Winter survival of immature instars of Mansonia indubitans Dyar & Shannon and Mansonia titillans Walker (Diptera: Culicidae), in Buenos Aires, Argentina

ARTICLES

Winter survival of immature instars of Mansonia indubitans Dyar & Shannon and Mansonia titillans Walker (Diptera: Culicidae), in Buenos Aires, Argentina

Juan P Torretta^I,1; Pablo R Mulieri^III,IV; Luciano D Patitucci^II; Valeria A Sander^IV; Patricia L Rodríguez^II,IV; Nicolás Schweigmann^II,IV

^ICátedra de Botánica Agrícola, Facultad de Agronomía

^IIDepartamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Av. San Martín 4453, C1417DSE Buenos Aires, Argentina

^IIIANLIS "Dr. C.G. Malbrán" Servicio de Vectores, Buenos Aires, Argentina

^IVConsejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina

ABSTRACT

We conducted a whole year research on the ecology of Mansonia indubitans and Ma. titillans in Macáes Pond, Costanera Sur Reserve, Buenos Aires, Argentina. The usage of different floating plants by immature instars and their overwintering was analized. The percentage of usage of the available floating macrophytes (Pistia, Limnobium, and Salvinia) by the larvae and pupae was studied. Also, we defined positivity (P+) as the percentage of plants with immature instars for each plant genus on a monthly basis. Ma. immature instars were captured throughout the year and Pistia was the resource most commonly exploited by the mosquitoes. The percentage of fourth-instar larvae and pupae on Pistia roots with respect to total immature instars captured was assessed on a monthly and seasonal basis. The proportion of fourth-instar larvae and pupae from both species of Mansonia on water lettuce roots, showed significant differences between months and seasons. Our results suggest that the populations of Ma. indubitans and Ma. titillans in Macáes Pond, survive during winter mainly as fourth-instar larvae.

Key words:Mansonia indubitans - Mansonia titillans - Pistia stratiotes - winter survival - plant-animal interaction

The widespread genus Mansonia Blanchard is associated with aquatic macrophytes in the larval and pupal stage. They both possess modifications of their respiratory organs (siphons and trumpets, respectively) which enable them to attach to the roots of aquatic plants and obtain air from the aerenchyma (Dyar 1928, Forattini 1965). Adult females lay their characteristic egg masses on the leafs of aquatic macrophytes (Mattingly 1972a, b, Gass et al. 1983, Linley et al. 1986, Lounibos & Dewald 1989, Ferreira & Nunes de Mello 1999). Numerous floating and rooted aquatic plant species are associated with many Mansonia species (Slaff & Haefner 1985, Krishnamoorthy et al. 1994).

Water lettuce (Pistia stratiotes L., Araceae), considered as the third aquatic weed of world importance (Barreto et al. 2000), is used by the immature stages of a great diversity of insects (Poi de Neiff 1983, Escher & Lounibos 1993) and supports numerous Mansonia species in Africa (Laurence 1959, 1960), Asia (Krishnamoorthy et al. 1994), and America (Lounibos & Escher 1985, Lounibos & Dewald 1989, Ferreira 1999). The use of floating macrophytes for wastewater treatment is recognized (Brix 1997) as representing a favourable breeding ground for Mansonia mosquitoes (Kengne et al. 2003).

Ma. indubitans Dyar & Shannon and Ma. titillans Walker share a wide geographic range in America with Buenos Aires province, Argentina, being the southern limit (Mitchell & Darsie 1985). Females of both species are aggresively haematophagous (Almirón 2002) and potential vectors of filariosis (Forattini 1965).

García et al. (1995) reported that the immature instars of these species disappear during the winter in a relict of marginal jungle in Punta Lara Reserve (Buenos Aires province, Argentina) due to the death of P. stratiotes. Garcia et al. (1995) suggested the survival of Mansonia by overwintering adults.

The objectives of this work were: (a) study the winter behavior of immature instars of Ma. indubitans and Ma. titillans in Buenos Aires City and (b) determine the use of different plant species during overwintering of both species in a heterogeneous habitat.

MATERIALS AND METHODS

Study area - Field studies were conducted at the Macáes Pond, located in Costanera Sur Reserve (S 34º36'26,7" O 58º20'54,4"), Buenos Aires City, Argentina.

This man-made, 1-ha, freshwater habitat was overgrown with floating plants, mainly Salvinia spp. (S. herzogii de la Sota and S. rotundifolia Wild), Pistia stratiotes L. and Limnobium laevigatum (Humb. & Bonpl. ex Wild.) Heine. Due to the difficulty in identifying to the species level in the field, individuals of Salvinia were considered at generic level (hereafter Salvinia).

Buenos Aires is characterized by a subtropical climate with cold winters (mean temperature of 11°C) and hot, rainy summers (mean temperature is 25°C, Servicio Meteo-rológico Nacional 1993).

Fig. 1 shows maximum and minimum monthly average temperatures in Buenos Aires during 2003. By the fact that this study was undertaken in just one year, we compared maximum and minimum monthly mean temperatures with the mean values for 1991-2002.

Field samples - Surveys for immature instars were made from January to December 2003. Samples were taken weekly (January-April and November-December) or fortnightly (May-October), except july (one visit), using a 1404 cm² quadrangular sampling device. On each survey date, individuals of three genera were examined in 2 to 6 sampling units. The coverage (percentage of sample units) and number of individuals of Pistia, Limnobium, and Salvinia were recorded and the immature instars were captured by shaking plants vigorously in a plastic container with water (Lounibos & Escher 1985), and collecting them with plastic pipettes. They were fixed in situ with 80% ethanol. Fourth-instar larvae and pupae were identified to species according to Ronderos and Bachmann (1963). The other instars were treated together (called early instar larvae). The presence and number of the Mansonia immature instars were determined for each individual plant. Each plant with a single differentiated root was considered as an individual. Plants connected with a stolon were separated before immature Mansonia were collected. This disturbance caused by the harvesting of the plants could cause the loosening of some larvae from the roots of the floating plants

Taking into account the number and coverage of individuals from each plant genus, we estimated the mean monthly size of Pistia, Limnobium, and Salvinia using the formula:

TP_i = [S(cov_i * # individuals/100) * 1404]/n

where cov_i: the coverage of the genus I; 1404: sampling unit area (in cm²); and n: number of monthly sampling units.

Exploitation of plant resourses - The percentage of usage (PU_i) was defined as the number of Mansonia individuals collected on each plant genus (i) related to the total number of individuals caught each month. In addition, we calculated the mean total plants per sampling units (PT), mean plants with immature instars (PCI) and calculated the positivity (P+ = [PCI/PT]*100) for each plant genus monthly along the year.

Proportions of collected immature instars on Pistia roots were compared using the independent proportions test (Fleiss 1981). To assess differences among seasons we analyzed together the months corresponding to summer (January-March), fall (April-June), winter (July-September), and spring (October-December).

From September 2003 to December 2003, we measured the length of the longest root from each Pistia and Limnobium individuals and registered the number of Mansonia immature instars on each plant.

RESULTS

The coverage of floating plants at Macáes Pond changed along the year. The estimated mean size and the number of individual from each plant genera per sampling unit are shown in Table I. During winter the number of Pistia individuals per sampling unit was the lowest, also these individuals had the major mean size estimated. Limnobium and Salvinia individuals did not show any pronounced seasonal pattern (Table I). Other macrophytes, such as Hydrocotile ranunculoides L., Lemna spp., and Wolffiella sp. were also found in lower ratio.

Mansonia immature instars were collected throughout the year. Among the 2759 larvae and pupae captured, 78.5% were on Pistia roots, 19.5% on Limnobium, and only 2% on modified leafs of Salvinia (Table II). The percentage of usage (PU) varied during the year, with Pistia being the plant most used during all months (Table II). Limnobium roots usage was higher in spring and summer months while Salvinia was little used throughout the year.

Overall, the positivity of Pistia (P+) was higher than for Limnobium (except for January and February) and Salvinia throughout the year (Fig. 2A), varying from 15 to 70%. The number of individuals of Pistia per sample unit had the highest values in summer and spring and lower in fall and winter.

The monthly density of Limnobium showed an increase in February, March, and April and then diminished from May, rising again in October, with a slight decrease in November-December (Fig. 2B).

The mean monthly number of Salvinia per sampling unit was highest during the whole year. The maximum values of fern densities were found in January-February and November-December (Fig. 2C), but the percentage with immature instars was very low throughout the entire study.

Maximum and minimum monthly temperatures during 2003 were similar to those of the preceeding 13 years (1991-2002) (Fig.1).

Of the 2167 larvae and pupae collected on the Pistia roots, 703 were Ma. indubitans, 419 were Ma. titillans, and the remaining 1045 were early instar larvae. The percentage of total individuals captured on Pistia roots increased during the months that exhibited the lowest temperatures. The total percentage of usage on Pistia (PU_P) is shown in Fig. 3A. Fig. 3B shows the PU_P of fourth-instar larvae and pupae from both species.

The highest exploitation of Pistia roots by these immature instars occurs from May to October, when the parameter values were above 70%. The proportion of the fourth-instar larvae and pupae from both species of Mansonia on water lettuce roots showed significant differences among months (c² = 953.29, d.f. = 11, p < 0.01) and among seasons (c² = 308.37, d.f. = 3, p < 0.01).

The number of Ma. indubitans and Ma. titillans individuals did not correlate with the length of the Pistia (r = 0.17, p > 0.05 and r = 0.24, p > 0.05, respectively) or Limnobium roots (r = -0.17, p > 0.05 and r = 0.46, p > 0.05, respectively).

DISCUSSION

The result that immature instars of Ma. indubitans and Ma. titillans in Macáes Pond, Costanera Sur Reserve were captured throughout the whole year differs from the results of others authors (Lounibos & Escher 1985, García et al. 1995). García et al. (1995) working in a nearby study area, found that Mansonia immatures disappeared in the absence of P. stratiotes. Lounibos and Escher (1985) reported a decline in Ma. titillans and Ma. dyari adult emergence in Florida (US) during January-February (boreal winter). They associated this fact with a decrease in water temperature and P. stratiotes dying off. Dewald and Lou-nibos (1990) have demonstrated that low temperature in winter was the most important variable affecting P. stratiotes growth. In Macáes Pond we observed a decrease in the number of water lettuce, although the population never disappeared completely.

Water temperature affects the longevity of the immature stages of Culicidae. Slaff and Haffner (1985) reported that Ma. titillans and Ma. dyari larvae diminish their metabolic activity when the temperature decreases. Moreover, under laboratory conditions, Tun-Lin et al. (2000) and Bayoh and Lindsay (2004) showed that the mortality of Aedes aegypti and Anopheles gambiae was 100% when bred at temperatures lower than 10 and 16ºC, respectively. Some species are clearly more cold tolerant than others (Bayoh & Lindsay 2004). Notwithstanding we did not measure water temperature, Young (1975) found that in a temperate pond the average air and water temperature followed the same pattern of seasonal fluctuations; therefore air temperature may reflect thermal conditions for the immatures instars.

With the arrival of the warm temperatures, fourth-instar larvae pupate and emerge as adults. This is consistent with our finding of pupae only during spring and summer. We suggest, that fourth-instar larvae better tolerate adverse temperature conditions than early instars, although the accumulation of fourth-instar may be solely a product of earlier instars molting to the final instar, where metamorphosis is suspended until temperatures rise up.

At Macáes Pond the reproduction of Mansonia species starts by the end of the spring (unpublished data). There are two factors that cause the proportion of fourth-instar larvae to diminish with rising temperature: (1) during the reproductive season there is an increase in early instar larvae and (2) fourth-instar larvae develop into pupae.

The association between Mansonia and Pistia in Macáes Pond is strong. Mulieri et al. (2005) found a selective P. stratiotes pattern of root utilization by Ma. indu-bitans and Ma. titillans in Macáes Pond during the warm season.

Nevertheless, during low temperature months, when the average number of Pistia individuals per sampling unit diminishes, the proportion of larvae on water lettuce roots increases until reaching maximum values during winter. The largest individuals of P. stratiotes are whom survive during winter. Based on our results, we suggest a special importance of P. stratiotes for the winter survival of Ma. indubitans and Ma. titillans in its southernmost distribution edge, corroborating with García et al. (1995).

We showed that populations of Ma. indubitans and Ma. titillans in Macáes Pond, Buenos Aires City, survive during the cold period principally as fourth-instar larvae. The existence of an overwintering adult population, as suggested by García et al. (1995), was not examined during our study.

The winter association between Mansonia larvae and water lettuce would allow control of these mosquitoes by careful management of this aquatic weed.

ACKNOWLEDGMENTS

To Marc Slaff and Phillip Lounibos for reviewing earlier drafts of this manuscript, the personnel of Reserva Ecológica Costanera Sur for logistic support, Victoria Botazzi for her valuable cooperation in the field work, and Ramiro Saurral for revising the language and anonymous reviewers, for their critical suggestions of this manuscript.

Received 5 December 2005

Accepted 11 July 2006

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Publication Dates

• Publication in this collection
  25 Oct 2006
• Date of issue
  Sept 2006

History

• Accepted
  11 July 2006
• Received
  05 Dec 2005