Population dynamics in changing environments: the case of an eruptive forest pest species

In recent decades we have seen rapid and co‐occurring changes in landscape structure, species distributions and even climate as consequences of human activity. Such changes affect the dynamics of the interaction between major forest pest species, such as bark beetles (Coleoptera: Curculionidae, Scolytinae), and their host trees. Normally breeding mostly in broken or severely stressed spruce; at high population densities some bark beetle species can colonise and kill healthy trees on scales ranging from single trees in a stand to multi‐annual landscape‐wide outbreaks. In Eurasia, the largest outbreaks are caused by the spruce bark beetle, Ips typographus (Linnaeus), which is common and shares a wide distribution with its main host, Norway spruce (Picea abies Karst.). A large literature is now available, from which this review aims to synthesize research relevant for the population dynamics of I. typographus and co‐occurring species under changing conditions. We find that spruce bark beetle population dynamics tend to be metastable, but that mixed‐species and age‐heterogeneous forests with good site‐matching tend to be less susceptible to large‐scale outbreaks. While large accumulations of logs should be removed and/or debarked before the next swarming period, intensive removal of all coarse dead wood may be counterproductive, as it reduces the diversity of predators that in some areas may play a role in keeping I. typographus populations below the outbreak threshold, and sanitary logging frequently causes edge effects and root damage, reducing the resistance of remaining trees. It is very hard to predict the outcome of interspecific interactions due to invading beetle species or I. typographus establishing outside its current range, as they can be of varying sign and strength and may fluctuate depending on environmental factors and population phase. Most research indicates that beetle outbreaks will increase in frequency and magnitude as temperature, wind speed and precipitation variability increases, and that mitigating forestry practices should be adopted as soon as possible considering the time lags involved.     

Prepupal diapause and instar IV developmental rates of the spruce beetle, Dendroctonus rufipennis (Coleoptera: Curculionidae, Scolytinae)

The spruce beetle, Dendroctonus rufipennis (Kirby), is an important mortality agent of native spruces throughout North America. The life-cycle duration of this species varies from 1 to 3 years depending temperature. The univoltine cycle (one generation per year) is thought to maximize outbreak risk and accelerate host mortality in established outbreaks. Prepupal diapause is associated with the semivoltine cycle (one generation per 2 years) and we investigated thermal conditions that result in diapause induction. Preliminary experiments used respirometry in an attempt to distinguish the diapause state of experimental insects but the technique was apparently confounded by low respiration before and during pupation, regardless of diapause status. Therefore, diapause induction was deduced using developmental delays. The observed developmental response was not a “switch”, with developmental delay either present or absent, but instead varied continuously. We found that temperatures <15 °C from instar III through mid-instar IV were associated with developmental delays beyond that expected from cool temperatures. Moreover, the duration of exposure to cool temperatures was important in determining the degree of developmental delay. Small, if any, delays were observed if the cumulative exposure to <15 °C was <20 d whereas >40 d cumulative exposure was associated with distinct developmental suppression. Intermediate exposure to cool temperatures resulted in minor developmental delays. We used our results to parameterize a maximum likelihood estimation model of temperature-dependent instar IV developmental rates, including the effect of diapause. This model can be included as part of a spruce beetle phenology model for predicting population dynamics.     

Seasonal cycles in stink bugs (Heteroptera,Pentatomidae) from the temperate zone: Diversity and control

The paper reviews the diversity of seasonal cycles known in stink bugs (Heteroptera, Pentatomidae) from the temperate zone and is based on the data of 43 pentatomid species studied in detail up to date (Saulich and Musolin, 2011). All the seasonal cycles realized by pentatomids in the temperate zone can be divided into two large groups: univoltine and multivoltine cycles. In univoltine cycles, only one generation is annually realized. However, univoltinism of a particular species or population can be ensured by different mechanisms: its control can be endogenous (involving an obligate diapause) or exogenous (environmental, involving a facultative diapause). Furthermore, endogenously controlled univoltine seasonal cycles can include obligate embryonic (egg) diapause (e.g., Picromerus bidens and Apateticus cynicus), obligate nymphal diapause (e.g., Pentatoma rufipes) or obligate adult (reproductive) diapause (e.g., Palomena prasina, Palomena angulosa, and Menida scotti). Exogenously controlled seasonal cycles are more flexible. Many species that are multivoltine in the subtropical or warm temperate zones are univoltine further polewards. In this case, their univoltinism is controlled exogenously, or environmentally. The mechanism often involves such seasonal adaptations as photoperiodic response of facultative winter diapause induction with a high thermal optimum (e.g., Arma custos and Dybowskyia reticulata) or a high critical threshold of winter diapause induction response (e.g., Graphosoma lineatum). The seasonal cycles of some species include not only winter diapause (hibernation) but also summer diapause (aestivation). The diapausing stage can be the same (e.g., Nezara antennata has facultative adult winter and summer diapauses) or different (e.g., Picromerus bidens survives winter in obligate embryonic diapause and spends the hottest period of summer in facultative adult aestivation). All the multivoltine cycles follow the same general pattern, with one, two, or even more directly breeding generation(s) followed by a generation that enters winter diapause. However, this sequence may be complicated by incorporation of specific seasonal adaptations such as aestivation, migrations, different forms of seasonal polyphenism or polymorphism (e.g., seasonal changes of body color), etc. Many stink bugs demonstrate geographic clines of voltinism, producing several generations in the subtropical regions (environmentally controlled multivoltine development) and two or only one generation(s) polewards (environmentally controlled bi- or univoltinism). However, some species demonstrate a strictly bivoltine seasonal cycle: they always produce two annual generations, each with either winter or summer diapause. An example is Nezara antennata which produces two generations and enters facultative winter and summer diapauses. Semivoltine seasonal cycles last more than one year. They are not very rare among insects and are known in true bugs, but have not yet been recorded among pentatomids. Examples of different seasonal cycles are described and discussed in detail. Further discussion is focused on the ecological importance of photoperiodic and thermal responses in cases of natural or artificial dispersal of pentatomids beyond their original ranges. The phytophagous Nezara viridula and the predatory Podisus maculiventris and Perillus bioculatus are used as examples. An attempt is made to compare the phylogeny of Pentatomidae and distribution of realized patterns of their seasonal development. However, it is concluded that reconstruction of phylogenetic relationships cannot yet provide a sufficient basis for prediction of realized seasonal cycles. It is suggested that the terms uni-, bi-, multi-, and semivoltinism should refer to populations rather than species, since the realized patterns of seasonal development often differ between the northern and southern populations of the same broadly distributed species.     

Diapause termination of Rhagoletis cerasi pupae is regulated by local adaptation and phenotypic plasticity: escape in time through bet‐hedging strategies

Persistence and thriving of univoltine, herbivore insect species of the temperate zone rely on obligate diapause response that ensures winter survival and synchronization with host phenology. We used a stenophagous fruit fly (Rhagoletis cerasi) with obligate pupae diapause to determine genetic and environmental effects on diapause intensity of geographically isolated populations with habitat heterogeneity. Pupae from two Greek and one German populations with various gene flow rates were exposed at five constant chilling temperatures (0–12 °C) for different durations and then incubated at a high temperature until all adults have emerged. Pupae diapause intensity differs among Greek and German populations, suggesting an adaptive response to habitat heterogeneity (mostly differences in phenology patterns of local host cultivars). Moderately warm winter temperatures, such as 8 °C, promote diapause termination in all three populations. Insufficient chilling (short duration or warmer temperatures) regulates the expression of prolonged dormancy. Interestingly, extended chilling (longer than required for terminating diapause) ‘return’ pupae to another (facultative) cycle of dormancy enabling adults to emerge during the next appropriate ‘window of time’; a strategy first time reported for univoltine insects. Consequently, diapause duration of R. cerasi is determined both by i) the adaptive response to local climatic conditions (annual dormancy) and ii) the plastic responses to interannual climatic variability resulting in two types of long life cycles within populations, prolonged and facultative dormancy as response to insufficient chilling and extended exposure to chilling, respectively. Long life cycles are expressed as a part of dormancy bet‐hedging strategies of R. cerasi populations.     

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.

     

Can spruce beetle (Dendroctonus rufipennis Kirky) pheromone trap catches or stand conditions predict Engelmann spruce (Picea engelmannii Parry ex Engelm.) tree mortality in Colorado?

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Mixed life-history strategies in a local population of the ectoparasitic fly Carnus hemapterus

A major issue for the proper understanding of the evolution of life-cycle histories is the regulation of voltinism and its variation. Diapause characteristics are known to regulate voltinism, but the underlying mechanisms are poorly understood. This paper studies diapause duration and voltinism variation in a haematophagous diptera parasitizing 2 sympatric hosts with very different breeding phenologies. We hypothesize that bivoltinism will be more frequent in carnid flies parasitizing an early breeding, multi-brooded species than in flies parasitizing a late breeder, single-brooded species. We obtained evidence of the co-occurrence of uni- and bivoltinism in both clutches of the multi-brooded Spotless starling (Sturnus unicolor) as well as in clutches of the single-brooded European roller (Coracias garrulus). Unexpectedly, the proportion of bivoltine flies was similar in both host species. A remarkable degree of host-parasite synchronization at the population level was found for bivoltine flies. Our findings reveal the facultative nature of diapause in Carnus. We discuss the influence of abiotic conditions and host availability on polymorphism in life-history cycles and the consequences both for the parasite and the host.     

Diapause development in the chestnut weevilCurculio elephas

Larval diapause development in the chestnut weevilCurculio elephas (Coleoptera, Curculionidae) was studied in the laboratory at different temperatures. The results proved that exposure to low temperatures (3–6°C) in the period December–February is not required to complete diapause. The diapause is terminated in December and from January on, the larvae can initiate post-diapause morphogenesis in the laboratory, if temperatures allow it. In the field developmental rates are negligible during winter cold (4–6°C) and only after March morphogenesis can proceed with no interruption until adult emergence. Diapause and post-diapause quiescence contribute to individual synchronization for initiation of development. The observed spread of adult emergence was 30 days in the laboratory. This variability produced during post-diapause development may be a response to annual variation in the phenology of the chestnuts.     

Adult diapause ofCybocephalus nigriceps nigriceps [Col.: Cybocephalidae]

During autumn and winter (October–February)Cybocephalus nigriceps nigriceps (J. Sahlberg) adults undergo a facultative diapause. The diapausing adults agregate in hiding places, development of the ovaries is arrested, and prey consumption is lowered. Diapause can be prevented by exposure to a long-day photoperiod (16 hr light) at high temperatures (29° and 32°C). Diapause is induced mainly by short-day conditions, but the termination of diapause is affected by both photoperiod (long-day conditions) and temperature (29°, 32° and 34°C). In mature females, diapause induction causes ovarian degeneration. The induction or inhibition of adult diapause is affected by photoperiods occurring during larval development as well as during adult life.     

Influence of temperature on spring flight initiation for southwestern ponderosa pine bark beetles (Coleoptera: Curculionidae, Scolytinae)

Determination of temperature requirements for many economically important insects is a cornerstone of pest management. For bark beetles (Coleoptera: Curculionidae, Scolytinae), this information can facilitate timing of management strategies. Our goals were to determine temperature predictors for flight initiation of three species of Ips bark beetles, five species of Dendroctonus bark beetles, and two genera of bark beetle predators, Enoclerus spp. (Coleoptera: Cleridae) and Temnochila chlorodia (Mannerheim) (Coleoptera: Ostomidae), in ponderosa pine forests of northcentral Arizona. We quantified beetle flight activity using data loggers and pheromone-baited funnel traps at 18 sites over 4 yr. Ambient air temperature was monitored using temperature data loggers located in close proximity to funnel traps. We analyzed degree-day accumulation and differences between minimum, average, and maximum ambient temperature for the week before and week of first beetle capture to calculate flight temperature thresholds. Degree-day accumulation was not a good predictor for initiation of beetle flight. For all species analyzed other than D. adjunctus Blandford, beetles were captured in traps only when springtime temperatures exceeded 15.0 degrees C. D. adjunctus was collected when maximum temperatures reached only 14.5 degrees C. Once initial flights had begun, beetles were often captured when maximum ambient air temperatures were below initial threshold temperatures. Maximum and average air temperatures were a better predictor for beetle flight initiation than minimum temperature. We establish a temperature range for effective monitoring of bark beetles and their predators, and we discuss the implications of our results under climate change scenarios.     

Survey and phylogenetic analysis of culturable microbes in the oral secretions of three bark beetle species

In a recent study, we reported a previously undescribed behavior in which a bark beetle exuded oral secretions containing bacteria that have antifungal properties, and hence defend their galleries against pervasive antagonistic Hyphomycete fungi. Actinobacteria, a group known for their antibiotic properties, were the most effective against fungi that invade the spruce beetle galleries. In the present study, we describe the isolation and identification of microorganisms from oral secretions of three bark beetles (Coleoptera: Curculionidae: Scolytinae): the spruce beetle, Dendroctonus rufipennis Kirby, the mountain pine beetle, Dendroctonus ponderosae Hopkins, and the pine engraver, Ips pini Say. Bacteria isolated from these three species span the major bacterial classes α-, β-, and γ-Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria, except for D. ponderosae , which yielded no α-proteobacteria or Bacteroidetes isolates. Spruce beetles and pine engraver beetles had similar numbers of α-proteobacteria isolates, but pine engravers yielded twice as many Bacteroidetes isolates as spruce beetles. In contrast, mountain pine beetles yielded more isolates in the β- and γ-proteobacteria than spruce beetles and pine engravers. The highest percentage of Actinobacteria was obtained from spruce beetles, followed by pine engravers and mountain pine beetles. All of the fungal isolates obtained from the three beetle species were Ascomycetes. The greatest fungal diversity was obtained in spruce beetles, which had nine species, followed by pine engravers with five, and mountain pine beetles with one.     

Threshold facilitations of interacting species

The dynamics of species interactions are of central importance for the understanding of ecological coexistence, community structure and the effects of biological invasions. Using bark beetles that colonize the same habitat as an example, we explore species interactions in a resource-based model system with positive feedback between insect abundance and resource availability. The net interspecies interaction was found to be highly dynamic and may alternate in time between competition and mutualism. When both bark beetle species were able to kill trees (“aggressive”), our simulations showed strong facilitations between beetle species. This may lead to escape from control by competition, and increase the frequency of outbreaks of tree-killing. The frequency of net positive interactions varied with interaction strengths and the relative aggressiveness of the species and was highest when both species were strongly aggressive; which predicts disastrous outbreaks if, e.g., the European spruce bark beetle Ips typographus and the North American spruce beetle Dendroctonus rufipennis should become interacting species due to introductions. In imbalanced pairs, the relatively less aggressive species was facilitated more often than the aggressive species. Net positive interactions did not occur for strongly inferior species, but their survival was an increasing function of interaction strength with aggressive species and availability of resources. The benefits for the inferior species in the model are consistent with the structure of one aggressive and several less aggressive or non-aggressive species, which is common in bark beetle communities in many parts of the world.     

Emergence patterns of univoltine and bivoltine Ips typographus (L.) populations and associated natural enemies

Control measures aiming at reducing bark beetle populations and preserving their natural enemies require a sound knowledge on their overwintering and emergence behaviour. These behavioural traits were investigated in univoltine and bivoltine populations of the European spruce bark beetle (Ips typographus [L.], Coleoptera: Scolytinae) and its predators and parasitoids over several consecutive years. In univoltine populations, roughly 50% of the bark beetles left their brood trees in fall together with most parasitoids and some significant predatory flies and beetles. In bivoltine populations, <10% of the second bark beetle generation emerged in fall and the remainder overwintered under the bark of their brood trees. Likewise, most predatory beetles and flies spent wintertime with their prey under the bark, while most parasitic wasps emerged in fall. The spring emergence of bivoltine predatory beetles was found to occur up to 3 weeks earlier than that of I. typographus, while that of the predatory flies and the parasitoids was delayed by up to 1 month. In univoltine populations, the bark beetles emerged several weeks prior to most antagonistic taxa. In the heat year 2003, three I. typographus generations were produced at the lower location, 36% of the third generation emerged in fall, while the proportions of overwintering predators remained largely the same as in previous years. Similar to their host, more parasitoids left their brood trees in fall after warm years. The results show that sanitation felling during winter probably kills most bark beetles in bivoltine populations, but also eliminates many natural enemies. In univoltine populations, sanitation felling might be less detrimental to both I. typographus and natural enemies because a fair fraction of their populations will already have left the trees before cutting. Warmer climates may affect the interactions of bark beetles and natural enemies and thus the impact of control measures.     

Photoperiod-dependent adult diapause within a geographical cline in the multivoltine bruchid Bruchidius dorsalis

Abstract The bruchid beetle Bruchidius dorsalis Fahraeus (Coleoptera: Bruchidae) has been known to undergo larval diapause during the final instar under short photoperiods ( Kurota & Shimada, 2001 ). This species has a multivoltine life cycle and the overwintering stages show a geographical variation across Japan ( Kurota & Shimada, 2002 ). In cooler areas, overwintering occurs during the final instar, whereas in warmer climates overwintering can occur during several developmental stages: non‐diapausing young instars, diapausing instars, and adults. In this study, we investigated the adult reproductive diapause in three populations from different geographical regions to clarify the role of geographical variation on overwintering strategies. We found that: (1) B. dorsalis entered reproductive diapause in addition to larval diapause under short photoperiods, (2) diapause propensity was higher and the critical photoperiod was longer in populations from cooler regions, and (3) the sensitive photoperiod range was the first 5 days after emergence. Predictions of the overwintering stage, derived from critical photoperiods, were consistent with actual overwintering stages observed in each population. The geographical variation in diapause induction is likely to reflect the adaptive overwintering strategy in each local environment.     

Spruce beetle outbreak was not driven by drought stress: Evidence from a tree‐ring iso‐demographic approach indicates temperatures were more important

Climate change has amplified eruptive bark beetle outbreaks over recent decades, including spruce beetle (Dendroctonus rufipennis). However, for projecting future bark beetle dynamics there is a critical lack of evidence to differentiate how outbreaks have been promoted by direct effects of warmer temperatures on beetle life cycles versus indirect effects of drought on host susceptibility. To diagnose whether drought‐induced host‐weakening was important to beetle attack success we used an iso‐demographic approach in Engelmann spruce (Picea engelmannii) forests that experienced widespread mortality caused by spruce beetle outbreaks in the 1990s, during a prolonged drought across the central and southern Rocky Mountain region. We determined tree death date demography during this outbreak to differentiate early‐ and late‐dying trees in stands distributed across a landscape within this larger regional mortality event. To directly test for a role of drought stress during outbreak initiation we determined whether early‐dying trees had greater sensitivity of tree‐ring carbon isotope discrimination (?¹³C) to drought compared to late‐dying trees. Rather, evidence indicated the abundance and size of host trees may have modified ?¹³C responses to drought. ?¹³C sensitivity to drought did not differ among early‐ versus late‐dying trees, which runs contrary to previously proposed links between spruce beetle outbreaks and drought. Overall, our results provide strong support for the view that irruptive spruce beetle outbreaks across North America have primarily been driven by warming‐amplified beetle life cycles whereas drought‐weakened host defenses appear to have been a distant secondary driver of these major disturbance events.     

Investigating the abundance and flight period of bark beetles (Coleoptera: Curculionidae: Scolytinae) over elevational gradients in Sitka spruce forests

1. A warming climate, as predicted under current climate change projections, is likely to influence the population dynamics of many forest insect species. Numerous bark beetle species in both Europe and North America have already responded to a warming climate by significantly expanding their geographical ranges.
2. The aim of the current study was to investigate how populations of bark beetles within stands of Sitka spruce, a widely planted non-native commercial plantation tree species in the U.K., were likely to respond to a warming climate. Experimental plots were established in stands of Sitka spruce over elevational gradients in two commercial forest plantations, and the abundance and emergence times of key bark beetle species were assessed over a 3-year period using flight interception traps. The air temperature difference between the lowest and highest experimental plot in each forest was consistently >1°C throughout the 3-year period.
3. In general, the abundance of the most dominant bark beetle species (e.g. Trypodendron, Dryocoetes, Hylastes spp.) was higher, and emergence times tended to be earlier in the year at the lower elevation plots, where temperatures were higher, although not all bark beetle species responded in the same manner.
4. The results of the study indicated that, under the projected future climate warming scenarios, monoculture Sitka spruce stands at low elevations may potentially be more vulnerable to significant outbreak events from existing or invasive bark beetle species. Hence, consideration of establishing more resilient forests of Sitka spruce by diversifying the species composition and structure of Sitka spruce stands is discussed.

     

Effect of variation in photoperiodic response on diapause induction and developmental time in the willow leaf beetle, Plagiodera versicolora

The willow leaf beetle, Plagiodera versicolora (Coleoptera: Chrysomelidae) overwinters in adult diapause. In this study, the photoperiodic responses for diapause induction and developmental time were examined in the Ishikari (Hokkaido, Japan) population of P. versicolora. All females entered reproductive diapause under short daylength (L10:D14), but 31.7% of females did not enter diapause under long daylength (L16:D8). The developmental time from oviposition to adult emergence was significantly longer at L10:D14 than that at L16:D8. Norm of reaction curves illustrated variation among families in the photoperiodic responses for diapause induction and for developmental time. ANOVA indicated significant family × photoperiod interactions in the developmental time. At L16:D8, developmental time was positively correlated with the incidence of diapause in females. This means that a female having a longer developmental time tends to have a longer critical photoperiod. Such variation may be maintained by differences in selection pressures on the growth rate and the critical photoperiod for diapause induction between univoltine and bivoltine genotypes because Ishikari is located in a transitional area between populations with univoltine and bivoltine life cycles.     

Diapause in the seasonal cycle of stink bugs (Heteroptera,Pentatomidae) from the Temperate Zone

The paper reviews the data on diapause and related phenomena in stink bugs (Heteroptera: Pentatomidae). Using stink bugs as examples, the consecutive stages of the complex dynamic process of diapause (such as diapause preparation, induction, initiation, maintenance, termination, post-diapause quiescence, and resumption of direct development) are described and discussed. Out of 43 pentatomid species studied in relation to diapause in the Temperate Zone up to date, the majority (38 species) overwinter as adults, two species—as eggs, and another two species—as nymphs. Pentatoma rufipes is believed to be able to overwinter at different stages of its life cycle. Less than 5 % of pentatomid species are probably able to overwinter twice. Only five species have obligate diapause, others have the facultative one. Day-length and temperature are the main diapause inducing factors in the majority of species. The role of food in the control of seasonal development is essential in the pentatomid species feeding on plant seeds. In different species, different stages are sensitive to day-length. Some pentatomids retain sensitivity to photoperiod even after diapause, others lose it and become photo-refractory (temporarily or permanently). In Pentatomidae, such seasonal adaptations as photoperiodic control of nymphal growth rates, seasonal body colour change, migrations, and summer diapause (aestivation) are widely represented, whereas wing and/or wing muscle polymorphism has not been reported yet. In the subfamily Podopinae, induction of facultative reproductive winter diapause is under the control of photoperiod and temperature. All species feed on seeds and their seasonal development to a great extent reflects availability of food. However, the same food preferences and pattern of seasonal development are also characteristic to many species from the subfamily Pentatominae. All species of the subfamily Asopinae are predators. Among them, Picromerus bidens and Apateticus cynicus have obligate embryonic winter diapause, which is rear among true bugs. At the same time, A. cynicus and Podisus maculiventris belong to the same tribe but have different types of diapause: obligate embryonic diapause in A. cynicus and facultative adult diapause in P. maculiventris. Other Asopinae species studied up to date have facultative adult diapause controlled by photoperiod and temperature with probably only one exception: in Andrallus spinidens, adult diapause is controlled by temperature, and photoperiod plays only a secondary role. Thus, in spite of the similar habits and feeding types among Asopinae, the species of this subfamily have different types of diapause and the latter is controlled by different factors. In the subfamily Pentatominae, most species overwinter as adults and induction of their diapause is controlled by the long-day type photoperiodic response, in spite of the differences in their feeding preferences (within phytophagy). However, there are some exceptions in this subfamily, too: Palomena prasina, P. angulosa and Menida scotti have obligate diapause, which conditions univoltinism in these pentatomids. In M. scotti, only females have obligate adult diapause, whereas males remain physiologically active through the whole winter, this pattern being unusual for Heteroptera. The univoltine seasonal cycle of this species with summer diapause (aestivation) and apparent migrations is similar to that of shield bugs (Scutelleridae). According to the analysis of seasonal development, the evolution of seasonal adaptations in Pentatomidae does not directly reflect their phylogeny. However, individual genera, small tribes or even subfamilies have similar complexes of seasonal adaptations. At the same time, Pentatominae is a large and apparently collected taxon, but most of species in this subfamily have the same facultative adult diapause.     

Geographic variation in diapause response of adult Rhipicephalus appendiculatus ticks

Diapause in adults of the African brown ear tick Rhipicephalus appendiculatus is dependent on latitude and stock origin. Lower latitude populations, that is, those nearer the equator, show no diapause response to long or short day exposure. Diapause response to short day lengths is found in higher latitude populations, further south, and increases with increasing latitude. Nearly all individuals in the southernmost population, used in this study, enter diapause under short and long day exposure which implies that the onset of diapause in this population is obligatory. Diapause termination in higher latitude populations is induced by increasing photo period, further north, but still south of the equator, diapause is terminated by ageing. The differences in diapausing behaviour of the different populations are discussed in relation to their climatic conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Banded Elm Bark Beetle, Scolytus schevyrewi Semenov (Coleoptera, Curculionidae, Scolytinae) in North America: a taxonomic review and modifications to the Wood (1982) key to the species of Scolytus Geoffroy in North and Central America

In 2003, an Asian bark beetle, Scolytus schevyrewi Semenov (Coleoptera: Curculionidae: Scolytinae), the banded elm bark beetle, was detected for the first time in North America. This paper modifies the Wood (1982) key to the species of Scolytus Geoffroy to enable identification of Scolytus schevyrewi in North and Central America. Variation of diagnostic characters in Scolytus schevyrewi is discussed.