or sapro- phagous to various combinations of feeding preferences. Some are detrimental to the SPB, others are neutral or beneficial (Kinn 1967, 1980, Moser, 1975, Wilson 1980). Many numerous and complex relationships exist among these mites and the SPB; hence mites should be excluded from studies to determine the capacity of SPB to attack trees and produce broods in the absence of selected as- sociates. At present there are no efficient methods for obtaining large numbers of SPB free of phoretic mites. Mechan- ically removing mites invariably injures the beetles be- cause the elytra have to be lifted. Rearing beetle adults from eggs on artificial media is tedious and time con- suming; very few adults are produced (Bridges 1979). Kinn (1979) first documented that desiccation of the phloem killed some mite species. Subsequent studies by the authors showed that desiccation might severely re- duce or eliminate all mite populations, resulting in vir- tually no phoretic mites on emerging beetles. In this study we decided to see whether mite-free populations of SPB could be reared from desiccated host material. Materials and Methods Four loblolly pines (Pinus tneda L.) infested with SPB were cut during February, May, August, and De- cember 1980. The trees were from a natural stand about 35 years old in Catahoula Ranger District of the Kis- atchie National Forest. Trees were felled when the SPB broods were pupae in the outer bark. Pupae do not carry phoretic mites (Roton 1978). Samples were taken from the bole area where the pupal stage was most concentrated and where compe- tition from other bark beetle species was least. Typi-

Effects of Subzero Fahrenheit Temperatures on Southern Pine Beetle Populations in the Southern Appalachians

Abstract Several areas throughout the southern Appalachians were sampled to determine the extent of southern pine beetle (Dendroctonus frontalis Zimmerman) brood mortality following a winter with subzero (F°) temperatures. Although 95-percent brood mortality may occur, results suggest that low winter temperatures may not be the sole reason for the decline of a southern pine beetle outbreak.

Area-wide Estimation of Southern Pine Beetle Populations 1

Aerial and ground survey methods are combined with within tree sampling procedures for the purpose of estimating absolute numbers of the southern pine beetle Dendroctonus fron-frontalis Zimm. (Coleoptera: Scolytidae) over a discrete forest stand containing a series of infestation spots. Using the combined survey and sampling technique, a procedure for determining the total area of infested bark within the stand is developed. Data collected over a 4- mo period on 800 ha of timber in southwestern Arkansas which contained a series of rapidly growing southern pine beetle infestations are used to illustrate the estimation procedure.

METHODS FOR AND COMPARISON OF SAMPLING SCHEMES FOR ESTIMATING WITHIN TREE SOUTHERN PINE BEETLE POPULATIONS (COLEOPTERA: SCOLYTIDAE)

AbstractNine sampling units for estimating southern pine beetle population within a tree are compared according to unit size, strata size, and number and type of sampling allocation. As a measure of performance of different sampling schemes or different unit sizes the ratio of the variances of the estimate is used, i.e. relative efficiency.Relative efficiency decreases as unit size increases, where equal surface area is sampled. Unequal stratification, which allocates smaller area to the upper and lower strata, results in higher relative efficiencies than equal stratification. Thus, unequal stratification is recommended. Samples should be allocated to these strata according to the optimal sampling ratios.

Parent Adult Reemergence in Southern Pine Beetle Populations 12

Reemergence of southern pine beetle parent adults was estimated from 34 shortleaf pines during a 14-mo period. The infested portion of the tree was divided into 5 equal sections, and a log containing 1000 cm2 of bark surface was removed from the middle of each section. The logs were held in the laboratory until reemergence was complete, at which time the parent adults were counted and sexed. Measurements were also made of attack density, gallery length, and numbers of reemergence holes. Overall percent reemergence ranged from 0–100, and averaged 65. Attack density and reemergence hole density were linearly related to the total number of beetles reemerging, and both independent variables showed significant predictive value.

The Need for Intensive Forest Management to Reduce Southern Pine Beetle Activity in East Texas

Abstract Since 1955 commercial forest land in east Texas has declined 6 percent while growing stock volume per acre has almost doubled. At the same time, the southern pine beetle population has increased tenfold. It is postulated that increasing numbers of high-density pine stands susceptible to southern pine beetle attack have become available for infestation. This trend will continue for at least 20 to 30 years. At present, the only method available to land managers to reduce the risk of timber loss to the southern pine beetle is to practice intensive forest management.

Optimization of Sampling Effort for Within-tree Populations of Southern Pine Beetle and Its Natural Enemies 1 , 2

As a pre-requisite to full-scale collection of southern pine beetle population data, determination of optimum sample unit size and numbers have been made. Optimum sample unit size was developed as a function of the variables to be measured (i.e., attack density, egg gallery length, total brood, parasite complex and predator complex). A series of permanent 2,000 cm2 X-ray maps, depicting southern pine beetle life stages plus natural enemies occurring at varying heights in trees of different sizes, were made. These map data were stored, as addressable grid cell values in the computer. Programs were written for randomly selecting a series of defined experimental sample unit sizes. From these observations, the relationship of sample number to sample unit size was determined, and a procedure was outlined for estimating density of the bark beetle and its natural enemy populations throughout an infested area.

Biochemical and biological variation of southern pine beetle populations in North and Central America1

AbstractPopulations of the southern pine beetle, Dendroctonus frontalis, from the loblolly pine region of the United States differed in pheromone production and did not interbreed with beetles attacking Pinus leiophylla in central Mexico. Beetles collected from loblolly pine in Virginia readily interbred with beetles from Texas and were essentially identical in the biosynthesis of insect‐produced volatiles. D. frontalis from Pinus oocarpa in Honduras also interbred with southern pine beetles collected in Texas, despite striking quantitative differences in pheromone production. Beetles from Pinus tenuifolia in Guatemala were close to the Mexican beetles morphologically but differed markedly in the chemistry of their pheromones. Differences in pheromone production, host preference and biology as well as the failure of some populations to interbreed indicate that the current taxonomic status of D. frontalis should be reviewed.ZusammenfassungBiochemische und biologische Unterschiede zwischen Dendroctonus‐frontalis‐Populationen in Nord‐ und MittelamerikaKäfer aus dem Verbreitungsgebiet der Loblolly‐Kiefer (Pinus taeda) in den Vereinigten Staaten ließen sich nicht mit Käfern aus der Pinus‐leiophylla‐Region in Mexiko kreuzen. Auch die Pheromonproduktion der beiden Populationen war unterschiedlich. Käfer aus der Pinus‐oocarpa‐Region in Honduras kreuzten sich mit Populationen aus den Vereinigten Staaten; zwischen diesen beiden Populationen bestanden nur quantitative Unterschiede in der Pheromonproduktion. Die gegenwärtig D. frontalis zugerechneten Populationen in Guatemala an Pinus tenuifolia erwiesen sich den mexikanischen Käfern nahestehend; sie wichen jedoch in der Pheromonproduktion erheblich von alien übrigen Populationen ab. Die Käferpopulationen in Mexiko und Guatemala lassen sich untereinander und von den übrigen D. frontalis zugerechneten Populationen nach morphologischen Merkmalen des männlichen Geschlechtsapparates trennen.

Impact of Insecticidal Control on the Southern Pine Beetle Population in East Texas12

Availability of synthetic attractants for Dendroctonus frontalis Zimmerman provided a means to evaluate effects of prescribed insecticidal control measures on the pest and its natural insect enemies. Olfactory and visual stimuli governing the behavior of these insects comprise an integral part of a complex system of colonization. Application of a persistent insecticidal formulation (benzene hexachloride-diesel oil) to suppress the target insect inadvertently becomes more deleterious to its natural enemies. A theory to explain the change in population dynamics of the southern pine beetle in east Texas from a periodically occurring problem to one of chronically high population levels is conceptualized.

Insects Associated with the Southern Pine Beetle in Texas, Louisiana, and Mississippi

Laboratory rearings designed to predict population trends of southern pine beetle, Dendroctonus frontalis Zimmerman, were conducted in Texas, Louisiana, and Mississippi during 1963–65. Insects found associated with the beetle were collected and identified. Forty-two families of insects containing 84 species were found associated with the beetle in the 3 States. Of these, 7 were known predators and 8 were known parasites of the southern pine beetle. A torymid, Roptrocerus eccoptogastri (Ratzeburg), was the most prevalent hymenopterous parasite collected. Cecidostiba dendroctoni Ashmead was identified for the first time as a parasite of the southern pine beetle in Mississippi. Medetera maura Wheeler, a dolichopodid fly, identified from Texas and Louisiana, was one of the most abundant predators of the southern pine beetle. Impact of these associated insects on southern pine beetle population dynamics was not determined. However, the combination of all species of predators, parasites, and competitors probably had an effect in lessening the intensity or causing the decline of beetle outbreaks.