Cold tolerance of the invasive larch casebearer and implications for invasion success

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Anomalous outbreaks of an invasive defoliator and native bark beetle facilitated by warm temperatures,changes in precipitation and interspecific interactions

Biotic disturbance agents such as insects can be highly responsive to climatic change and have widespread ecological and economic impacts on forests. Quantifying the responses of introduced and native insects to climate, including how dynamics of one agent may mediate those of another, is important for forecasting disturbance and associated impacts on forest structure and function. We investigated drivers of outbreaks by larch casebearer Coleophora laricella, an invasive defoliator, and eastern larch beetle Dendroctonus simplex, a native, tree‐killing bark beetle, on tamarack Larix laricina from 2000 to in Minnesota, USA. We evaluated the utility of temporal, spatial and climatic variables in predicting the presence/absence of outbreaks of each insect in cells of rasterized aerial survey data. The role of defoliation by larch casebearer in outbreaks of eastern larch beetle was also investigated. For both species, the most important predictors of outbreak occurrence were proximity of conspecific outbreaks in space and time. For larch casebearer, outbreak occurrence was positively associated with spring precipitation and warmer growing seasons. Outbreak occurrence of eastern larch beetle was positively associated with warmer and dryer years and was more likely in cells with prior defoliation by larch casebearer. Our results demonstrate that climate can drive large scale outbreaks of introduced and non‐native disturbance agents on a single host species, and that interactions at the tree level between such agents may scale up to manifest across large temporal and spatial scales.     

The Effect of Larch Casebearer on Larch Growth and Reproductive Organ Formation

Dependencies of larch (Larix sibirica Ledeb.) growth and reproductive organ formation on its infestation by a phyllophagous miner, larch casebearer (Coleophora sibiricella Flkv., Lepidoptera, Coleophoridae) were studied. The work was performed in the sustained outbreak of the larch casebearer in the forest-steppe-larch forest at Kuznetsk Alatau. For the representative tree groups, we determined shoot linear growth, the number of axillary buds, trunk radial growth, the number of micro- and macrostrobili produced, and the contents of water, monoses, starch, and nitrogenous compounds in needles as dependent on the infestation density. We found that infestation exceeding 0.5 larvae per brachyblast significantly suppressed tree growth and reproductive organ formation. At this threshold value of infestation, a significant inhibition of shoot linear growth can serve a marker for suppressed trunk radial growth and reproductive organ formation. The lower degree of infestation did not suppress larch growth and even stimulated it. This observation, along with our data on the changes in the composition of nitrogenous and carbohydrate compounds in damaged needles, demonstrates the active character of interaction between the host tree and the phyllophagous insect, a transit of the source function from damaged needles to healthy ones, and the stimulated reparation of the assimilation apparatus.     

Influence of temperature on the reproductive success,brood development and brood fitness of the eastern larch beetle Dendroctonus simplex LeConte

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The behavior of the larch casebearer (<Emphasis Type="Italic">Protocryptis sibiricella</Emphasis>, Lepidoptera,Coleophoridae) in sustained outbreak sites: Communication 1

The effects of the trophic behavior of the larch casebearer Protocryptis sibiricella (Falkovitsh, 1964) on the existence of sustained outbreak sites were monitored in 1995–1998 in a forest-steppe larch forest of the Kuznetsk Alatau foothills. The trophic behavior of the larch casebearer (partial and multiple damage to needles, coincidence of intense feeding of adult larvae with active regeneration in the larch crown) facilitates stable resistance of trees to secondary pests even at maximum densities of the miner.     

Seasonal changes in supercooling capacity and major cryoprotectants of overwintering Asian longhorned beetle (Anoplophora glabripennis) larvae

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Overwintering strategy of endoparasitoids in Chilo suppressalis: a perspective from the cold hardiness of a host

Although parasitoids ultimately kill their host, koinobiont parasitoids must protect not only themselves but also their hosts against extreme environments. In this study, the parasitism rate of Chilo suppressalis Walker (Lepidoptera: Pyralidae) was investigated, and the average body weights, supercooling points, and concentrations of glycerol (acting as a cryoprotectant) in the hemolymph were compared between parasitized and non‐parasitized larvae. Five species of koinobiont endoparasitoids parasitized the overwintering C. suppressalis larvae and the total parasitism rate was 47.6% (n = 1 537). Average body weight of parasitized larvae was significantly lower than that of non‐parasitized larvae, and the parasitism rate of the lighter group (20–30 mg) was highest. The supercooling point of parasitized C. suppressalis larvae (?15.7 ± 0.3 °C) was significantly lower than that of the non‐parasitized larvae (?14.3 ± 0.2 °C). In addition, supercooling points were not correlated with body weights between parasitized and non‐parasitized larvae, indicating that cold hardiness of parasitized larvae was enhanced by endoparasitoids. Furthermore, the concentration of glycerol in the hemolymph was significantly higher in parasitized larvae (205.0 ± 7.1 μmol ml^?1) than in non‐parasitized larvae (169.8 ± 14.4 μmol ml^?1), which suggests that the mechanism that decreases the supercooling point of parasitized larvae was associated with glycerol. All these results indicated that the cold hardiness of parasitized C. suppressalis larvae was enhanced by their endoparasitoids, which benefitted overwintering endoparasitoids.     

Occurrence of the Root Rot Pathogen,Fusarium commune,in Forest Nurseries of?the Midwestern and Western United States

Fusarium commune can cause damping‐off and root rot of conifer seedlings in forest nurseries, and this pathogen has been previously reported from Oregon, Idaho, and Washington, USA. We collected Fusarium isolates from additional nurseries in the midwestern and western USA to more fully determine occurrence of this pathogen. We used DNA sequences of the mitochondrial small subunit gene to identify F. commune. In addition to confirming the occurrence of F. commune in Oregon, Idaho, and Washington, USA, we also discovered that F. commune is even more widespread with this first report of F. commune occurring in Nevada, Montana, Nebraska, and Michigan, USA.     

Metabolism and cold tolerance of Chinese white pine beetle Dendroctonus armandi (Coleoptera: Curculionidae: Scolytinae) during the overwintering period

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Effects of acclimation and diapause on the cold tolerance of Trogoderma granarium

Khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae), is a pest of stored grain in Africa, Asia, and Europe. It is a quarantine insect for much of the rest of the world. Control of T. granarium can be achieved with methyl bromide, but this fumigant is an ozone‐depleting substance and is being phased out worldwide. Thus, there is an urgent need to find new methods of control, including the use of low temperatures. Here, we assess the effects of diapause and cold acclimation on the cold tolerance of T. granarium. The percentage of larvae in diapause increased with larval density, reaching 57.3% when reared at a density of 73 larvae g^?1 diet. The cold tolerance of T. granarium was assessed by the supercooling points (SCPs) of various life stages. The SCP of non‐acclimated insects ranged from ?26.2 ± 0.2 °C (mean ± SEM) for eggs to ?14.4 ± 0.4 °C for larvae. The lowest SCP for larvae, ?24.3 ± 0.3 °C, was obtained for diapausing‐acclimated larvae. Based on mean LT₅₀ values, the most cold‐tolerant stage at ?10 °C was the diapausing‐acclimated larvae (87 days) followed by non‐diapausing‐acclimated larvae (51 days), diapausing non‐acclimated larvae (19 days), adults (4 days), non‐diapausing non‐acclimated larvae (2 days), pupae (0.4 days), and eggs (0.2 days). The estimated times to obtain 99.9968% mortality (Probit 9) for diapausing‐acclimated larvae are 999, 442, 347, 84, and 15 days at 0, ?5, ?10, ?15, and ?20 °C, respectively. Probit 9 is an estimated value used by quarantine experts to estimate conditions that are required to kill all insects. In light of the long exposure time needed to control T. granarium even at ?20 °C, cooling to below ?27 °C (i.e., below the SCP of eggs) will quickly kill all life stages and may be the best way to control this insect with low temperatures.     

A method for quantitative determination of ice nucleating agents in insect hemolymph

The relationship between the concentration of insect hemolymph ice nucleators in samples of 0.9% NaCl solution and the supercooling points of the samples was determined by using a dilution technique. The supercooling points were only moderately reduced following dilution by a factor of up to 10³, whereas dilution beyond this point caused a marked drop in the supercooling points. The dilution factor corresponding to a 50% reduction in the nucleating activity of native hemolymph is taken as a measure of the concentration of ice nucleators in native hemolymph.This method was used to determine the concentration of ice nucleators in the hemolymph of Eurosta solidaginis larvae from Minnesota and Texas, acclimated to different temperatures. Significant levels of nucleators were found only in larvae from Minnesota, and +5 °C was found to be the optimal temperature for nucleator formation. This comparatively high temperature optimum is interpreted as a physiological adaptation, ensuring sufficient nucleator levels in the hemolymph by the time of the first exposure to freezing temperatures in the winter.     

Comparative off‐host survival of larval Rocky Mountain Wood Ticks (Dermacentor andersoni) collected from ecologically distinct field populations

Dermacentor andersoni (Ixodida: Ixodidae) Stiles, also known as the Rocky Mountain Wood Tick (RMWT), is found throughout the western United States and transmits pathogens of importance to human and animal health. The distributions and activity patterns of RMWTs are shaped by regional climatic variation. However, it is unknown if responses to climatic variation differ across the tick's geographical range. The objective of this narrow study was to test the hypothesis that the responses of RMWTs to abiotic conditions [e.g. temperature and RH (relative humidity)] vary among populations. We collected RMWTs from ecologically distinct field sites in the states of Montana and Oregon (USA). In the laboratory, we tracked weekly survival of tick larvae under four combinations of RH (75% and 98%) and temperature (26 and 32 °C) that reflected the range of conditions observed in the source habitats during spring‐summer. For both populations, larval survival time decreased at the higher ambient temperature (50% mortality 1–2 weeks earlier). Differences in RH did not affect the survival time of larvae from Oregon. By contrast, the survival time of larvae from Montana decreased at the lower RH (50% mortality 1 week earlier). These data suggest that the tolerance limits for water stress may differ among populations of D. andersoni.     

News & Notes: Double-Stranded RNA in Isolates of Discula destructiva from the Pacific Northwestern United States and British Columbia,Canada

Twenty-nine isolates of Discula destructiva Redlin from the Pacific northwestern United States and British Columbia, Canada were extracted for the presence of double-stranded RNA by use of phenol-chloroform and CF-11 chromatography. Seven/29 were positive. Agarose-gel banding profiles of the western isolates were different from those found previously in 80/80 isolates from the eastern U.S. These results suggest that isolates of D. destructiva responsible for dogwood anthracnose in Washington, Oregon, Idaho, and British Columbia may have an origin different from that of the eastern isolates. Received: 26 July 1996 / Accepted: 7 August 1996     

The influence of fire on carbon distribution and net primary production of boreal Larix gmelinii forests in north-eastern China

The boreal larch forest of Eurasia is a widespread forest ecosystem and plays an important role in the carbon budget of boreal forests. However, few carbon budgets exist for these forests, and the effects of wildfire, the dominant natural disturbance in this region, on carbon budgets are poorly understood. The objective of this study was to quantify the effects of wildfire on carbon distribution and net primary production (NPP) for three major Dahurian larch (Larix gmelinii Rupr.) forest ecosystems in Tahe, Daxing'anling, north‐eastern China: Larix gmelinii–Ledum palustre, Larix gmelinii–grass and Larix gmelinii–Rhododendron dahurica forests. The experimental design included mature forests (unburned), and lightly and heavily burned forests from the 1.3‐million‐ha 1987 wildfire. We measured carbon distribution and above‐ground NPP, and estimated fine root production from literature values. Total ecosystem carbon content for the mature forests was greatest for Larix–Ledum forests (251.4 t C ha^?1) and smallest for Larix–grass forests (123.8 t C ha^?1). Larix–Ledum forests contained the smallest vegetation carbon (13.5%), while Larix–Rhododendron contained the largest vegetation carbon (63.1%). Fires tended to transfer carbon from vegetation to detritus and soil. Total NPP did not differ significantly between the lightly burned and unburned stands, and averaged 1.58, 1.29 and 1.01 t C ha^?1 year^?1 for Larix–grass, Larix–Rhododendron and Larix–Ledum lightly burned stands, respectively. Above‐ground net primary production (ANPP) of heavily burned stands was 92–95% less than unburned and lightly burned stands. The estimated carbon loss during the 1987 fire showed substantial variability among forest types and fire severity levels. Depending upon the assumptions made about the fraction of the landscape occupied by the three larch forest types, the 1987 conflagration in north‐east China released 2.5 × 10⁷?4.9 × 10⁷ t C to the atmosphere. This study illustrates the need to distinguish between the different larch forests for developing general carbon budgets.     

Some effects of freezing temperatures on overwintering larvae of the large elm bark beetle (Scolytus scolytus)

Overwintering final instar larvae of Scolytus scolytus were tested for coldhardiness by exposure to a range of sub-zero temperatures (–7°Cto –31 °C) in a frost-gradient apparatus for 7 days. The Lt₅₀ for larvae removed from the bark (– 20.5 °C) was significantly different (P < 0.01) from the Lt50 (–18.3 °C) for larvae insulated by the bark (thickness of 7 mm ± 2 mm). Larvae with food in their digestive tract were more susceptible to freezing than the overwintering final instars which had voided their stomach contents. Duration and intensity of cold did not cause any adverse long-term effects. Most of the larvae which survived the sub-zero treatments were able to pupate or reach the adult stage. The mean supercooling point of the overwintering larvae (–30.85 °C) confirmed their cold-hardiness.     

Phylogeography of a post-glacial colonizer: Microtus longicaudus (Rodentia: muridae)

The molecular phylogeography of Microtus longicaudus was investigated with DNA sequences of the mitochondrial cytochrome b gene. We used phylogenetic and pairwise distance methods to reconstruct the history of the species with particular emphasis on the Pacific Northwest. Genetic variation across the species was consistent with vicariant events during the Pleistocene and subsequent northern postglacial expansion following the receding Laurentide and Cordilleran ice sheets. The largest break (> 6% uncorrected sequence divergence) was found to exist between populations found southeast of the Colorado River (eastern Arizona, Colorado, Wyoming and New Mexico) and all other western populations. Other well-supported subclades were composed of samples from: (i) the islands and north coast of southeast Alaska; (ii) eastern Alaska, British Columbia, Washington and Oregon; and (iii) northern California, Idaho and Montana. Within subclades, divergence was low. Our results suggest that the close relationships among haplotypes within northern subclades are a result of recent colonization, whereas higher among-subclade divergence is caused by genetic differentiation during prolonged periods of isolation, possibly as a result of mid-Pleistocene climatic events.     

After-Ripening in Festuca idahoensis Seeds: Adaptive Dormancy and Implications for Restoration

Festuca idahoensis (Idaho fescue) was a common native perennial bunchgrass in the sagebrush steppe of the western United States until the introductions of domestic livestock and alien plants. Restoration of Idaho fescue to degraded sites will likely involve reseeding, and one of the factors affecting reseeding success is germinability of the seeds employed. We investigated effects of after-ripening and storage temperature on germinability of Idaho fescue seeds collected from a central Oregon site. Six months of after-ripening were required before maximum germination was obtained. Storage of dry seeds at either room temperature (20°C) or at cooler, alternating temperatures (5/15°C) did not alter the rate at which dormancy was lost. Storage at the warmer temperature promoted rapid germination in seeds that had broken dormancy. Seed longevity varied greatly from year to year. Seeds produced in a very dry year had poorer germination and shorter longevity than seeds produced during a year with near normal precipitation. Because seed dispersal occurs in late July and early August for Idaho fescue in central Oregon, a six-month after-ripening requirement ensures that the greatest potential germination coincides with the spring period most likely to provide sufficient moisture for seedling establishment.     

Isolation, characterization, and inheritance of microsatellite loci in alpine larch and western larch.

Microsatellite loci or simple sequence repeat loci (SSRs) were isolated in alpine larch (Larix lyallii Parl.) and western larch (Larix occidentalis Nutt.). In total, 14 SSR loci were characterized; two [(TCT)4, A7] came from published Larix DNA sequence data, one (CA)17 was obtained from a partial non-enriched alpine larch total genomic DNA library, and the remaining 11 loci were obtained from larch genomic DNAs enriched for (CA)n repeats. The SSR regions in these clones could be divided into three categories: perfect repeat sequences without interruption, imperfect repeat sequences with interruption(s), and compound repeat sequences with adjacent tandem simple dinucleotides. Eight of the 14 loci analyzed were found to be polymorphic and useful markers after silver-staining polyacrylamide gel electrophoresis. In addition, several SSR primers developed for alpine larch were able to successfully amplify polymorphic loci in its related species, western larch, and among other closely related taxa within the Larix genus. The inheritance of microsatellite loci was verified by analysis of haploid megagametophyte and diploid embryo tissues of progeny obtained from controlled crosses between western larch and alpine larch. All microsatellite loci analyzed had alleles that segregated according to expected Mendelian frequencies. Two species-specific markers (UAKLly10a and UAKLla1) allow easy and rapid identification of specific genetic entry of alpine larch and western larch at any stage in the sporophyte phase of the life cycle. Therefore, these markers are efficient in identifying the parental species and to validate controlled crosses between these two closely related species. These results are important in tree improvement programs of alpine larch and western larch aimed at producing genetically improved hybrid stock for reforestation in Western Canada and U.S.A.     

Root–shoot communication of the seedlings of Japanese larch and a hybrid species grown in different soil-temperature regimes

We studied the effects of soil temperature (7, 15, and 25°C) on the growth and photosynthesis of seedlings of the Japanese larch (Larix kaempferi) and its hybrid larch (L. gmelinii × L. kaempferi) to simulate early stages of regeneration after disturbance. At a soil temperature of 7°C, the root length per unit root biomass, chlorophyll concentration, and photosynthetic nitrogen-use efficiency (PNUE) were markedly lower in the Japanese larch than in the hybrid larch, which may indicate that the hybrid larch is better at acquiring water and nutrients. At ambient temperatures of 17–25°C, the light-saturated photosynthesis rate (P _sat) of both seedlings grown at a soil temperature of 7°C was lower than at 15 or 25°C. By the 16th week, the needle area, root area, and biomass in seedlings of both types were lower at a soil temperature of 7°C than at soil temperatures of 15 or 25°C. At a soil temperature of 25°C, P _sat and nitrogen uptake were lower in both larch species than at 15°C. The growth of the Japanese larch declined sharply from 15 to 25°C; however, the growth of the hybrid larch decreased only slightly from 15 to 25°C. We conclude that an increased soil temperature may retard larch growth in cold regions, especially in the case of the Japanese larch.     

Physiological traits suggest limited diapause response in false codling moth,Thaumatotibia leucotreta (Lepidoptera: Tortricidae)

The ability of a pest insect species to enter diapause, a physiological state of dormancy, has significant implications for population dynamics and pest management practises in agricultural landscapes. The false codling moth Thaumatotibia leucotreta is a major pest of deciduous and citrus fruit in southern Africa and a quarantine pest of international concern. Apart from an early field assessment that may have been compromised by taxonomic uncertainty surrounding cryptic developing life stages, no studies have investigated diapause induction within an experimental framework for this species, and none to date have used a suite of physiological traits potentially indicative of the diapause state. Here, we subjected larvae to cooling and shortening day length over a period of 14 days [Diapause Treatment (DT) group] relative to a similar‐aged control (CON) group held at optimal rearing conditions (25°C, 12 : 12 L : D) and tested if physiological traits, including resting metabolic rate, body freezing temperature (=supercooling point, equivalent to the low‐temperature mortality threshold) and body condition (body mass, body lipid and water content) varied in a direction that may be reflective of diapause induction. Mean metabolic rate in DT larvae was 0.044 ml CO₂/h (mean mass: 52.7 mg), which was significantly higher than in CON larvae [0.025 ml CO₂/h, mean mass: 51.5 mg (P = 0.04)]. Supercooling points were not statistically lower in the CON group than in DT larvae (DT:?15.6 ± 1.5°C; CON: ?16.4 ± 2.8°C; P = 0.33). Measures of body size, body condition and resting water loss rates remained similar between groups. These results support the conclusion of early field observations that T. leucotreta does not undergo diapause that has significant implications for the management of the species.