Impact of climatic variation on populations of pine processionary moth Thaumetopoea pityocampa in a core area of its distribution

• 1 There is growing appreciation of climatic effects on insect population dynamics at the margins of distribution limits. Climatic effects might be less important and/or involve different drivers and processes near the centre of distributions.
• 2 We evaluated the effects of interannual variation in temperatures, radiation and precipitation on populations of pine processionary moth Thaumetopoea pityocampa in Central–South Portugal, a low altitude area with a relatively mild Mediterranean climate near the centre of the north–south range of the species.
• 3 We tested for effects of climate on mortality of young larvae, growth rates, final larval mass and fecundity.
• 4 Results indicated high mortality of early instars associated with low minimum and maximum daily temperatures and low precipitation. Low minimum temperatures were further associated with high parasitism by the larval parasitoid Phryxe caudata (Rondani) (Diptera, Tachinidae). Furthermore, larval growth rates were higher with high solar radiation during December and January, which was itself negatively related to precipitation and air temperature. Slow larval growth rates led to lower final mass at pupation in the spring, and smaller egg masses and smaller initial colony sizes during the next autumn.
• 5 Thus climatic factors, and temperature in particular, apparently contributed to population dynamics of T. pityocampa in the core of its distribution, as well as at its northern limits. The most specific climatic parameters of importance, however, and the connections between climate, physiology and insect demographics in the core area were clearly different from northern areas.

     

The effect of temperature on the development and life cycle regulation of the pine weevil Hylobius abietis and the potential impacts of climate change

• 1 The pine weevil Hylobius abietis is widely distributed in the Palaearctic region where it is a major pest. Although predominantly semi‐voltine, with a 2‐year life cycle, the generation time across its range can vary from 1 to 4 years. The duration of the life cycle and the seasonal timing of weevil activity affect the economic impact and management of this pest, all of which are likely to change in a warming climate.
• 2 To determine the effect of temperature and tree species on weevil growth and development, laboratory experiments were performed with eggs, larvae, prepupae, pupae and adults, using, as appropriate, the host species Scots pine Pinus sylvestris L. and Sitka spruce Picea sitchensis (Bong.) Carr. under constant or alternating temperatures.
• 3 The development rate was linearly related to temperature, with developmental thresholds for eggs, larvae and pupae of 8, 4.5 and 7.3 °C, respectively. Day‐degrees were estimated for each life stage. Larval development was affected by tree species, being slower on Sitka spruce than on Scots pine, and was faster under alternating than constant temperatures.
• 4 The development time for prepupae was highly variable, with an apparent facultative prepupal diapause initiated by temperature. The temperature range 20–17.5 °C marked the transition between median prepupal development times of approximately 25 and 90 days. The prepupal stage may serve to minimize the risk of overwintering mortality in the pupal stage and help to synchronize the life cycle.
• 5 Larval and adult mass was positively related to developmental temperature, demonstrating an inverse temperature size rule, and weevils were heavier when developing on Scots pine than Sitka spruce. Development in alternating temperatures reduced weevil mass on Scots pine. The influence of temperature on weevil mass is likely to have a positive effect on fecundity and overwintering survival. The effects of climate change on development, voltinism and weevil mass are discussed.

     

Climate change and the outbreak ranges of two North American bark beetles

Abstract

• 1 One expected effect of global climate change on insect populations is a shift in geographical distributions toward higher latitudes and higher elevations. Southern pine beetle Dendroctonus frontalis and mountain pine beetle Dendroctonus ponderosae undergo regional outbreaks that result in large‐scale disturbances to pine forests in the south‐eastern and western United States, respectively.
• 2 Our objective was to investigate potential range shifts under climate change of outbreak areas for both bark beetle species and the areas of occurrence of the forest types susceptible to them.
• 3 To project range changes, we used discriminant function models that incorporated climatic variables. Models to project bark beetle ranges employed changed forest distributions as well as changes in climatic variables.
• 4 Projected outbreak areas for southern pine beetle increased with higher temperatures and generally shifted northward, as did the distributions of the southern pine forests.
• 5 Projected outbreak areas for mountain pine beetle decreased with increasing temperature and shifted toward higher elevation. That trend was mirrored in the projected distributions of pine forests in the region of the western U.S. encompassed by the study.
• 6 Projected outbreak areas for the two bark beetle species and the area of occurrence of western pine forests increased with more precipitation and decreased with less precipitation, whereas the area of occurrence of southern pine forests decreased slightly with increasing precipitation.
• 7 Predicted shifts of outbreak ranges for both bark beetle species followed general expectations for the effects of global climate change and reflected the underlying long‐term distributional shifts of their host forests.

     

Potential effects of climate warming on the survivorship of adult Monochamus galloprovincialis

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Temperature as a predictor of survival of the pine processionary moth in the Italian Alps

1 The pine processionary moth Thaumetopoea pityocampa is expanding its geographical range in Europe, as a consequence of enhanced winter survival under a warmer climate. A combination of daytime nest temperatures and night air temperatures determines the number of hours larvae are able to feed (hours above realized feeding threshold, RFT). 2 We tested the RFT‐based model for survival across multiple areas of the insect’s range in the Italian Alps over a 2‐year period. In a series of translocation experiments using natural temperature gradients as spatial analogues for global warming, we transferred colonies of T. pityocampa larvae to sites within zones of historical distribution, recent distribution, and outside the present range. The sites included traditional (Pinus nigra, Pinus sylvestris) as well as novel (Pinus mugo, Pinus uncinata, Pinus cembra) hosts. 3 Survival during precold (August to November), cold (December to February) and postcold (March to May) periods were analysed against climatic variables (temperature and rainfall) and predictors developed by the model. 4 Host species did not significantly affect final survival, with the exception of slower larval development, and resulting lower cold tolerance, on P. cembra than on P. mugo at the same site. 5 Across all the sites and hosts, final survival of colonies depended on the number of feeding hours during the cold period (RFT), which explained 82% of the variance in a regression model. We recommend using RFT, or its surrogate daily mean minimum temperature when nest temperature is not available, in predictive models of range expansion of T. pityocampa under climate change scenarios.     

Solar radiation directly affects larval performance of a forest insect

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The effects of chilling and forcing temperatures on spring synchrony between larch casebearer and tamarack

1. Spring phenological synchrony can be important for tree-insect interactions. Depending on the magnitude and direction of phenological shifts, overwintering insects could be affected in many ways, for example, facing starvation or having to contend with increased chemical or physical defences of host trees. If temperature has different influences on the phenology of trees and insects, climate change can alter spring phenological synchrony.
2. In this experiment, we exposed tamarack seedlings and larch case bearer larvae from Minnesota, USA, to a variety of chilling and forcing temperatures and measured spring phenology (twig bud break and larval activation). We additionally measured case bearer performance on seedlings that were exposed to different forcing × chilling levels, tracking larval survivorship to adulthood.
3. Warmer forcing enhanced larval activation and bud break, but larval development slowed down past 21°C. Higher chilling temperatures accelerated bud break, but the effect was inconclusive for larvae. There was no chilling × forcing interaction for either species. Spring activity accelerated more quickly with increases in temperature for larvae than for seedlings, resulting in increased phenological synchrony at warmer temperatures. Activation rates for overwintering larvae were highest at 27°C, while survivorship to adulthood following spring activation was highest at 21°C. At temperatures at or beyond 27°C, no larvae reached adulthood.
4. Warmer winters and springs will likely initially increase spring synchrony between tamarack and larch case bearer, exposing larvae to younger, potentially more nutritious foliage, but extremely warm spring temperatures may decrease survivorship of larvae to adulthood.

     

The role of low-level ozone exposure and mycorrhizas in chemical quality and insect herbivore performance on Scots pine seedlings

The effects of low‐level ozone exposure and suppression of natural mycorrhizas on the above‐ground chemical quality of Scots pine (Pinus sylvestris L.) needles and insect herbivore performance were studied in a two‐year field experiment. Seedlings were fumigated with the ozone doses 1.5–1.7 times the ambient, and natural mycorrhizal infection level was about 35% reduced in roots with fungicide propiconazole. On ozone‐exposed seedlings the mean relative growth rate (MRGR) of Lygus rugulipennis Popp. nymphs was lower than on ambient ozone seedlings, but Gilpinia pallida Klug sawfly larvae grew better on elevated ozone seedlings than on ambient ozone seedlings. MRGR of Schizolachnus pineti Fabr. and Cinara pinea L. aphid nymphs or Neodiprion sertifer Geoffr. sawfly larvae or the oviposition of L. rugulipennis and N. sertifer were not affected by ozone exposure. Although ozone exposure did not affect total phenolics, total terpene, total or individual resin acid, total free amino acid, nutrient or sugar concentrations in needles, MRGR of L. rugulipennis positively correlated with total terpenes and MRGR of G. pallida positively with total amino acids. In addition, ozone exposure increased serine and proline concentration and marginally also starch concentration in needles. When mycorrhizas were reduced with fungicide, only MRGR of L. rugulipennis nymphs increased, but performance of other insect herbivores studied was not changed. However, number of L. rugulipennis eggs correlated positively with mycorrhizal infection level and also with total sugars. Reduction of mycorrhizas did not strongly affect the concentrations of analysed compounds in needles, because only phosphorus and potassium and some individual resin acids were reduced by fungicide treatment. These results suggest that low‐level ozone exposure and moderately declined mycorrhizal infection do not drastically affect either the above‐ground chemical quality of Scots pine seedlings or performance of studied insect herbivores.     

Cold treatment enhances low‐temperature flight performance in false codling moth,Thaumatotibia leucotreta (Lepidoptera: Tortricidae)

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Warming erodes individual-level variability in life history responses to predation risk in larvae of the mayfly Cloeon dipterum

1. Warming and predation risk are ubiquitous environmental factors that can modify life histories and population dynamics of aquatic ectotherms. While separate responses to each of these factors are well understood, their joint effects on individual life histories and population dynamics remain largely unexplored. Current theory predicts that the magnitude of prey behavioural, physiological, and life history responses to predation risk should diminish with warming due to the reduced metabolic scope. However, empirical support for this prediction remains equivocal, and experiments covering a substantial proportion of individual prey ontogeny until maturation are lacking.
2. To fill these gaps, we ran a laboratory experiment to investigate how warming and non-consumptive predation risk influence life history responses in the larvae of the mayfly Cloeon dipterum, an aquatic insect with highly plastic development. We reared larvae of varying initial sizes at three temperatures (21, 24, and 27°C) in a risk-free environment and under predation risk signalled by chemical cues from dragonfly larvae (Aeshna cyanea), and followed their individual survival, growth, and development until emergence.
3. Some C. dipterum larvae substantially prolonged their development and the proportion of these slow individuals declined rapidly with temperature and increased with predation risk. We attribute this response to cohort splitting, a common life history strategy of aquatic insects and other taxa in unpredictable environment.
4. Growth, development, and maturation varied predictably with temperature in the fast larvae that did not prolong their development. They grew and developed faster but matured at smaller sizes with increasing temperature. Predation risk tended to slow down individual growth and development in line with the reduced metabolic scope hypothesis, but the differences were relatively minor and observable only at 21°C.
5. Survival to subimago increased with predation risk, possibly due to indirect effects mediated by dissolved micronutrients, but did not vary significantly with temperature. Survival also tended to be higher in the slow individuals. This partly compensated for a smaller final size relative to the fast individuals and made both strategies comparable in overall fitness.
6. Our results show that warming may erode individual-level variability in life history responses to predation risk. This implies that warming can synchronise population dynamics and consequently make such populations more vulnerable to unpredictable disturbances.

     

Performance of pine looper Bupalus piniarius larvae under population build-up conditions

In the first year of an outbreak, Bupalus piniarius larvae, encounter intense crowding. In the later stages of larval development, they are forced to feed on the non-preferred current-year needles of Scots pine or even on alternative hosts. It was hypothesized that larval feeding on a non-preferred resource (current-year needles) will negatively affect B. piniarius performance. It was also hypothesized that larval mutual interference (crowding without competition for food) will have negative additive effects. These hypotheses were tested in laboratory and field experiments. Fourth instar larvae were reared singly and under crowded conditions in cohorts of ten. Larvae in both situations were reared on control branches (containing both mature and current-year shoots) and branches containing only current-year shoots. Crowded larvae were reared also on Norway spruce, an alternative host. Crowding and feeding on a non-preferred resource had opposite effects on B. piniarius larval performance. Crowding in the late larval instars enhanced larval performance while absence of the preferred resource had negative effects. Larval growth rate was higher and development time was shorter for larvae exposed to crowded conditions than for solitary larvae. There was, however, no difference between the groups in final pupal weights or survival. Survival was 25% lower for larvae feeding on non-preferred current-year needles and pupal weights 9% lower, compared with results for larvae feeding on mature needles. Larvae feeding on Norway spruce suffered greatly extended development time, 82% lower survival, and resulted in 60% lighter pupae compared with conspecifics on Scots pine. It was concluded that not only quantity but also quality of the available food resource is critical for B. piniarius development.     

Importance of temperature for the performance and biocontrol efficiency of the parasitoid Perilitus brevicollis (Hymenoptera: Braconidae) on Salix

With the prospect of warmer temperatures as a consequence of ongoing climate change, it is important to investigate how such increases will affect parasitoids and their top-down suppression of herbivory in agroecosystems. Here we studied how the performance and biocontrol efficiency of the willow “bodyguard” Perilitus brevicollis Haliday (Hymenoptera: Braconidae) were affected at different constant temperatures (10, 15, 20, 25°C) when parasitizing a pest insect, the blue willow beetle (Phratora vulgatissima L., Coleoptera: Chrysomelidae). Parasitism did not reduce herbivory at all at 10°C, indicating poor biocontrol efficiency at low temperatures. At higher temperatures, however, parasitism reduced herbivory substantially, implying that biocontrol may be promoted by a warmer climate. Parasitoid performance (survival and development rate) generally increased with increasing temperature up to 20°C. The only exception was body size, which followed the temperature–size rule and decreased with increasing temperature. Our results indicate that a warmer climate may enhance the biocontrol of the blue willow beetle in environments that currently are cooler than the parasitoid’s optimal temperature for development.     

Occurrence of two different development patterns in Lobesia botrana (Lepidoptera: Tortricidae) larvae during the second generation

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Effect of temperature on the reproductive success,developmental rate and brood characteristics of Ips sexdentatus (Boern.)

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Evergreen foliage allows early hatching in a pine processionary moth and escape from predation

1. Seasonal variation in leaf quality and climate conditions often imposes constraints on the temporal occurrence of tree‐feeding insect larvae, but the seasonal effects of predation have received limited attention. In temperate climate zones, both the abundance and activity of predators can be expected to vary over time. 2. The study reported herein examined the impact of temporal variation in predator activity levels on the life history of an herbivorous insect feeding on a constant food source: previous‐year needles of Scots pine (Pinus sylvestris L.). In field experiments, the survival and growth rates of colonies of Thaumetopoea pinivora Treitschke larvae that had been manipulated to hatch at three different dates were compared. Eggs of T. pinivora usually hatch by mid‐April in southern Sweden, which is earlier than most other herbivorous insects that overwinter as eggs in this region. 3. Predator exclusion experiments indicated that larvae which hatched later than April experienced a higher level of predator activity, mainly by ants. The final larval size and the timing of pupation were not affected by hatching date. First instar larvae were more extensively preyed on than second instars. 4. The life history of herbivore species can be affected by seasonal variation in predation pressures. This study suggests that early hatching in a lepidopteran species can allow a temporal escape from predation during the vulnerable early life stages.     

The effects of drought and herbivory on plant–herbivore interactions across 16 soybean genotypes in a field experiment

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Predicting the distribution of the two bark beetles Tomicus destruens and Tomicus piniperda in Europe and the Mediterranean region

• 1 Various factors such as climate and resource availability influence the geographical distributions of organisms. Species sensitive to small temperature variations are known to experience rapid distribution shifts as a result of current global warming, sometimes leading to new threats to agriculture and forests. Tomicus piniperda and Tomicus destruens (Coleoptera, Curculionidae, Scolytinae) cause economic damage to pines in Europe and around the Mediterranean Basin. However, their respective potential distributions have not yet been studied at a large scale. The present study aimed to investigate the influence of climatic and host factors on the geographical distributions of both Tomicus species in Europe and around the Mediterranean Sea, and to establish maps of suitable areas.
• 2 Using 114 published localities where the presence or absence of both species was unambiguously recorded, we gathered WorldClim meteorological records to correlate the occurrence of insects with bioclimatic variables and to build potential distribution maps.
• 3 The two studied Tomicus species presented parapatric distributions and opposite climate demands, with T. destruens occurring in locations with warmer temperatures, whereas T. piniperda occurs under a colder climate. Amongst the investigated climate variables, temperature appeared to be most correlated with both species distributions.
• 4 The potential ranges of both species were further restricted by the availability of pine hosts. It appeared that setting new pine plantations in regions where T. destruens or T. piniperda are still absent could favour a rapid expansion of their distributions. Our data will be useful when aiming to apply management strategies adapted to each species, and to forecast their potential range expansions/contractions as a result of climate warming.

     

Effects of increased water temperature on leaf litter quality and detritivore performance: a whole‐reach manipulative experiment

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Carry over effects of the entomopathogen Bacillus thuringiensis ssp. Kurstaki on Choristoneura fumiferana (Lepidoptera: Tortricidae) progeny under various stressful environmental conditions

Abstract

• 1 In the present study, we documented the lethal and sublethal effects of the entomopathogen Btk on spruce budworm and its progeny under various environmental conditions. We hypothesized that aerial spray of Bacillus thuringiensis ssp. kurstaki (Btk) could affect the biological performance of the surviving spruce budworm (Choristoneura fumiferana Clem.) populations and their progenies and that Btk sublethal effects could be widened by other types of stress (i.e. temperature conditions and changes in food suitability from year to year).
• 2 The results from a 3‐year field experiment indicated that Btk treatments decreased the fitness of the surviving larvae whatever the prevailing temperature and nutritional conditions.
• 3 The detrimental Btk effects on the parental generation carried over to the offspring. The percent of egg hatch and first‐instar survival were negatively affected by Btk whatever other stress spruce budworm parents underwent.
• 4 The present study also highlighted the fact that the effects of temperature and nutritional stress suffered by the parents could carry over to the next generation. Balsam fir flowering, which provided larvae with pollen rich in nitrogen, favoured both the parental generation and the fitness of their offspring. Spruce budworm mothers allocated to their progenies large amounts of energy reserves (triglycerides and glycogen) that greatly enhanced the survival of the early stages.
• 5 Egg hatch and the survival of first‐instar larval progeny were drastically affected when their parents had reduced larval growth as a result of exposure to cool temperatures that had desynchronized insect and bud phenology.
• 6 Budworm mothers submitted to negative impacts of previous defoliation allocated low amounts of energy reserves to their progeny. This lack of energy associated with unfavourable temperature conditions (i.e. high temperatures in late summer and in early fall and an extended cool period in spring) drastically reduced survival of diapausing second‐instar larvae.
• 7 These results highlight the importance of considering the various sources of stress when attempting to evaluate the impact of a control agent on an insect pest population and its progenies.

     

Effect of forest stand type on host plant quality and direct and indirect effects on pine sawfly performance

1. Observed lower levels of herbivory in mixed compared with monoculture stands have been hypothesized to depend on top-down forces, through higher predation pressure by natural enemies or through bottom-up mechanisms through plant quality effects on herbivore performance.
2. In this study, we compared the performance measured as host plant induced mortality, cocoon weight, and predation mortality of the European pine sawfly Neodiprion sertifer (Geoffroy) (Hymenoptera, Diprionidae) in mixed and monoculture forest stands.
3. We did not observe a difference in host plant induced mortality, cocoon weight, or predation mortality between mixed and monoculture forest stands. We did find an effect of local conditions around each experimental tree on pine sawfly performance. For example, the nitrogen content of pine needles is negatively affected by the proportion of pine around the experimental tree, which in turn increases the survival of sawfly larvae.
4. The results suggest that local conditions around individual trees are more important for the performance of the European pine sawfly than stand type, i.e. mixed or monoculture plant stands.
5. We conclude that the ongoing trend for diversification within commercial forestry calls for more research where the effects of both bottom-up and top-down effects are studied at several spatial scales.