Toxicity of non-pyrethroid insecticides against Triatoma infestans (Hemiptera: Reduviidae)

Triatoma infestans (Klug) is the main vector of Chagas disease, which is a public health concern in most Latin American countries. The prevention of Chagas disease is based on the chemical control of the vector using pyrethroid insecticides. In the last decade, different levels of deltamethrin resistance have been detected in certain areas of Argentina and Bolivia. Because of this, alternative non-pyrethroid insecticides from different chemical groups were evaluated against two T. infestans populations, NFS and El Malá, with the objective of finding new insecticides to control resistant insect populations. Toxicity to different insecticides was evaluated in a deltamethrin-susceptible and a deltamethrin-resistant population. Topical application of the insecticides fenitrothion and imidacloprid to first nymphs had lethal effects on both populations, producing 50% lethal dose (LD50) values that ranged from 5.2-28 ng/insect. However, amitraz, flubendiamide, ivermectin, indoxacarb and spinosad showed no insecticidal activity in first instars at the applied doses (LD50 > 200 ng/insect). Fenitrothion and imidacloprid were effective against both deltamethrin-susceptible and deltamethrin-resistant populations of T. infestans. Therefore, they may be considered alternative non-pyrethroid insecticides for the control of Chagas disease.     

Insecticide resistance in Brazilian Triatoma infestans and Venezuelan Rhodnius prolixus

Resistance levels to insecticides used in control of Chagas Disease vectors were assessed in two species of bugs (Hemiptera: Reduviidae): Triatoma infestans (Klug) from Brazil and Rhodnius prolixus Stål from Venezuela. The resistance ratios, compared to susceptible laboratory strains, were determined by topical application bioassays. The T. infestans PA strain exhibited resistance ratios of 7× to deltamethrin, 3.6× to β‐cyfluthrin and 3.3× to cypermethrin, but was susceptible to β‐cypermethrin and lambda‐cyhalothrin. Rhodnius prolixus CA strain showed resistance to all the pyrethroids evaluated, the resistance ratios ranging between 4.5× to lambda‐cyhalothrin and 12.4× to cypermethrin. Deltamethrin resistance in both strains was decreased by piperonyl butoxide, suggesting oxidative metabolism as cause of resistance.     

The effects of five organophosphorus insecticides on thermal stress in tadpoles of the Pacific tree frog, Hyla regilla

Three week old tadpoles of Hyla regilla were exposed to the insecticides temephos, fenthion, methyl parathion, chlorpyrifos, and malathion for 24 h at concentrations less than and at field applied rates as recommended for mosquito control in California. The thermal tolerance was significantly lowered by all chemicals and concentrations tested. Chlorpyrifos and methyl parathion were the most active compounds tested whereas malathion was the least toxic to H regilla tadpoles. Activity was depressed, in comparison to control tadpoles, by the chemicals methyl parathion (100 ppb) and malathion (500 ppb).     

Integumental esteratic activity in Triatoma infestans and its contribution to the degradation of organophosphorus insecticides

1. Esteratic activity was found in the integument of adult Triatoma infestans, principally located in the epidermis. Specific activity was on average 29.3 nM phenylthioacetate/hr per anatomical unit.2. Esteratic activity depends on the insect's age and its starvation state. A dramatic increase was observed 1 week after moulting followed by a slight decay as a function of the age. A significant decrease of the activity was observed with a longer fasting time.3. Integumental esterases were characterized as carboxylesterases, butyrylcholinesterases and aryl plus acetylesterases by using eserine and paraoxon as inhibitors and acetylthiocholine, butyrylthiocholine and phenylthioacetate as substrates.4. Epidermis homogenates were able to hydrolyse OP insecticides when incubated in vitro. Hydrolysis of the carboxyester linkage by malathion was established and cleavage of the P—S—C bond by parathion.     

Hydrocarbon biosynthesis in Triatoma infestans eggs

Triatoma infestans eggs are shown to synthesize hydrocarbons. Radio-gas chromatography was used to demonstrate metabolism of [1-¹⁴C]propionate into precursor methyl-branched fatty acids and into methyl-branched hydrocarbons in T. infestans eggs. These reactions have not been demonstrated previously in insect eggs. An in vivo study showed that hydrocarbons are also transported to eggs by the hemolymph. Inhibition of hydrocarbon synthesis by sodium trichloroacetate (NaTCA) was correlated with reduced oviposition, reduced hatchability, and reduced insect survival. Scanning electron microscopy showed impoverishment of the eggs' epicuticular waxes following NaTCA treatment. © 1994 Wiley-Liss, Inc.     

The occurrence of arachidonic acid in Triatoma infestans

1. Gas-liquid chromatography and mass spectrometry were used to verify the presence of arachidonic acid (20:4 omega 6) in adult male and female organs of the blood-suckling bug Triatoma infestans. Fat body, gonad and head lipids were analyzed. 2. Male gonads contained the highest percentage of phospholipids and the highest percentage of arachidonic acid. 3. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine and sphingomyelin were isolated from gonads and heads of sexed insects and the fatty acid composition analyzed. 4. All the phospholipids analyzed contained relevant percentages of linoleic acid and fair percentages of arachidonic acid, except for sphingomyelin where no 20:4 omega 6 acid was found. 5. However, arachidonic acid was specifically incorporated in phosphatidylinositol since this phospholipid contained very large percentages of the acid. 6. In male gonads, phosphatidylcholine also contained a similar high percentage of the acid. Arachidonic acid is apparently incorporated, selectively, into these lipids from the blood of the host.     

Brindley's gland exocrine products of Triatoma infestans

olatile exocrine products of the metathoracic Brindley's glands in Triatoma infestans (Hemiptera: Reduviidae) were obtained by dissection and by sampling the air passed over agitated live adults. Isobutyric acid was the main component in the glands, together with isobutyl, isoamyl and amyl alcohols, 2-phenylethanol and other carboxylic acids and esters. Isobutyric acid, isobutyl, isoamyl and amyl alcohols and esters were also found to be emitted into the air, apparently for defence. No volatile products were detected in the metasternal glands.     

The biting rate of Triatoma infestans in Argentina

1. The daily proportion of fed individuals in a population of the reduviid bug Triatoma infestans (Klug), maintained under natural climatic conditions in experimental chicken houses in central Argentina, was estimated from the proportion of bugs retaining colourless urine in the rectum. 2. From the estimates of feeding frequency throughout a 1 year period, it was shown that temperature has a dominant effect on biting rate, but density-dependent effects became apparent during the warmest months. 3. These and other data on the determinants of blood consumption by T. infestans, were incorporated into a detailed hypothesis of density regulation in this species.     

Degradation of serum proteins in Triatoma infestans.

It has been previously described (Perassi 1972, 1973)that no serum proteins from the animal on which the insects were fed can be detected in the insects' hemolymph of either male or female Triatoma infestans. In other words, the absorption of the proteins does not take place from the gut content to the hemolymph. Furthermore, according to the present investigation, the degradative processes of the gut content has revealed that, at least from an immunological point of view, an almost complete degradation of the rat serum proteins is carried out in the wide and slender midgut of T. infestans. These findings support the conclusion that in T. infestans the absorption of serum proteins from the gut to the hemolymph does not occur unless the proteins have been degradated previously.     

Population dynamics and control of Triatoma infestans

Analysis of field populations of Triatoma infestans (Hemiptera: Reduviidae), after a 3-year study, shows that population growth rate is affected by both density-dependent and density-independent mortality. Although an equilibrium exists, apparently as a consequence of a density dependent-mechanism, population density fluctuates throughout the year because of the effect of monthly mean minimum temperature as a density-independent source of mortality. Simulation studies based on Moran curves shows that high population densities have an approximately constant extinction probability (around 0.20), independently of the season the population starts growing. However, at very low population densities, the extinction probability depends strongly on the season when the population begins to grow. Very low density populations beginning in winter or autumn have the highest extinction probability. The outcome of the simulation studies coincides with results observed in field populations affected by insecticide application at different seasons.     

Temporal change of the aggregation response in Triatoma infestans

Adults and larvae of Triatoma infestans spend daylight hours assembled in shaded places. An assembling factor has been demonstrated in the excrement of this species. We analysed different aspects of the dynamics of the response of bugs. Recently fed insects do not aggregate around faeces. They start to show a significant assembling response from the 8th hour after feeding onwards. Just deposited faeces do not evoke assembling, but a significant rejection instead. This reaction switches 3 h after deposition, when the faeces become attractive to the insects. The attractiveness of faeces persists for about 10 days and can be recovered after this time by rehydration. These findings are discussed in relation to the biological role of faeces and the dynamics of the use of refuges by T. infestans.     

Repellency of deet to nymphs of Triatoma infestans

The repellency of N,N-diethyl-3-methylbenzamide (deet) to Triatoma infestans (Klug) (Hemiptera: Reduviidae) was evaluated using third-instar nymphs and a video tracking technique. Three experimental designs were used: (a) the test arena floor was divided into two halves, only one of which was treated with deet; (b) the arena floor was divided into an inner circle and an outer ring, only the latter treated with deet; (c) half of the test arena was covered by a filter paper roof treated with deet (out of reach of the nymphs). In all three types of experiment, a repellent effect was demonstrated proportional to the dose of deet. When a host (pigeon) was shielded by deet-treated gauze, the rate of blood-feeding by the nymphs was inhibited. Topical pre-treatment of the nymphs with N-ethylmaleimide, to block chemoreception, inhibited the repellency.     

Kinetics of expression of the salivary apyrases in Triatoma infestans

Apyrases are nucleoside triphosphate-diphosphohydrolases that remove Pi from ATP and ADP. The blood feeding reduviid Triatoma infestans, which transmits the Trypanosoma cruzi agent of Chagas disease to animals and man, presents in its salivary glands five apyrases with molecular masses of 88, 82, 79, 68 and 67 kDa. These triatomine apyrases have been associated with the prevention of ADP induced platelet aggregation in the host. Here we provide biochemical data showing that these apyrases are stored in the lumen of the salivary gland D1 pairs, and that about one half of the pool of the enzyme is consumed during feeding. After the feeding recovery of apyrases to maximal activity level takes days, thus suggesting de novo protein synthesis. This hypothesis is supported by quantitative RT-PCR analysis which shows an upregulation of the 79 kDa apyrase mRNA level after feeding.     

Fatty acid biosynthesis in the integument tissue of Triatoma infestans

1. The biosynthesis of lipids and their distribution in several tissues were investigated by injection of 1-14C acetate in females and 5th instar nymphs of the hematophagous hemiptera T. infestans. 2. Biosynthesis of palmitic, palmitoleic, stearic, oleic and very long chain fatty acids up to 26 carbons and hydrocarbons, was shown. They were found in haemolymph, fat body, integument, epicuticle and oocytes with special distribution. 3. Epicuticular hydrocarbon labelling was shown to precede that of haemolymph hydrocarbons. 4. Radioactivity incorporation into each lipid class depends on the developmental stage and the time after injection. 5. "In vitro" incubation of integument tissue with 1-14C acetate demonstrated the biosynthesis of palmitic and stearic acids, a low desaturation to oleic and palmitoleic and an elongation to acids of up to 34 carbons. Hydrocarbons were also synthesized. 6. Haemolymph in the incubation medium has a positive effect on the release of newly-synthesized fatty acid and unsaponifiable material from the integument.     

Chronobiological basis of thermopreference in the haematophagous bug Triatoma infestans

The chronobiological basis of the daily dynamics of thermopreference was tested in adults of Triatoma infestans, continuously registering the preferred temperature of insects released over a temperature gradient for 13 days. We found that the thermopreference in T. infestans is a dynamic process that depends on the time of the day and the post-feeding time. When submitted to a 12:12 h light/darkness cycle (L/D), the preferred temperature reached the highest and the lowest values at the end of the light and dark phases, respectively. This daily rhythm persisted under constant conditions of illumination (D/D and L/L), suggesting the existence of an internal oscillator controlling this behaviour. Statistical analysis revealed that the thermopreference of insects kept under L/D exhibited a ca. 24 h periodicity, while insects kept in D/D and L/L showed free-running periods of tau((D/D))=23.35 and tau((L/L))=27.35 h, respectively. The persistence of a cyclic pattern of thermopreference under constant conditions, and free-running periods, close to, but different from 24 h, demonstrate the existence of an endogenous control of the thermopreference in this species. The biological relevance of these results is discussed in the light of the hypothesis that both the length of time elapsed since feeding and the time of the day modify thermopreference in these bugs. The gradual decline in preferred temperature following feeding may be associated with energy conservation during starvation. The rhythmic modulation of thermopreference may be associated with the daily rhythm of locomotion activity shown by these bugs.     

Molecular phylogeography of the Chagas' disease vector Triatoma infestans in Argentina

Triatoma infestans is the main vector of Chagas' disease in South America between latitudes 10°S and 46°S. A multilocus microsatellite data set of 836 individuals from 27 populations of T. infestans, from all its range of distribution in Argentina, was analyzed. Our results favor the hypothesis of two independent migration events of colonization in Argentina and secondary contacts. The majority of the populations of the western provinces of Catamarca, La Rioja, San Juan and the west of Cordoba province, had almost no shared ancestry with the rest of the populations analyzed. Probably those populations, belonging to localities close to the Andean region, could have been established by the dispersal line of T. infestans that would have arrived to Argentina through the Andes, whereas most of the rest of the populations analyzed may have derived from the dispersal line of T. infestans in non-Andean lowlands. Among them, those from the provinces of Formosa, Chaco, Santiago del Estero and Santa Fe shared different percentages of ancestry and presented lower degree of genetic differentiation. The migratory movement linked to regional economies and possibly associated with passive dispersal, would allow a higher genetic exchange among these populations of T. infestans. This study, using microsatellite markers, provides a new approach for evaluating the validity of the different hypotheses concerning the evolutionary history of this species. Two major lineages of T. infestans, an Andean and non-Andean, are suggested.