Limnology (2007) DOI 10.1007/s10201-006-0197-6
© The Japanese Society of Limnology 2007
ASIA/OCEANIA REPORT Nabaneeta Saha · Gautam Aditya · Animesh Bal Goutam Kumar Saha
A comparative study of predation of three aquatic heteropteran bugs on Culex quinquefasciatus larvae
Received: July 27, 2006 / Accepted: January 4, 2007
Abstract The aquatic bugs Anisops bouvieri Kirkaldy 1704 (Heteroptera: Notonectidae), Diplonychus (=Sphaerodema) rusticus Fabricius 1781, and Diplonychus annulatus Fabricius 1781 (Heteroptera: Belostomatidae) are common members of the freshwater insect communities of the East Calcutta Wetlands along the eastern fringe of Kolkata, India. These insects are established predators of dipteran larvae and other organisms. A comparative account of their predatory efficiency was made using larvae of Culex quinquefasciatus Say 1823 in the laboratory. It was revealed that a single adult of A. bouvieri could consume 2–34 fourthinstar mosquito larvae per day, D. rusticus 11–87 fourthinstar mosquito larvae per day, and D. annulatus 33–122 fourth-instar mosquito larvae per day, depending upon the prey and predator densities. The mean predation rate of A. bouvieri and D. annulatus remained stable over a 6-day feeding period but varied for D. rusticus. The predatory impact (PI) values were 14.77–17.31, 46.9–55.73, and 61.74– 72.72 larvae/day for A. bouvieri, D. rusticus, and D. annulatus, respectively. Compared to these, the clearance rate (CR) value range was 9.06–13.25 for A. bouvieri, 13.64– 15.99 for D. rusticus, and 13.50–16.52 larvae l/day/predator for D. annulatus. The values of mutual interference, “m,” remained 0.06–0.78 for A. bouvieri, 0.003–0.25 for D. rusticus, and 0.09–0.27 for D. annulatus, and did not vary between the days. The difference in predatory efficiency, CR, and PI values varied significantly among the three predators, indicating the possible difference in the function as predators occupying the same guild. It can be assumed that these predators play an important role in larval population
N. Saha · G. Aditya · G.K. Saha (*) Department of Zoology, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 00019, India Tel. +91-33-24753681; Fax +91-33-24614849 e-mail:
[email protected] N. Saha · A. Bal Zoological Survey of India, New Alipore, Kolkata, India G. Aditya Department of Zoology, The University of Burdwan, Golapbag, Burdwan, India
regulation of mosquitoes and thereby impart an effect on species composition and interactions in the aquatic insect communities of the wetlands and other similar habitats where they occur. Key words Water bugs · Diplonychus (=Sphaerodema) annulatus · Diplonychus rusticus · Anisops bouvieri · Culex quinquefasciatus · Predation · Predatory interference · Wetlands
Introduction The backswimmer Anisops bouvieri Kirkaldy 1704 (Heteroptera: Notonectidae) and the giant water bugs Diplonychus (=Sphaerodema) rusticus Fabricius 1781 and Diplonychus (=Sphaerodema) annulatus Fabricius 1781 (Heteroptera: Belostomatidae) are common inhabitants and major predators in the aquatic ecosystems of Kolkata and its surrounding areas (Nandi et al. 1993; Mukherjee et al. 1998; Khan and Ghosh 2000), especially the wetlands adjacent to the eastern fringe of the city, designated as East Calcutta wetlands (Institute of Wetland Management and Ecological Design 2004). The region harbors a rich biodiversity and is an internationally acclaimed Ramsar site. The notonectids and belostomatids are true bugs and feed by piercing the prey with the rostrum, injecting digestive juices, and sucking the liquefied contents from the prey. They hunt for their prey by ambush, remaining still until the prey is detected by mechanoreception or vision (Gittleman 1974; Streams 1987; Cloarec 1990; Dieguez and Gilbert 2003). Within freshwater ecosystems, notonectids are primarily associated with shallow littoral clear water areas among vegetation or deeper open water regions whereas the large-sized belostomatids are often found along the littoral zone associated with macrophytes (Rao 1981; Bhattacharya 1998; Pal et al. 1998; Gilbert and Burns 1999). These aquatic macroinvertebrate predators are reported to coexist abundantly with other organisms, including the aquatic stages of several species of mosquito in rice fields (Victor et al. 1991; Mogi
et al. 1995; Victor and Reuben 1999; Sunish and Reuben 2002) and shallow water pools (Das et al. 2006), and have an extremely broad food niche among which mosquito larvae is the most abundantly available prey (Panickar and Rajagopalan 1977; Tawfik et al. 1986; Hati 1988; Pal and Ghosh 1988; Nishi and Venkatesan 1989; Pramanik and Raut 2003; Aditya et al. 2004, 2005). The present study was designed to evaluate the predatory efficiency of the water bugs A. bouvieri, D. rusticus, and D. annulatus on immature stages of Culex quinquefasciatus Say 1823 (Diptera: Culicidae) as prey. Assessment of predation of these top predators will reflect upon the possible intraguild species interactions that play an important role in organizing the structure of freshwater aquatic communities (Murdoch et al. 1984; Blaustein 1998; Gilbert et al. 1999; Hampton and Gilbert 2001; Hampton 2004). It will also strengthen the idea of utilizing natural enemies for controlling vector mosquitoes, an alternative option to chemical pesticides, which often interfere with nontarget organisms, thus adversely affecting the biodiversity of the system.
Methods Study insects The adult water bugs A. bouvieri, D. rusticus, and D. annulatus were collected from the wetlands along Eastern Metropolitan Bypass, Kolkata, and the pond located in the Ballygunge Science College campus, University of Calcutta, Kolkata. An insect net of 200-µm mesh size was swept through the vegetation along the edge of the pond as well as through the open water areas. Adult morphs of the predators (irrespective of sex) were segregated and maintained within glass aquaria, 28 000 cm3 in volume and containing pond water up to 23 cm height, in the laboratory. Mosquito larvae were provided as food from time to time, and a few specimens of aquatic plants such as Chara and Vallisneria were placed inside the aquarium to simulate natural conditions. The insects were collected 10 days before the commencement of the experiments and maintained in the laboratory for acclimatization with Culex (Cx). quinquefasciatus larvae as food. The backswimmers A. bouvieri that were used in the experiments were on average 6.22 mm (range, 5.8–6.9 mm) in body length, measured from tip of the head to the end of the abdomen. The average body lengths of the belostomatids used in the experiments were 15.32 mm (range, 14.8–16.1 mm) and 22.3 mm (range, 21.9–23.1 mm) for D. rusticus and D. annulatus, respectively. Mosquito larvae were collected from drains of the same campus. The fourth-instar larvae to be used in the experiments were separated from the heterogeneous mixture containing different size-classes of larvae by appropriate sieving and kept within enamel trays with an adequate amount of food (1 Leviest capsule/300 larvae, approximately). The smaller instars obtained were maintained in the laboratory following specifications of Jones and Schreiber (1998) to obtain the fourth-instar stage as and when required.
Laboratory studies Two experiments were carried out to determine the rate of predation of the water bugs. Determination of predatory impact (PI) One adult predator was placed within a plastic tray, and 100 or 200 fourth-instar Cx. quinquefasciatus larvae per 5 l were provided as food. The number of prey consumed after 24 h was noted, and the prey density was reset. In this way, the feeding rate (numbers of prey killed per day) of the same individual predator was observed every 24 h for 6 consecutive days. Nine repetitions were followed for each predator species and prey density. Data obtained were used to calculate predatory impact (PI) following Hampton et al. (2000) with certain modifications, using the following formula: PI = ΣPe/ T where PI = predatory impact (numbers of larvae consumed/ day); Pe = number of prey eaten or killed; and T = time in days (here, T = 6). To evaluate the consumed number of prey, two-way repeated-measures analysis of variance (ANOVA) was performed with “predator species” and “prey density” as between-subject factors and “days” as within-subject factors. Data of the first day were excluded from the two-way repeated measures ANOVA as the predators were starved before this and were expected to predate at a higher rate. With the values of PI calculated, two-way ANOVA between predator species and prey density was done to find significant interactions, if any. This analysis was followed by a separate one-way ANOVA and post hoc Tukey test when the interactions were noted to be significantly different. Determination of clearance rate (CR) and mutual interference (m) Here, one or five adult morphs of the predators were exposed to 200 fourth-instar Cx. quinquefasciatus larvae within plastic tubs 40 cm in diameter containing 16 l pond water. The number of prey killed was noted, and the prey density was reset every 24 h for 3 consecutive days with the same set of predators. Six replicates were carried out with each predator species. Data obtained on predation by five predators were used to calculate clearance rate (CR) following Gilbert and Burns (1999) with required modifications, using the following formula: CR = VlnPe/NT where CR = number of prey killed volume/day/predator; Pe = number of prey killed/day; V = volume of water (in liters); N = number of predators; and T = time (in days). The clearance rate was considered as an overall indicator of prey–predator interaction, including the space available and predation in unit time. This indicator was used for comparing the efficacy of different predators. The data ob-
tained on CR values were subjected to one-way repeatedmeasures ANOVA with predator species as between-subject factor and days as within-subject factor; this was followed by a separate one-way ANOVA and Tukey test among the three predator species. Also, from the data obtained on predation from this experiment, the mutual interference constant between predators when present in multiple numbers was calculated using the following equation (Elliott 2003): Na/PN = Q P
−m
where m = mutual interference constant; Na = mean number of prey consumed; P = number of predators; N = prey density; and Q = value of Na/P N when P = 1. The “m” values of the predators were subjected to oneway repeated-measures ANOVA to judge the difference between the days and the predator species, if any. When multiple predators of the same species are considered, the value of mutual interference is expected to remain stable for a particular time period, provided other factors influencing predation remain unchanged. In the experimental setup, the volume of water and the number of prey provided between the 3 days were the same and thus the value of “m,” if it differed, was the result of predatory capability of the conspecific water bugs. However, “m” can be used as an indicator of the difference in the level of aggregation by the predator species and their predation capability against the mosquito larvae. Statistical analyses on the data obtained on the prey consumption and the indices of predation, PI, CR, and “m” were carried out following Zar (1999). The predators were fed to satiation and then starved for 24 h before their utilization in the experiments to equalize the hunger level approximately. In both experiments, controls without predators were set with equal number of replicates as those of the test. The pond water collected from the Ballygunge Science College campus, Kolkata, that was used as the experimental medium had a pH range of 8.5–9.0 and temperature 25°–30°C. In all the experimental and control sets, a few sticks and leaves of the aquatic plants
Vallisneria spiralis and Jussiaea repens were added to serve as sites for prey and predator refuge. All experiments were conducted from June 21 to July 10, 2005.
Results During the course of the experiment, observations were made on the predatory behavior of the bugs. The belostomatids hunted fiercely for their prey. After a successful encounter, the predator grasped a mosquito larva with its pro- and mesothoracic legs, subdued the struggling larva by puncturing its body with the help of the sharp rostrum, and then fed on its internal body fluid. Finally, the dead remains of the larva, mainly including the sclerotized head, were discarded and the predator hunted for its next prey. However, it was noted that not all encounters led to successful grasping and restraining of the larvae. In comparison to D. annulatus, D. rusticus seemed to kill more numbers of mosquito larvae than those from which they sucked out the body fluid. The belostomatids were found to alternate between active foraging and ambushing. The backswimmers also exhibited a similar pattern, but they preyed mostly in the central open water portion of the containers. These observations are in support of earlier studies (Cloarec 1990; Venkatesan and D’Sylva 1990; Aditya et al. 2004). Results of two-way repeated-measures ANOVA showed that the number of prey consumed varied significantly between the three predator species, two prey densities, and days (i.e., from day 2 to day 6, day 1 was avoided because the predators were starved before this and are expected to consume at a higher rate). Although the interaction days*predator species was statistically significant, days*prey density and days*prey density*predator species were not found to be significant (Table 1). Because days-by-predator species interaction showed significant variations, a separate one-way ANOVA followed by a post hoc Tukey test was carried out to justify the differences in the numbers of prey
Table 1. Results of two-way repeated measures analysis of variance (ANOVA) with days as within-subject factor and predator species, and prey density as between-subject factors on the number of prey consumed by Anisops bouvieri, Diplonychus rusticus, and Diplonychus annulatus (n = 9 replicates per species per density per days) Source of variation
SS
df
MS
A. Tests of within-subject contrast Total Days Days*prey density Days*predator species Days*prey density*predator species Error
25 701.81 11 943.24 0.457 6 557.01 549.07 6 652.04
53 1 1 2 2 47
11 943.24 0.457 3 278.5 274.53 141.53
84.38 0.003 23.16 1.94
0.001 0.955 0.001 0.155
486 213 4 297.72 1 087.53 935.16 7 957.75
1 1 2 2 47
4 297.7 54 376.65 467.58 169.31
25.38 321.15 2.76
0.001 0.001 0.07
B. Tests of between-subject effects Total Prey density Predator species Prey density*predator species Error SS, sum of squares; MS, mean square
F
P value
consumed by the predator species per day. The backswimmers A. bouvieri (at 100-prey density: F = 4.33, P < 0.001, at 200-prey density: F = 10.15, P < 0.001, df = 4 for both, by one-way ANOVA) did not exhibit any statistically significant (P > 0.13) variations in majority of the between-days comparisons (except between day 2 and 3, P < 0.004, at 100-prey density; day 2 and 3, P < 0.001, day 3 and 4, P < 0.01, day 3 and 6, P < 0.001, at 200-prey density). For D. annulatus (F = 2.91, P < 0.03, for prey density 100; F = 3.37, P < 0.01, for prey density 200, df = 4 for both, by one-way ANOVA), the variations were not significant (P > 0.05) in any of the between-days comparisons at both the prey densities. In contrast, D. rusticus (F = 25.65, P < 0.001, for prey density 100; F = 11.03, P < 0.001, for prey density 200, df = 4 for both, by one way ANOVA) showed significant variations (P < 0.05) for maximum number of the between-days comparisons (except between day 3 and 4, P > 0.92; day 5 and 6, P > 0.94, at 100-prey density; day 2 and 3, P > 0.64,
Fig. 1. Mean number (mean ± SE) of prey (fourth-instar Culex quinquefasciatus) killed per day at 100-prey (I) and 200-prey (II) densities per 5 l pond water by the water bugs Anisops bouvieri, Diplonychus rusticus, and Diplonychus annulatus with results of between-days post hoc tests where days sharing a common alphabetical designation exhibited no significant variations in feeding (sets of letters are considered individually for each graph)
day 2 and 4, P > 0.2, day 2 and 5, P > 0.08, day 3 and 4, P > 0.92, day 5 and 6, P > 0.99, at 200-prey density). A single adult morph of A. bouvieri could consume 2–34 fourth-instar larvae per day depending upon the prey density. The average number of prey killed during the 6-day period was 88.66 ± 4.49 for 100- and 103.87 ± 2.94 for 200prey densities. After being starved for 24 h, the notonectid bug could kill 19.11 ± 1.28 larvae on the first day, in the presence of 100 prey. For the 200-prey density, the value was 20.5 ± 1.38. D. rusticus consumed 11–87 larvae whereas the mean number of prey killed during the entire experimental tenure was 281.44 ± 10.87 for 100 prey and 335.66 ± 10.07 for 200 prey. On the first day, it could consume 60.55 ± 5.68 larvae at the 100-prey density after 24-h starvation; the value was 68.88 ± 3.70 for 200 prey. D. annulatus was found to predate upon 33–122 fourth-instar larvae/day, the numbers being 370.44 ± 19.20 and 436.33 ± 9.58 for 100 and 200 prey, respectively (Fig. 1).
18
A. bouvieri D. rusticus
80
c
70
D. annulatus
b d
60
b
40 30 20
D. rusticus
b
16
D. annulatus e
50
A. bouveri
f
Clearence Rate
Number of prey killed/predator/day (PI)
90
d
e
e
14 a
12
e
c
10
d
a
8
10
1
0
2
3
Days
100
200 Prey density
Fig. 2. Predatory impact (PI) values (mean ± SE) with results of Tukey test between A. bouvieri, D. annulatus, and D. rusticus (n = 9 replicates/predator species/prey density). The PI value does not vary significantly between predators if they share a letter (a, b, c, d, e, f) in common
Determination of predatory impact (PI) The predatory impact (PI) values were 14.77 ± 0.7, 46.90 ± 1.18, and 61.74 ± 3.20 at 100-prey density and 17.31 ± 0.49, 55.73 ± 2.69, and 72.72 ± 1.6 larvae per day at 200-prey density for A. bouvieri, D. rusticus, and D. annulatus, respectively (Fig. 2). PI varied significantly among the three predator species [F = 389.07, P < 0.001, df(v1,v2) = 2, 24] and two prey densities [F = 17.96, P < 0.001, df(v1,v2) = 1, 24], but the interaction between predator species and prey density was not significant [F = 2.21, P > 0.13, df(v1,v2) = 2, 24, by two-way ANOVA]. Because the PI values between the predator species differed significantly, a separate one-way ANOVA followed by post hoc Tukey test was carried out. The results confirmed that the PI value was significantly higher in the belostamatids compared to the notonectid bug (P < 0.001, by post hoc Tukey test; see Fig. 2). Also, the PI values varied significantly with prey density in case of each predator species [F = 6.96, P < 0.05, for A. bouvieri; F = 7.38, P < 0.05, for D. rusticus, and F = 9.27, P < 0.05, for D. annulatus; df(v1,v2) = 1, 17 for all, by one-way ANOVA]. Determination of clearance rate (CR) and mutual interference (m) Collectively, five adults of A. bouvieri consumed between 22 and 63 fourth-instar Cx. quinquefasciatus larvae per day. Equivalent values for the belostomatid bugs D. rusticus and D. annulatus were 71–148 and 81–175 larvae per day, respectively. The clearance rate (CR) values for A. bouvieri ranged from 9.06 to 13.25, for D. rusticus from 13.64 to 15.99, and for D. annulatus from 13.50 to 16.52 larvae l/day/ predator. Mean CR values varied significantly between the predator species [F = 183.82, P < 0.001, df(v1,v2) = 2, 15] and days [F = 41.75, P < 0.001, df(v1,v2) = 2, 14] as well as with the interaction between predator species*days [F = 2.96, P < 0.03, df(v1,v2) = 4, 30; by one-way repeated-measure ANOVA]. Subsequent one-way ANOVA followed by
Fig. 3. Clearance rate (CR) values (mean ± SE) of A. bouvieri, D. rusticus, and D. annulatus with results of Tukey test (SE omitted for values 0.05, by post hoc Tukey Test) for the first and the second day of the experiment; however, the variations were not significant on the third day (Fig. 3). The values of mutual interference (Table 2) of predation among the conspecific predators differed significantly between predator species [F = 12.06, P < 0.01, df(v1,v2) = 2, 15], days [F = 110.09, P < 0.001, df(v1,v2) = 1, 15] and their interaction [F = 27.75, P < 0.001, d (v1,v2) = 2, 15]. The rate of predation of A. bouvieri, D. rusticus, and D. annulatus was 8.67–23.83, 25.33–35.88, and 26.0–34.33, respectively, within a period of 24 h. These data were used to calculate the Q values for respective predators and subsequent “m” values, when the predator density was five. Not a single larva of Cx. quinquefasciatus (prey) died in any of the control sets, with respect to both experiments. However, a major percentage of the larvae pupated by the end of the long-term experiments.
Discussion From the results it is evident that all the three heteropteran bugs preyed effectively upon mosquito larvae, although the rate varied significantly between them. The number of prey consumed varied significantly between the days, but the overall predation pattern of D. annulatus and A. bouvieri showed less variation over the 6-day feeding trials. This result reflects their capability to induce a stable prey mortality, although the number of prey consumed remained considerably low for A. bouvieri. The belostomatid bug D. rusticus, on the other hand, showed significant variation in prey consumption between the days. On reaching the maximum satiation level there was a sudden decline in predation, a characteristic predation rhythm of D. rusticus (Pramanik and Raut 2005), in the 6-day experiment, at both
Table 2. Mutual interference constant (“m”) of three predators for 3 continuous days (A) Rate of predation of a single predator for 24 h when the prey density was 200 fourth-instar Culex(Cx.) quinquefasciatus larvae/16 l pond water (n = 6 trials per predator)
A. bouvieri D. rusticus D. annulatus
1
Day 2
3
11.83 ± 1.68 25.33 ± 2.96 32.16 ± 4.76
8.66 ± 1.28 35.88 ± 3.37 34.33 ± 2.89
23.83 ± 2.25 27.83 ± 2.08 26.0 ± 1.71
(B) “Q” values for a single predator obtained from the above predation rate
A. bouvieri D. rusticus D. annulatus
1
Day 2
3
0.05 ± 0.01 0.12 ± 0.01 0.16 ± 0.02
0.04 ± 0.01 0.17 ± 0.01 0.17 ± 0.01
0.11 ± 0.01 0.13 ± 0.01 0.13 ± 0.01
(C) “m” values for the predator species at a predator density of 5
A. bouvieri D. rusticus D. annulatus
1
Day 2
3
0.06 ± 0.04 0.01 ± 0.03 0.09 ± 0.04
0.24 ± 0.07 0.35 ± 0.02 0.31 ± 0.01
0.78 ± 0.06 0.25 ± 0.03 0.27 ± 0.02
prey densities. Because these predators belong to the same trophic level with considerable degree of niche overlap, this variation could be attributed to the difference in energy requirements resulting from the variations in body size, effectiveness in restraining and killing the struggling prey, or preference for a particular type of prey. Predatory impact (PI), which represents the prey consumption capability of the predator through a time period, however, varied significantly with prey density, indicating their capability to consume more prey at higher densities (see Fig. 2). In general, arthropod predation is regulated by certain common factors related to prey and predator densities (Hassell et al. 1978; Fox and Murdoch 1978) or body size of the prey and predator (Blios and Cloarec 1983; Scott and Murdoch 1983) as well as habitat area (Sunahara et al. 2002). The difference in the predation pattern of the water bugs observed here might be caused by these factors. Clearance rate (CR) quantifies the ability of the predator to search, capture, and predate within a specific space and time. When the predators were present in multiple numbers within a larger surface area, the predation rate increased, which is similar to natural situations. CR varied significantly between the notonectid and belostomatids, but not between the belostomatids (see Fig. 3). Thus, it can be assumed that the predation strategy of A. bouvieri is different from that of the belostomatid bugs, although they coexist in similar habitats. The predatory strategy of a number of aquatic insects is known to depend upon the complexity of the physical environment (Nishi and Venkatesan 1997; Ikeda and Nakasuji 2002; Hampton 2004; Alto et al. 2005) and the presence of multiple prey types (Blaustein 1998; Hampton et al. 2000; Lundkvist et al. 2003). Among the three aquatic bugs considered here, D. annulatus consumed the maximum numbers of fourth-instar larvae of Cx. quinquefascia-
tus, followed by D. rusticus and A. bouvieri, when present in multiples. Because in nature these bugs reside within a heterogeneous environment with a complex species composition and are open to choice of a broad variety of food items, these results may vary under natural situations with multiple prey species or in a structurally complex habitat system with a single predator species or different coexisting predators in combination. The mutual interference constant for the three water bugs was different, indicating that the intensity of aggregation is dissimilar and species specific. The significant difference in “m” values between the predator species and days (as well as the interaction) reveals that the degree of suppression of prey consumption resulting from the presence of other conspecific predators fluctuates with time. Perhaps the differences in the predatory efficiency and satiation levels of individual predators contributed to the daily variation in “m” values. Also, this finding can be viewed as the difference in the adaptation as predators and segregation at niche level. In natural situation, the “m” values are expected to differ because the abundance of prey is homogeneous and the space available is much larger. Nonetheless, as all the three predators considered here, i.e., A. bouvieri, D. rusticus, and D. annulatus, are found in clusters, irrespective of the distribution of prey in patches or uniformly in space, interference is obvious. Considering these water bugs as biological resources to combat the mosquito population, the present study ascertains their competence, at least at the primary level of qualifying as biological control agents. Several other invertebrates that have been identified as potential predators of mosquito larvae include Triops newberryi (Notostraca: Triopsidae), Macrocyclops albidus (Crustacea: Copepoda), odonate larvae (Odonata), and the coleopteran beetles Hydroporus sp. and Rhantus sp. (Coleoptera: Dytiscidae) (von Kögel 1987; Fincke et al. 1997; Su and Mulla 2002; Lundkvist et al. 2003; Rey et al. 2004; Aditya et al. 2006). The crustacean T. newberryi has been introduced and established successfully against mosquito immatures in a date garden in California (Su and Mulla 2002). The larval stages of the odonates Gynacantha membranalis, Megaloprepus coerulatus, and Mecistogaster spp. depressed mosquito larvae survivorship to pupation (Fincke et al. 1997). In water-filled tree holes of Panama, the cyclopoid M. albidus was effective against 1- to 4-day-old mosquito immatures in a subtropical environment in Florida (Rey et al. 2004). In all these instances, habitat specificity and prey preference played a major role in population regulation of target mosquito species. The dytiscid beetle Colymbetes paykulli and the notonectid bug Notonecta maculata showed preference for mosquito larvae over Daphnia sp. and Chironomus sp. larvae, respectively, in their preferred habitats (Lundkvist et al. 2003; Blaustein et al. 2004). Rhantus consputus were observed to prefer larvae of Aedes vexans to cladocerans and ostracods (von Kögel 1987). The results of the present laboratory experiments indicate a similar possibility of exploiting these hemipteran bugs as predators against mosquito immatures in the different freshwater habitats of Kolkata and its surroundings areas. However, considering the vast range of
probable alternative prey, the prey choice by these predators needs to be evaluated. Acknowledgments The authors are thankful to the Director, Zoological Survey of India and the respective Heads of the Department of Zoology, University of Calcutta, Kolkata, and The University of Burdwan, Burdwan, India for the facilities provided. The fellowship granted to N.S. by ZSI, Kolkata, is thankfully acknowledged. The authors are grateful to two anonymous reviewers for modifications of the earlier manuscripts.
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