Long-term dynamics of a predator used for biological control and decoupling from mosquito prey in a subtropical treehole ecosystem

We identified, staged and counted the immature stages of mosquitoes from 1,826 censuses (with replacement) of the aquatic contents of ten treeholes surveyed every 2 weeks between 1978 and 1993. These time series were used to examine the population dynamics and effect on prey of the predatory mosquito Toxorhynchites rutilus. The mean annual frequency of occurrence of T. rutilus ranged from 0.02 to 0.67 among holes, and no fourth instars were recovered during a 30-month dry period. Oviposition and pupation by this species were recorded in all months, but most commonly in the spring. Overwintering larvae of the predator increased in weight during the prolonged fourth instar that preceded pupation in the spring. Time series analyses showed that the presence of a fourth instar T. rutilus significantly reduced the abundances of late-stage Aedes triseriatus mosquitoes. Pupal numbers of this prey species were more negatively affected by T. rutilus than were numbers of fourth instar A. triseriatus. Long-term declines in mean annual abundance of A. triseriatus prey during 16 years of observations on two holes were not correlated with increases in the mean annual frequencies of T. rutilus. Local extinctions of the aquatic stages of A. triseriatus within treeholes were common, but in most holes not significantly associated with the presence of T. rutilus, suggesting that predation does not routinely drive mosquito prey locally extinct in this ecosystem. The decoupling of T. rutilus and A. triseriatus, as revealed through these complete and long-term censuses, is contrasted with other reports of generalist predators causing extinctions of mosquito prey. Discrepancies among reported outcomes probably result from differences in duration of sampling periods and statistical procedures along with real differences in the intensity of predation among systems and sites. Received: 14 August 1996 / Accepted: 24 February 1997     

Feeding behaviour of the ladybeetle,Pseudoscymnus kurohime

A laboratory study was made of the feeding behaviour of the ladybeetlePseudoscymnus kurohime (Miyatake) when attacking the sugar cane woolly aphidCeratovacuna lanigera Zehntner. The 1st-instar ladybeetle larva was smaller than the 1st instar aphid nymph. All larval stages of the ladybeetle sucked out the body fluids of aphids and left their emptied corpses. The 1st, 2nd, and 3rd instar ladybeetle larvae mostly attacked 1st instar aphids, whereas the 4th-instar ladybeetle larvae attacked all stages of aphids. Ladybeetle adults ate mostly 1st-instar aphids. Young larvae attacked aphids in several different ways: (1) They crawled under an aphid, seized it by its underside and lifted it up. (2) They attacked new born nymphs at birth or shortly afterwards. (3) They fed on an aphid that had been captured by an older larva. The larvae preferred to seize with their mandibles the head or thorax of an aphid, while adults seized their prey by the abdomen. When attacked by an adult, 82% of the aphids secreted droplets from their abdominal cornicles, whereas only 7.2–12% secreted droplets when attacked by larvae. The 4th instar larvae more voracious than the younger larvae.     

Winter cold influences the spatial and age distributions of the North American treehole mosquito Anopheles barberi

Summary We conducted experiments to assess the importance of winter cold and photoperiod as factors affecting the spatial and age distributions of overwintering larvae of the treehole mosquito Anopheles barberi. Larval dormancy in A. barberi was induced by photoperiods with 14.75 h of light or less per 24 h cycle. About 75% of the larvae entering dormancy were in the second instar regardless of photoperiod. Dormant second instar larvae survived freezing at-15° C for 24 h better than dormant third instar larvae. Larvae were more likely to survive freezing at-15° C in water from treeholes in which they were commonly found in nature than in water from treeholes in which they were unlikely to occur. Female oviposition was significantly higher into water from treeholes in which larvae were likely to be found than in either water from treeholes in which larvae were not commonly found or distilled water. These findings suggest that, in the northern part of its range, the distribution of A. barberi and the age structure of overwintering cohorts are influenced by extreme winter cold. The mechanisms responsible for the distribution of larvae and the overwintering age structure are, respectively, female oviposition behavior and larval photoperiodism.     

Predation on Mosquito Larvae by Mesocyclops thermocyclopoides (Copepoda: Cyclopoida) in the Presence of Alternate Prey

The cyclopoid copepod Mesocyclops thermocyclopoides, a dominant invertebrate predator in many shallow ponds and temporary water bodies in northern India, feeds on cladocerans, rotifers, ciliates and when present, on mosquito larvae also. We studied in the laboratory the prey consumption rates of the copepod on first and fourth instar larvae of two species of mosquito (Anopheles stephensi and Culex quinquefasciatus) in relation to their density. We also studied its prey selectivity with mosquito larvae in the presence of an alternate prey (the cladocerans‐either Moina macrocopa or Ceriodaphnia cornuta) in different proportions. With either mosquito species, the copepod actively selected Instar‐I larvae, avoiding the Instar‐IV larvae, and with either instar, selected Anopheles stephensi over Culex quinquefasciatus. When prey choice included the cladoceran as an alternate prey, the copepod selected the cladoceran only when the other prey was Instar‐IV mosquito larvae. Our results point to the potential and promise of M. thermocyclopoides as a biological agent for controlling larval populations of vectorially important mosquito species.     

Selection of healthy and nuclear polyhedrosis virus infectedAnticarsia gemmatalis [Lep.: Noctuidae] as prey by nymphalNabis roseipennis [Hemiptera: Nabidae] in laboratory and on soybean

Nabis roseipennis Reuter nymphs demonstrated a preference for nuclear polyhedrosis virus (NPV) — infected over healthyAnticarsia gemmatalis Hübner larvae when offered a choice of larval prey in Petri dishes and on soybean. In Petri dishes, small (second-third instar) and large (fifth-sixth instar) nymphs attacked a significantly greater number of diseased than healthy larvae at all larval instars tested (first-fifth instars) and exposure periods (2, 5 and 24 h), except that at 2 h the number of 1^st and 3^rd instar larvae attacked by large nymphs did not differ significantly (P≤0.05). Nabis roseipennis caged with larvae on individual soybean plants in the greenhouse resulted in a generally low percentage of attack by small and large nymphs after 2 days, ranging from 5.6 to 36.7%. As in the Petri dishes, the nabids showed a significant preference for diseased larvae over healthy larvae attacked for all nabid and larval sizes on soybean, with the percentage of diseased larvae attacked ranging from 28.0 to 65.4% (P≤0.05). This preference for diseased larvae on soybean as well as in Petri dishes demonstrates that the preference was not due to the close proximity in which the host and prey were found in the Petri dishes. The preference for diseased larvae may be due to a reduction in a defensive response in late stages of disease. This material is based upon work supported in part by USDA Grant No. 83-CRCR-1-1212.     

Cannibalism of parasitoid‐attacked conspecifics in a non‐carnivorous caterpillar

Cannibalism, the killing and consumption of conspecifics, can even occur in insect species typically considered to be non‐carnivorous. Of particular interest is the cannibalism of parasitoid‐attacked conspecifics, which could reduce parasitism levels in subsequent generations for that conspecific population. This study reports on the occurrence and some of the consequences of cannibalism in parasitoid‐attacked obliquebanded leafroller, Choristoneura rosaceana (Harris) (Lepidoptera: Tortricidae). We show that larvae of C. rosaceana, which is considered to be an herbivorous caterpillar species, did not prey upon live conspecifics, but readily consumed conspecifics attacked by Habrobracon gelechiae Ashmead (Hymenoptera: Braconidae). Further examination found that C. rosaceana larvae feeding on parasitoid‐attacked conspecifics, since their fourth instar, suffered a higher mortality and reduction in body size than those fed on plant material only. The cannibalism of attacked conspecifics did not appear to offer any nutrient benefits for the cannibal. To our best knowledge, this is the first empirical example of the occurrence and some of the consequences of cannibalism by a non‐carnivorous insect on its parasitoid‐attacked conspecifics. We discuss the adaptive significance of such cannibalism on parasitoid‐attacked conspecifics with respect to a trans‐generational fitness gain for the population through the killing of the parasitoids, thereby reducing parasitism in subsequent generations.     

Morphology and development of <Emphasis Type="Italic">Pteromalus cerealellae</Emphasis> (Ashmead) (Hymenoptera: Pteromalidae) on <Emphasis Type="Italic">Callosobruchus maculatus</Emphasis> (F.) (Coleoptera: Chrysomelidae)

Pteromalus cerealellae (Ashmead) (Hymenoptera: Pteromalidae) is an ectoparasitoid of several stored-product insect pests. Very little information has been published on its biology and development in host larvae, which typically are concealed within seeds. We documented the development of P. cerealellae within fourth instar larvae of its concealed host, Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae) infesting cowpea seeds. The preimaginal life stages of the parasitoid were characterized for the first time using morphological structures revealed by microscopic techniques including scanning electron microscopy. Pteromalus cerealellae produces hymenopteriform eggs and larvae. Eggs hatch into 13-segmented first instar larvae with peripneustic condition of spiracles. The larvae have simple, tusk-like mandibles, whereas the mandibles of the pupae and the adults are of the conventional toothed types. Using statistical analyses of the sizes of the larval mandibles and head capsules in conjunction with reliable characters such as the number of exuviae on the body of parasitoid larvae, cuticular folding, and excretion of the meconium, we recorded four larval instars for P. cerealellae. The data showed significant positive correlations between larval mandible lengths and widths of larval head capsules, as well as between mandible lengths and larval instars, suggesting that mandible length is a good predictor of the number of instars in P. cerealellae. Developmental time from egg to adult emergence was ∼12 d for females and ∼11 days for males at 30 ± 1°C, 70 ± 5% r.h. and 12L:12D photoperiod.     

Selective and frequency dependent predation of aquatic mosquito predator Diplonychus indicus Venkatesan & Rao (Hemiptera: Belostomatidae) on immature stages of three mosquito species

Frequency dependent mosquito larval size (II and IV instars) and species selection by the water bug Diplonychus indicus against three mosquito species Culex quinquefasciatus, Aedes aegypti and Anopheles stephensi was studied in the laboratory. The different frequencies used for each species selection were 20:30:50, 30:50:20, 50:20:30, 25:35:40, 35:40:25 and 40:25:35 of fourth instars of the respective three prey species. All nymphal water bugs (I–V instars) selected IV instar mosquito larvae and the mean proportion of late (larger) larvae eaten by the predator instars was significantly higher than the mean proportion of early (smaller) larvae eaten (F= 2.28; P < 0.001). In all six ratios used to determine the frequency dependent mosquito species selection, all the stages of the water bug selected Ae. aegypti over the other two species (F= 452.43; P < 0.001). The mean number of mosquito larvae eaten increased as its density increased based on various ratios of larvae offered. The study indicated that the predatory efficiency of D. indicus was high when Ae. aegypti was offered as prey, suggesting the utility of this mosquito predator in the control of dengue vectors.     

Temperature: Diet Interactions Affect Survival through Foraging Behavior in a Bromeliad‐Dwelling Predator

Temperature, food quantity and quality play important roles in insect growth and survival, influencing population dynamics as well as interactions with other community members. However, the interaction between temperature and diet and its ecological consequences have been poorly documented. Toxorhynchites are well‐known biocontrol agents for container‐inhabiting mosquito larvae. We found that Toxorhynchites haemorrhoidalis larvae (Diptera: Culicidae) inhabiting water‐filled rosettes of tank bromeliads catch and eat prey of both aquatic (mosquito larvae) and terrestrial origin (ants), using distinct predatory methods. They carried out frontal attacks on ants, but ambushed mosquito larvae. In choice tests, T. haemorrhoidalis favored terrestrial prey. Temperature had a significant effect on predator development and survival through its interaction with diet, but did not alter the preference for ants. T. haemorrhoidalis larvae emerged quickly when fed only mosquito larvae, whereas all individuals died before pupation when fed only ants. We conclude that behavioral factors (i.e., attraction to ants that disturb the surface of the water) overtake physiological factors (i.e., the adverse outcome of elevated temperature and an ant‐based diet) in determining a predator's response to temperature:diet interactions. Finally, because T. haemorrhoidalis larvae preferentially feed on terrestrial insects in tank bromeliads, mosquito larvae may indirectly benefit from predation release.     

Dytiscus latissimus and D. circumcinctus (Coleoptera,Dytiscidae) larvae as predators on three case-making caddis larvae

The predacious behaviour of Dytiscus circumcinctus and D. latissimus larvae was studied experimentally. When offered different prey simultaneously, D. latissimus larvae preferred cased caddis larvae relative to mayfly nymphs and isopods, whereas in D. circumcinctus the preference order was reversed. Notonectid nymphs and tadpoles were consumed in higher numbers by D. circumcinctus than by D. latissimus larvae. D. circumcinctus larvae and instar III larvae of D. latissimus most frequently captured caddis larvae through the case wall, whereas the instar I and II larvae of D. latissimus normally attacked the thorax of the exposed larva from above the front opening of the case. Limnephilus borealis, L. nigriceps and L. rhombicus caddis larvae differed in case structure, and they were all successfully captured by D. latissimus and D. circumcinctus instar II and III larvae. Neither capture success nor ingestion efficiency varied significantly between the two Dytiscus species or between different prey species. Instar II and III D. circumcinctus larvae had shorter reaction times than those of D. latissimus. The larger L. borealis and L. rhombicus larvae were preferred by the two last Dytiscus larval instars, and the handling time of these two prey was longer than that of L. nigriceps larvae.     

Habitat segregation among African treehole mosquitoes

Abstract. 1. The division of macrohabitats, seasons and microhabitats among mosquitoes which inhabit water-filled treeholes was investigated in a wet/dry seasonal environment near the coast of Kenya. Twenty-two species from four genera (Aedes fourteen, Culex three, Eretmapodites three, Toxorhynchites two) were identified from treeholes and bamboo traps. These included five which were more abundant in other microhabitats: leaf axils, fruit husks, or abandoned snail shells. 2. Trapping in different vegetation zones or heights in forests revealed that all seventeen species examined showed opposition preferences for one or more macrohabitats. Segregation of related species by macrohabitat was particularly apparent among members of the subgenus Stegomyia of Aedes which accounted for 57% of individuals identified from one locality. 3. Seasonal overlap was low between Aedes, which tracked rainfall patterns, and Culex and Eretmapodites. Aedes bromeliae differed from other Aedes found in treeholes in that maximal larval numbers did not follow heavy rainfall. 4. Negative associations between Aedes fulgens and two congeners which co-occur in the same forest and season were attributed to preferences for tree-holes of different sizes. Positive associations between Aedes aegypti and several other Aedes were reduced when root buttresses, a subclass of larger treeholes containing more species, were removed. 5. Separation of species by growth stages may be accomplished by two schemes of egg hatching which desynchronize larval cohorts in the same tree-hole: the early pattern of Stegomyia whereby 50–95% of all eggs hatch at first immersion, and the delayed pattern of other subgenera (Diceromyia, Finlaya, Pseudarmigeres) in which less than one-third of all eggs hatch at first immersion. 6. The number of wet treeholes changed seasonally. During one year five or six periods occurred when 50% or more of the censused holes contained sufficient water for larval survival. The number of individuals and species of mosquitoes in treeholes were positively correlated with treehole volume. In a comparison of treeholes and their mosquito fauna from two forests, treehole volume and mean number of species per hole differed significantly, but mosquito density did not. 7. Ecotonal trapping sites of mixed grasses, trees, and scrub yielded the highest mean numbers of species per sample, and traps placed outside a village, the lowest. Specific dominance was significantly less in the ecotone than elsewhere. The predatory larvae of Toxorhynchites brevipalpis in ecotonal and forested traps were not correlated with species richness.     

Habitat complexity and sex-dependent predation of mosquito larvae in containers

Studies in aquatic systems have shown that habitat complexity may provide refuge or reduce the number of encounters prey have with actively searching predators. For ambush predators, habitat complexity may enhance or have no effect on predation rates because it conceals predators, reduces prey detection by predators, or visually impairs both predators and prey. We investigated the effects of habitat complexity and predation by the ambush predators Toxorhynchites rutilus and Corethrella appendiculata on their mosquito prey Aedes albopictus and Ochlerotatus triseriatus in container analogs of treeholes. As in other ambush predator-prey systems, habitat complexity did not alter the effects of T. rutilus or C. appendiculata whose presence decreased prey survivorship, shortened development time, and increased adult size compared to treatments where predators were absent. Faster growth and larger size were due to predator-mediated release from competition among surviving prey. Male and female prey survivorship were similar in the absence of predators, however when predators were present, survivorship of both prey species was skewed in favor of males. We conclude that habitat complexity is relatively unimportant in shaping predator-prey interactions in this treehole community, where predation risk differs between prey sexes.     

Testing Predictions of Displacement of Native Aedes by the Invasive Asian Tiger Mosquito Aedes Albopictus in Florida, USA

The Asian Tiger Mosquito Aedes albopictus arrived in the USA in 1985 in used automobile tires from Japan and became established in Texas. This species has since spread to become the most abundant container-inhabiting mosquito in the southeastern USA, including Florida, where it has reduced the range of another non-indigenous mosquito, Aedes aegypti. To assess the accuracy of predictions that A. albopictus would competitively exclude the native Eastern Treehole Mosquito Aedes triseriatus from tires but not from treeholes (Livdahl and Willey (1991) Science 253: 189–191), we extensively monitored the abundances of mosquito immatures before and after the Asian Tiger invaded these habitats in south Florida. These field data failed to demonstrate exclusion of A. triseriatus from treeholes following the establishment of A. albopictus in this microhabitat in 1991. However, A. albopictus had significantly higher metamorphic success and showed a significant increase in mean crowding on A. triseriatus in treeholes monitored from 1991 to 1999. In urban and suburban sites, A. triseriatus was uncommon in abandoned tires even before the arrival of A. albopictus. In some wooded sites, there is evidence for a decline in numbers of A. triseriatus in used tires and cemetery vases, but the native species has not been excluded from these habitats. Overall, the negative effect of A. albopictus on A. triseriatus has been less severe than that on A. aegypti. Experiments outdoors in surrogate treeholes showed that A. albopictus was more successful than A. triseriatus in survival to emergence in the presence of predatory larvae of the native mosquito Toxorhynchites rutilus when first instar predators encountered both prey species shortly after their hatch. Eggs of A. albopictus also hatched more rapidly than those of A. triseriatus, giving larvae of the invasive species an initial developmental advantage to escape predation. Biological traits that may favor A. albopictus are offset partly by greater treehole occupancy by A. triseriatus and the infrequency of the invasive mosquito species in undisturbed woodlands, which mitigates against displacement of the native mosquito in these habitats.     

Larval habitat for the avian malaria vector Culex quinquefasciatus (Diptera: Culicidae) in altered mid‐elevation mesic‐dry forests in Hawai'i

Effective management of avian malaria (Plasmodium relictum) in Hawai'i's endemic honeycreepers (Drepanidinae) requires the identification and subsequent reduction or treatment of larval habitat for the mosquito vector, Culex quinquefasciatus (Diptera: Culicidae). We conducted ground surveys, treehole surveys, and helicopter aerial surveys from 2001–2003 to identify all potential larval mosquito habitat within two 100+ ha mesic‐dry forest study sites in Hawai'i Volcanoes National Park, Hawai'i; ‘Ainahou Ranch and Mauna Loa Strip Road. At ‘Ainahou Ranch, anthropogenic sites (43%) were more likely to contain mosquitoes than naturally occurring (8%) sites. Larvae of Cx. quinquefasciatus were predominately found in anthropogenic sites while Aedes albopictus larvae occurred less frequently in both anthropogenic sites and naturally‐occurring sites. Additionally, moderate‐size (≈20–22,000 liters) anthropogenic potential larval habitat had >50% probability of mosquito presence compared to larger‐ and smaller‐volume habitat (<50%). Less than 20% of trees surveyed at ‘Ainahou Ranch had treeholes and few mosquito larvae were detected. Aerial surveys at ‘Ainahou Ranch detected 56% (95% CI: 42–68%) of the potential larval habitat identified in ground surveys. At Mauna Loa Strip Road, Cx. quinquefasciatus larvae were only found in the rock holes of small intermittent stream drainages that made up 20% (5 of 25) of the total potential larval habitat. The volume of the potential larval habitat did not influence the probability of mosquito occurrence at Mauna Loa Strip Road. Our results suggest that Cx. quinquefasciatus abundance, and subsequently avian malaria, may be controlled by larval habitat reduction in the mesic‐dry landscapes of Hawai'i where anthropogenic sources predominate.     

Predator–prey interactions and the cannibalism of larvae of Armigeres subalbatus (Diptera: Culicidae)

Predation and cannibalism can affect the co-existence of mosquito species or the assemblage of mosquito species (i.e., community structure). In this study, predatory feeding patterns and cannibalism of larvae of Armigeres subalbatus mosquitoes were quantified under laboratory conditions using Aedes albopictus and Culex uniformis larvae as prey organisms. Rate of consumption of prey larvae and the rate of cannibalism of 1st to 4th instar larvae of Armigeres subalbatus were reported at 24 h intervals with and without alternative food supplement. Both the 3rd and 4th instar larvae of Ar. subalbatus showed a substantial predation on the larvae of Ae. albopictus (33.6 ± 4.4) and Cx uniformis (47.3 ± 7.6). The cannibalism of the predatory larvae was strongly correlated to the larval density (r = 0.9). The predator and the prey density and the given food type were significant factors that determined the rate of pupation, death and the emergence as adults. A significant relationship was shown for the co-occurrence of predatory larvae and the prey larvae at the 85 natural breeding habitats observed at the field (χ² = 74.4, p < 0.001). Nine breeding sites (10.6%) were positive for both prey and predatory larvae. Our study showed robust information about the predatory and cannibalistic behavior of Ar. subalbatus larvae emphasizing their role in managing the population structure of mosquito communities.     

Attracted to the enemy: Aedes aegypti prefers oviposition sites with predator-killed conspecifics

Oviposition habitat choices of species with aquatic larvae are expected to be influenced by both offspring risk of mortality due to predation, and offspring growth potential. Aquatic predators may indirectly influence growth potential for prey by reducing prey density and, for filter-feeding prey, by increasing bacterial food for prey via added organic matter (feces, partially eaten victims), creating the potential for interactive effects on oviposition choices. We tested the hypothesis that the mosquito Aedes aegypti preferentially oviposits in habitats with predatory Toxorhynchites larvae because of indirect effects of predation on chemical cues indicating bacterial abundance. We predicted that A. aegypti would avoid oviposition in sites with Toxorhynchites, but prefer to oviposit where bacterial food for larvae is abundant, and that predation by Toxorhynchites would increase bacterial abundances. Gravid A. aegypti were offered paired oviposition sites representing choices among: predator presence; the act of predation; conspecific density; dead conspecific larvae; and bacterial activity. A. aegypti preferentially oviposited in sites with Toxorhynchites theobaldi predation, and with killed conspecific larvae, but failed to detect preferences for other treatments. The antibiotic tetracycline eliminated the strongest oviposition preference. Both predation by Toxorhynchites and killed larvae increased bacterial abundances, suggesting that oviposition attraction is cued by bacteria. Our results show the potential for indirect effects, like trophic cascades, to influence oviposition choices and community composition in aquatic systems. Our results suggest that predators like Toxorhynchites may be doubly beneficial as biocontrol agents because of the attraction of ovipositing mosquitoes to bacterial by-products of Toxorhynchites feeding.     

Ontogenetic shifts in the functional response and interference interactions of Rhyacophila dorsalis larvae (Trichoptera)

1. Ontogenetic shifts in predator behaviour can affect the assessment of food‐web structure and the development of predator–prey models. Therefore, it is important to establish if the functional response and interference interactions differ between life‐stages. These hypotheses were tested by (i) comparing the functional response of second, third, fourth and fifth larval instars of Rhyacophila dorsalis, using three stream tanks with one Rhyacophila larva per tank and one of 10 prey densities between 20 and 200 larvae of Chironomus sp.; (ii) using other experiments to assess interference within instars (two to five larvae of the same instar per tank), and between pairs of different instars (one, two or three larvae per instar; total predator densities of two, four or six larvae per tank). 2. The first hypothesis was supported. The number of prey eaten by each instar increased with prey density, the relationship being described by a type II model. The curvilinear response was stronger for fourth and fifth instars than for second and third instars. Mean handling time did not change significantly with prey density, and increased with decreasing instar number from 169 s for fifth instars to 200 s for second instars. Attack rate decreased progressively with decreasing instar number. Handling time varied considerably for each predator–prey encounter, but was normally distributed for each predator instar. Variations in attack rate and handling time were related to differences in activity between instars, fourth and fifth instars being more active and aggressive than second and third instars, and having a higher food intake. 3. The second hypothesis was partially supported. In the interference experiments between larvae of the same instar or different instars, mean handling time did not change significantly with increasing predator density, and attack rate did not change for second and third instars but decreased curvilinearly for fourth and fifth instars. Interference between some instars could not be studied because insufficient second instars were available at the same time as fourth and fifth instars, and most third instars were eaten by fourth and fifth instars in the experiments. Prey capture always decreased with decreasing attack rate. Therefore, interference reduced prey consumption in fourth and fifth instars, but not in second and third instars. The varying feeding responses of different instars should be taken into account when assessing their role in predator–prey relationships in the field.     

The life history of the diving beetle, Lancetes angusticollis (Curtis) (Coleoptera: Dytiscidae), on sub-Antarctic South Georgia

Two populations of the world's most southerly diving beetle (Lancetes angusticollis) were studied on sub-Antarctic South Georgia between November 1995 and April 1996. Parallel observations were made on laboratory cultures of each life stage. All juvenile stages of L. angusticollis are voracious predators, preying mainly on the herbivorous copepod Boeckella poppei. Adult beetles additionally predate benthic ostracods. Laboratory predation rates and field population densities found in this study suggest that L. angusticollis has a more significant impact on its prey than recognised in recent studies of South Georgian lakes. Spring field samples contained only a few individuals of larval instar IV and adults. Young larvae were present from December onwards, with a rapid progression through juvenile instars during the summer months. Fourth instar larvae left the water for pupation between mid-December and at least mid-February. Combining these observations with known developmental threshold temperatures for each life stage implies that L. angusticollis has a complex biennial (minimum) life-cycle, with overwintering possible in three life stages [aquatic larvae, terrestrial pupae (not proven) and aquatic adults]. L. angusticollis may be a suitable indicator species in the context of climate warming studies: a small (1°C) rise in mean environmental temperatures, comparable to that already observed at several sub-Antarctic and maritime Antarctic localities, would allow completion of an annual (univoltine) life-cycle, with concomitant rapid population increase, and serious implications for trophic interactions in these simple lake ecosystems. Received: 24 November 1997 / Accepted: 12 April 1998     

Demographic Consequences of Predators on Prey: Trait and Density Mediated Effects on Mosquito Larvae in Containers

Predators may affect prey population growth and community diversity through density mediated lethal and trait mediated non-lethal effects that influence phenotypic traits of prey. We tested experimentally the roles of thinning the density of prey (lethality) in the absence of predator cues and density and trait mediated effects (lethality + intimidation) of predatory midge Corethrella appendiculata on competing native and invasive mosquito prey. Predator-mediated reductions in prey and density reductions in the absence of C. appendiculata resulted in lower percent survivorship to adulthood and estimates of the finite rate of increase (λ′) for invasive mosquito Aedes albopictus relative to that of controls. In most instances, thinning the density of prey in the absence, but not in the presence, of C. appendiculata cues resulted in lower survivorship to adulthood and λ′ for native mosquito Aedes triseriatus relative to that of controls. Together, these results suggested trait mediated effects of C. appendiculata specific to each species of mosquito prey. Release from intraspecific competition attributable to density reductions in the absence, but not in the presence, of C. appendiculata enhanced growth and lengthened adult lifespan relative to that of controls for A. albopictus but not A. triseriatus. These results show the importance of predator-mediated density and trait mediated effects on phenotypic traits and populations of invasive and native mosquitoes. Species-specific differences in the phenotypic responses of prey may be due, in part, to longer evolutionary history of C. appendiculata with A. triseriatus than A. albopictus.     

Analysis of molecular markers for metamorphic competency and their response to starvation or feeding in the mosquito, Aedes aegypti (Diptera: Culicidae)

The nutritional condition of fourth instar larvae of the yellow fever mosquito, Aedes aegypti, governs female longevity and egg production, both are key determinants of pathogen transmission. As well, nutrition provisions larval growth and development and attains its greatest pace in the last larval instar in preparation for metamorphosis to an adult. These developmental processes are regulated by a complex endocrine interplay of juvenile hormone, neuropeptides, and ecdysteroids that is nutrition sensitive. We previously determined that feeding for only 24 h post-ecdysis was sufficient for fourth instar Ae. aegypti larvae to reach critical weight and accumulate sufficient nutritional stores to commit to metamorphosis. To understand the genetic basis of metamorphic commitment in Ae. aegypti, we profiled the expression of 16 genes known to be involved in the endocrine and nutritional regulation of insect metamorphosis in two ways. The first set is a developmental profile from the beginning of the fourth instar to early pupae, and the second set is for fourth instars starved or fed for up to 36 h. By comparing the two sets, we found that seven of the genes (AaegCYP302, AaegJHE43357, AaegBrCZ4, AaegCPF1-2, AaegCPR-7, AaegPpl, and AaegSlif) were expressed during metamorphic commitment in fourth instars and in fed but not starved larvae. Based on these results, the seven genes alone or in combination may serve as molecular indicators of nutritional and metamorphic status of fourth instar Ae. aegypti larvae and possibly other mosquito species in field and laboratory studies to gauge sub-lethal effects of novel and traditional cultural or chemical controls.