Pathogens of Ips amitinus: new species and comparison with Ips typographus

Ips amitinus and I. typographus are two serious pests of spruce in Europe, have similar bionomics and are likely to occur and meet on the same host trees. We therefore hypothesized that the two species support similar levels of similar pathogens. To test this hypothesis, we collected mature beetles from three trap trees at each of eight study sites and determined beetle numbers and pathogen infection levels. In total, 938 mature I. amitinus beetles and 3435 of I. typographus were dissected; five pathogens, as well as intestinal nematodes and endoparasitoids, were detected. The neogregarine Mattesia schwenkei is reported here for the first time as a new pathogen in 9.4% of I. amitinus individuals at one site. Average infection levels of most pathogens (Chytridiopsis typographi, Gregarina typographi, Mattesia schwenkei and parasitoids) were significantly higher in I. typographus than in I. amitinus. Metschnikowia typographi was confirmed only in Ips amitinus, while the microsporidium of Nosema typographi occurred only in I. typographus. Within‐season increases in G. typographi infection levels were documented in Ips amitinus.     

Nematodes associated with Ips cembrae (Coleoptera: Curculionidae): comparison of generations,sexes and sampling methods

A study of nematodes associated with the large larch bark beetle Ips cembrae (Heer 1836) was carried out at three locations in the Czech Republic. The proportion of beetles infested by endoparasitic nematodes (representatives of genera Contortylenchus, Parasitylenchus, Cryptaphelenchus and Parasitorhabditis) ranged from 29.9 to 50.9%. Significant differences were determined in nematode infestation levels among locations, generations and sampling methods. No differences were found in infestation rates between males and females. The percentage of bark beetles with phoretic nematodes ranged from 18 to 42.9%. Phoretic nematodes directly found under elytra, on wings and between body segments of the bark beetles belong to the genus Micoletzkya. However, adults and juveniles of other two phoretic species Laimaphelenchus penardi and Bursaphelenchus sp. were found in the gallery frass of I. cembrae. Infestation by phoretic nematodes positively correlated with the presence of mites under elytra.     

Occurrence of pathogens in Ips typographus (Coleoptera: Curculionidae) and in other spruce bark beetles from the wilderness reserve Dürrenstein (Lower Austria)

The study presents new data on spatial distribution of bark beetle pathogens, on changes in frequency over several years and on their prevalence during different time periods within a year from several locations within the wilderness reserve Dürrenstein (Lower Austria). The occurrence of pathogens was investigated in Ips typographus (during five years), in Pityogenes chalcographus (during two years) and in Ips amitinus (in one year). In total, seven pathogen species could be detected in I. typographus. The most dominant pathogen species were the Ips typographus-Entomopoxvirus (ItEPV), the sporozoan species Gregarina typographi and the microsporidium Chytridiopsis typographi; the latter two pathogen species were recorded every year and at about similar high (G. typographi) or low (C. typographi) rates, the ItEPV in strongly varying rates. The neogregarine Mattesia cf. schwenkei and the two microsporidia Nosema typographi and Unikaryon montanum were found in I. typographus only sporadically and the rhizopodan species Malamoeba scolyti was found once. The number of infected males and females was relatively similar with almost all pathogen species in most of the years except U. montanum, which occurred exclusively in females. Three pathogen species were recorded in P. chalcographus which were Gregarina typographi, Mattesia cf. schwenkei and Chytridiopsis typographi. Two pathogen species were observed in I. amitinus, Gregarina typographi and Chytridiopsis typographi.     

The comparative prevalence and demographic impact of two pathogens in swarming Ips typographus adults: a quantitative analysis of long term trapping data

• 1 Adult Ips typographus were collected using pheromone traps at a locality in the eastern part of Austria between 1995 and 2004. The occurrence of two pathogens, Gregarina typographi and Chytridiopsis typographi, was determined throughout the period of beetle swarming activities. Weekly and annual data sets were then analysed by smoothing statistical techniques and epidemiological models.
• 2 The pathogens spread differently within the beetle population with respect to their biological characteristics: infectious forms of C. typographi were immediately available after their development in the insect's gut, but not those of G. typographi.
• 3 Both of the pathogens had a low prevalence in swarming beetles, especially C. typographi, and there was no evidence of a between‐year or within‐year epidemiological process. Conversely, it was shown that G. typographi has a positive effect on the rate of increase of trapped beetles.
• 4 Fitting a nonlinear model to the data suggested that: (i) this was due to a higher catch ability of beetles infected with G. typographi than of healthy beetles; (ii) when this effect is taken into account, G. typographi induces a specific within‐year low mortality in beetle populations; and (iii) beetle populations increase naturally within a year, despite their infection by both pathogens. No clear effect of C. typographi was detected in the trapped data set when the prevalence of this species was high in beetle populations collected from trees.
• 5 It is hypothesized that both pathogens induce different behavioural effects on their host, resulting in: (i) favouring the trapping of G. typographi‐infected beetles and (ii) hindering the capture of C. typographi‐infected individuals. This could be the result of both of the pathogens having an opposite effect on the flight abilities of beetles and/or on the beetles' response to the aggregation pheromones used in the traps.

     

Nematodes associated with the double‐spined bark beetle Ips duplicatus (Coleoptera: Curculionidae) in central Europe

Ips duplicatus (Sahlberg) is an important forest pest in central Europe, but its nematode associates have seldom been studied. Therefore, nematodes associated with I. duplicatus were determined at three localities in the Czech Republic. The percentage of beetles with phoretic nematodes ranged from 18 to 65%. Micoletzkya buetschlii and other phoretic nematodes were found under elytra, on wings, and between body segments. The percentage of beetles with nematodes in the haemocoel ranged from 3 to 30%, and the nematodes included Contortylenchus diplogaster and Parasitylenchus cf. aculeatus. Juveniles of Parasitorhabditis obtusa (Fuchs 1915) were found in the intestines of 0–16% of the beetles. The most abundant species in I. duplicatus galleries were P. obtusa and M. buetschlii. Cryptaphelenchus sp., Parasitaphelenchus sp. and unidentified tylenchid juveniles also were found in the galleries. The percentage of beetles with nematodes was greater in the overwintering than in the offspring generation, and numbers of nematodes per gallery increased with gallery development.     

Pathogens of the spruce bark beetles Ips typographus and Ips duplicatus

Pathogens of two important bark beetles, Ips typographus and Ips duplicatus, both in outbreaks connected with infestation of spruces by the fungus Armillaria ostoyae, were compared at four localities in the eastern Czech Republic. Low infestations of Chytridiopsis typographi, Nosema typographi, Menzbieria chalcographi, and Gregarina typographi were detected in I. typographus. In I. duplicatus, only C. typographi and G. typographi were found and with low infection levels. The microsporidium, Larssoniella duplicati, was not detected in I. typographus, but was detected in I. duplicatus at all localities in almost 80% of the samples (a sample consisted of 40–50 beetles collected at one locality in one period) and often with a very high infection level (up to 57% of the beetles infected in a sample). The infection level of L. duplicati did not differ between generations of I. duplicatus. I. duplicatus overwinters mainly in the adult stage, and no decrease in the number of infected overwintering I. duplicatus was observed. The relatively constant infection level of L. duplicati suggests that transmission is unlikely to be horizontal via oral ingestion.     

Pathogen's level and parasitism rate in Ips typographus at high population densities: importance of time

We compared the levels of pathogen infection in parental beetles, parasitism of the offspring, abundance of predators and breeding performance success of univoltine populations of Ips typographus in plots characterized by short‐term (2–3 years) outbreaks vs. those with long‐term (>10 years) outbreaks on two localities at ca. 1100 m altitude in the ?umava Mts. The numbers of I. typographus were high in all plots, whether the plots were characterized by long‐term outbreaks or short‐term outbreaks. The numbers of maternal galleries in the sample areas ranged from 300 to 400 per m². The density of parental beetle galleries, abundance of surviving specimens of I. typographus, and length of maternal galleries did not differ between plots. The short‐term outbreaks had only fewer ectoparasitoids of I. typographus and a lower percentage of parasitism and infection level of Mattesia schwenkei than the long‐term outbreaks even though the maternal gallery densities and beetle production were the same. The most mortality appeared to be caused by intraspecific larval competition, and the identical reproductive success in plots with short‐term and long‐term outbreaks indicates that the negative feedback resulting from parasitoids and entomopathogens does not substantially reduce beetle numbers. Although entomopathogenic fungi as Beauveria bassiana occur naturally in the galleries of spruce bark beetles, there was no evidence of its presence in the studied population. The low levels of predation and/or parasitism in both kinds of plots indicate that natural enemies did not play a significant role in reducing outbreak numbers of I. typographus.     

Temperature‐dependent development of the European larch bark beetle,Ips cembrae

The temperature‐dependent development of the European larch bark beetle, Ips cembrae, was studied under long‐day conditions L:D 16:8 at three temperature regimes, 15°C, 20°C and 25°C, using the sandwich plate method. By observing the individual developmental progress, we calculated the developmental times and lower developmental thresholds of one entire generation and various ontogenetic stages. The mean developmental time of one generation was about 120, 64 and 37 days at 15°C, 20°C and 25°C, respectively. The egg stage comprised about 9% of the total development or about 16% of the pre‐imaginal development. The larval stages took about 39% of the entire and about 66% of the pre‐imaginal development. The pupal stage needed about 11% of the total or about 18% of the pre‐imaginal development. The lower developmental threshold for one generation was 11.2°C. The egg stage had the highest lower developmental threshold of 12.0°C, the pupa the lowest of 9.8°C and the total larval stages showed a value of 11.2°C. The thermal requirements for I. cembrae have never been studied in detail before. The results will be a valuable contribution for monitoring and risk assessment models to estimate the beetle's phenology and its potential impacts on forest ecosystems under changing climate conditions.     

Using the SSU,ITS, and Ribosomal DNA Operon Arrangement to Characterize Two Microsporidia Infecting Bruce Spanworm,Operophtera bruceata (Lepidoptera: Geometridae)

Research pertaining to the two closely‐related microsporidian genera Nosema and Vairimorpha is hindered by inconsistencies in species differentiation within and between the two clades. One proposal to better delimit these genera is to restructure the Nosema around a “True Nosema” clade, consisting of species that share a characteristic reversed ribosomal DNA operon arrangement and small subunit (SSU) ribosomal DNA sequences similar to that of the Nosema type species, N. bombycis. Using this framework, we assess two distinct microsporidia recovered from the forest insect Bruce spanworm (Operophtera bruceata) by sequencing their SSU and internal transcribed spacer regions. Phylogenetic analyses place one of our isolates within the proposed True Nosema clade close to N. furnacalis and place the other in the broader Nosema/Vairimorpha clade close to N. thomsoni. We found that 25% of Bruce spanworm cadavers collected over the four‐year study period were infected with microsporidia, but no infections were detected in cadavers of the Bruce spanworm's invasive congener, the winter moth (O. brumata), collected over the same period. We comment on these findings as they relate to the population dynamics of the Bruce spanworm‐winter moth system in this region, and more broadly, on the value of ribosomal DNA operon arrangement in Nosema systematics.     

Feeding by Scolytus bark beetles to test for differently susceptible elm varieties

Dutch elm disease (DED), caused by the fungi Ophiostoma ulmi and O. novo‐ulmi, has reduced elm populations severely in Europe and North America. Breeding programmes are in action to find less susceptible elm varieties suitable for re‐establishing elm stands. Bark beetles, mainly Scolytus spp., are the only known natural vectors of DED. During twig feeding, beetles transfer Ophiostoma spores to healthy elms. Thus, less palatable elms should run a lower risk of DED infections. In feeding preference bioassays, we offered twigs from elms exhibiting different degree of susceptibility to O. novo‐ulmi, together with non‐host trees to Scolytus beetles. Scolytus multistriatus preferred wych elm, Ulmus glabra, to 100% in two‐choice tests, whereas S. laevis did not discriminate between a tolerant and a susceptible variety of field elm, U. minor. We suggest that the feeding assay is useful as a low‐tech method in breeding programmes for evaluating the suitability of promising elm genotypes to vector insects.     

Evaluation of attractants and traps for monitoring small banded pine weevil Pissodes castaneus

This study aimed to develop a semiochemical‐baited trapping system to monitor the populations of small banded pine weevil, Pissodes castaneus, a serious pest in Pinus sylvestris young stands that are weakened by biotic and abiotic factors. The scope of the work included the development of a dispenser for compounds (ethanol and α‐pinene) emitted by P. sylvestris and the pheromones of P. castaneus: grandisol and grandisal. Additionally, the effectiveness of beetle catches in different types of traps (unitrap, cross‐unitrap and long and short pipe traps) baited with a dispenser was assessed. The olfactometric studies showed that most of the newly hatched beetles that had not fed were attracted by a mixture of grandisol and grandisal. However, in the group of feeding beetles, half were attracted by a mixture of ethanol and α‐pinene. These results indicated that both pheromones and α‐pinene plus ethanol should be useful for capturing P. castaneus beetles. In the field trials, the highest efficiency was found in baited unitraps that caught up to several hundred P. castaneus beetles, while the baited cross‐unitraps caught up to a few dozen beetles. No insects were found in either type of baited pipe trap or in any of the unbaited control traps. The baited unitraps and cross‐unitraps also collected, with varied intensity, Hylobius abietis beetles, a serious pest of reforestations. These results indicate the possibility of using a unitrap baited with a 4‐component attractant for monitoring P. castaneus in integrated pest management for the protection of young forests.     

Population genetic analysis of a parasitic mycovirus to infer the invasion history of its fungal host

Hymenoscyphus fraxineus mitovirus 1 (HfMV1) occurs in the fungus Hymenoscyphus fraxineus, an introduced plant pathogen responsible for the devastating ash dieback epidemic in Europe. Here, we explored the prevalence and genetic structure of HfMV1 to elucidate the invasion history of both the virus and the fungal host. A total of 1298 H. fraxineus isolates (181 from Japan and 1117 from Europe) were screened for the presence of this RNA virus and 301 virus‐positive isolates subjected to partial sequence analysis of the viral RNA polymerase gene. Our results indicate a high mean prevalence (78.7%) of HfMV1 across European H. fraxineus isolates, which is supported by the observed high transmission rate (average 83.8%) of the mitovirus into sexual spores of its host. In accordance with an expected founder effect in the introduced population in Europe, only 1.1% of the Japanese isolates were tested virus positive. In Europe, HfMV1 shows low nucleotide diversity but a high number of haplotypes, which seem to be subject to strong purifying selection. Phylogenetic and clustering analysis detected two genetically distinct HfMV1 groups, both present throughout Europe. This pattern supports the hypothesis that only two (mitovirus‐carrying) H. fraxineus individuals were introduced into Europe as previously suggested from the bi‐allelic nature of the fungus. Moreover, our data points to reciprocal mating events between the two introduced individuals, which presumably initiated the ash dieback epidemic in Europe.     

Population structure of the invasive forest pathogen Hymenoscyphus pseudoalbidus

Understanding the genetic diversity and structure of invasive pathogens in source and in introduced areas is crucial to the revelation of hidden biological features of an organism, to the reconstruction of the course of invasions and to the establishment of effective control measures. Hymenoscyphus pseudoalbidus (anamorph: Chalara fraxinea) is an invasive and highly destructive fungal pathogen found on common ash Fraxinus excelsior in Europe and is native to East Asia. To gain insights into its dispersal mechanisms and history of invasion, we used microsatellite markers and characterized the genetic structure and diversity of H. pseudoalbidus populations at three spatial levels: (i) between Europe and Japan, (ii) in Europe and (iii) at the epidemic's front in Switzerland. Phylogenetic and network analysis demonstrated that individuals from both regions are conspecific. However, populations from Japan harboured a higher genetic diversity and were genetically differentiated from European ones. No evident population structure was found among the 1208 European strains using Bayesian and multivariate clustering analysis. Only the distribution of genetic diversity in space, pairwise population differentiation (G_ST) and the spatial analysis of principal components revealed a faint geographical pattern around Europe. A significant allele deficiency in most European populations pointed to a recent genetic bottleneck, whereas no pattern of isolation by distance was found. Our data suggest that H. pseudoalbidus was introduced just once by at least two individuals. The potential source region of H. pseudoalbidus is vast, and further investigations are required for a more accurate localization of the source population.     

Effects of heat on the dispersal performance of Ips typographus

Under present climate conditions, Ips typographus (L.) is Europe's most critical disturbance agent for mature Norway spruce (Picea abies). With ongoing climate change, the bark beetle will most probably become more prominent as a pest. The aim of this study was to analyse the dispersal performance of I. typographus under various weather conditions, especially hot days with a maximum air temperature above 30°C. In a field study, marked bark beetles were released from breeding logs and could be retraced in traps distributed across the survey area. With daily collections of the trapped beetles, it was possible to analyse the flight activity and the average flight distance of the bark beetles during hot, moderate and cool days. The numbers of daily catches and the average flight distance during the hottest days (air temperature maximum ≥30°C) did not significantly differ from the moderate days (air temperature maximum ≥22°C and <30°C). The numbers of daily catches and the average flight distance during the cool days (air temperature maximum <22°C) were significantly lower than during hot and moderate days. The results give an insight on the dispersal capacity of I. typographus under climate change driven future conditions. Increased air temperatures do not seem to impair the flight performance of I. typographus. A small proportion of cool days during the swarming period even seems to favour dispersal of I. typographus.     

Fluorescent prey traps in carnivorous plants

Carnivorous plants acquire most of their nutrients by capturing ants, insects and other arthropods through their leaf‐evolved biological traps. So far, the best‐known attractants in carnivorous prey traps are nectar, colour and olfactory cues. Here, fresh prey traps of 14 Nepenthes, five Sarracenia, five Drosera, two Pinguicula species/hybrids, Dionaea muscipula and Utricularia stellaris were scanned at UV 366 nm. Fluorescence emissions of major isolates of fresh Nepenthes khasiana pitcher peristomes were recorded at an excitation wavelength of 366 nm. N. khasiana field pitcher peristomes were masked by its slippery zone extract, and prey capture rates were compared with control pitchers. We found the existence of distinct blue fluorescence emissions at the capture spots of Nepenthes, Sarracenia and Dionaea prey traps at UV 366 nm. These alluring blue emissions gradually developed with the growth of the prey traps and diminished towards their death. On excitation at 366 nm, N. khasiana peristome 3:1 CHCl₃–MeOH extract and its two major blue bands showed strong fluorescence emissions at 430–480 nm. Masking of blue emissions on peristomes drastically reduced prey capture in N. khasiana pitchers. We propose these molecular emissions as a critical factor attracting arthropods and other visitors to these carnivorous traps. Drosera, Pinguicula and Utricularia prey traps showed only red chlorophyll emissions at 366 nm.     

Diet‐related gut bacterial dysbiosis correlates with impaired development,increased mortality and Nosema disease in the honeybee (Apis mellifera)

Dysbiosis, defined as unhealthy shifts in bacterial community composition, can lower the colonization resistance of the gut to intrinsic pathogens. Here, we determined the effect of diet age and type on the health and bacterial community composition of the honeybee (Apis mellifera). We fed newly emerged bees fresh or aged diets, and then recorded host development and bacterial community composition from four distinct regions of the hosts’ digestive tract. Feeding fresh pollen or fresh substitute, we found no difference in host mortality, diet consumption, development or microbial community composition. In contrast, bees fed aged diets suffered impaired development, increased mortality and developed a significantly dysbiotic microbiome. The consumption of aged diets resulted in a significant reduction in the core ileum bacterium Snodgrassella alvi and a corresponding increase in intrinsic pathogen Frischella perrara. Moreover, the relative abundance of S. alvi in the ileum was positively correlated with host survival and development. The inverse was true for both F. perrara and Parasacharibacter apium. Collectively, our findings suggest that the early establishment of S. alvi is associated with healthy nurse development and potentially excludes F. perrara and P. apium from the ileum. Although at low abundance, establishment of the common midgut pathogen Nosema spp. was significantly associated with ileum dysbiosis and associated host deficiencies. Moreover, dysbiosis in the ileum was reflected in the rectum, mouthparts and hypopharyngeal glands, suggesting a systemic host effect. Our findings demonstrate that typically occurring alterations in diet quality play a significant role in colony health and the establishment of a dysbiotic gut microbiome.     

Diversity of fungal latent pathogens and true endophytes associated with fruit trees in Uruguay

We have explored the fungal diversity in asymptomatic twigs of apple, peach, pear and blueberry trees, with the objective of discerning between true endophytes and latent pathogens. Several fungal genera containing known bark pathogens were found. Seven Diaporthe species—D. oxe, D. infecunda, D. serafiniae, D. phaseolorum, D. terebinthifolii, D. foeniculina and D. brasiliensis—were identified, along with Botryosphaeria dothidea, Neofusicoccum parvum, Neofusicoccum australe, Cytospora sp., Cytospora acaciae and Pestalotiopsis spp. A pathogenicity trial was undertaken to determine the role of these species on apple, pear, blueberry and peach shoots. Diaporthe brasiliensis, D. foeniculina, Diaporthe inconspicua, D. terebinthifolii, Diaporthe sp.1, Cytospora‐like isolates and Pestalotiopsis spp. isolates produced no lesions on inoculated shoots, suggesting that they could be considered true endophytes on their respective hosts. Meanwhile, some of the isolates of Diaporthe—D. oxe, Diaporthe sp.2, D. infecunda and D. serafiniae, B. dothidea, N. parvum and N. australe could be regarded as latent pathogens in their respective hosts as they produced sunken cankers and necrosis on inoculated shoots. These results demonstrate that apple, pear, blueberry and peach healthy shoots can host many known endophytic fungi along with potential wood disease‐causing fungi that should be regarded as latent pathogens.     

Survival of Phytophthora cinnamomi and Fusarium verticillioides in commercial potting substrates for ornamental plants

Live plants, particularly when accompanied by soil or potting substrates, are considered the main pathway for international spread of plant pathogens. Modern, rapid shipping technologies for international plant trade increase the probability of plant pathogen survival during transport and the subsequent chances of disease outbreaks in new locations. The survival of two model pathogens, an Oomycete, Phytophthora cinnamomi, and a filamentous fungus, Fusarium verticillioides, was studied in two different commercial potting substrates (peat and peat‐free) under glasshouse conditions in the absence of a plant host. Survival rates were analysed at 2, 7, 12 and 17 months after substrate inoculation. Fusarium verticillioides had the longest survival rate, and was still present at 17 months. In contrast, P. cinnamomi survived up to 7 months but was not recovered after 12 or 17 months. There was no significant difference in the number of colony‐forming units (CFUs) of either pathogen in the two substrates, except at 2 months, when higher numbers were recovered from peat substrates.     

The Lactoperoxidase System: a Natural Biochemical Biocontrol Agent for Pre‐ and Postharvest Applications

Controlling pests in pre‐ and postharvest crops using natural and low‐impact products is a major challenge. The lactoperoxidase system is an enzymatic system that exists in all external secretions in mammals and is part of the non‐immune system. We tested its efficacy in in vitro microplates on Phytophthora infestans, Penicillium digitatum, Penicillium italicum, Penicillium expansum and Botrytis cinerea to determine the most suitable concentrations for use. Then, we verified its efficacy in planta under controlled conditions. Solutions prepared with 5.4 mm iodide and 1.2 mm thiocyanate and diluted threefold inhibited pathogen growth in vitro by 63–100%. Twofold‐diluted solutions protected potato plants against P. infestans by 60–74% under controlled conditions. Undiluted solution inhibited orange's and apple's postharvest pathogens in curative application with efficacy levels ranging between 84 and 95% in orange and between 63 and 74% in apple. 1.5‐fold concentrated solutions inhibited postharvest pathogens of apple in curative application with efficacy levels ranging between 84 and 92%. Our results also show that the oxidative stress response of fruit following wounding could interfere with ion efficiency. Our tests demonstrate for the first time that this biochemical method is as efficient as a conventional synthetic chemical method under controlled conditions.     

Host‐specific thermal profiles affect fitness of a widespread pathogen

Host behavior can interact with environmental context to influence outcomes of pathogen exposure and the impact of disease on species and populations. Determining whether the thermal behaviors of individual species influence susceptibility to disease can help enhance our ability to explain and predict how and when disease outbreaks are likely to occur. The widespread disease chytridiomycosis (caused by the fungal pathogen Batrachochytrium dendrobatidis, Bd) often has species‐specific impacts on amphibian communities; some host species are asymptomatic, whereas others experience mass mortalities and population extirpation. We determined whether the average natural thermal regimes experienced by sympatric frog species in nature, in and of themselves, can account for differences in vulnerability to disease. We did this by growing Bd under temperatures mimicking those experienced by frogs in the wild. At low and high elevations, the rainforest frogs Litoria nannotis, L. rheocola, and L. serrata maintained mean thermal regimes within the optimal range for pathogen growth (15–25°C). Thermal regimes for L. serrata, which has recovered from Bd‐related declines, resulted in slower pathogen growth than the cooler and less variable thermal regimes for the other two species, which have experienced more long‐lasting declines. For L. rheocola and L. serrata, pathogen growth was faster in thermal regimes corresponding to high elevations than in those corresponding to low elevations, where temperatures were warmer. For L. nannotis, which prefers moist and thermally stable microenvironments, pathogen growth was fastest for low‐elevation thermal regimes. All of the thermal regimes we tested resulted in pathogen growth rates equivalent to, or significantly faster than, rates expected from constant‐temperature experiments. The effects of host body temperature on Bd can explain many of the broad ecological patterns of population declines in our focal species, via direct effects on pathogen fitness. Understanding the functional response of pathogens to conditions experienced by the host is important for determining the ecological drivers of disease outbreaks.