Suppression of Aedes aegypti by predatory Toxorhynchites moctezuma in an island habitat

Larval populations of the mosquito Aedes aegypti were suppressed by predatory Toxorhynchites moctezuma mosquito larvae released systematically in a village on Union Island (Saint Vincent and the Grenadines) during March-December 1988. Eggs and larvae of Tx.moctezuma were transported from Trinidad and introduced into all semi-permanent and permanent water-holding containers in the experimental village at Clifton. The semi-isolated village of Ashton served as control. Base-line Ae.aegypti indices (house, ovitrap, Breteau, cistern/tank, drum/barrel, small containers) were obtained for the two villages over a 4-month period prior to the introduction of the predatory Tx.moctezuma mosquito larvae. After sustained releases of predators for 5 months, all indices of Ae.aegypti were lower in the treated village than in the untreated village during the last 3 months of the year.     

A laboratory investigation of mosquito larval predation by Toxorhynchites moctezuma on Aedes aegypti

1. Functional responses of predatory Toxorhynchites moctezuma (Dyar & Knab) larvae feeding on Aedes aegypti (L.) larvae (Diptera: Culicidae) were found to be type II of Holling (1959) and Rogers (1972). 2. Estimates of searching rate were generally higher for later instar predators. The search rate of second instar predators declined as prey instar increased, but fourth instar Tx. moctezuma had the highest search rate for second instar Ae. aegypti. 3. Prey handling times were higher for early instar predators and late instar prey. 4. When presented with mixtures of two instars of Ae. aegypti, second instar Tx. moctezuma showed frequency independent selectivity for the early instars, whereas fourth instar predators showed frequency independent selectivity for the late instars of Ae. aegypti. There was no evidence of frequency dependent predation. Preferences appeared to be transitive. 5. Extended random predator equations, using parameters derived from the functional responses, did not adequately describe the outcome of predation in the prey mixture experiment, even when the possibility of optimal switching behaviour was accounted for.     

Laboratory evaluation of Toxorhynchites splendens (Diptera: Culicidae) for predation of Aedes albopictus mosquito larvae

Biology of the mosquito Toxorhynchites splendens (Wiedemann) was studied in the laboratory to provide baseline data for using the predatory larvae of this species against those of Aedes albopictus (Skuse) in a biological control programme. The mean incubation time of Tx.splendens eggs was 43.8 h and the time required for newly-hatched larvae to initiate predation was 2.5 h. Mean numbers of prey larvae consumed and killed by each Tx.splendens larva totalled 389 and 345 respectively. The larval period of Tx.splendens was not significantly different for rearing individually or in groups of nine, with equal prey density, and duration of larval development was proportional to prey density. In mass rearing, larval cannibalism was usually observed during days 1-3 post-eclosion. The incidence of cannibalism decreased sharply on the fourth day after hatching when some larvae became fourth-instar. Adult female Tx.splendens usually commenced oviposition on day 4 after emergence. The number of eggs laid daily increased on day 7 and the peak oviposition of 6.3 eggs/female/day occurred on day 11. When oviposition containers were provided only intermittently, gravid females of Tx.splendens scattered most of their eggs on the dry floor of the cage. Viability of eggs laid by females aged 4-14 days was high (60-90%) but decreased to less than 40% as the females aged.     

Pathogenicity of Spiroplasma taiwanense for larval Aedes aegypti mosquitoes

Helical replicative forms, but not the persistent non-replicative forms, of Spiroplasma taiwanense Abalain-Colloc et al. (isolated from the mosquito Anopheles sinensis Wiedemann in Taiwan) were shown to reduce significantly the survival of Aedes aegypti (L.) mosquito larvae reared in 10 ml of water with 0.3 ml of S.taiwanense suspensions added on days 0 and 3. The suspensions contained, respectively, helical forms at a concentration of 10(9) Colour Change Units (CCU)/ml and persistent forms at 10(6) CCU/ml. It is suggested that S.taiwanense, or toxins produced from it, are potentially useful for use in integrated mosquito control programmes.     

Oviposition habitat preferences of Toxorhynchites moctezuma mosquitoes in four types of tropical forest in Trinidad

1. Oviposition of the mosquito Toxorhynchites moctezuma Dyar & Knab was investigated in four types of tropical forest in Trinidad, West Indies, using surrogate and natural ovitraps. Larvae of Tx. moctezuma are obligate predators that might be useful for the biological control of Aedes aegypti (L). 2. Significantly more oviposition occurred in seasonal-deciduous forest than in either montane or evergreen-seasonal forest. 3. Oviposition in surrogate containers (black-painted polystyrene cups, 90 mm diameter) was compared with that occurring in typical natural containers (nutpots of Lecythis zapucajo Aublet). Surrogate ovipots were relatively insensitive indicators of oviposition activity, and would be an inefficient means of harvesting Tx. moctezuma eggs. 4. Implications for the collection, culture and mass release of Tx. moctezuma are discussed.     

Electroantennogram studies of potential oviposition attractants for Toxorhynchites moctezuma and T. amboinensis mosquitoes

Abstract.Ether extracts of water samples taken from tyre sections in the field were shown to be active in electroantennogram studies with female Toxorhynchites moctezuma (Dyar & Knab) and Toxorhynchites amboinensis (Doleschall). In addition, electroantennograms were recorded from both Toxorhynchites species for seven compounds found commonly in water containing decaying leaves and known to be oviposition attractants for other mosquito species. The compounds were 4‐methylcyclohexanol, phenol, indole, 3‐methylindole, m‐ cresol, o‐ cresol and p‐ cresol. The response thresholds for these compounds ranged between 1 × 10^–4μg and 1 μg for female antennae (100 μg for male antennae). These results provide a basis for behavioural studies of oviposition attractants for the Toxorhynchites species.     

The functional response of Toxorhynchites rutilus rutilus to changes in the population density of its prey Aedes aegypti

We present an analysis of the functional response of the predator Toxorhynchites rutilus rutilus (Coquillett) to changes in the density of the larvae of Aedes aegypti (L.) (Diptera: Culicidae). The experiment was replicated for five different ages, and at three different densities of the predator. The data were fitted to Rogers' (1972) random predator equation by non-linear least-squares in order to estimate searching efficiency and handling time for each experimental treatment. The data show that estimated searching efficiencies are highest at intermediate ages of the predator for all predator densities tested. Handling time declines exponentially with increasing predator age. There is a marked interference effect; searching efficiency decreases with increased predator density, and this is most pronounced at intermediate prey ages. Estimated handling times increase with predator density at a rate which declines with increasing predator age.     

The relative importance and distribution of Aedes polynesiensis and Ae.aegypti larval habitats in Samoa

Abstract. In preparation for a Filariasis Control programme in Samoa, during 1978 monthly larval surveys of the vector mosquito Aedes polynesiensis were carried out in four study villages in the main island of Upolu. A more extensive survey of larval habitat distribution was then made in twenty-two villages of Upolu and eighteen of Savai'i island, to determine the importance of habitat types according to their abundance, volume of water and whether their productivity was permanent or seasonal. Ae.aegypti larval densities and habitat distribution were also monitored and the occurrence of predatory Toxorhynchites amboinensis larvae in northern Upolu was recorded from forty-one collections. Aedes Breteau and container indices fluctuated with the pattern of rainfall in two coastal villages and an inland bush village, but not in a coconut plantation community. The five main Aedes larval habitat types encountered were: 200 litre water-storage drums, discarded tins and bottles, coconut shells, automobile tyres and treeholes. Aedes immatures occurred perennially in drums and tree holes, but breeding discontinued in tins, bottles and coconut shells during the driest month of July. For Ae.polynesiensis in Upolu the Breteau and container indices of 104.5 ± SD 80.9 and 35.3 ± 12.4 respectively were significantly higher than those in Savai'i: 33.1 ±25.0 and 24.3 ±20.0 respectively. Likewise for Ae.aegypti the Breteau and container indices of 50.8 ± 32.5 and 23.9 ± 15.6, respectively, were also significantly higher than those in Savai'i: 12.7 ± 17.1 and 9.4 ±13.2 respectively. Habitat types greater or lesser importance were determined by plotting the percentage of each type of container utilized for Aedes breeding against the percentage of each type amongst all larva-positive containers. Ae.polynesiensis preferred tree-holes but not water-storage drums. Ae.aegypti preferred drums and tyres; mixed populations of larvae of both species were commonest in these two types of habitat. Ae.polynesiensis occurred in every village. Ae.aegypti was encountered in all twenty-two villages surveyed in Upolu and nine of eighteen villages in Savai'i. Total larval surveys revealed that drums and tree-holes contained the highest numbers of Aedes larvae. The study provided criteria for planning a control programme.     

Laboratory experiments on factors affecting oviposition site selection in Toxorhynchites amboinensis (Diptera: Culicidae), with a report on the occurrence of egg cannibalism

Laboratory experiments tested the effects of water quality and the presence of conspecific and heterospecific immatures on oviposition by Toxorhynchites amboinensis (Doleschall). The females showed a highly significant preference for oak leaf infusion water over distilled water. When twenty starved third and fourth instar Tx.amboinensis larvae were present in the water, substantially fewer eggs were counted from pots containing these conspecifics, than from controls in which no larvae were present. Numbers of eggs from pots containing starved second instar larvae did not differ significantly from controls. Observations of larval behaviour while oviposition was occurring suggested that egg numbers were reduced in containers because of egg cannibalism with third and fourth instar larvae, and not because the larvae caused a deterrent effect. Subsequent experiments confirming the occurrence of substantial egg cannibalism by third and especially fourth instar larvae are described. As with larvae, the presence of Tx.amboinensis pupae in the water had little effect on oviposition. If placed in the water 24 h prior to test, pupae very slightly enhanced its attractiveness, but if introduced immediately before test there appeared to be a slight deterrent effect. With heterospecific larvae, twenty fourth instar Ae.aegypti larvae introduced into infusion water 24 h before test rendered the water slightly attractive, while water in which Ae.aegypti larvae had been reared for 48 h proved highly repellent.     

Mesocyclops longisetus effects on survivorship of Aedes aegypti immature stages in car tyres

Abstract .The effect of the introduction of the entomophagous copepod Mesocyclops longisetus (Acuacultura F.C.B. strain) on the survival of Aedes aegypti immature stages in car tyres was evaluated under semi-natural conditions in the municipality of Merida, Yucatan, Mexico. Life tables were constructed for the immature stages of the mosquito in the presence and absence of M. longisetus , and the survival data were compared using log–linear models. The data set was adjusted using the GLIM statistical package and the quality of adjustment was evaluated with a chi-squared test . Survivorship curves were constructed for each treatment.
In the absence of M. longisetus , the survivorship of Ae. aegypti immature stages averaged 9%. The highest mortality rate was observed during the fourth larval instar (54%) and the resulting survival pattern corresponded to a type II survivorship curve. The mortality rate of Ae. aegypti first-instar larvae (fifty per tyre) increased more than 200-fold in the presence of M. longisetus (twenty per tyre) and the highest mortality was during the first two larval instars, where it reached 98.9%, with a resulting survivorship of 0.2%. Overall mortality was sixfold greater in the presence of the copepod than in its absence. The survival pattern of immature stages of Ae. aegypti in the presence of the copepod corresponded to a type III survivorship curve. As M. longisetus was so effective against Ae. aegypti immature stages in tyres under semi-natural conditions, its long-term effectiveness should be evaluated under socially and ecologically realistic field conditions in Mexico.     

Laboratory testing of a lethal ovitrap for Aedes aegypti

Laboratory tests were conducted to determine the feasibility of making the mosquito ovitrap lethal to Aedes aegypti (L.) when they attempt to oviposit in the trap. Heavy-weight velour paper strips (2.54 x 11 cm) were used as an alternative to the wooden paddle normally provided as a substrate for mosquito oviposition. The paper strips were pretreated with insecticide solutions and allowed to dry before being used in oviposition cups of 473 ml capacity, filled with water initially to within 2.5 cm of the brim. Insecticides chosen for their quick knock-down efficacy were bendiocarb 76% WP (1.06 mg a.i./strip) and four pyrethroids: permethrin 25% WP (0.16 mg a.i./strip), deltamethin 4.75% SC (0.87 mg a.i./strip), cypermethrin 40% WP (2.81 mg a.i./strip), and cyfluthrin 20% WP (0.57 mg a.i./ strip). For experimental evaluation, two oviposition cups (one with an insecticide-treated strip and one with an untreated strip) were placed in cages (cubic 30 cm) with gravid female Ae. aegypti mosquitoes (aged 6-8 days) from a susceptible laboratory strain. Mortality-rates of female mosquitoes were 45% for bendiocarb, 47% for permethrin, 98% for deltamethrin, 100% for cypermethrin, and 100% for cyfluthrin. Young instar larvae added to the treated cups died within 2h. After water evaporation from the cups for 38 days, fresh mosquito females had access to previously submerged portions of the velour paper paddle, and mortality rates of 59% or more occurred. Cups that had water (360 ml) dripped into them, to simulate rain, produced female mosquito mortality rates of > 50% and all larvae died within 3 h of being added. These tests demonstrate that the ovitrap can be made lethal to both adults and larvae by insecticidal treatment of the ovistrip. Field efficacy trials are underway in Brazil to access the impact of this simple, low-cost, environmentally benign approach on populations of the dengue vector Ae. aegypti.     

Anti-mosquito antibodies decrease the reproductive capacity of Aedes aegypti

Aedes aegypti (L.) fed on rabbits immunized with mosquito antigens showed a reduction in fecundity in the first oviposition and decreased viability of the progeny. Feeding behaviour of mosquitoes was not affected and no significant mortality was observed due to the presence of anti-mosquito antibodies in the bloodmeal. Antibodies were detected in the oocytes of mosquitoes 48 h after the bloodmeal. The role of specific antibodies in influencing fecundity is discussed.     

Effects of mosquito larval feeding behavior on Bacillus sphaericus efficacy

Three species of mosquito larvae, representing three genera, were exposed from 10 min to 2 hr to the pathogen Bacillus sphaericus. Culex quinquefasciatus rapidly ingested the bacterium with resulting high mortality. Anopheles albimanus ingested it at a much lower rate initially with correspondingly low mortality. At the longest time interval they had accumulated approximately the same number of bacterial cells as C. quinquefasciatus but achieved a lower mortality. Aedes aegypti ingested and accumulated bacterium at the same rate as C. quinquefasciatus but was resistant to all time intervals. Utilizing ¹⁴C-labeled bacteria, we demonstrated that these differences were attributable to larval behavior in the case of A. albimanus but not in the case of A. aegypti.     

Adaptive larval thermotolerance and induced cross-tolerance to propoxur insecticide in mosquitoes Anopheles stephensi and Aedes aegypti

Abstract. Fourth-instar larvae of mosquitoes Anopheles stephensi and Aedes aegypti normally died within 90 min at 43C. Pre-exposure to high but sublethal temperatures conferred adaptive thermotolerance, dependent on the temperature and the duration of pre-exposure. Adaptive cross-tolerance to propoxur (a carbamate insecticide) was also induced in larvae by pre-exposing them to sublethal temperatures. Pre-exposure to sublethal concentrations of propoxur was found to confer cross-thermotolerance to a lower extent. These results suggest that the shock proteins (e.g. heat shock proteins) induced by unrelated stress factors play an important role in the development of adaptive cross-protection (stress response) to other stress conditions.     

Genetic structure among Thai populations of Aedes aegypti mosquitoes

Thirty‐one field populations of Aedes aegypti (L.) were compared using isozyme starch gel electrophoresis to characterize genetic variation between populations. Ae. aegypti were collected from seven provinces in Thailand. Thirty‐one isozyme encoding loci, including 19 polymorphic loci, were characterized. Only small levels of genetic differentiation were observed among the 31 district populations in the seven provinces. Isolation by distance among populations from the seven provinces showed no correlation between genetic variation and geographical distance.     

Proportions of eggs laid by Aedes aegypti on different substrates within an ovitrap in Trinidad, West Indies

Abstract. Ovitraps were exposed outdoors during wet and dry seasons in Trinidad to determine the proportions of eggs laid on three substrates: the widely-used hardboard 'paddle' (P), the water surface (W) and the inside surface of the ovitrap wall (S) in three types of blackened ovitrap: round, glass (RG), round, plastic (RP) and rectangular, plastic (SP). Of the 3556 eggs obtained, most (86.4%) were laid during the wet season when statistically significant differences were detected between substrates but not between ovitrap types. Most eggs (>80%) were laid on P, significantly fewer ( P < 0.005) on W, on S and on W + S, thus confirming the superiority of the paddle as a device for monitoring oviposition activity. Implications of these findings for the interpretation of Aedes studies using ovitraps are discussed.     

Physiological and biochemical response of Aedes aegypti tolerance to Bacillus thuringiensis

Bacillus thuringiensis subsp. israelensis (Bti) represents the only eco-friendly bio-degradable insecticide for mosquito-borne disease control. Our research aims to identify if mosquito detoxification enzymes play an important role in Bti tolerance mechanisms in the dengue vector, Aedes aegypti. Several enzymes, such as amylase, cytochromes P450, Na⁺/K⁺-ATPase, acetylcholinesterase, protease and glutathione S-transferase (GST) were analysed and level of activity determined in Ae. aegypti larvae after Bti treatment. Bti exposure significantly increased the level of amylase (183.2%) as well as the activity of cytochromes P450 (177.5%), and Na⁺/K⁺-ATPase (142.9%). On the other hand, there was a decrease of 8.6% and 11.4% in acetylcholinesterase and GST activity, and no significant effect in the total level of protease activity. We suggest that the variation in amylase, cytochromes P450, Na⁺/K⁺-ATPase, acetylcholinesterase, protease and GST activity may be associated with the Bti insecticidal mechanism. This study provides the basis of detoxifying enzymes in Bti tolerance.     

Screening of Brazilian Bacillus sphaericus strains for high toxicity against Culex quinquefasciatus and Aedes aegypti

Abstract:  In this work, 246 Bacillus sphaericus strains were evaluated against Aedes aegypti and Culex quinquefasciatus larvae to select the most effective ones to be used as the basis of a national product. All strains were isolated from different regions of Brazil and they are stored in a Bacillus spp. collection at Embrapa Genetic Resources and Biotechnology. The selected strains were characterized by biochemical and molecular methods. Based on selective bioassays, 87 strains were identified as toxic to one or both target species. All of these strains contain genes that encode the 42, 51 kDa proteins that constitute the binary toxin and the 100 kDa Mtx1 toxin. All toxic strains presented a very high LC₅₀ against A. aegypti , so, a product based on any of these B. sphaericus strains would not be recommended for use in programmes to control A. aegypti . S201 had highest activity against C. quinquefasciatus , presenting the lowest LC₅₀ and LC₉₀ in bioassays.     

Electronic mosquito repellers induce increased biting rates in Aedes aegypti mosquitoes (Diptera: Culicidae)

Studies have demonstrated that electronic mosquito repellers are useless and that some of them could even increase the attraction of mosquitoes. While testing some electronic repellers, we noted that they also promoted an increase in biting rates. The present work has evaluated three commercial devices and a computer program working on five different sound frequencies. In a test chamber, the number of Aedes aegypti L. bite attempts was computed during four cycles of 3 min each by alternately turning the devices off and on. The mosquito biting rates for five sound frequencies (ranging from 9.6 kHz to 18.2 kHz) initially demonstrated a significant increase (ranging from around 20% to 50%), which decreased from 8.3% to 25.1% when the repellers were turned off. The biting rate significantly increased at 11.8 kHz (33.7%) when the device was turned on again. The danger of using electronic repellers and the role of sound frequencies stimulating mosquito biting are discussed.