Is there any alternative insecticide to control pyrethroid-resistant Triatoma infestans populations (Hemiptera: Reduviidae) in the Gran Chaco ecoregion?

The kissing bug Triatoma infestans (Klug) is the main vector of Chagas disease, which is a public health concern in most Latin American countries, where 10 million people are infected and over 90 million people are at risk of infection. The prevention of Chagas disease is mainly based on the chemical control of the vector using pyre­throid insecticides. In the last decade, high resistance levels to these insecticides have been detected in the geographic ecoregion of the Gran Chaco. Actually, the alternative insecticides to control pyrethroid populations are based in the use of organophosphate compounds. Thus, there is a need in the finding of new synthetic insecticides against T. infestansresistant populations.

The aims of the present study were: a) to analyze the toxicity of eight non-pyrethorid insecticides against two T. infestans populations, a susceptible (S) and a deltamethrin-resistant (R) population with a resistant level (RL) of 1031, b) to evaluate the delayed toxicity and the influence of the nutritional state, and c) to test the effectiveness of different commercial formulations (emulsifiable concentrate (EC) 35%, wettable granule (WG) 70%, and 10% pour-on) against T.infestans.

Topical application of the insecticides amitraz, flubendiamide, ivermectin, indoxacarb and spinosad showed no insecticidal activity (lethal dose 50% (LD₅₀) > 200 ng/insect). Nevertheless, imidacloprid had lethal effect on both populations (LD₅₀values of 5.2 and 9.2 ng/insect). Based on these data, imidacloprid may be considered as an alternative to non-pyrethroid insecticide for the control of pyrethroid-resistant populations.

There was no significant variation in the LD₅₀s after 24, 48 and 72 h of initial topical application on both S (LD₅₀=5.2 ng/insect) and R (LD50= 9.2 ng/insect) populations. This could indicate that the toxicological effect of the active ingredient remains stable through the time. The nutritional condition (starvation/feeding) of the insects had no significant influence on the insecticidal activity of the imidacloprid on either populations S (LD₅₀starved= 5.2 ng/i and LD₅₀feeded=3.9 ng/i) or R (LD₅₀starved= 9.2 ng/I and LD₅₀feeded=10.8 ng/i). Thus, the rate of penetration associated with physicochemical modifications of cuticle after feeding seems not to alter the relatively fast-acting effect after topical application.

Concerning commercial formulations of imidacloprid, EC and WP were ineffective on glass surface LC₅₀ >1g/m², while the pour-on formulation applied at the recommended dose of the 0.1µl/g to pigeons showed a high effect of 100% mortality against T.infestans. The above results indicate that the neonicotinoid imidacloprid seems to be a possible alternative to pyrethroid insecticides in areas of Gran Chaco where T. infestans developed resistance. However, the adequate formulation of the active ingredient is essential in their effectiveness. Finally, the implementation of an integrative control strategy is also discussed.