Influence of intraguild predation between Episyrphus balteatus and Hippodamia variegata on their prey

Intraguild predation (IGP) is an interaction that frequently occurs in natural enemy communities, especially aphidophagous predators. This research investigated IGP intensity between Episyrphus balteatus De Geer (Diptera: Syrphidae), with Hippodamia variegata Goeze (Coleoptera: Coccinellidae). Five predator combinations including second and third larvae of H. variegata and third instar larvae of E. balteatus plus control treatment (totally six treatments) were tested. The effect of IGP on cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae) population density was investigated on sweet pepper seedlings under laboratory microcosms. In most combinations, the third instar larvae of E. balteatus alone reduced an A. gossypii population more efficiently than ladybird larvae and their combinations. Furthermore, IGP between third instar of E. balteatus and second larvae of H. variegata was asymmetrical; second instar H. variegata larvae were always the intraguild prey for third instar E. balteatus. The obtained result showed that outcome of IGP interaction on cotton aphid density was non-additive.     

Optimization and evaluation of microencapsulated artificial diet for mass rearing the predatory ladybird Propylea japonica (Coleoptera: Coccinellidae)

Artificial diet optimization is a key aspect in mass rearing of natural enemies since it influences the quality and feeding effectiveness, and thus the success of the biological control program. Here, we introduced the microencapsulation method to package liquid artificial diet for feeding of the ladybird Propylea japonica. An orthogonal test of the quality of microencapsulated artificial diets (ADMs) was performed on key variables in production; Ca‐alginate concentration, chitosan concentration and weight ratio of wall material to inner diet. We compared the development and reproduction of P. japonica fed on the ADMs under different cold‐stored periods with those fed on fresh aphids and liquid artificial diets, in addition to a comparison of respiration, locomotion and predation. Our results indicated that chitosan concentration and ratio of shell to core significantly influence the quality of ADMs. The optimal recipe is 1.0% Ca‐alginate, 1.6% chitosan and shell : core = 1 : 2. Insects reared on fresh optimized ADMs were similar to those fed on fresh prey in all developmental and reproductive characteristics except for survival ratio and female fertility. ADMs appeared more beneficial than using a liquid artificial diet, although this may decrease with the prolonging of the cold‐storage period. P. japonica fed either on fresh ADMs or fresh prey showed improved respiration and predation abilities compared to where liquid artificial diet was used. Our study indicates advantages of microencapsulation in the production of artificial diet for predatory ladybird rearing. A microencapsulated diet can directly increase the efficiency and stability of mass rearing.     

Predation and avoidance behaviour in aphid‐ladybird interactions of native and invasive ladybirds in Europe

1. While detrimental effects of invasive predators on native species are well documented, we often lack a mechanistic understanding of the invasion success. Lack of prey avoidance behaviour can lead to higher consumption rates by invasive predators compared to native predators. This competitive advantage is expected to contribute to the invasion success of non‐native predators.
2. We compared aphid consumption and cue avoidance behaviour of aphids between four native ladybird species (Coccinella septempunctata, Adalia bipunctata, Propylea quatuordecimpunctata, and Hippodamia variegata) and the invasive Asian ladybird Harmonia axyridis.
3. The invasive H. axyridis and the native C. septempunctata consumed more aphids than the three smaller native ladybird species. In line with our expectations, aphids avoided leaves bearing cues of most native ladybird species but not of the invasive H. axyridis.
4. Our results indicate that body size rather than ladybird origin determined aphid predation rates. The lack of aphid avoidance behaviour towards cues of H. axyridis indicates that they were not able to recognise the chemical cues of the invasive predator.
5. Relatively large body size and the absence of cue avoidance in aphids might benefit the invasive H. axyridis, particularly in comparison to smaller native ladybird species. The absence of avoidance behaviour in aphids might lead to even higher predation rates of H. axyridis under more natural conditions.

     

Environmental factors affecting the behavior of Coenosia attenuata,a predator of Trialeurodes vaporariorum in tomato greenhouses

Adults of the predatory fly Coenosia attenuata Stein (Diptera: Muscidae) catch their prey while in flight. I investigated this activity over two seasons in a tomato greenhouse naturally infested with Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae). The flight of C. attenuata individuals was affected by environmental factors and was increased in response to increases in temperature, the number of prey flights, and conspecific density. Predator and prey flights were distributed throughout the day, but there was a regular daily trend, each with two partially overlapping activity peaks. The possibility of predation was limited by differences in daily flight‐activity times. Predatory flights comprised a small percentage (ca. 6%) of the total flights, with a predation success rate of 61%. Overall, the predatory activity of C. attenuata depended on the selection of hunting sites with good visibility to ensure a clear view before take‐off and allow the capture of prey in flight. Similar numbers of overall flights were made by both sexes, but C. attenuata females performed more predation flights and territorial defense activities than males. The ecological role of C. attenuata and its limited adaptability to greenhouses is discussed in light of its possible use in biological control of whiteflies.     

Interaction between female mating preferences and predation may explain the maintenance of rare males in the pentamorphic fish Poecilia parae

Variation in mating preferences coupled with selective predation may allow for the maintenance of alternative mating strategies. Males of the South American live‐bearing fish Poecilia parae fall in one of five discrete morphs: red, yellow, blue, stripe‐coloured tail (parae) and female mimic (immaculata). Field surveys indicate that the red and yellow morphs are the rarest and that their rarity is consistent across years. We explored the role of variable female mating preference and selective predation by visual predators in explaining the rarity of red and yellow males, and more generally, the maintenance of this extreme colour polymorphism. We presented wild‐caught P. parae females and Aequidens tetramerus, the most common cichlid predator, with the five male colour morphs in separate trials to determine mating and prey preferences, respectively. We found that a large proportion of females shared a strong preference for the rare carotenoid‐based red and yellow males, but a distinct group also preferred the blue and parae morphs. The cichlid predator strongly preferred red and yellow males as prey. Together, these results suggest that the interaction between premating sexual selection favouring and predation acting against the red and yellow morphs may explain their rarity in the wild. The trade‐off between sexual and natural selection, accompanied by variation in female mating preferences, may therefore facilitate the maintenance of the striking colour polymorphism in P. parae.     

Light emitting diode(LED)‐based trapping of the greenhouse whitefly (Trialeurodes vaporariorum)

Visual traps like yellow sticky card traps are used for monitoring and control of the greenhouse whitefly (Trialeurodes vaporariorum). However, reflected intensity (brightness) and hence, attractiveness depend on the ambient light conditions, and the colour (wavelength) might not fit with the sensitivity of whitefly photoreceptors. The use of light emitting diodes (LEDs) is a promising approach to increase the attractiveness, specificity and adaptability of visual traps. We constructed LED‐based visual traps equipped with blue and green high‐power LEDs and ultraviolet (UV) standard LEDs according to the putative spectral sensitivities of the insects' photoreceptors. In a series of small‐scale choice and no‐choice recapture experiments, the factors time of day as well as light intensity and light quality (colour) of LED traps were studied in terms of attractiveness compared to yellow traps without LEDs. Green LED traps (517 nm peak wavelength) were comparably attractive in no‐choice experiments but clearly preferred over yellow traps in all choice experiments. The time of day had a clear effect on the flight activity of the whiteflies and thereby on the trapping success. Blue LEDs (474 nm) suppressed the attractiveness of the light traps when combined with green LEDs suggesting that a yet undetected photoreceptor, sensitive for blue light, and an inhibiting interaction with the green receptor, might exist in T. vaporariorum. In choice experiments between LED traps emitting green light only or in combination with UV (368 nm), the green‐UV combination was preferred. In no‐choice night‐time experiments, UV LEDs considerably increased whitefly flight activity and efficacy of trapping. Most likely, the reason for the modifying effect of UV is the stimulating influence on flight activity. In conclusion, it seems that the use of green LEDs alone or in combination with UV LEDs could be an innovative option for improving attractiveness of visual traps.     

Visual orientation of the black fungus gnat,Bradysia difformis,explored using LEDs

Fungus gnats occur worldwide with more than 1 700 described species. They can cause serious damages on ornamentals, crop plants, and edible mushrooms, and are considered to be a serious pest in the last years. Bradysia difformisFrey (Diptera: Sciaridae) represents a common species in Europe. Usually, yellow sticky traps are used for monitoring and control in greenhouses and fluorescent tube‐based light traps are additionally applied for control in mushroom cultivation. The importance of such visual trapping measures for efficient monitoring or alternative control increases in biological and integrated plant protection. However, detailed color preferences of fungus gnats are mostly unknown. We studied the visual orientation of B. difformis with light‐emitting diodes (LEDs) in a broad range of peak wavelengths from 371 nm (ultraviolet, UV) to 619 nm (amber). We determined attractive wavelengths in consecutive choice experiments in daylight and darkness. Highest numbers of adult B. difformis were attracted to UV radiation (382 nm) followed by green‐yellow light (532–592 nm). The responses to UV and the green‐yellow range were relatively unspecific and mostly independent from intensity. Combination of UV and yellow LEDs improved trapping efficacy compared to a single UV or yellow LED trap, as well as compared to a common yellow sticky trap. When both wavelengths were compared to a black surface to increase contrasts, the black surface was preferred over yellow, but was less attractive than UV. Thus, B. difformis displays two, probably wavelength‐specific, behaviors to UV radiation and green‐yellow light, with UV being the most attractive stimulus. These behaviors might be directly related to underlying photoreceptors, suggesting dichromatic vision in B. difformis.     

Biotic interactions between Eriopis connexa and Hippodamia variegata,a native and an exotic coccinellid species associated with alfalfa fields in Chile

Human‐assisted introductions, including those in the context of biological control, are considered to be one of the most important factors of global environmental change. However, the mechanisms underlying environmental changes, such as a decrease in the relative abundance of native species, are poorly understood. Since the introduction of the ladybird beetle, Hippodamia variegata (Goeze) (Coleoptera: Coccinellidae), in Chile in the 1970s for biological control of aphids, a reduction in the relative abundance of the native ladybird beetle Eriopis connexa (Germar) has been noticed. To explore the role of cannibalism, intraguild predation (IGP) or competition as possible mechanisms that might increase dominance of H. variegata over E. connexa, several laboratory experiments were carried out. The native and the exotic species were very similar in their voracity and biotic interactions. Although H. variegata was able to maintain constant reproductive performance, E. connexa reproduction decreased at lower densities of aphids, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae). The impacts of cannibalism, IGP, and competition exerted by these species were very symmetrical in both larvae and adults. We conclude that these biotic factors may not fully explain the increase in relative abundance of H. variegata. The higher sensitivity of E. connexa to prey patch size, along with other factors such as chemical‐mediated negative interspecific interactions may be operating in alfalfa fields, changing the composition of associated coccinellid assemblages.     

Experimental assessment of predation risk for juvenile green sturgeon,Acipenser medirostris,by two predatory fishes

Predation is a common cause of early life stage mortality in fishes, with reduced risk as individuals grow and become too large to be consumed by gape-limited predatory fishes. Large-bodied species, such as sturgeon, may reach this size-refuge within the first year. However, there is limited understanding of what this size threshold is despite the value of this information for conservation management. We conducted laboratory-based predation experiments on juvenile green sturgeon, Acipenser medirostris, to estimate vulnerability to predation during outmigration from their natal reaches in California to the Pacific Ocean. Two highly abundant and non-native predatory fish species (largemouth bass, Micropterus salmoides, and striped bass, Morone saxatilis) were captured in the wild to be tested with developing juvenile green sturgeon from the UC Davis Green Sturgeon Broodstock Program. Experimental tanks, each containing five predators, received thirty prey for 24-hr exposures. Between sturgeon prey trials, predators were exposed to alternative prey species to confirm predators were exhibiting normal feeding behaviors. In addition to green sturgeon mortality data, trials were video recorded and predatory behaviors were quantified. Overall, these predator species displayed much lower rates of predation on juvenile green sturgeon than alternate prey. Predation decreased with green sturgeon size, and predation risk diminished to zero once sturgeon reached a length threshold of roughly 20–22 cm total length, or between 38% and 58% of predator total length. Behavioral analyses showed low motivation to feed on green sturgeon, with both predators attempting predation less frequently as sturgeon grew. Results of this study imply that optimizing growth rates for larval and juvenile sturgeon would shorten the time in which they are vulnerable to predation. Future experiments should assess predation risk of juvenile green sturgeon by additional predator species common to the Sacramento-San Joaquin watershed.     

多异瓢虫与七星瓢虫、大草蛉不同虫态间的集团内捕食

多异瓢虫、七星瓢虫和大草蛉是控制蚜虫重要的广谱捕食性天敌昆虫,为明确这些昆虫间的集团内捕食效应与个体发育阶段的关系,观察了多异瓢虫与七星瓢虫、大草蛉各虫态(蛹除外)之间的集团内捕食效应。设置30个虫态(虫龄)组合处理,于温室盆栽棉花(无蚜虫)上接种等量的试虫,观察24 h后的存活数量。结果表明,当参与双方的虫态不同时,晚期虫态的存活数量显著大于早期虫态;当参与双方的幼虫龄期不同时,高龄幼虫的存活数量显著大于低龄幼虫;成虫对卵、1龄和4龄幼虫的捕食程度随虫龄增大而呈抛物线增大,存在显著的二次函数关系。当参与双方的虫态(龄)相同时,多异瓢虫成虫的存活数量显著小于七星瓢虫,但大于大草蛉成虫;多异瓢虫幼虫(1龄和4龄)的存活数量显著小于大草蛉,而与七星瓢虫没有显著差异。研究结果说明,在没有集团外猎物(蚜虫)存在的情况下,多异瓢虫与七星瓢虫、大草蛉之间的集团内捕食效应只在参与者的虫态或虫龄不同时取决于发育阶段,而当参与者虫态或虫龄相同时,还取决于其他因素。     

A comparative study on the efficacy of a pest-specific and prey-marking enzyme-linked immunosorbent assay for detection of predation

The efficacy of two different antigen–antibody combinations to detect predation on eggs of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) was compared. The first method was an indirect enzyme‐linked immunosorbent assay (ELISA) using monoclonal antibody‐based gut content analysis that detects H. armigera egg protein. The second method was a sandwich ELISA that detects an exotic protein [rabbit immunoglobulin G (IgG)] applied as an external marker to H. armigera eggs. The target predators were the predatory beetles Dicranolaius bellulus (Guerin‐Meneville) (Coleoptera: Melyridae) and Hippodamia variegata (Goeze) (Coleoptera: Coccinellidae). Beetles were fed with H. armigera eggs that had been marked with rabbit IgG and then held at various intervals after prey consumption. Each individual beetle was then assayed by both ELISA techniques to identify the prey remains in their guts. The two ELISA methods were further tested on field‐collected predators. Specifically, protein‐marked egg masses were strategically placed in a cotton field. Then, predators from surrounding cotton plants were collected at various time intervals after the marked eggs were exposed and assayed by both ELISAs to detect the frequency of predation on the marked eggs. The rabbit IgG‐specific sandwich ELISA had a higher detection rate than the H. armigera‐specific indirect ELISA under controlled and field conditions for both predator species. Moreover, a greater proportion of field‐collected D. bellulus tested positive for predation than H. variegata. The advantages and disadvantages of using prey‐marking ELISAs instead of pest‐specific ELISA assays are discussed.     

Biological traits and life table parameters of variegated lady beetle, <Emphasis Type="Italic">Hippodamia variegata</Emphasis> (Coleoptera: Coccinellidae) on three aphid species

The aphidophagous ladybird beetle, Hippodamia variegata (Goeze) (Coleoptera: Coccinellidae), is a dominant predator in various crop systems. Its life history was studied when fed on three aphid prey, viz. Aphis gossypii Glover, Aphis fabae Scopoli and Macrosiphum rosae (L.), under laboratory conditions, 25 ± 1 °C, 65 ± 5 % RH and a photoperiod of 16L:8D. The immature development period was shortest (15.2 days) when A. gossypii was used as prey and longest (18.9 days) on A. fabae. Adult coccinellids had the shortest longevity on A. fabae (64.8 days). The higher and lower mean daily fecundity was recorded for A. fabae (12.4 eggs) and M. rosae (6.2 eggs), respectively. The highest (291.0 eggs) and lowest (183.2 eggs) net reproduction rates (R ₀) were observed on A. fabae and M. rosae, respectively. Mean generation time (T) on rose aphid was significantly longer (33.4 days) than on the two other preys. The intrinsic rate of increase (r _m) was affected by the R ₀ value, and it was highest on A. fabae (0.183) and lowest on M. rosae (0.156). Based on r _m as an index of suitability of prey species, A. fabae was the most suitable prey for H. variegata. Our finding may provide basic information for developing aphid biological control programs.     

Studies of the effect of environmental factors on the rotifer predator–prey system in freshwater

The aim of this work is to evaluate the effect of environmental factors: temperature and photoperiod on the zooplankton predator–prey system. Rotifers, an important and cosmopolitan group of zooplankton in freshwater, were used in our study. We investigated the effect of temperature (20, 23, and 30°C) and of photoperiod (L:D = 12:0 and 0:12) on the predatory rotifer Asplanchna brightwelli consuming rotifer Brachionus calyciflorus as prey. Under A. brightwelli predation, populations of B. calyciflorus prey were consumed more slowly at 20 ± 1 and 30 ± 1°C as compared to 23 ± 1°C. Prey consumption by A. brightwelli increased from 0.63 ± 0.09 ind. predator⁻¹ at 20°C to a peak of 1.22 ± 0.12 ind. predator⁻¹ at 23°C, then decreased significantly to 0.93 ± 0.14 ind. predator⁻¹ at 30 ± 1°C. In addition, predation responded to temperature changing sensitively and rapidly. Statistical analysis showed that the prey consumption were significant different under altered temperature periods during 12 h. Photoperiod also significantly influenced the rate of A. brighwelli predation. B. calyciflorus suffered less predation in darkness than in light. The rate of prey consumption in light (1.06 ind. predator⁻¹) was twice the average of that in darkness (0.51 ind. predator⁻¹). Furthermore, predation rate varied under changing photoperiod but predators moved back into the light did not resume their original consumption rate. Our results demonstrate that whether the predation in rotifer successfully or not is strongly influenced by temperature and photoperiod.     

Attacks by predators on artificial cryptic and aposematic insect larvae

Colour and colour patterns seem to be especially important visual warning signals for predators, which might have innate or learned ability to avoid aposematic prey. To test the importance of larval colour pattern of the aposematic ladybird Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), an invasive alien species in Europe, we presented the plasticine models of aposematic larvae to wild and naïve birds. We studied the attacks on aposematic larvae of various patterns and colours in nature and in an outdoor aviary. The larvae were cryptic (green), aposematic (resembling those of the H. axyridis larvae), and semi-aposematic (i.e., black but missing the typical orange patches of H. axyridis larvae). We detected attacks on 71 larvae out of 450 (i.e., 2.6% daily predation). Twenty-nine attacks were made by birds, 37 by arthropods, and five by gastropods. Wild birds attacked green and black larvae significantly more often than aposematic larvae. Colour did not have an effect on attacks by arthropods. The experiment with naïve birds was conducted in an outdoor aviary, where naïve great tits, Parus major L., were offered the same artificial larvae as in the first experiment. In total, 57 of 90 exposed larvae were attacked by birds (i.e., 28% daily predation), and green larvae were attacked significantly more than the aposematic larvae (but not more than black larvae). Our results imply that aposematic larvae of H. axyridis are more than 12× less likely to be predated by birds than green larvae in nature. The aposematic pattern represented a more effective signal than the semi-aposematic signal. The ability to reject aposematic prey seemed to be innate in our birds.     

Asymmetrical intraguild predation between natural enemies of the citrus mealybug

The ability of a predator to discriminate against parasitized prey determines the extent of asymmetrical intraguild predation, which is often crucial for the outcome of biological control. Anagyrus nr. pseudococci (Girault) (Hymenoptera: Encyrtidae), a parasitoid of the citrus mealybug, Planococcus citri (Risso) (Hemiptera: Pseudococcidae), suffers from intraguild predation by coccinellids occurring in the same habitat. The level of intraguild predation on A. nr. pseudococci by Nephus includens (Kirsch) (Coleoptera: Coccinellidae) at different immature stages has been investigated with and without simultaneous offer of extraguild prey. Larvae of A. nr. pseudococci appeared to face increased intraguild predation at early developmental stages, whereas mummification provided adequate protection against the predatory coccinellid. Different predation levels on unparasitized vs. parasitized hosts at various developmental stages in choice assays indicated that N. includens preferences might be determined not solely by palatability of the prey but also by its ability to protect itself.     

The photokinesis of oriental fruit flies,Bactrocera dorsalis,to LED lights of various wavelengths

The oriental fruit fly, Bactrocera dorsalis Hendel (Diptera: Tephritidae), is an invasive pest of orchards around the world, particularly in Asian countries such as China. Light traps offer a potential means for pest monitoring and management. This study aimed to evaluate the sensitivity of the fly to light and investigate the impact of monochromatic light in the sensitivity spectrum on B. dorsalis. Six light wavelengths in LEDs – green (522 nm), yellow (596 nm), blue (450 nm), red (633 nm), purple (440 nm), and white (compound light) – were adapted to test responses of 5‐, 10‐, and 20‐day‐old B. dorsalis adults kept in laboratory conditions. We also tested the effects of green and red lights on pupal development and adults’ life activities. The results indicated a phototaxis preference rank in B. dorsalis adults to monochromatic LEDs with, in decreasing order, green, yellow, purple, blue, and red. Moreover, positive phototaxis significantly increased with age. Male adults are more sensitive than female adults to test lights, mainly at the age of 10 and 20 days. Emergence rates of pupae exposed to 12 and 24 h green light daily were 42 and 67%, respectively, whereas controls held in red light emerged at 33 and 37%, respectively. Furthermore, body weight, female fecundity, and mortality of B. dorsalis in night‐time exposure of green light (from 21:00 to 09:00 hours; during daytime flies were illuminated by white LED light) were significantly higher than in red‐light test groups and dark controls. In conclusion, B. dorsalis displayed preference toward green light, and fly age and gender seemed to significantly impact the phototactic behavior. Green LED light exposure during nighttime remarkably improved the emergence rates of B. dorsalis, and it enhanced growth, development, and ovipositing peak period, but decreased adult lifespan. This research lays a foundation for the development of new trap models, e.g., with green sticky cards or green light, for monitoring and control of B. dorsalis in the field.     

Predation of cotton bollworm by green lacewings in the presence of cotton aphid as alternative prey on transgenic Bt and conventional cotton

Experiments were conducted in small arenas and on whole plants to explore the effect of cotton aphids, Aphis gossypii Glover (Hemiptera: Aphididae), as alternative prey on the predation of Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) larvae by green lacewing larvae, Mallada signatus Schneider (Neuroptera: Chrysopidae). Transgenic Bt (Bollgard II^®) and conventional cotton plants were included to explore potential differences in the predator's performance on these cotton types. In small arenas, the presence of 20 aphids reduced predation on H. armigera larvae by 22% (from 5.5 to 3.3 of 10) by a single lacewing larva over a 24‐h period. The presence of H. armigera reduced predation on aphids by ca. 29% (from 16.8 to 11.0 of 20) over 24 h. On whole plants, the presence of alternative prey had no effect on the number of H. armigera larvae or aphids remaining after 3 days. The presence of H. armigera larvae alone, without the predator, caused a 24% reduction in the numbers of aphids on conventional, but not on Bt cotton plants. The combination of Bt cotton and lacewing larvae caused a 96.6% removal of early‐stage H. armigera larvae, a statistically significant increase over the addition of the proportions (91.6%) removed by each factor measured separately, providing evidence of synergism. These studies suggest that the presence of aphids as alternative prey would not necessarily disrupt the predation by green lacewing on larvae of H. armigera, especially on Bt cotton.     

Fishing spiders, green sunfish, and a stream-dwelling water strider: male-female conflict and prey responses to single versus multiple predator environments

Many studies have experimentally addressed the effects of a particular predator species on prey behavior. In nature, however, prey frequently face multiple species of predators that often vary in their predatory mode and in their level of predation risk. Relatively few studies have considered prey responses under these complex conditions. In Kentucky, the stream-dwelling water strider (Aquariusremigis) coexists with many potentially dangerous predators, two of which are the green sunfish (Lepomiscyanellus) and the fishing spider (Dolomedesvittatus). Green sunfish occupy stream pools and attack water striders from below. In contrast, fishing spiders hunt along stream shorelines where they perch on overhanging vegetation or rocks and attack water striders near shore. We compared how A. remigis individuals respond to these two very different predators in pools with one or both predators. The presence of sunfish in pools had strong effects on male water strider behavior, including increased use of three types of refuge from sunfish (riffles, climbing out of the water, sitting on the water but at the edges of pools), decreased activity and a decreased number of aggressive males on the water. Spiders also influenced water strider behavior; male water striders avoided spiders by shifting away from the edges of pools. Comparisons of the effects of the two predator species showed that in general, antipredator responses by male water striders were stronger in pools with fish alone than in those with spiders alone. In the presence of both predators, male water strider behavior (microhabitat use and activity) was generally similar to behavior in the presence of fish alone. In contrast, female water striders showed no significant response to the presence of sunfish, and little response to the presence of spiders. This lack of response could be because females spent much of their time in refuges even in the absence of predators (apparently hiding from harassment by males). Both spiders and fish caused decreases in water strider mating activity. The presence of fish reduced both the number of matings per pool (mating frequency), and mean mating durations. Spiders induced a decrease in mean mating duration, but not in mating frequency. The largest reductions in mating activity occurred in pools with both predators present. Pools with either spiders or fish alone suffered 15–20% water strider mortality during our experiment (versus no mortality in predator-free pools). Extant theory suggests that when prey face conflicting microhabitat responses to two predators (as in this study), the predators should have facilitative effects on predation rates (i.e., prey that avoid one predator are often killed by the other and vice versa). Mortality rates in pools with both predators present, however, were not significantly different from that predicted by a null model of multiple predator effects. The lack of predator facilitation can be explained by the compensatory reductions in water strider activity and mating activity in the presence of both predators. Received: 26 August 1996 / Accepted: 12 June 1998     

Prey capture performance in hatchlings of two sibling Harmonia ladybird species in relation to maternal investment through sibling cannibalism

1. To elucidate the factors responsible for the evolution of host specialisation, prey capture performance in hatchlings of two sibling ladybird species, Harmonia yedoensis and H. axyridis, feeding on four aphid species was examined. Harmonia yedoensis is a specialist predator that preys only on pine aphids in the field, whereas H. axyridis is a generalist predator with a broad prey range. 2. In H. yedoensis, sibling cannibalism in each clutch was intense and predation against pine aphid as well as other aphid species was moderately successful. In contrast, the predation success rate of H. axyridis against pine aphid was quite low. 3. Moreover, it was experimentally shown that increased maternal investment enhanced prey capture performance against pine aphid in H. yedoensis but not in H. axyridis hatchlings, despite their increased body size due to maternal investment. 4. In addition, morphological and behavioural analysis showed that hatchlings of H. yedoensis had longer legs and a larger head capsule size and could walk faster than H. axyridis. 5. In summary, the interactive effects between a large amount of maternal investment and morphological specialisation of the first instars may enable H. yedoensis to capture the pine aphid efficiently, a highly elusive prey for ladybird hatchlings. The ability of H. yedoensis to utilise the three other aphid species in addition to the pine aphid suggests that a trade‐off in prey capture performance is not the main factor in the host specialisation of H. yedoensis.     

Influence of light and habitat on predation of Culex quinquefasciatus (Diptera: Culicidae) larvae by the waterbugs (Hemiptera: Heteroptera)

The influence of light and habitat structure on the predation of Culex quinquefasciatus larvae by the common heteropteran water bug, Diplonychus (=Sphaerodema) annulatus, D. rusticus and Anisops bouvieri was assessed in the laboratory. It was revealed that water bugs predate more in presence of light than in dark conditions. While A. bouvieri consumed more prey in structured conditions, D. annulatus and D. rusticus consumed more prey in open conditions. The selection of prey size as well as the respective numbers varied between predators and treatments significantly. Prey vulnerability (PV), an indicator of predatory efficiency, was highest for D. annulatus, moderate for D. rusticus and low for A. bouvieri. Prey consumption and PV values under different treatment conditions indicate that for belostomatid water bugs D. annulatus and D. rusticus, the order of prey consumption under different habitat and light/dark combinations is light open > dark open > light vegetated > dark vegetated. In the case of the backswimmer, A. bouvieri, the order of prey consumption appeared to be light structured > dark structured > light open > dark open. These findings were consistent with resource‐partitioning by water bug species sharing the same guild. If the observed results are extended to natural settings the efficiency of these predators in controlling mosquito populations will vary with the structural complexity of habitats and the intensity of light.