Effects of Anisops sardea (Hemiptera: Notonectidae) on oviposition habitat selection by mosquitoes and other dipterans and on community structure in artificial pools

We investigated the effects of the predatory backswimmer, Anisops sardea, on oviposition habitat selection of mosquitoes and other dipterans, and on community structure in experimental pools. We predicted that those dipteran species whose larvae were shown to be highly vulnerable to predation by Anisops would avoid Anisops pools when choosing an oviposition site. We established the following treatments in plastic tubs: (1) Control (without Anisops); (2) Free Anisops (ten Anisops within pool but not caged); (3) Caged Anisops (ten Anisops in cage). The pools were open to colonization by insects. We added resting stages of crustaceans and first instar larvae of the mosquitoes Culiseta longiareolata and Culex laticinctus. Among the dipteran species, Culiseta longiareolata, Culex laticinctus, Chironomus riparius (Chironomidae) and Forcipomyia sp. (Ceratopogonidae), only Culisetalarvae were highly vulnerable to predation. As predicted, based on larval vulnerability, Culiseta, but not the other species, avoided Anisops pools when ovipositing. Free Anisops reduced taxon richness. This reduction resulted largely from the elimination of the cladoceran Ceriodaphniasp. and Culiseta in most free Anisops pools. Thus, Anisops sardea structures the community, both by a behavioral response of prey to its presence and by consumption of prey.     

Direct and indirect effects of a predatory backswimmer (Notonecta maculata) on community structure of desert temporary pools

Abstract.

• 1 We assessed experimentally the effects of the predatory backswimmer, Notonecta maculata, on naturally colonizing mosquito populations in artificial outdoor pools in the Negev Desert, Israel. A single Notonecta adult per pool (8–15 litres water) had a very large negative impact on populations of Culiseta longiareolata, the most common species found in natural local pools. Notonecta caused large reductions of Culiseta egg rafts and early-instar larvae (instars I and II) and virtually 100% reductions of late-instar Culiseta larvae (instars III and IV) and pupae.
• 2 Notonecta also caused a trophic cascade in the experimental pools; by preying on periphyton-feeding Culiseta larvae, Notonecta indirectly caused significantly higher densities of diatoms, the major component of the periphyton.
• 3 Surveys of nearby natural pools taken between March and May supported the experimental results: a strong negative association between Notonecta and Culiseta among pools occurred as Notonecta increased in numbers and became more widely distributed. Anopheles (occurring only in May) and Culex mosquito immatures were not negatively associated with Notonecta. Culiseta was not associated with surface vegetation whereas both Culex and Anopheles showed strong positive associations with surface vegetation both among and within pools. We attribute the negative association between the predator and Culiseta to local prey extinctions caused by Notonecta in individual pools. Culiseta, being an open water species, is apparently more prone to predation by Notonecta than the vegetation-dwelling Culex and Anopheles.

     

Oviposition habitat selection by mosquitoes in response to predator (Notonecta maculata) density

Abstract.  Some species of mosquitoes can detect the presence of predatory notonectid bugs and avoid oviposition in predator pools. The oviposition response of two mosquito species, Culiseta longiareolata Macquart and Culex laticinctus Edwards (Diptera: Culicidae), to a range of densities of the predator, Notonecta maculata Fabricius (Heteroptera: Notonectidae), was tested here. Densities of 0, 1, 2 or 4 Notonecta were established in 30-L artificial pools. Both mosquito species oviposited less in predator pools, but the response was unrelated to predator density, whereas vulnerability of Culiseta immatures to predation was density-dependent. Thus, although mosquitoes can detect Notonecta at any density within the range tested, they may be unable to discriminate among predator densities. The avoidance of predator pools by Culiseta , as well as its vulnerability to predation, occurred to a lesser degree than in earlier studies. This may have been due to the mitigating effects of components of the biotic community.     

Modelling evolutionarily stable strategies in oviposition site selection, with varying risks of predation and intraspecific competition

Many ovipositing mosquitoes, as well as other species, can detect biotic factors that affect fitness. However, a female mosquito seeking a high quality oviposition site (e.g. one with low risk of predation and competition to her progeny) must often balance the competing risk of increasing probability of mortality to herself while she continues to search, against increased probability of finding a high quality site. Such oviposition site selection may affect adult population size. We examined a female mosquito’s expected strategy of oviposition site selection under conditions of varying predator prevalence and adult mortality risk, by combining a detailed structured population model with a Markov chain implementation of the adult behavioural process. We used parameter values from the specific mosquito-predator system, Culiseta longiareolata-Notonecta maculata, although the overall results can be generalised to many mosquito species. Our model finds the evolutionarily stable strategy of oviposition site selection for different parameter combinations. Our model predicts that oviposition strategy does not vary smoothly with varying environmental risk of adult mortality, but that certain oviposition strategies become unstable at some parameter values. Mosquitoes will distribute their reproductive effort between breeding sites of varying predation risk only when adult mortality is low or larval competition high. Our model predicts that females will continue searching for predator-free pools, rather than oviposit in the first site encountered, regardless of the risk of mortality to the adult. The ecological basis for a reproductive strategy with alternative behaviours is important for understanding the effect of biotic factors on the population dynamics of mosquitoes, and for the development of biological control strategies, such as the dissemination of predator-cue chemicals.     

Predation-dependent oviposition habitat selection by the mosquito Culiseta longiareolata: a test of competing hypotheses

We investigated the mechanism underlying oviposition habitat selection (OHS) in the mosquito Culiseta longiareolata. The putative outcome of a trade‐off between the risk of predation and detrimental density dependence, OHS in this species presents an opportunity to test two competing alternatives: (1) a polymorphic scenario, in which a fixed proportion of females constantly avoid ‘predator pools’, while the remainder oviposits at random; and (2) a monomorphic scenario, in which all females oviposit in predator pools with a certain probability. We present a conceptual framework that demonstrates how a simple experimental design – whereby predator incidence in artificial pools is alternated between 0.25 and 0.75 – can distinguish between, or refute, the two scenarios. Given the proportional use, by ovipositing females, of predator‐free pools observed under each treatment, and a bootstrap estimate of the ratio of daily oviposition rates, we find the monomorphic scenario twice as likely as the polymorphic.     

Experimental evidence for predation risk sensitive oviposition by a mosquito, Culiseta longiareolata

1. Females should choose to oviposit in habitats where risk of predation and competition are low. The ovipositional responses of a mosquito, Culiseta longiareolata, to a predator and to species sharing the same trophic level as this mosquito (controphic species) were assessed experimentally in outdoor artificial pools. 2. The predator, larval Anax imperator, which strongly reduced larval C. longiareolata survival, resulted in a 52% reduction of C. longiareolata egg rafts. The controphic species (primarily Daphnia magna), which had a small but statistically significant negative effect on the survival of C. longiareolata larvae, did not have a statistically significant influence on the number of egg rafts. 3. Laboratory trials indicated that only a small fraction of the reduced number of egg rafts seen in predator pools may be due to consumption of the egg rafts by A. imperator. 4. The experimental evidence indicates that the reduced number of C. longiareolata egg rafts found in the presence of A. imperator is due largely to oviposition habitat selection, i.e. C. longiareolata females choose pools with low risk of predation for their offspring.     

Oviposition habitat selection by the mosquito Culiseta longiareolata in response to risk of predation and conspecific larval density

Abstract. 1. Most female Culiseta longiareolata (Diptera: Culicidae) avoid ovipositing in pools that contain the predatory backswimmer Notonecta maculata . Such oviposition habitat selection has been suggested to reflect a trade-off between the risk of predation on larvae and potential density-dependent fitness costs. This putative trade-off was examined. In particular, evidence was sought in support of direct female response to local heterogeneity in habitat quality.
2. Three habitat types were established using artificial outdoor pools: predator pools, and non-predator pools with either low or high densities of Culiseta larvae. During each experimental night, females were offered one of the three possible pair-wise treatment combinations.
3. The majority (≈88%) of females oviposited in low-density pools rather than in the predator- or high-density pools. Furthermore, a substantially higher proportion of females oviposited in predator pools when faced with the high-density alternative, however this was due largely to fewer females ovipositing in high- vs low-density pools.
4. Females of a second mosquito species ( Culex laticinctus ), the larvae of which are at a lower risk of predation, were predicted to exhibit weaker aversion to N. maculata ; this prediction was supported only weakly.
5. Oviposition habitat selection by female C. longiareolata does not appear to involve a behavioural response that is based on individual assessment of local heterogeneity in relative pool quality, at least not at the spatial scale examined here; alternative explanations are discussed.     

Chemical detection of the predator Notonecta irrorata by ovipositing Culex mosquitoes.

We tested the oviposition response of Culex mosquitoes to the predator Notonecta irrorata in an outdoor artificial pool experiment employing equal numbers of control and predator pools. There was a strong oviposition avoidance by Culex of Notonecta pools; 83% of egg rafts were found in control pools during the period in which Notonecta were present. After removing Notonecta, mosquitoes continued to avoid ovipositing in the former Notonecta pools for two additional days suggesting a predator-released kairomone as the cue used by the mosquitoes to detect the presence of this predator.     

Influence of the predatory backswimmer, Notonecta maculata, on invertebrate community structure

1. The impact of the predator, Notonecta maculata , on invertebrate community structure was investigated by a combination of a natural pool survey, an outdoor artificial pool experiment, and a laboratory prey preference experiment.
2. In a survey of forty natural pools in the Negev Desert, Israel, pelagic/neustonic species richness and overall species richness, but not benthic species richness, were negatively associated with N. maculata density.
3. In the artificial pool experiment, in which the presence or absence of Notonecta was manipulated, Notonecta caused a reduction in the richness of species colonizing the pools. Notonecta strongly reduced densities of the pelagic dipterans, Culiseta longiareolata (often 100% reduction of later-instar larvae and pupae) and ephydrids. Fewer Culiseta egg rafts were found in Notonecta pools, indicating that part of the observed reduction of Culiseta is not due to consumption of Culiseta larvae by Notonecta, but to habitat selection by ovipositing Culiseta females in response to risk of predation by Notonecta . Notonecta did not affect densities of (benthic) chironomids or the small cladoceran, Ceriodaphnia cf. quadrangula . Laboratory assessments of selectivity by Notonecta for the various prey matched patterns found in the field experiment.
4. This work provides strong evidence that N. maculata is an important organizer of community structure: it strongly reduces or even eliminates larger pelagic or neustonic species, but does not affect densities of small or benthic species.     

Effects of predator density and duration of predator occupancy on crustacean abundance and diversity in experimental pools

Keystone predators, by reducing the abundance of competitively superior prey, may have indirect positive effects on weak competitors, possibly increasing their abundance or preventing local competitive exclusion. By analogy to the Intermediate Disturbance Hypothesis, we would expect species diversity to peak at intermediate predator densities. In a replicated artificial pool experiment, we examined the relationships between density of the backswimmer Notonecta maculata (0, 1, 2, and 4 per 30 l pool) and invertebrate taxon diversity over an 11-week period of predator occupancy. Diversity reached high levels at high predator density sooner than at intermediate density. At the end of the experiment, taxon diversity was greatest at densities of 2 and 4 Notonecta per pool. While the overall predator density–diversity curve was in line with the intermediate disturbance hypothesis, the reduction in diversity from intermediate to high predator density was not statistically significant. Density of the preferred prey Daphnia magna decreased with Notonecta density, while densities of the smaller cladocerans Moina brachiata and Ceriodaphnia spp. increased. Suppression of Daphnia at high Notonecta densities may partially explain the increase in Moina and Ceriodaphnia densities. However, most of the relationship between Notonecta and the smaller cladocerans appears to be independent of Daphnia, suggesting complex interactions within the community. Our results suggest that keystone predation plays a strong role in structuring this community. Although diversity did not decrease significantly at the highest predator density as predicted, such a decrease may be more likely for pools with longer durations of predator occupancy or with higher predator densities.     

Interactive effects of salinity and a predator on mosquito oviposition and larval performance

Oviposition habitat selection (OHS) is increasingly being recognized as playing a large role in explaining mosquito distributions and community assemblages. Most studies have assessed the role of single factors affecting OHS, while in nature, oviposition patterns are most likely explained by multiple, interacting biotic and abiotic factors. Determining how various factors interact to affect OHS is important for understanding metapopulation and metacommunity dynamics. We investigated the individual and interactive effects of three water salinities (0, 15 and 30 p.p.t. NaCl added) and the aquatic predator Anisops debilis Perplexa (Hemiptera: Notonectidae) on OHS and larval performance of the mosquitoes Ochlerotatus caspius Pallas and Culiseta longiareolata Macquart (Diptera: Culicidae) in outdoor-artificial-pool and laboratory experiments. C. longiareolata inhabited only freshwater pools, strongly avoided pools containing A. debilis, and larvae experienced lower survival in the presence of A. debilis. Salinity concentration interacted strongly with the predator in affecting OHS and larval survival of O. caspius; oviposition increased with increasing salinity in the absence of the predator and decreased with increasing salinity in the presence of the predator. O. caspius larval survival in predator-free pools was lowest in freshwater and highest at intermediate salinity. In predator pools, survival was highest at high salinity, where predation rate was shown to be lowest in the laboratory. Our results highlight that assessing the role of single factors in affecting mosquito distributions can be misleading. Instead, multiple factors may interact to affect oviposition patterns and larval performance.     

Can adults of the New Zealand mosquito Culex pervigilans (Bergorth) detect the presence of a key predator in larval habitats?

The influence of predators on mosquito populations may be direct through predation or indirect through sub‐lethal responses of adult mosquitoes in life history traits such as oviposition behavior. In New Zealand, the backswimmer, Anisops wakefieldi, is a common predator of mosquito larvae found in temporary and permanent water bodies. We predicted that the New Zealand native mosquito, Culex pervigilans, whose larvae are vulnerable to predation of Anisops, would likely avoid the containers with the presence of Anisops or its kairomone. We established temporary water containers without predators, free‐roaming predators, caged predators (which were unable to eat mosquitoes), or containers from which predators were removed immediately prior to the experiment (these containers would have remnant kairomones from the predators). Each treatment with Anisops had predator densities of one, three, or nine Anisops. Contrary to our predictions, when choosing oviposition habitats, Cx. pervigilans appeared to ignore the presence of free‐roaming Anisops, caged Anisops, and water with Anisops kairomone. We thus observed no significant differences between the numbers of egg rafts laid by Cx. pervigilans in the different predator treatments nor were the number of egg rafts significantly affected by the density of predators. Rather than the presence of predators, environmental factors including temperature, humidity, and pressure were significantly correlated with mosquito oviposition. These mosquitoes appeared to either ignore the presence of the predator, had no ability to detect the presence of predators, or perhaps the cues from Anisops predators were not sufficiently strong enough to alarm these mosquitoes. We argue that the mosquito has not evolved the ability to detect the presence of these predators while ovipositing.     

Mosquito female response to the presence of larvivorous fish does not match threat to larvae

1. Several species with complex life‐history traits such as amphibians and insects with aquatic immature stages and terrestrial adults avoid ovipositing in pools containing larvivorous fish. This avoidance response was assumed to be a general one for most fish species. 2. The generality of ovipositing Culex to the presence of three, widespread larvivorous fish species was tested in a set of field experiments with artificial oviposition pools using caged fish. 3. Larval performance was further examined under actual predation by these three fish species. 4. Results show that ovipositing females responded strongly to the presence of caged mosquitofish, Gambusia affinis, while showing no significant response to the presence of caged green sunfish, Lepomis cyanellus, or the pirate perch, Aphredoderus sayanus. All three fish species consumed similar amounts of larvae. 5. This is the first example of species‐specific response differences to predators during mosquito oviposition habitat selection. These results point to the existence of predator‐released kairomones affecting mosquito behaviour. These kairomones may either be species‐specific or vary in concentration among fish, and probably have an important role in the understanding of mosquito spatial distribution.     

Rapid habituation by mosquito larvae to predator kairomones

Larvae of some species of mosquitoes have been shown to respond to water‐borne kairomones from predators by reducing bottom‐feeding and replacing it with surface filter‐feeding, which uses less movement and is thus less likely to attract a predator. However, if no predator attack takes place, then it would be more efficient to use a risk allocation strategy of habituating their response depending on the predator and the overall risk. The larvae of Culiseta longiareolata Macquart live in temporary rain‐filled pools, where they are exposed to a high level of predation. Within one hour, they responded to kairomones from dragonfly or damselfly nymphs, or to the fish Aphanius, by significantly reducing bottom‐feeding activity. Continued exposure to the predator kairomones resulted in habituation of their response to damselflies, a slower habituation to fish, but no habituation to dragonflies even after 30 h. In contrast, the larvae of Culex quinquefasciatus Say normally live in highly polluted and thus anaerobic water, where the predation risk will be much lower. They also showed a significant reduction in bottom‐feeding after 1 h of exposure to predator kairomones but had completely habituated this response within 6 h of continuous exposure. Some species of mosquito larvae can thus show a very rapid habituation to predator kairomones, while others only habituate slowly depending on the predator and overall predation risk.     

Influence of fire salamander larvae on among-pool distribution of mosquito egg rafts: oviposition habitat selection or egg raft predation?

Many amphibian populations are in decline worldwide. Surprisingly, few studies have examined how such declines may benefit mosquitoes. Amphibian larvae may compete with and prey upon mosquito larvae, and may alter oviposition habitat selection (OHS) of mosquito adults. However, often overlooked, observed among-pool egg distributions attributed to OHS may additionally or alternatively be explained by egg predation. Temporary pools of mountainous areas of the Mediterranean serve as larval habitat for both the mosquito, Culiseta longiareolata, and the salamander, Salamandra infraimmaculata. We found Culiseta larvae and egg rafts to be highly vulnerable to predation by pre-metamorphosing Salamandra larvae, but not to metamorphosing ones. In outdoor mesocosm experiments, oviposition avoidance by Culiseta females in response to caged Salamandra was not demonstrated regardless of salamander developmental stage. Egg raft abundance was significantly reduced in free-roaming, pre-metamorphosing Salamandra but not by metamorphosing ones. Thus, Salamandra larvae may have little deterrence on Culiseta oviposition. Instead, fewer egg rafts are attributed largely to egg predation. This study highlights the importance of egg raft predation in addition to OHS when interpreting the influence of predators on prey egg distributions. It also highlights that a cost of declining amphibian populations is their reduced impacts on mosquito populations.     

Environmental correlates of the intrinsic rate of natural increase in primates

Morphological and life history traits of two clones of the cladoceran Daphnia pulex were measured in the presence and absence of size-selective insect predators, the midge larva Chaoborus flavicans, which preys on small Daphnia, and the water bug Notonecta glauca, which preys on large Daphnia. The aim was to detect predator-induced phenotypic changes, particularly the effect of simultaneous exposure to both types of predators. Other work has shown that in the presence of Chaoborus americanus, Daphnia pulex produce a socalled neck spine which may carry several teeth. The morphological modifications are supposed to serve as an anti-predator device. Furthermore, females exposed to Chaoborus often delay their maturation; this has been interpreted as a cost that balances the benefits of the neck teeth. In this investigation, females of both clones produced fewer but larger offspring than control animals when reared in the presence of Chaoborus flavicans. The offspring showed the typical neck spine and delayed first reproduction. In the presence of Notonecta glauca, one of the clones produced more and smaller offspring, and maturation occurred at earlier instars. The other clone also produced more offspring than the control but there was no size difference. When both predators were present, in most cases the reactions of the daphnids were similar to those in the Notonecta experiment. The response to Chaoborus appeared to be suppressed. The observed modifications are interpreted as evolved strategies that reduce the impact of size-selective predation. They are consistent with predictions of life-history theory.     

Multi‐scale oviposition site selection in two mosquito species

1. In organisms characterised by complex life cycles, habitat selection often occurs at multiple spatial scales. For instance, female mosquitoes searching for an appropriate aquatic habitat to oviposit their eggs should also consider the characteristics of the terrestrial landscape in which it is embedded. 2. In this study, a field experiment was conducted to test for multi‐scale oviposition site selection in two mosquito species. Artificial pools were placed in two adjacent landscapes, olive plantations and a citrus orchard, mainly differing in their blooming periods and nectar availability. Pools were organised in three pairs: predatory caged fish were present in both pools, in one pool, or in none. 3. Early during the season, most of the egg rafts were laid by Culiseta longiareolata females in pools located within the blooming citrus orchard. When blooming shifted to the olive plantation, C. longiareolata become opportunistic. Culex pipiens females appeared later on during the season, when egg rafts of C. longiareolata were scarce, and they exhibited a higher selectivity to the olive plantation, although its blooming ended. In addition, the selectivity of C. pipiens to fish‐free pools was stronger than that of C. longiareolata. 4. Culex pipiens was more selective, possibly due to its high dispersal ability, which can lower movement cost and enhance the ability to gather environmental information. A trade‐off among gonotrophic cycles, combined with a shorter breeding season and limited recognition ability of the predatory fish may have reduced C. longiareolata selectivity. These differential oviposition patterns can strongly affect the population and community dynamics of both species.     

Cannibalism as the cause of an ontogenetic shift in habitat use by fry of the threespine stickleback

Summary In Crystal Lake, British Columbia, small fry (15 mm SL) of the threespine stickleback (Gasterosteus aculeatus) are concentrated in vegetation while larger fry are not. Because fry in all size classes feed primarily on zooplankton, even when in vegetation, we hypothesized that size-limited predation was responsible for the observed shift in habitat use with size. The major predators on fry in Crystal Lake are adult threespine stickleback, the water scorpion, Ranatra sp. (Hemiptera: Nepidae), backswimmers, Notonecta spp. (Hemiptera: Notonectidae), and dragonfly naiads of the genus Aeshna (Odonata: Aeschnidae). On the basis of distribution and hunting behavior we excluded the insects Ranatra sp., and Notonecta sp. as causal agents for this shift in resource by fry in water >0.25 m deep. Ranatra was found almost exclusively near the shoreline in water <0.25 m deep, and both insects hunted primarily as ambush predators within vegetation. Such predators seemed more likely to drive vulnerable fry from vegetation than to restrict them to it. In contrast, Aeshna naiads and adult stickleback frequently hunted outside of vegetation. In prey preference experiments the naiads did not show the decline in predation efficiency on fry >15 mm SL that would be expected if size-limited predation by this insect was responsible for the observed shift in resource use by fry. Adult stickleback only fed on fry <15 mm SL, and in an experimental situation, consumed fry at a rate 10 times greater than that exhibited by any of the insects. Predation experiments demonstrated that small fry (11–15 mm) spent more time in vegetation in the presence of adult conspecifics than they did in control pools, as would be expected if size-limited cannibalism caused small, vulnerable fry to be restricted to vegetation. Fry >15 mm SL were found outside of vegetation more often than in control treatments. The probable cause of this result is that adults become aggressive toward fry at this size, and often could be seen chasing large fry from vegetation during the experiments. Dragonfly naiads (Aeshna spp.) spent most of their time in vegetation in the experimental pools. Both size classes of fry spent less time in vegetation in the presence of dragonfly naiads than they did in control treatments, an apparent reflection of their similar vulnerabilities to these naiads. The presence of vegetation in pools reduced predation rates by adult stickleback on small fry. Because the experiments presented here indicate that fry are capable of rapidly assessing predation risk and of altering their behavior adaptively, we conclude that small fry occupy vegetation as a refuge from cannibalism. Once fry have reached the size-threshold at which they are no longer vulnerable to adult conspecifics they are able to forage farther from vegetation thereby reducing risk of predation by insects in vegetation and possibly acquiring more abundant food resources.     

Predation potential of three flatworm species (Platyhelminthes: Turbellaria) on mosquitoes (Diptera: Culicidae)

We conducted a field survey for flatworms to select species as potential biological control agents against Aedes aegypti and Culex pipiens (Diptera, Culicidae) breeding in artificial containers. Laboratory experiments were performed to determine the daily predation rate, differential predation on each mosquito larval instar, selective predation on either A. aegypti or C. pipiens, and predator tolerance to water from artificial containers. Girardia anceps (Tricladida, Paludicola, Dugesiidae), Mesostoma ehrenbergii and Bothromesostoma cf. evelinae (Rhabdocoela, Typhloplanoida, Typhloplanidae) were found in temporary puddles and permanent pools. In the laboratory, they killed between 52% and 100% of immature mosquitoes coexisting in the same habitat. No preference of flatworms for mosquito preys was detected. Predation rate was related to predator size and instar of preys. Girardia anceps and B. evelinae survived after a dry period and when re-flooding occurred, they laid eggs. Tolerance to water from artificial containers was highest in G. anceps and this species could be a suitable predator to reduce mosquito populations from artificial containers using an inoculative approach.     

Anti-predator strategies in oviposition site selection of Pyrrhosoma nymphula (Zygoptera: Odonata)

Summary Selection of sites for oviposition when the risk of predation by green frogs was variable studied in tandems of the damselfly Pyrrhosoma nymphula. In the absence of predators the number of tandems landing increased with the size of the oviposition site and with the number of pairs already present. Pairs approaching an oviposition site landed promptly and preferred locations near other ovipositing pairs. Pairs which had landed near others left the landing site and flew to another water plant after a shorter period than those landing alone. They stayed in groups for only 16.5% of the total duration of stay at the oviposition site. With predators present fewer tandems landed. There was no relationship between the landing site of solitary tandems and the locations of the perching frogs. Pairs landing in groups were disturbed by frogs after a shorter period than pairs landing alone. Damselfly aggregation did not affect the predation success of the frogs, but the predation risk to individual tandem pairs was reduced.