Climate change and the potential for range expansion of the Lyme disease vector Ixodes scapularis in Canada

We used an Ixodes scapularis population model to investigate potential northward spread of the tick associated with climate change. Annual degree-days >0 degrees C limits for I. scapularis establishment, obtained from tick population model simulations, were mapped using temperatures projected for the 2020s, 2050s and 2080s by two Global Climate Models (the Canadian CGCM2 and the UK HadCM3) for two greenhouse gas emission scenario enforcings 'A2'and 'B2' of the Intergovernmental Panel on Climate Change. Under scenario 'A2' using either climate model, the theoretical range for I. scapularis establishment moved northwards by approximately 200 km by the 2020s and 1000 km by the 2080s. Reductions in emissions (scenario 'B2') had little effect on projected range expansion up to the 2050s, but the range expansion projected to occur between the 2050s and 2080s was less than that under scenario 'A2'. When the tick population model was driven by projected annual temperature cycles (obtained using CGCM2 under scenario 'A2'), tick abundance almost doubled by the 2020s at the current northern limit of I. scapularis, suggesting that the threshold numbers of immigrating ticks needed to establish new populations will fall during the coming decades. The projected degrees of theoretical range expansion and increased tick survival by the 2020s, suggest that actual range expansion of I. scapularis may be detectable within the next two decades. Seasonal tick activity under climate change scenarios was consistent with maintenance of endemic cycles of the Lyme disease agent in newly established tick populations. The geographic range of I. scapularis-borne zoonoses may, therefore, expand significantly northwards as a consequence of climate change this century.     

Horizontal transfer of the insect growth regulator pyriproxyfen to larval microcosms by gravid Aedes albopictus and Ochlerotatus triseriatus mosquitoes in the laboratory

The insect growth regulator (IGR) pyriproxyfen is highly active against mosquitoes (Diptera: Culicidae). Through continuous emersion of large larvae (instars 3-4) the concentration causing 50% inhibition of adult emergence (EI50) was determined as 0.200 p.p.b. for Aedes albopictus (Skuse) and 3.5 to 7 times less for Ochlerotatus triseriatus (Say): IE50 0.0288 p.p.b. As a possible method of application to larval microcosms of these species that oviposit in water containers and phytotelmata, the horizontal transfer of pyriproxyfen to larval microcosms by adult mosquitoes was evaluated under laboratory conditions. Gravid females were forced to walk on surfaces treated with pyriproxyfen (tarsal contact exposure) and then allowed to oviposit in larval microcosms. Using replicate bioassay cages, each with an oviposition container, and a factorial experimental design, we assessed Ae. albopictus for the effects of (i) pyriproxyfen concentration (0.2, 0.3 and 0.4 mg/cm2) contacted by gravid females, and (ii) the number of treated gravid females added to bioassay cages (one, three or five females/cage), on the mortality of larvae in oviposition containers. Only 0.2 mg/cm2 treatment rate was tested on Oc. triseriatus. A significant (P < 0.05) curvilinear response in inhibition of emergence (IE) was achieved on both species. Densities of one or three treated Oc. triseriatus females/cage yielded IE rates of only 21-27%, whereas five treated females/cage resulted in 70% inhibition. With Ae. albopictus, densities of three or five treated females/cage yielded 48-67% and 59-73% IE, respectively, whereas one treated female/cage gave only 4-30% inhibition. Use of IGR-treated oviposition containers to achieve horizontal transfer of pyriproxyfen to mosquito oviposition sites could be a field management technique based on mosquito biology and behaviour. In binary choice tests with Ae. albopictus, horizontal transfer of pyriproxyfen from a container with a treated ovistrip (0.3 or 0.4 mg/cm2) to an untreated microcosm resulted in 14-38% inhibition. In larval bioassays, pyriproxyfen activity declined markedly within 10 days. Forcibly exposing gravid female mosquitoes to pyriproxyfen-treated paper surface did not affect their fecundity. However, from the 1st to 2nd gonotrophic cycles the egg hatch rate declined by 30% (P < 0.05). Some variation of results could be due to interactions between females at the oviposition site, possibly causing disproportionate transfer of pyriproxyfen to larval microcosms. Comparative studies of the oviposition behaviour of each mosquito are warranted and would potentially provide information needed to improve the technique.     

Cover Caption

Females of the common green blow fly, Lucilia sericata, ovipositing on rat carrion. When females oviposit in aggregations, even‐aged larval offspring develop faster and fewer are preyed upon. These benefits imply that females engage in coordinated, pheromone‐mediated oviposition behavior. Yet, new data show that oviposition site‐seeking females do not respond to pheromones but to semiochemicals associated with gravid or non‐gravid feeding flies, taking chances that resources are suitable for oviposition and that ovipositing flies are present (see pages 651–660). Image: Sean McCann.     

Suboptimal Larval Habitats Modulate Oviposition of the Malaria Vector Mosquito Anopheles coluzzii

Selection of oviposition sites by gravid females is a critical behavioral step in the reproductive cycle of Anopheles coluzzii, which is one of the principal Afrotropical malaria vector mosquitoes. Several studies suggest this decision is mediated by semiochemicals associated with potential oviposition sites. To better understand the chemosensory basis of this behavior and identify compounds that can modulate oviposition, we examined the generally held hypothesis that suboptimal larval habitats give rise to semiochemicals that negatively influence the oviposition preference of gravid females. Dual-choice bioassays indicated that oviposition sites conditioned in this manner do indeed foster significant and concentration dependent aversive effects on the oviposition site selection of gravid females. Headspace analyses derived from aversive habitats consistently noted the presence of dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS) and 6-methyl-5-hepten-2-one (sulcatone) each of which unitarily affected An. coluzzii oviposition preference. Electrophysiological assays across the antennae, maxillary palp, and labellum of gravid An. coluzzii revealed differential responses to these semiochemicals. Taken together, these findings validate the hypothesis in question and suggest that suboptimal environments for An. coluzzii larval development results in the release of DMDS, DMTS and sulcatone that impact the response valence of gravid females.     

Utilisation of morphological features in life table studies of Liriomyza huidobrensis (Dipt., Agromyzidae) developing in lettuce

Abstract: A method of distinguishing different larval instars of Liriomyza huidobrensis morphologically, using measurements of the cephalopharyngeal skeleton was developed. The growth ratios of cephalopharyngeal skeletons between first and second and second and third instar larvae were 1.80 and 1.47, respectively, enabling clear separation to be achieved for experimental work. Using this method the development rates of the immature stages of L. huidobrensis feeding on Lactuca sativa were determined under constant temperatures of 11, 16, 19, 26 and 28 ± 1°C and were shown to increase linearly with temperature over the range investigated. The theoretical lower threshold temperatures for development from oviposition to the end of each larval instar or pupal stage were 5.35, 6.30, 6.20 and 5.70°C, respectively. The overall threshold temperature for development from oviposition to 50% adult emergence (5.70°C) was used to calculate degree‐day (DD) requirements for development from oviposition to each larval instar or pupal eclosion, which were 84.3, 30.1, 58.9, 143.7 DD, respectively. The use of these data for optimizing the timing of application of control agents which are effective against specific developmental stages is discussed.     

Forecasting emergence and movement of overwintering hazelnut big bud mites from big buds

Eriophyoid big bud mites are key pests of hazelnut throughout the world, but they are difficult to control with chemicals or other methods because they are protected inside the bud. The most effective time for control is during the relatively short emergence period which is difficult for growers to predict. The key objectives of this study were to monitor mite emergence from big buds in spring, determine the phenology of mites in relation to tree phenology and weather, and identify the optimum timing for control measures. Mite emergence was found to occur between early and late spring in Canterbury, New Zealand. Mite emergence and movement occurred when daily maximum temperatures were >15 degrees C and when mean temperatures were >9 degrees C, with mite emergence increasing with temperature. The developmental status of new buds during mite emergence was a crucial factor in the infestation of new buds. An accumulated heat sum model (DD), starting at Julian date 152 and using a lower threshold temperature of 6 degrees C, predicted the onset of emergence on two cultivars and at two sites at approximately 172 DD. A regression model based on leaf number, bud length, bud width, DD and Julian date provided a more satisfactory prediction of percent accumulated mite emergence. It is recommended both peak mite emergence and the developmental status of hazelnut buds be used to optimise the time to apply control measures. The optimum time to apply a control was predicted to be before buds measure 0.5 x 0.5 mm (width x length), are enclosed within the axil, and have a rounded tip, or, when 50% accumulated mite emergence has occurred, whichever occurs first.     

Laboratory evaluation of Toxorhynchites splendens (Diptera: Culicidae) for predation of Aedes albopictus mosquito larvae

Biology of the mosquito Toxorhynchites splendens (Wiedemann) was studied in the laboratory to provide baseline data for using the predatory larvae of this species against those of Aedes albopictus (Skuse) in a biological control programme. The mean incubation time of Tx.splendens eggs was 43.8 h and the time required for newly-hatched larvae to initiate predation was 2.5 h. Mean numbers of prey larvae consumed and killed by each Tx.splendens larva totalled 389 and 345 respectively. The larval period of Tx.splendens was not significantly different for rearing individually or in groups of nine, with equal prey density, and duration of larval development was proportional to prey density. In mass rearing, larval cannibalism was usually observed during days 1-3 post-eclosion. The incidence of cannibalism decreased sharply on the fourth day after hatching when some larvae became fourth-instar. Adult female Tx.splendens usually commenced oviposition on day 4 after emergence. The number of eggs laid daily increased on day 7 and the peak oviposition of 6.3 eggs/female/day occurred on day 11. When oviposition containers were provided only intermittently, gravid females of Tx.splendens scattered most of their eggs on the dry floor of the cage. Viability of eggs laid by females aged 4-14 days was high (60-90%) but decreased to less than 40% as the females aged.     

A dynamic population model to investigate effects of climate on geographic range and seasonality of the tick Ixodes scapularis

A dynamic population model of Ixodes scapularis, the vector of a number of tick-borne zoonoses in North America, was developed to simulate effects of temperature on tick survival and seasonality. Tick development rates were modelled as temperature-dependent time delays, calculated using mean monthly normal temperature data from specific meteorological stations. Temperature also influenced host-finding success in the model. Using data from stations near endemic populations of I. scapularis, the model reached repeatable, stable, cyclical equilibria with seasonal activity of different instars being very close to that observed in the field. In simulations run using data from meteorological stations in central and eastern Canada, the maximum equilibrium numbers of ticks declined the further north was the station location, and simulated populations died out at more northerly stations. Tick die-out at northern latitudes was due to a steady increase in mortality of all life stages with decreasing temperature rather than a specific threshold event in phenology of one life stage. By linear regression we investigated mean annual numbers of degree-days >0 degrees C (DD>0 degrees C) as a readily mapped index of the temperature conditions at the meteorological stations providing temperature data for the model. Maximum numbers of ticks at equilibrium were strongly associated with the mean DD>0 degrees C (r2>0.96, P<0.001), when the Province of origin of the meteorological station was accounted for (Quebec>Ontario, beta=103, P<0.001). The intercepts of the regression models provided theoretical limits for the establishment of I. scapularis in Canada. Maps of these limits suggested that the range of southeast Canada where temperature conditions are currently suitable for the tick, is much wider than the existing distribution of I. scapularis, implying that there is potential for spread. Future applications of the model in investigating climate change effects on I. scapularis are discussed.     

Salicylic Acid Induces Changes in Mango Fruit that Affect Oviposition Behavior and Development of the Oriental Fruit Fly,Bactrocera dorsalis

The Oriental fruit fly, Bactrocera dorsalis (Hendel) is an important quarantine pest around the globe. Although measures for its control are implemented worldwide through IPM and male annihilation, there is little effect on their population. Hence, there is a need for new strategies to control this minacious pest. A strategy that has received negligible attention is the induction of ‘natural plant defenses’ by phytohormones. In this study, we investigated the effect of salicylic acid (SA) treatment of mango fruit (cv. Totapuri) on oviposition and larval development of B. dorsalis. In oviposition choice assays, gravid females laid significantly less eggs in SA treated compared to untreated fruit. Headspace volatiles collected from SA treated fruit were less attractive to gravid females compared to volatiles from untreated fruit. GC-MS analysis of the headspace volatiles from SA treated and untreated fruit showed noticeable changes in their chemical compositions. Cis-ocimene and 3-carene (attractants to B. dorsalis) were reduced in the headspace volatiles of treated fruit. Further, reduced pupae formation and adult emergence was observed in treated fruit compared to control. Increased phenol and flavonoid content was recorded in treated fruit. We also observed differential expression of anti-oxidative enzymes namely catalase (CAT), polyphenoloxidase (PPO) and peroxidase (POD). In summary, the results indicate that SA treatment reduced oviposition, larval development and adult emergence of B. dorsalis and suggest a role of SA in enhancing mango tolerance to B. dorsalis.     

Temperature thresholds and degree-day model for Marmara gulosa (Lepidoptera: Gracillariidae)

The developmental thresholds for Marmara gulosa Guillén & Davis (Lepidoptera: Gracillariidae) were investigated in the laboratory by using 17, 21, 25, 29, and 33 degrees C. The lowest mortality occurred in cohorts exposed to 25 and 29 degrees C. Other temperatures caused >10% mortality primarily in egg and first and second instar sap-feeding larvae. Linear regression analysis approximated the lower developmental threshold at 12.2 degrees C. High mortality and slow developmental rate at 33 degrees C indicate the upper developmental threshold is near this temperature. The degree-day (DD) model indicated that a generation requires an accumulation of 322 DD for development from egg to adult emergence. Average daily temperatures in the San Joaquin Valley could produce up to seven generations of M. gulosa per year. Field studies documented two, five, and three overlapping generations of M. gulosa in walnuts (Juglans regia L.; Juglandaceae), pummelos (Citrus maxima (Burm.) Merr.; Rutaceae), and oranges (Citrus sinensis (L.) Osbeck; Rutaceae), for a total of seven observed peelminer generations. Degree-day units between generations averaged 375 DD for larvae infesting walnut twigs; however, availability of green wood probably affected timing of infestations. Degree-day units between larval generations averaged 322 for pummelos and 309 for oranges, confirming the laboratory estimation. First infestation of citrus occurred in June in pummelo fruit and August in orange fruit when fruit neared 60 mm in diameter. Fruit size and degree-day units could be used as management tools to more precisely time insecticide treatments to target the egg stage and prevent rind damage to citrus. Degree-day units also could be used to more precisely time natural enemy releases to target larval instars that are preferred for oviposition.     

Oviposition deterrent, ovicidal and gravid mortality effects of ethanolic extract of Andrographis paniculata Nees against the malarial vector Anopheles stephensi Liston (Diptera: Culicidae)

Oviposition is an important phenomenon of mosquitoes and has recently become the focus in the concept of integrated vector control management. In the present study, we evaluated oviposition deterrent, ovicidal and mortality effects of ethanolic extract of Andrographis paniculata Nees against gravid and oviposited females of Anopheles stephensi Liston. Water treated with the ethanolic extract had a high deterrent activity in ovipositing females: oviposition activity index values for the test species were –0.28, –0.45, –0.49 and –0.59 for extract concentrations of 29, 35, 41 and 46 p.p.m., respectively. High degrees of mortality were observed with various concentrations of extract: 1.12 (control) to 11.70 for gravid females, and 0.65 (control) to 10.25 for oviposited females. The highest mortality in both gravid and oviposited females was observed soon after they came in contact with oviposition medium treated with the extract, and this was found to be significant at doses higher than 35 p.p.m., suggesting possible contact toxicity of the extract. The extract caused moderate ovicidal activity against various age groups of A. stephensi, but it inflicted delayed effects such as high larval, pupal and adult mortality. The age of the eggs and the duration of the extract treatment influenced the ovicidal activity observed. It is clear that ethanolic extract of A. paniculata Nees can affect the oviposition cycle of A. stephensi Liston, thereby suppressing the vector population and adversely influencing transmission of the disease pathogen.     

Effect of low temperatures on mortality and oviposition in conjunction with climate mapping to predict spread of the root weevil Diaprepes abbreviatus and introduced natural enemies

The tropical root weevil, Diaprepes abbreviatus (L.), has been a pest of citrus and ornamental plants since its introduction into Lake County, FL, in 1964. Since then, it has colonized the Florida peninsula to the south of its point of introduction but has not expanded its range to the north. A lower threshold for oviposition by D. abbreviatus was estimated as 14.9 degrees C. Eggs were highly susceptible to cold, with 95% mortality (LTime95) occurring in 4.2 d at 12 degrees C. Relative susceptibility of life stages to cold was eggs > pupae > larvae > adults. Archived weather data from Florida were examined to guide a mapping exercise using the lower developmental threshold for larvae (12 degrees C) and the lower threshold for oviposition (15 degrees C) as critical temperatures for mapping the distribution of D. abbreviatus and the potential for establishment of egg parasitoids. Probability maps using the last 10 yr of weather data examined the frequency of at least 10, 15, 20, 25, or 30 d per winter when soil temperature was 相似文献   

Is aggregated oviposition by the blow flies Lucilia sericata and Phormia regina (Diptera: Calliphoridae) really pheromone‐mediated?

When female blow flies Lucilia sericata and Phormia regina (Diptera: Calliphoridae) oviposit in aggregations on carrion, even‐aged larval offspring reportedly develop faster, and fewer are parasitized or preyed upon. The benefits of aggregated oviposition equally affect con‐ and heterospecific larvae sharing a resource. The benefits imply that female blow flies engage in coordinated, pheromone‐mediated oviposition behavior. Yet, repeated attempts to identify oviposition pheromones have failed invoking doubt that they exist. Simply by regurgitating and feeding on carrion, flies may produce attractive semiochemicals. If flies were to aggregate in response to feeding flies rather than ovipositing flies, then the semiochemical cue(s) may be associated with the salivary gland. Working with L. sericata and P. regina and using liver as a surrogate oviposition medium, we test the hypotheses, and present data in their support, that (i) gravid or nongravid females ovipositing and/or feeding on liver enhance its attractiveness to gravid and nongravid females; (ii) females respond to semiochemicals from feeding heterospecific females; (iii) females respond equally well to semiochemicals from feeding con‐ and heterospecific females; (iv) macerated head tissues of females applied to liver enhance its attractiveness; and (v) females in direct contact with and feeding on liver, but not when next to yet physically separated from liver, enhance attraction of flies. We conclude that oviposition site‐seeking females do not respond to an oviposition pheromone. Instead, they appear to coopt semiochemicals associated with feeding flies as resource indicators, taking chances that resources are suitable for oviposition, and that ovipositing flies are present.     

An autodissemination station for the transfer of an insect growth regulator to mosquito oviposition sites

A prototype autodissemination station to topically contaminate oviposition-seeking container-dwelling mosquitoes with the insect growth regulator, pyriproxyfen, was developed and tested in the laboratory. Our test subject was the Asian tiger mosquito, Aedes albopictus (Skuse) (Diptera: Culicidae), an urban species that colonizes small-volume cryptic larval habitats and is a skip ovipositor that visits multiple containers. The station consists of a water reservoir to attract gravid females, which is joined to a transfer chamber designed to contaminate visiting mosquitoes. The unit is easily constructed by moulding wet shredded cardboard using corn starch as a binder. The essential criteria that must be met to prove the efficacy of an autodissemination station require it to demonstrate effectiveness in attracting the target insect, in transferring the toxicant to the insect that will disperse the agent, and in facilitating the subsequent transfer of the toxicant from the insect to target habitats at a lethal concentration. Cage experiments demonstrated that the unit was readily accepted by gravid females as an oviposition site. A powder formulation of pyriproxyfen-impregnated silica particles adhered to visiting Ae. albopictus females (mean: 66 particles/female), although particles were lost over time. In cage (2.2 m(3) ) trials, pyriproxyfen-charged stations resulted in 100% inhibition of adult emergence, whereas in small-room (31.1 m(3) ) trials, 81% emergence inhibition was recorded. The venereal transfer of pyriproxyfen from contaminated males to virgin females was also observed, and pyriproxyfen was subsequently transferred to water-holding containers at concentrations that inhibited emergence. Key autodissemination station features include lack of maintenance requirements, biodegradable construction, low cost and low risk.     

Modeling mountain pine beetle (Dendroctonus ponderosae) oviposition

Mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae, Scolytinae), is a significant forest disturbance agent with a widespread distribution in western North America. Population success is influenced by temperatures that drive phenology and ultimately the adult emergence synchrony required to mass attack and kill host trees during outbreaks. In addition to lifestage‐specific developmental rates and thresholds, oviposition timing can be a source of variance in adult emergence synchrony, and is a critical aspect of mountain pine beetle phenology. Adaptation to local climates has resulted in longer generation times in southern compared to northern populations in common gardens, and the role of oviposition rate in these differences is unclear. Oviposition rates and fecundity in a northern population have been described, although data are lacking for southern populations. We assessed southern mountain pine beetle oviposition rates and fecundity in a range of temperatures using a non‐destructive technique that included frequent X‐ray imaging. We found that oviposition rate and fecundity vary independently such that a female with high oviposition rate did not necessarily have high fecundity and vice versa. Observed fecundity within the 30‐day experimental period was lowest at the lowest temperature, although estimated potential fecundity did not differ among temperatures. Females at varying temperatures have the potential to lay similar numbers of eggs, although it will take longer at lower temperatures. Southern mountain pine beetle reared in Pinus strobiformis Engelm. (Pinaceae) had a higher upper threshold for oviposition, a similar lower threshold, and slightly greater potential fecundity compared to a northern population reared in Pinus contorta Douglas. A comparison of modeled oviposition rates between the two populations, which could be influenced by host tree, suggests that differences in oviposition rate do not explain observed differences in total generation time. Our oviposition model will facilitate development of a phenology model for southern mountain pine beetle populations.     

Stochastic dynamic population model for northern corn rootworm (Coleoptera: Chrysomelidae).

A stochastic dynamic population model for the complete life cycle of northern corn rootworm, Diabrotica barberi Smith & Lawrence, is described. Adult population dynamics from emergence to oviposition are based on a published single-season model for which temperature-dependent development and age-dependent advancement determine adult population dynamics and oviposition. Randomly generated daily temperatures make this model component stochastic. Stochastic hatch is 50+/-8%. A stochastic nonlinear density-dependent larval survival model is estimated using field data from artificial infestation experiments. A regional model of corn phenology is estimated to incorporate the effect of dispersal on adult mortality. Random daily weather is generated using parameters for Brookings, SD. Model performance is evaluated with deterministic simulations, which show that the population converges to zero unless adult mortality is reduced by the availability of corn pollen from the regional model of corn phenology. Stochastic model performance is evaluated with stochastic daily weather, egg hatch, and larval survival in various combinations. Sensitivity analysis is conducted to evaluate model responsiveness to each parameter. Model results are generally consistent with published data.     

The question of fluctuating asymmetry in the blacklegged tick Ixodes scapularis (Acari: Ixodidae)

An analysis of fluctuating asymmetry was conducted on populations of the blacklegged tick Ixodes scapularis. The eight groups used in this study consisted of larvae and nymphs and males and females from the states of Minnesota, Massachusetts, Maryland, Missouri, North Carolina and Georgia and the F1 progenies of reciprocal crosses between ticks from Massachusetts and Georgia. Measurements included 16 larval, 19 nymphal, ten female and 12 male bilateral characters. Only five differences between the right and left bilateral characters had normal distributions with means of zero and differences in variances between the groups. These five characters included three setal lengths of the larvae, the spiracular plate length of females and the coxa I internal spur widths of males. Bivariate plots of character size ((R+L)/2) and asymmetry (R-L) showed no correlation. In the spiracular plate lengths of females and one of the setal lengths, ticks from Massachusetts had significantly less within-group variance than all the other groups. The only character in which fluctuating asymmetry was observed was the coxa I internal spur width of males, in which ticks from Minnesota, Missouri and North Carolina had significantly greater variance than the remaining groups; fluctuating asymmetry in this character may be explained by sexual selection. The cross progeny did not demonstrate any fluctuating asymmetry, as would be expected if the northern and southern forms of I. scapularis were true species. The virtual lack of fluctuating asymmetry in the characters used in this study further supports the conclusions of other studies which concluded that I. scapularis is a species with clinal variation and a broad geographic distribution.     

The effect of a predatory leech,Nephelopsis obscura,on mortality,growth, and production of chironomid larvae in a small pond

Summary The effect of a predatory leech, Nephelopsis obscura, on survivorship, growth, and production of chronomid larvae was studied by enclosure experiments carried out in a small pond. The prey population was composed almost entirely of the tubiculous, microphagous chironomid larvae, Chironomus riparius and Glyptotendipes paripes. Nephelopsis significantly reduced chironomid survivorship within the enclosures, and accounted for most of the measured mortality of fourth instar larvae. The cropping by Nephelopsis was not significantly biased toward either prey species. In long-term experiments (66 d) chironomid biomass in enclosures without leeches reached much higher levels than in enclosures containing Nephelopsis. This increase in biomass was due to growth of surviving larvae, rather than recruitment, since emergence and oviposition were not going on during the course of the experiments. The enhanced survivorship of larvae within leech-free enclosures was eventually accompanied by reduced growth and specific production (daily production/biomass) for C. riparius, which made up about 90% of the larval population. Growth and specific production of G. paripes (10% of larval population) was not affected. Short-term experiments (25 d) involving manipulation of densities and species ratio (9:1 CR:GP and 1:9 CR:GP) of larvae revealed that growth of the majority species was strongly influenced by larval density, whereas growth of the minority species was not. The same pattern was observed both in the presence and in the absence of Nephelopsis and was a result of differences in resource utilization between the two species. In the shortterm experiments, growth rates estimated for larvae exposed to leeches were significantly less than those for larvae in leech-free enclosures. This could be due either to size-biased consumption of larvae by Nephelopsis, or possibly a disturbance factor leading to reduced larval food intake and/or increased metabolic costs.     

The effect of temperature on the egg incubation period of Capnia bifrons (Plecoptera: Capniidae) from Windermere (English Lake District)

Gravid females of Capnia bifrons (Newman) from Windermere (English Lake District) were almost completely ovoviviparous, the eggs hatching within 15 min after oviposition in the water. When kept in the laboratory at constant temperatures between 3.8 and 19.8°C, few females survived to lay eggs at temperatures above 12.1°C. The relationship between air temperature (T°C) and the egg incubation period (Y days between fertilisation and oviposition) was given by the regression equation: Y = 316.4 T^−0.9996 (r²= 0.957, p < 0.001). This equation successfully predicted egg incubation periods for gravid females kept in cages in the field.
Comparisons with similar studies on four non-ovoviviparous species of Plecoptera showed that egg development was rarely more rapid in C. bifrons . It was also shown that the hypothesis of ovoviviparity being an adaptation to combat low water temperatures could be rejected for C. bifrons from Windermere.