Univoltine Life Cycle Research Articles

Environmental drivers influencing stonefly assemblages along a longitudinal gradient in karst lotic habitats

Stoneflies are among the most sensitive aquatic insect taxa and therefore arguably the best indicator of the excellent, i.e. pristine, ecological status of surface streams. Karst habitats are one of the most exciting freshwater habitats in terms of biological-geological interplay. They, in turn, support a biodiversity far superior to surrounding freshwater habitats and hence these habitats are designated as biodiversity hotspots. Our study deals with both of these crucial ecological players. We studied stonefly assemblages, their microhabitat preferences and emergence patterns along a karst oligotrophic hydrosystem. The sampling was conducted monthly from March 2007 to December 2008 using pyramid-type emergence traps set in various habitats and associated microhabitats (e.g. springs, rivers, streams, tufa barriers × moss, angiosperm, cobble, sand, silt substrates). Favorable environmental conditions, such as a wide range of karst habitat types with low water temperature and high oxygen concentration, resulted in high stonefly species richness (31 recorded species). Water temperature and pH had the highest influence on stonefly assemblages. Species richness and diversity decreased in a downstream direction. We recorded a longitudinal shift from crenal-epirhithral to epirhithral-metarhithral assemblages with some hyporhithral and potamal elements. Upstream sites were dominated by shredders, while downstream sites had a higher proportion of gatherers-collectors. Several species showed a significant preference for a specific microhabitat type in accordance with their feeding strategies and food availability. The majority of recorded species exhibited univoltine life cycles slow or fast.

Summer egg diapause in a matchstick grasshopper synchronizes the life cycle and buffers thermal extremes.

The phenological response is among the most important traits affecting a species' sensitivity to climate. In insects, strongly seasonal environments often select for a univoltine life cycle such that one seasonal extreme is avoided as an inactive stage. Through understanding the underlying mechanisms for univoltinism, and the consequences of its failure, we can better predict insect responses to climate change. Here we combine empirical data and simulation studies to investigate the consequences of an unusual diapause mechanism in a parthenogenetic matchstick grasshopper, Warramaba virgo, from arid southern Australia. Our field body temperature measurements indicate that this species is a thermoconformer and our laboratory studies of the thermal response of feeding rate imply strong constraints on winter activity. However, the species exhibits no obligate winter diapause, and eggs can develop in 1 month under constant temperatures approximating the mean soil temperature at the time of oviposition (summer). We show that diurnal temperature cycles exceeding a peak of 36 °C inhibit egg development in summer, and that this is sufficient to prevent autumnal hatching of eggs. Development is also strongly retarded below 24 °C. Microclimate-driven simulation studies of egg development show that these thermal responses provide robust maintenance of a univoltine life cycle, thereby resulting in survival of heat stress as an egg (due to limited developmental state) and avoidance of cold stress as a nymph and adult (due to overwintering in the soil as an egg).

Establishment of Striacosta albicosta (Lepidoptera: Noctuidae) as a Primary Pest of Corn in the Great Lakes Region

Western bean cutworm, Striacosta albicosta Smith (Lepidoptera: Noctuidae), is a pest of corn, Zea maize L., and dry edible beans, Phaseolus sp. L., native to the western United States. Following the range expansion into the U.S. Corn Belt, pheromone trap monitoring began in the Great Lakes region in 2006. The first S. albicosta was captured in Michigan in 2006 and in Ontario, Canada in 2008. Pheromone traps were used to document spread and increasing captures of S. albicosta across Michigan and Ontario until 2012. Trapping confirmed the univoltine life cycle of S. albicosta in this region and identified peak flight, typically occurring in late July. Overwintering of S. albicosta in this region was confirmed by emergence from infested fields and overwintering experiments. Multiple soil textures were infested with prepupae, and recovery was assessed throughout the winter. Overwintering success was not affected by soil texture; however, prepupae were found at greater depths in coarse-textured soils. Soil temperatures at overwintering depths did not reach the supercooling point. Injury to corn by S. albicosta increased in incidence, severity and geographic range from 2010 to 2014 in field plots. Decreasing control of injury by Cry1F corn hybrids was observed over time. These findings show that S. albicosta has established as a perennial corn pest in the Great Lakes region due to observations of overwintering success and unmanaged injury. We recommend S. albicosta obtain primary pest status in this region within regulatory framework and a resistance management plan be required for traits targeting this pest.

Spatial spread of Wolbachia in Rhagoletis cerasi populations.

The bacterial endosymbiont Wolbachia has been used to control insect pests owing to its ability to manipulate their life history and suppress infectious diseases. Therefore, knowledge on Wolbachia dynamics in natural populations is fundamental. The European cherry fruit fly, Rhagoletis cerasi, is infected with the Wolbachia strain wCer2, mainly present in southern and central European populations, and is currently spreading into wCer2-uninfected populations driven by high unidirectional cytoplasmic incompatibility. Here, we describe the distribution of wCer2 along two transition zones where the infection is spreading into wCer2-uninfected R. cerasi populations. Fine-scale sampling of 19 populations in the Czech Republic showed a smooth decrease of wCer2 frequency from south to north within a distance of less than 20 km. Sampling of 12 Hungarian populations, however, showed a sharp decline of wCer2 infection frequency within a few kilometres. We fitted a standard wave equation to our empirical data and estimated a Wolbachia wave speed of 1.9 km yr−1 in the Czech Republic and 1.0 km yr−1 in Hungary. Considering the univoltine life cycle and limited dispersal ability of R. cerasi, our study highlights a rapid Wolbachia spread in natural host populations.

Anatomical and phenological implications of the relationship between Schinus polygama (Cav.) (Cabrera) and the galling insect Calophya rubra (Blanchard).

The success of galling insects could be determined by synchronisation with host plant phenology and climate conditions, ensuring suitable oviposition sites for gall induction and food resources for their survival. The anatomical, histochemical and phenological synchronisation strategies between Calophya rubra (Blanchard) (Hemiptera: Psylloidea) and its host, the evergreen plant Schinus polygama (Cav.) (Cabrera) (Anacardiaceae), in the Mediterranean climate of southern Chile was evaluated and compared to that of the congeneric C. cf. duvauae (Scott) from Brazil and closely related host plant S.engleri in a subtropical climate. Anatomical, histometric, histochemical and vegetative phenology studies of the stem and galls were conducted from June 2015 to December 2016. Based on the anatomical, histometric and histochemical analysis, the conical stem gall traits imply gains over the non-galled stem toward the galling insect survival, but the maintenance of phellem, secretory ducts and pith indicate conservative developmental traits that cannot be manipulated by C.rubra. Our results indicate that the conditions of the Mediterranean climate zone limit C.rubra immature activity during unfavourable periods, probably determining a diapause period and a univoltine life cycle, which are peculiarities of the S.polygama- C.rubra system. The synchronisation between development and seasonality confers peculiarities to the S.polygama- C. rubra system in the Mediterranean climate zone.

Effects of resource-dependent cannibalism on population size distribution and individual life history in a case-bearing caddisfly.

Resource availability often determines the intensity of cannibalism, which has a considerable effect on population size distribution and individual life history. Larvae of the caddisfly Psilotreta kisoensis build portable cases from sedimentary sands and often display cannibalism. For this species, the availability of preferable case material is a critical factor that affects larval fitness, and material is locally variable depending on the underlying geology. In this study, we investigated how sand quality as a case material determines cannibalism frequency among larvae and, in turn, how the differential cannibalism frequency affects the body-size distribution and voltinism. Rearing experiments within a cohort revealed that a bimodal size distribution developed regardless of material quality. However, as the preferable material became abundant, the proportion of larger to smaller individuals increased. Consecutive experiments suggested that smaller larvae were more frequently cannibalized by larger ones and excluded from the population when preferable smooth material was abundant. This frequent cannibalism resulted in a bimodal size distribution with a significantly higher proportion of larger compared to smaller individuals. The size-dependent cannibalism was significantly suppressed when the larvae were raised in an environment with a scarcity of the preferable case material. This is probably because larvae cannot enjoy the benefit of rapid growth by cannibalism due to the difficulties in enlarging their case. At low cannibalism the growth of smaller individuals was stunted, and this was probably due to risk of cannibalism by larger individuals. This growth reduction in small individuals led to a bimodal size-distribution but with a lower proportion of larger to smaller individuals compared to at high cannibalism. A field study in two streams showed a similar size distribution of larvae as was found in the rearing experiment. The bimodal ratio has consequences for life history, since a size-bimodal population causes a cohort splitting: only larvae that were fully grown at 1 year had a univoltine life cycle, whereas larvae with a stunted growth continued their larval life for another year (semivoltine). This study suggests that availability of preferable case building material is an important factor that affects cannibalism, which in turn affects larval population size structure and cohort splitting.

Life History, Growth, Survival, and Secondary Production of Ancyronyx variegatus (Germar, 1824) (Coleoptera: Elmidae) in Elk Creek, Erie County, Pennsylvania, USA

Abstract Larvae and adults of Ancyronyx variegatus (Germar, 1824) were collected monthly from Elk Creek, Erie County, Pennsylvania, USA. Ancyronyx variegatus has a univoltine life cycle with six instars. Primary recruitment months were April, May, and June, and adults were present throughout the year. The annual secondary production of A. variegata in Elk Creek was 0.55784 g/m2/yr. The mean standing stock biomass was 0.14296 g/m2, and the cohort production/biomass ratio was 3.902. The estimate of secondary production is important because it combines individual growth and population survivorship into a single measurement, which leads to a better understanding of the ecological importance of A. variegatus in the wood-dwelling aquatic insect community.

Three-year lifecycle, large body, and very high threshold temperature in the cricket Gryllus argenteus for special adaptation to desiccation cycle in Malawi.

In temperate climates, the initiation and termination of diapause synchronize the stress-tolerant stage with the stressful season and reproduction with the non-stressful season in many insects. Synchronization is often regulated by photoperiodism.Voltinism and the ultimate size of adults are also important determinants for their lifecycle, and different diapause stages and voltinism patterns are known in crickets.Here, we investigated the life history of the African cricket Gryllus argenteus from Malawi, which is a typical arid tropical highland. The climate is characterized by alternating arid and wet seasons, each of which lasts for half a year, and where the available heat mass is much less than lowlands at the same latitude. We first measured the nymphal duration at each rearing temperature and calculated the lower developmental threshold (t 0) to be 20.19°C based on Ikemoto and Takai (2000) and 19.38°C based on a conventional line-fitting method. These values are very high relative to many other insects. The local temperature in winter does not fall below 15°C, but this is much higher than the lethal limit. This suggested that critical stress in this locality was not coldness but low precipitation in winter. We estimated, based both on local temperature change and the Ikemoto and Takai's t 0, that G. argenteus required 3years to complete its lifecycle unlike wet lowland species, where univoltinism or multi-voltinism are commonplace. Photoperiodism was observed in this species, but due to a lag between annual cycles in photoperiod, temperature, and humidity, photoperiodism alone cannot atune their lifecycle with local conditions.Synchronization in this species was achieved by three different adaptations: photoperiodism, high t 0, and large body size, which give it a long lifecycle. Although the species cannot achieve a univoltine lifecycle because of its high t0 value, it can escape from dry season by entering diapause at moderate temperatures, probably thereby achieving adaptive synchrony of lifecycle with both favorable and unfavorable seasons. A comparison between a conventional photothermogram and a newly formulated photohydrogram or photohygrogram demonstrates that even though sufficient heat is available, scarcity of water and thus scarcity of foliage should force the cricket to maintain diapause at intermediate temperature. The results suggested that high t 0, large body size, and multi-ennial lifecycle mutually affect each other and formulate a unique adaptation under such an extreme environment.

Diapause and overwintering of two spruce bark beetle species.

Diapause, a strategy to endure unfavourable conditions (e.g. cold winters) is commonly found in ectothermic organisms and is characterized by an arrest of development and reproduction, a reduction of metabolic rate, and an increased resistance to adversity. Diapause, in addition to adaptations for surviving low winter temperatures, significantly influences phenology, voltinism and ultimately population growth. We review the literature on diapause and overwintering behaviour of two bark beetle species that affect spruce‐dominated forests in the northern hemisphere, and describe and compare how these strategies can influence population dynamics. The European spruce bark beetle Ips typographus (L.) (Coleoptera, Curculionidae) is the most important forest pest of Norway spruce in Europe. It enters an adult reproductive diapause that might be either facultative or obligate. Obligate diapausing beetles are considered strictly univoltine, entering this dormancy type regardless of environmental cues. Facultative diapausing individuals enter diapause induced by photoperiod, modified by temperature, thus being potentially multivoltine. The spruce beetle Dendroctonus rufipennis (Kirby) (Coleoptera: Curculionidae) infests all spruce species in its natural range in North America. A facultative prepupal diapause is averted by relatively warm temperatures, resulting in a univoltine life cycle, whereas cool temperatures induce prepupal diapause leading to a semivoltine cycle. An adult obligate diapause in D. rufipennis could limit bi‐ or multivoltinism. We discuss and compare the influence of diapause and overwinter survival on voltinism and population dynamics of these two species in a changing climate and provide an outlook on future research.

Ground crickets singing in volcanic warm “islets” in snowy winter: Their seasonal life cycles, photoperiodic responses and origin

AbstractThe ground cricket Dianemobius nigrofasciatus overwinters as an egg in Japan, being univoltine in Hokkaido and northern Honshu and bivoltine farther south. In Hokkaido, however, this cricket is heard singing in winter in several fumarolic fields covered with moss and grasses locally known as “bokke”. In such warm “islets” the adult density was high in early summer and again in autumn, indicating that the cricket is bivoltine in contrast to the univoltine life cycle outside the bokke habitats in Hokkaido. Eggs laid by females collected at regular intervals from a bokke habitat showed a clear seasonal cycle of diapause incidence. At 26°C, the bokke strains produced non‐diapause eggs under long days and diapause eggs under short days as in the southern bivoltine populations, although the critical day‐length was longer than in the south. Several strains derived from non‐bokke habitats in Hokkaido and northern Honshu produced high percentages of diapause eggs under long days as well as short days as expected for the univoltine life cycle. Winter adults singing in bokke habitats could be either survivors of the autumn generation or individuals derived from eggs laid in autumn and then matured in response to the high soil temperature. In the laboratory, the proportion of egg diapause in short days was decreased by selection only for several generations. Phylogenetic trees of bokke and non‐bokke populations, based on both the nucleotide sequence of the mitochondrial COI gene and four allozyme loci, suggest that bokke populations have not been isolated from non‐bokke populations for an evolutionarily significant time.

Life cycles of dominant mayflies (Ephemeroptera) on a torrent of the high Bolivian Andes

The mayflies of the temperate and cold zones have well-synchronized life cycles, distinct cohorts, short emergence and flight periods. In contrast, aquatic insects from the tropical zones are characterized by multivoltine life cycles, “non-discernible cohorts” and extended flight periods throughout the year. This report is the first observation of life cycle patterns made of two species of mayflies on a torrent in the high elevation Bolivian Andes. The samples were taken from four sites and four periods during a hydrological season. The life cycle of each species was examined using size-class frequency analysis and a monthly modal progression model (von Bertalanffy’s model) to infer the life cycle synchrony type. These first observations showed a moderately synchronized univoltine life cycle for Andesiops peruvianus (Ulmer, 1920), whereas Meridialaris tintinnabula Pescador and Peters (1987), had an unsynchronized multivoltine life cycle. These results showed that the generalization of all aquatic insects as unsynchronized multivoltine species in the Andean region may not be entirely accurate since there is still a need to further clarify the life cycle patterns of the wide variety of aquatic insects living in this high elevation tropical environment.

A large‐scale latitudinal pattern of life‐history traits in a strictly univoltine damselfly

1. Variation in thermal conditions and season length along latitudinal gradients affect body size‐related traits over different life stages. Selection is expected to optimise these size traits in response to the costs and benefits.2. Egg, hatchling, larval and adult size in males and females were estimated along a latitudinal gradient of 2730 km across Europe in the univoltine damselfly Lestes sponsa, using a combination of field‐collection and laboratory‐rearing experiments. In the laboratory, individuals were grown in temperatures and photoperiod simulating those at the latitude of origin, and in common‐garden conditions.3. The size of adults sampled in nature was negatively correlated with latitude. In all populations the females were larger than the males. Results from simulated and common‐garden rearing experiments supported this pattern of size difference across latitudes and between sexes, suggesting a genetic component for the latitudinal size trend and female‐biased size dimorphism. In contrast, hatchling size showed a positive relationship with latitude, but egg size, although differing between latitudes, showed no such relationship.4. The results support a converse Bergmann cline, i.e. a negative body size cline towards the north. This negative cline in body size is probably driven by progressively stronger seasonal time and temperature constraints towards the higher latitudes and by the obligate univoltine life cycle of L. sponsa. As egg size showed no relationship with latitude, other environmental factors besides temperature, such as desiccation risk, probably affect this trait.

Overwintering biology and limits of cold tolerance in larvae of pistachio twig borer, Kermania pistaciella.

Pistachio twig borer, Kermania pistaciella is an important pest of pistachio trees. It has an univoltine life-cycle and its larvae tunnel and feed inside pistachio twigs for almost 10 months each year. The last larval instars overwinter inside the twigs. Survival/mortality associated with low temperatures during overwintering stage is currently unknown. We found that overwintering larvae of the Rafsanjan (Iran) population of K. pistaciella rely on maintaining a stably high supercooling capacity throughout the cold season. Their supercooling points (SCPs) ranged between -19.4 and -22.7°C from October to February. Larvae were able to survive 24 h exposures to -15°C anytime during the cold season. During December and January, larvae were undergoing quiescence type of dormancy caused probably by low ambient temperatures and/or changes in host tree physiology (tree dormancy). Larvae attain highest cold tolerance (high survival at -20°C) during dormancy, which offers them sufficient protection against geographically and ecologically relevant cold spells. High cold tolerance during dormancy was not associated with accumulation of any low-molecular mass cryoprotective substances. The SCP sets the limit of cold tolerance in pistachio twig borer, meaning that high mortality of overwintering populations can be expected only in the regions or years where or when the temperatures fall below the average larval SCP (i.e., below -20°C). Partial mortality can be expected also when temperatures repeatedly drop close to the SCP on a diurnal basis.

Development time plasticity of the pine processionary moth (Thaumetopoea pityocampa) populations under laboratory conditions

The pine processionary moth Thaumetopoea pityocampa (Lepidoptera, Notodontidae) is a univoltine defoliator that is active over a wide range of latitudes and elevations, being largely influenced by temperature variations, especially during larval development across the winter. This work compares field development time with that observed in the laboratory rearing under controlled conditions, in four Th. pityocampa populations characterized by different life history phenology: two populations from the Italian Alps characterized by early and late adult emergence, and two populations from Portugal, the first characterized by winter feeding and late adult emergence, the second by a switch of the larval feeding from winter to summer. The rearing started from the egg stage and was maintained in the laboratory at 20-25°C under natural light in transparent boxes. In spite of the different geographic origins and asynchrony of the period of larval development, all populations maintained an annual life cycle under laboratory conditions, as well as a phenology similar to that of the field populations. Such an outcome was possible due to a trade-off in the duration of the larval and pupal stages, the latter being identified as the phase of development when an efficient regulatory mechanism is acting to maintain the univoltine life cycle.

Does nutritional behavior affect gut endosymbiont prevalence? The case of Paramoebidium avitruviense (Mesomycetozoa) within Siphonoperla torrentium (Plecoptera) nymphs

Aquatic macroinvertebrates harbor a broad spectrum of gut endosymbionts, among them trichomycetes species (including fungi and protozoa) are predominantly associated with shredders, collector or scraper hosts, which only rarely may ingest some animal tissues. An exception to this rule may be the protozoan Paramoebidium avitruviense , documented within opportunistic-predator nymphs of the stonefly Siphonoperla torrentium . In the present work, we studied a population of S. torrentium bearing P. avitruviense during 21 months to describe the life cycle and feeding habits of S. torrentium . We tested for associations between host diet, process of infestation and the prevalence of P. avitruviense , and provide information about the temporal dynamics of both the stonefly and the protozoan. Siphonoperla torrentium presented a univoltine life cycle and displayed an omnivorous diet which changed toward an invertebrate-based diet in older nymphs. The endosymbiont P. avitruviense showed fluctuations in relation to the host nymphal size, and prevalence peaks concurred with the highest host growth rate. Binary logistic regressions revealed that the diet influenced the prevalence of the protozoan, at least for some of the gut content fractions. Whereas an increment of the animal fraction decreased the probability of being infested, the consumption of other fractions related to a gatherer–collector activity increased the probabilities of infestation. However, the most decisive factor affecting endosymbiont prevalence was the melanization grade of S. torrentium , whereby the higher the melanization of the nymph (= longer post-ecdysis period), the higher the infestation probability by P. avitruviense .

Diapause induction and relaxed selection on alternative developmental pathways in a butterfly.

Seasonal phenotypic plasticity entails differential trait expression depending on the time of season. The facultative induction of winter diapause in temperate insects is a developmental switch mechanism often leading to differential expression in life-history traits. However, when there is a latitudinal shift from a bivoltine to univoltine life cycle, selection for pathway-specific expression is disrupted, which may allow drift towards less optimal trait values within the non-selected pathway. We use field- and experimental data from five Swedish populations of Pararge aegeria to investigate latitudinal variation in voltinism, local adaptation in the diapause switch and footprints of selection on pathway-specific regulation of life-history traits and sexual dimorphism in larval development. Field data clearly illustrated how natural populations gradually shift from bivoltinism to univoltinism as latitude increases. This was supported experimentally as the decrease in direct development at higher latitudes was accompanied by increasing critical daylengths, suggesting local adaptation in the diapause switch. The differential expression among developmental pathways in development time and growth rate was significantly less pronounced in univoltine populations. Univoltine populations showed no significant signs of protandry during larval development, suggesting that erosion of the direct development pathway under relaxed selection has led to the loss of its sex-specific modifications.

Life cycle of Huarpea fallax (Hymenoptera: Sapygidae) in a xeric forest in Argentina

This study describes the life cycle of Huarpea fallax (Hymenoptera: Sapygidae) in a xeric forest in La Pampa province, Argentina. This cleptoparasitic wasp attacks the nests of two species of leaf-cutter bees: Megachile catamarcensis and Anthidium vigintipunctatum, both belonging to the family Megachilidae. Nests of these bee species were obtained during a trap-nesting programme. Adult emergence showed a unimodal pattern indicating a univoltine life cycle. The period from egg-laying to adult emergence lasted for 10–13 months; however, one female took about 2 years to emerge, suggesting parsivoltinism. Most females attack one cell per host nest, the outermost cells being the ones most parasitized. However, the position of the attacked cells was variable. In this paper, although there were insufficient data to prove a correlation, the data suggest a positive trend between body size of sapygid wasps and their host bees.

Nesting ecology of Megachile (Chrysosarus) catamarcensis Schrottky (Hymenoptera: Megachilidae), a Prosopis-specialist bee

SummaryNests of the leafcutting bee Megachile (Chrysosarus) catamarcensis Schrottky (Hymenoptera: Megachilidae) were obtained during a 24 month trap-nesting programme in a xeric forest in the Province of La Pampa, Argentina. Nests were constructed in December and January; females made an average of 4.0 ± 2.2 cells, and used mud with flower petals and/or leaf pieces to build their nests. Provisions of cells were practically pure pollen mass of Proposis caldenia. Adult emergence showed a unimodal pattern, suggesting a univoltine life cycle. However, two individuals that emerged a month after the traps were collected indicate the existence of a small early-summer generation. This could indicate that M. catamarcensis, in some years (warmer) or in some populations (at lower latitudes), could exhibit a bivoltine life cycle. Approximately 15.3 % of all offspring failed to complete development to the adult stage, and an additional 13.9 % were killed by natural enemies. These included diverse parasitic wasps (Chrysididae, Sapygidae, and Leucospidae), a cleptoparasite bee (Megachilidae: Coelioxys), and a bee fly (Bombyliidae: Antrhax).

Density- and Size-Dependent Winter Mortality and Growth of Late Chaoborus flavicans Larvae

Winter processes such as overwinter survival and growth of individuals can have wide-ranging consequences for population dynamics and communities within and across seasons. In freshwater organisms winter processes have been mainly studied in fish despite that invertebrates also have substantial impacts on lake and pond food webs. One of the major invertebrate consumers in lake and ponds is the planktonic larvae of the dipteran insect Chaoborus spec. However, while much is known about Chaoborus feeding ecology, behaviour and structuring role in food webs, its winter ecology and how it affects its populations are poorly understood. Here size- and density-dependent winter mortality and body growth of late Chaoborus flavicans larvae were quantified over naturally occurring size and density ranges in autumn and under natural winter conditions using two field enclosure experiments. Winter mortality increased with autumn density but decreased with autumn body size while winter growth rates decreased with autumn density and body sizes. There was also a density- and size-independent background mortality component. The proportion of pupae found in spring decreased strongly and exponentially with autumn density. These results may explain the commonly observed univoltine life cycle and multi-annual density fluctuations in northern Chaoborus populations. They further demonstrate the relevance of winter processes and conditions for freshwater invertebrates and ecosystems.

Host testing, establishment and biology of the gorse soft shoot moth, Agonopterix umbellana (Fabricius) (Lepidoptera: Oecophoridae), a potential biological control agent for gorse, Ulex europaeus L. (Fabaceae), in Australia

Host specificity tests on over 250 species or cultivars of plants including 37 Australian native and introduced plant species or cultivars showed that the gorse soft shoot moth, Agonopterix umbellana, would be unlikely to survive on any plant other than gorse in Australia. The first field release was at Jericho, Tasmania during spring 2007. By the summer of 2013 high larval densities of A. umbellana were evident at the release point, with dispersal recorded to a maximum of 200m over ca. 2ha. Recoveries at other release sites in Tasmania as well as in South Australia and Victoria suggest that the species will eventually establish across south-eastern Australia. The life cycle of A. umbellana is closely synchronised with gorse phenology. Population increase and dispersal is expected to be slow as a consequence of its univoltine life cycle incorporating the narrow larval feeding period of 3months when gorse is a suitable food source. At Jericho, oviposition commenced in late winter which enabled hatching to coincide with the presence of new gorse growth for developing larvae. Larvae fed over a 3month period on new growth available from mid-October to mid-January passing through six instars before pupation. The first adults were recorded in early February emerging in an obligate reproductive diapause which was completed by late July. Controlled temperature cabinets held at 12, 15, 18, 21 and 24°C were used to estimate lower development thresholds (LDT) and thermal constants. The LDT for eggs larvae, pupae and total development were 8.7, 7.5, 7.9 and 7.9°C, respectively and the thermal constants were 143.0°, 526.3°, 278.0° and 971.0° degree days respectively. In the long term, it is hoped that A. umbellana will contribute to the biological control of gorse in Australia. This will be in combination with the gorse seed weevil, Exapion ulicis, the gorse spider mite, Tetranychus lintearius and the gorse thrips, Sericothips staphylinus, even though earlier studies have shown these other agents are constrained either by predation in the case of T. lintearius or the effects of gorse phenology in the case of S. staphylinus and E. ulicis.