Aedes aegypti survival in the presence of Toxorhynchites violaceus ( Diptera : Culicidae ) fourth instar larvae

The mosquito Aedes aegypti (Linnaeus, 1762) is the main vector of dengue and yellow fever viruses. Different methods have been used to control A. aegypti, including chemical and biological tools. However, chemical control can lead to a subsequent increase in the mosquitoes’ insecticide resistance, and biological control represents an important method as an alternative to insecticide usage. Larvae from the Toxorhynchites genus (Diptera: Culicidae) are predators of other mosquitoes and represent a potential natural biocontrol agent of A. aegypti larvae. In the present work, A. aegypti larval survival was studied in the presence of the neotropical Toxorhynchites violaceus (Wiedemann, 1821) fourth instar larvae. Toxorhynchites violaceus consumption of A. aegypti increased during the 192 hours of the experiment and was more marked in the intervals between 96 and 120 hours and between 168 and 192 hours, when the A. aegypti survival reached 0%. During the fourth instar, T. violaceus increased its predation on A. aegypti larvae, possibly in order to increase its nutrient storage prior to pupation. Otherwise, low prey consumption can lead to a nutritional deficit for the larvae, delaying the adult’s sexual development and reducing its egg production. Here we show that A. aegypti survival can be reduced by the T. violaceus fourth larvae predation under laboratory conditions.

The mosquito Aedes aegypti (Linnaeus, 1762) is an important vector of dengue and yellow fever viruses.Aedes aegypti is geographically widespread in Brazil.Larvae can grow in natural and artificial small water-filled containers.To control A. aegypti natural populations, chemical products such as larvicides are widely used.However, the evolutionary pressure of indiscriminate insecticide usage may increase the frequency of alleles that provide insecticide resistance in A. aegypti (BESERRA et al. 2007).
Several studies have shown that the A. aegypti populations can be reduced by Toxorhynchites spp.predation (TIKASINGH & EUSTACE 1992, FOCKS 2007).Even though Toxorhynchites spp.predate during their entire larval lifespan, fourth instar larvae are the most efficient predators (CHOWANADISAI et al. 1984).However, little is known about the interactions of A. aegypti with the fourth instar larvae of the Neotropical Toxorhynchites violaceus (Wiedemann, 1821).In view of this gap, we have studied the larval survival of A. aegypti in the presence of T. violaceus fourth instar larvae.
Toxorhynchites violaceus larvae were collected in bromeliads from 'Serra da Piedade' (43°40'N, 19°49'S, altitude 1746 m), Caeté, Minas Gerais, Brazil, by removing the water between bromeliad leaves.Only third instar larvae were transferred to 250 ml plastic pots and taken to the laboratory.They were fed on 20 A. aegypti fourth instar larvae per day, until the T. violaceus larvae became pupae (experiment was ended at this point).identification.The experiment was conducted at Universidade Federal de Viçosa, Minas Gerais, Brazil.
The experiment was conducted in an incubator in the same light and temperature conditions described above.Thirtytwo T. violaceus fourth instar larvae divided into eight groups (with four replicates/group) were used in the experiment.Each T. violaceus larva was placed individually in 100 ml plastic pots with 50 ml of dechlorinated water.After 48 hours of T. violaceus starvation, 40 fourth instar A. aegypti larvae were offered to each T. violaceus larva (total of 160 per group) at the same time.
The experiment was set with the following observation periods: 24, 48, 72, 96, 120, 144, 168 and 192 hours ( named A,  B, C, D, E, F, G, and H groups, respectively).After each 24-hours period, predated larvae were counted and one group was discarded.At the end of the experiment, there was only one remaining group (H) that corresponded to 192 hours.Also, after each 24-hours period, the non-predated A. aegypti larvae were removed from the plastic pots, counted and replaced by 40 new ones.During this period A. aegypti larvae did not receive any food.We analyzed these data using survival analyses with a Weibull distribution, since prey mortality increased with time (CRAWLEY 1993), using the number of unpredated larvae as the response variable.The analyses were done using free software developed by the R DEVELOPMENT CORE TEAM (2006) with p < 0.05.
The survival curve of A. aegypti larvae was plotted as a function of T. violaceus predation (Survival A. aegypti = e -160.7741^- 2.898551x^2.898551, p < 0.05) and it showed that the predation increased with time (ϱ = 2.90, Fig. 1).Aedes aegypti fourth instar larvae survival was almost 98% in the first 24 hours and decreased subsequently, reaching 0% by 192 hours.In addition, A. aegypti fourth instar larvae number reduction was more marked between 96 and 120 hours (from 72 to 47%), and between 168 and 192 hours (from 38 to 0%, Fig. 1).
Our result showed that predation of A. aegypti by T. violaceus increases as the fourth instar larvae age.This was also reported for other Toxorhynchites spp.and it is known that an increase in predation assures that sufficient amounts of nutrients are stored prior to pupation (CAMPOS & LOUNIBOS 2000).Insufficient prey consumption can lead to a nutritional deficit in the larvae, delaying sexual development in the adult and reducing egg production (AMALRAJ et al. 2005).
The potential usage of neotropical Toxorhynchites spp. in vector control has been poorly studied until now.In the present work it was shown that the predator T. violaceus is capable of killing several A. aegypti larvae within the same water container in 24 hours.Even though this was tested in laboratory condi-tions, this study constitutes the first attempt to investigate A. aegypti larvae survival in the presence of the T. violaceus fourth instar larvae.The use of this predator in the control of the invasive vector A. aegypti larvae needs to be further tested in the field.