Desenvolvimento de formulação seca à base de Metarhizium humberi com terra diatomácea para controle de Aedes aegypti

Abstract

Entomopathogenic fungi are promising biological agents for controlling populations of Aedes aegypti, the main vector of medically important arboviruses in tropical and subtropical regions. Prolonged periods of low humidity, common during the dry season in regions where this mosquito occurs, impair the development of the extracuticular stages. After adhering to the cuticle, conidia require high humidity to germinate and infect the host. Conidia formulations with specific additives aim to increase the insecticidal effect, in addition to improving the conditions for propagule development. The definition of additives is crucial for the development and success of a formulation. Diatomaceous earth (DE) is an inert powder obtained from microalgae fossils, which, when in contact with the insect's cuticle, causes abrasive damage and adsorbs lipids, leading to the evaporation of liquids from the insect and, consequently, death by dehydration. Because of this, it is possible that the damage caused to the cuticle, together with the extravasated moisture, favors the penetration and germination of conidia, even in low humidity environmental conditions. The present study sought to define an appropriate concentration of TD as an additive in a formulation of Metarhizium humberi conidia for the control of A. aegypti adults in low humidity conditions. Under laboratory conditions of 35 ± 2 or > 98% RH (positive control), TD was tested at six concentrations ranging from 0.05 to 1 mg/cm2 , and based on the results found, a sublethal operational concentration was determined for testing with conidia. TD had a high adulticide effect at low humidity. The LC30 of TD at 3 and 5 days were 0.04 (0.006‒0.08) mg/cm2 and 0.03 (0.007‒0.06) mg/cm2 , respectively, and the TL50 ranged from 1.5 (0.9‒2) days for 1 mg/cm2 to 6.5 (4.1‒9) days for 0.05 mg/cm2 . At high humidity, the LC30 at 10 days was 0.46 (0.13‒4.30) mg/cm2 , and the TL50 was 11 (6.4‒24.5) days for 1 mg/cm2 . Based on applicability in a powder formulation, 0.02 mg TD/cm2 was defined as the sublethal concentration of the additive. Under the same conditions, conidia were tested at three concentrations ranging from 3.3 x 105 to 3.3 x 107 conidia/cm2 , formulated or not with 0.02 mg TD/cm2. At low humidity, the additive enhanced the adulticide effect of conidia. The average mortality caused only by conidia in 3 and 5 days was 0% and 0% for 3.3 x 105 conidia/cm2, 0% and 30 ± 34.6% for 3.3 x 106 conidia/cm2, and 25 ± 17.3% and 90 ± 11.5% for 3.3 x 107 conidia/cm2. For conidia + TD, the average mortality rates in the same period were 12.5 ± 9.5% and 35 ± 17.3% for 3.3 x 105 conidia/cm2 , 0% and 50 ± 20% for 3.3 x 106 conidia/cm2 , and 20 ± 8.1% and 77.5 ± 17% for 3.3 x 107 conidia/cm2 . At high humidity, conidia without TD showed high mortality rates, and the addition of TD also increased theadulticide effect. The mortality rates for conidia alone at 3 and 5 days were 0% and 10 ± 11.5% for 3.3 x 105 conidia/cm2 , 0% and 55 ± 28.8% for 3.3 x 106 conidia/cm2 , and 65 ± 28.8% and 90 ± 11.5% for 3.3 x 107 conidia/cm2 . For conidia + TD, the mean mortality rates in the same period were 0% and 22.5 ± 12.5% for 3.3 x 105 conidia/cm2 , 0% and 67.5 ± 27.5% for 3.3 x 106 conidia/cm2 , and 67.5 ± 22.1% and 100% for 3.3 x 107 conidia/cm2 . Based on biorationality, mortality averages, LC, and LC, the most appropriate conidia concentration + 0.02 mg TD/cm2 for the formulation was 1.5 x 106 conidia/cm2 . This formulation is effective in both low and high humidity conditions in laboratory conditions, and in future studies, it will be tested in semi-field and field conditions.