Phloem Temperatures Research Articles

A mountain pine beetle (Coleoptera: Curculionidae) adult development rate model confirms evolved geographic differences.

Insects live in a wide range of thermal environments and have evolved species- and location-specific physiological processes for survival in hot and cold extremes. Thermally driven dormancy strategies, development rates and thresholds are important for synchronizing cohorts within a population and to local climates and often vary among populations within a species. Mountain pine beetle (MPB), Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae, Scolytinae),is a widely distributed forest insect native to North America with clinal genetic differentiation in thermally dependent traits. MPB development occurs in Pinus phloem beneath the bark, and its cryptic habitat makes experimentation difficult, particularly for the adult stage. We describe a novel method for modeling MPB adult development following pupation and terminating in emergence from a brood tree. We focus on an Arizona (southern) MPB population with previously described preadult development rates. Field-observed tree attack, adult emergence, and phloem temperature data are combined in a parameterized cohort model and candidate rate curves are evaluated to describe adult emergence timing. Model competition indicates that the Brière rate curve provided the best fit to field data and performed well under cross-validation. Results confirm that the development of Arizona MPB adults is slower than the previously described development rate of more northern Utah adults. Using the estimated adult rate curve in a scenario of increasing mean temperatures, we show that the timing of second-generation adult emergence in the same year would result in cold-intolerant lifestages during winter, limiting the success of bivoltine populations.

Oviposition Model for a Southern Population of Mountain Pine Beetle.

We develop a predictive oviposition model for a southern population of mountain pine beetle (MPB) using a previously developed rate curve, incorporating variation in both oviposition rate and fecundity. We also introduce a method for determining the time delay before oviposition. The model describes the probability of oviposition for a season of MPB attacks using hourly phloem temperature and adult MPB attack data. We also develop an asymptotic approximation of MPB oviposition that is much less computationally taxing. The detailed oviposition model and its asymptotic approximation are compared with other ovipositional models for MPB; the predictive capacity of each model is evaluated using previously published observations.

Cold Tolerance of Pityophthorus juglandis (Coleoptera: Scolytidae) From Northern California.

Winter survivorship of insects is determined by a combination of physiological, behavioral, and microhabitat characteristics. We characterized the cold tolerance of the walnut twig beetle, Pityophthorus juglandis Blackman, a domestic alien invasive bark beetle that vectors a phytopathogenic fungus. The beetle and fungus cause thousand cankers disease in species of Juglans and Pterocarya. The disease is spreading in the United States of America (USA) and Italy. Contact thermocouple thermometry was used to measure the supercooling points of adults and larvae and lower lethal temperatures of adults from a population from northern California. Supercooling points ranged from -12.2 °C to - 25.0 °C for adults and -13.6 °C to - 23.5 °C for larvae; lower lethal temperatures of adults ranged from -14 °C to - 23 °C. We found seasonal changes in adult supercooling points in fall, winter, and spring. The supercooling point for males was 0.5 °C colder than for females over all months and 1 °C colder in the winter than in other seasons. The cold-tolerance strategy shifted in P. juglandis adults from freeze intolerance (December 2013 and January 2014) to partial freeze tolerance (February 2014). An intermediate level of cold tolerance with a plastic response to cold partially explains survival of P. juglandis outside of its native range in the southwestern USA. In addition, we characterized the relationship between minimum air temperatures and minimum phloem temperatures in two Juglans spp. in northern California and Colorado and characterized portions of the native geographic range of eastern black walnut, J. nigra L., that may be too cold currently for this insect to persist.

Metabolism and cold tolerance of overwintering adult mountain pine beetles (Dendroctonus ponderosae): Evidence of facultative diapause?

We sought evidence for a distinct diapause in adult overwintering mountain pine beetles (Dendroctonus ponderosae Hopkins) by measuring metabolic rate and supercooling ability of field collected beetles throughout the year. Metabolic rates measured at 0, 5, and10°C declined significantly from October through November, then rose slowly, reaching levels as high as those recorded in October by late May. From December to February metabolic rates were not correlated with minimum weekly phloem temperatures (R2=0.0%, P=0.592), but were correlated with phloem temperatures as winter advanced to spring (R2=44.8%, P=0.010), a pattern consistent with progression through the maintenance and termination phases of diapause. Supercooling points were also significantly lower in winter compared to fall and spring (F(8,143)=32.6, P<0.001) and were closely correlated with metabolic rates (R2>79% for all three temperatures). Dry mass declined linearly with winter progression (F(8,150)=8.34, P<0.001), explained by catabolism of metabolic reserves, with a concomitant accumulation of metabolic water (F(8,147)=35.24, P<0.001). The strong mid-winter metabolic suppression correlated with improved supercooling ability, coupled with their lack of response to variation in environmental temperature, are evidence of possible diapause in adult overwintering mountain pine beetles.

Breeding performance of the second generation in some bivoltine populations of Ips typographus (Coleoptera Curculionidae) in the south-eastern Alps

The spruce bark beetle, Ips typographus, is one of the most extensively studied European forest pests. Gaps exist in the knowledge about second generation breeding performance in bivoltine populations. In this study, the breeding performance of the second generation was eval- uated in three bivoltine populations of I. typographus in the SE Alps. Length of the maternal galleries (from 40.5 to 44.8 mm), population growth rate (PGR; from 0.7 to 3.6), and emerged adults per m 2 (from 669 to 1,570 insects/m 2 ) varied among populations and were negatively correlated with bark colonisation density. Pheromone traps set up in the three investigated forests differed in the number of trapped beetles, with mean captures ranging between 5,310 and 19,850 insects per trap. The populations giving the highest captures in the traps showed the lowest bark col- onisation density (248 vs. 489 maternal galleries per m 2 ) and the best breeding performance. The populations of parasitoids and predators corresponded to just 1-9 and 2-10% of the emerging I. typographus adults, respectively, and phloem temperature never reached thresholds lethal to I. typographus. Interspecific competition was negligible, whereas intraspecific competition was found to be the main factor affecting the breeding performance of the second generation, although with different intensity according to the colonisation density. It is hypothesised that competition with the first generation and spring precipitation influence the number of suitable hosts available to the second generation.

Modeling the Effects of Developmental Variation on Insect Phenology

Phenology, the timing of developmental events such as oviposition or pupation, is highly dependent on temperature; since insects are ectotherms, the time it takes them to complete a life stage (development time) depends on the temperatures they experience. This dependence varies within and between populations due to variation among individuals that is fixed within a life stage (giving rise to what we call persistent variation) and variation from random effects within a life stage (giving rise to what we call random variation). It is important to understand how both types of variation affect phenology if we are to predict the effects of climate change on insect populations.We present three nested phenology models incorporating increasing levels of variation. First, we derive an advection equation to describe the temperature-dependent development of a population with no variation in development time. This model is extended to incorporate persistent variation by introducing a developmental phenotype that varies within a population, yielding a phenotype-dependent advection equation. This is further extended by including a diffusion term describing random variation in a phenotype-dependent Fokker-Planck development equation. These models are also novel because they are formulated in terms of development time rather than developmental rate; development time can be measured directly in the laboratory, whereas developmental rate is calculated by transforming laboratory data. We fit the phenology models to development time data for mountain pine beetles (MPB) (Dendroctonus ponderosae Hopkins [Coleoptera: Scolytidae]) held at constant temperatures in laboratory experiments. The nested models are parameterized using a maximum likelihood approach. The results of the parameterization show that the phenotype-dependent advection model provides the best fit to laboratory data, suggesting that MPB phenology may be adequately described in terms of persistent variation alone. MPB phenology is simulated using phloem temperatures and attack time distributions measured in central Idaho. The resulting emergence time distributions compare favorably to field observations.

Effects of sunlight exposure and log size on pine engraver (Coleoptera: Curculionidae) reproduction in ponderosa pine slash in Northern Arizona, U.S.A.

Abstract1 Abiotic conditions within logs can affect pine engraver Ips pini (Say) reproductive success, and slash management techniques have been developed that exploit these relationships to reduce brood production. In the present study, we investigated the affect of sunlight exposure on phloem temperature and moisture in logs of two diameters and the subsequent effects on pine engraver reproduction.2 Logs, 30 cm in length, with diameters of 10 and 15 cm, were cut, left in the field for natural colonization by I pini, and then placed in an open meadow and under shade cloth, providing 27% and 66% shade, until offspring beetles had left the logs. Phloem temperature and moisture were recorded over the duration of the experiment and, at the end of the field experiment, logs were dissected and galleries were measured to gauge beetle reproductive success.3 As sunlight exposure increased, phloem temperatures increased and potentially lethal temperatures were often reached in the high‐sunlight exposure but seldom in the low‐sunlight. Smaller diameter logs had drier phloem than larger diameter logs. All logs dried with time but sunlight level did not affect desiccation rates. Ips pini preferred attacking larger logs and the bottom side of logs. Sunlight exposure had a significant effect on net reproductive success in smaller diameter logs, with very little net reproductive success in high‐sunlight exposed logs, and the highest reproductive success was found in small diameter logs in the low‐sunlight treatments.4 Management implications of these results are discussed.

Connecting phenological predictions with population growth rates for mountain pine beetle, an outbreak insect

It is expected that a significant impact of global warming will be disruption of phenology as environmental cues become disassociated from their selective impacts. However there are few, if any, models directly connecting phenology with population growth rates. In this paper we discuss connecting a distributional model describing mountain pine beetle phenology with a model of population success mea- sured using annual growth rates derived from aerially detected counts of infested trees. This model bridges the gap between phenology predictions and population viability/growth rates for mountain pine beetle. The model is parameterized and compared with 8 years of data from a recent outbreak in central Idaho, and is driven using measured tree phloem temperatures from north and south bole aspects and cumulative forest area impacted. A model driven by observed south-side phloem temperatures and that includes a correction for forest area previously infested and killed is most predictive and generates realistic parameter values of mountain pine beetle fecundity and population growth. Given that observed phloem temperatures are not always available, we explore a variety of methods for using daily maximum and minimum ambient temper- atures in model predictions.

Modeling cold tolerance in the mountain pine beetle, Dendroctonus ponderosae

Cold-induced mortality is a key factor driving mountain pine beetle, Dendroctonus ponderosae, population dynamics. In this species, the supercooling point (SCP) is representative of mortality induced by acute cold exposure. Mountain pine beetle SCP and associated cold-induced mortality fluctuate throughout a generation, with the highest SCPs prior to and following winter. Using observed SCPs of field-collected D. ponderosae larvae throughout the developmental season and associated phloem temperatures, we developed a mechanistic model that describes the SCP distribution of a population as a function of daily changes in the temperature-dependent processes leading to gain and loss of cold tolerance. It is based on the changing proportion of individuals in three states: (1) a non cold-hardened, feeding state, (2) an intermediate state in which insects have ceased feeding, voided their gut content and eliminated as many ice-nucleating agents as possible from the body, and (3) a fully cold-hardened state where insects have accumulated a maximum concentration of cryoprotectants (e.g. glycerol). Shifts in the proportion of individuals in each state occur in response to the driving variables influencing the opposite rates of gain and loss of cold hardening. The level of cold-induced mortality predicted by the model and its relation to extreme winter temperature is in good agreement with a range of field and laboratory observations. Our model predicts that cold tolerance of D. ponderosae varies within a season, among seasons, and among geographic locations depending on local climate. This variability is an emergent property of the model, and has important implications for understanding the insect's response to seasonal fluctuations in temperature, as well as population response to climate change. Because cold-induced mortality is but one of several major influences of climate on D. ponderosae population dynamics, we suggest that this model be integrated with others simulating the insect's biology.

Ecology of Mountain Pine Beetle (Coleoptera: Scolytidae) Cold Hardening in the Intermountain West

The mountain pine beetle, Dendroctonus ponderosae Hopkins, spends the majority of its life cycle within the phloem of pine trees, experiencing exposure to temperatures below −30°C in many parts of their expansive range. To better understand cold tolerance capabilities of this insect, seasonal patterns of cold-hardiness, as measured by supercooling points in the laboratory, were compared with seasonal patterns of host tree phloem temperatures at several geographic sites for 2 beetle generations. Larvae were found to be intolerant of tissue freezing, and supercooling points measured appear to be a reasonable estimate of the lower limit for survival. Of the compounds analyzed, glycerol was found to be the major cryoprotectant. No differences in supercooling points were found among instars or between larvae collected from the north and south aspect of tree boles. Both phloem temperatures and supercooling points of larvae collected from within the phloem were found to be different among the geographic sites sampled. Mountain pine beetle larvae appear to respond to seasonal and yearly fluctuations in microhabitat temperatures by adjusting levels of cold hardening.

Modelling micro-habitat temperature for Dendroctonus ponderosae (coleoptera: scolytidae)

We evaluate two landscape-scale microhabitat temperature prediction models suitable for the mountain pine beetle, Dendroctonus ponderosae Hopkins (coleoptera: scolytidae). Both models are based on maximum and minimum air temperatures measured at meteorological stations. The first, ‘lapse’ model employs temperature observations for a single nearby weather station, adjusting maximum and minimum temperatures based on elevation differences and appropriate historical vertical lapse rates. The second, ‘geographic trend surface’ model is based on the locations, elevations, and daily temperature measurements of surrounding weather stations. Predicted air temperatures are adjusted with a radiance-based exposure index to estimate sub-cortical phloem temperatures. Model parameters were estimated using original field measurements at three sites, and using 25 years of regional temperature measurements for sites in the western United States. Stand air temperature and phloem predictions were validated in comparisons with four withheld weather stations, and against one year of independently measured temperatures from four forest stands. Mean errors (observed minus predicted) of daily maximum stand air temperature ranged from −3.9 to 1.5°C, while mean prediction errors for daily minimum air temperatures ranged from −6.3 to 1.7°C. The geographic trend surface model performed slightly better across a range of sites, both for maximum and minimum air temperatures. Phloem temperature predictions were generally more variable, particularly for maximum temperatures on south sides of trees. Standard deviations in prediction errors were generally lower for minimum temperatures and did not differ by model form or tree exposures.

TEMPERATURE-DEPENDENT DEVELOPMENT OF THE MOUNTAIN PINE BEETLE (COLEOPTERA: SCOLYTIDAE) AND SIMULATION OF ITS PHENOLOGY

AbstractTemperature-dependent development of the egg, larval, and pupal life-stages of the mountain pine beetle (Dendroctonus ponderosae Hopkins) was described using data from constant-temperature laboratory experiments. A phenology model describing the effect of temperature on the temporal distribution of the life-stages was developed using these data. Phloem temperatures recorded in a beetle-infested lodgepole pine (Pinus contorta Douglas) were used as input to run the model. Results from model simulations suggest that inherent temperature thresholds in each life-stage help to synchronize population dynamics with seasonal climatic changes. This basic phenological information and the developed model will facilitate both research and management endeavors aimed at reducing losses in lodgepole pine stands caused by mountain pine beetle infestations.

Factors influencing generation times of spruce beetles in Alaska

Direct solar radiation to the bark surface of white spruce, Piceaglauca (Moench) Voss, is the primary environmental factor influencing the developmental rate of spruce beetles, Dendroctonusrufipennis (Kirby), in Alaska. A phloem threshold temperature of 16.5 °C is required to initiate the development of 1 year life cycle beetles. Tree location within a stand and stand aspect in relation to direct solar radiation determine which trees or areas of the tree support beetles with 1- and 2-year life cycles. Two-year cycles normally developed on the north and west sides of standing trees and the north and bottom sides of felled trees, which were characterized by an average phloem temperature of 10.6 °C. One-year cycles normally developed on the south sides of standing trees and the south and top sides of felled trees, which were characterized by an average phloem temperature of 16.5 °C.

GALLERY CONSTRUCTION AND OVIPOSITION BY IPS CALLIGRAPHUS (COLEOPTERA: SCOLYTIDAE) IN RELATION TO SLASH PINE PHLOEM THICKNESS AND TEMPERATURE

AbstractReproductive performance of Ips calligraphus (Germar) in typical slash pine, Pinus elliottii Engelm. var. elliottii, bark slabs with phloem thicknesses ranging from 0.5 to 4.0 mm was studied at 20°, 25°, and 30 °C during the fall (1981) and summer (1982) seasons by means of radiography and slab dissection. Oviposition rate (eggs/day) and egg density (eggs/cm) were positively correlated with both phloem thickness and temperature, being greatest with a combination of thick phloem (3–4 mm) and warm temperature (30 °C). Gallery construction rate (cm/day) was positively correlated, and length of initial egg-free gallery was negatively correlated, with temperature. Depth of xylem-etching was negatively correlated with phloem thickness. Reproductive performance, as measured by oviposition rate and egg density, was greater in the summer study than in the fall study under similar conditions of phloem thickness and temperature. Nutritional, physical, and seasonal characteristics of xylem, phloem, and outer bark are discussed in relation to the above findings.