Effect of temperature and relative humidity on egg hatching and pyrethroid toxicity in susceptible and resistant Triatoma infestans

Triatoma infestans is the primary vector of Chagas disease in the Southern Cone, and the emergence of pyrethroid resistance impairs control effort. While macro-ecological models suggest a link between resistance distribution and climatic factors, there is limited experimental evidence regarding the impact of temperature and relative humidity (RH) on embryonic development and subsequent nymphal susceptibility. This study examined the effect of different temperature (18 °C and 30 °C) and RH (40% and 70%) combinations on egg hatching in pyrethroid-susceptible and -resistant T. infestans colonies, and the subsequent toxicity of deltamethrin in first-instar nymphs. Both colonies exhibited a hatching rate of 0% at 18 °C and high RH (70%) over 81 days. However, the hatching rate of the resistant colony was 12.86% at 18 °C and low RH (40%), rising to 82.86% at 30 °C and high RH (70%). Eggs from the resistant strain that were exposed to an extremely low temperature (12 °C) prior to being transferred to a high temperature reached a 30% hatching rate, which may indicate a reversal of cold-induced inhibition. The lethal dose required to kill 50% of individuals (LD₅₀) remained stable for the resistant strain when insects exhibiting reversal inhibition were included in the toxicological analysis, suggesting that cold stress experienced during the egg stage does not alter subsequent nymphal susceptibility to deltamethrin. These results indicate that, although thermal and hydric regimes influence T. infestans egg hatching, pyrethroid resistance phenotypes remain consistent despite environmental variations during embryogenesis under the specific experimental conditions tested.