Increased Attraction and Stability of Beauveria bassiana-Formulated Microgranules for Aedes aegypti Biocontrol

Abstract

Aedes aegypti (Linn.) incidence has increased in recent years, causing human viral diseases such as dengue, which are often fatal. Beauveria bassiana (Bals., Vuillemin) efficacy for Ae. aegypti biological control has been evidenced but it relies on host susceptibility and strain virulence. We hypothesized that B. bassiana conidia microgranular formulations (MGF) with the additives acetone, lactic acid, and sugar increase Ae. aegypti adult exposure, thus improving their biocontrol effectiveness. Beauveria bassiana strain four (BBPTG4) conidia stability was assessed after 0 d, 5 d, and 30 d storage at 25 °C ± 2 °C with additives or in MGF after 91 d of storage at 25 °C ± 2 °C or 4 °C ± 1 °C, whereas mortality was evaluated after adult exposure to MGF + conidia, using home-made traps. Additives did not show toxicity to conidia. In addition, we observed that sugar in MGF increased Ae. aegypti adults’ attraction and their viability resulted in a 3-fold reduction after 5 d and 1- to 4-fold decrease after 30 d of storage, and formulations were less attractive (p < 0.05). Conidia stability was higher on MGF regardless of the storage temperature, losing up to 2.5-fold viability after 91 d. In conclusion, BBPTG4 infected and killed Ae. aegypti, whereas MGF attracting adults resulted in 42.2% mortality, increasing fungus auto dissemination potential among infected surviving adults. It is necessary to further evaluate MGF against Ae. aegypti in the field.

Keywords: conidia viability; mosquito control; propagules stabilizers.

Figure 1

Beauveria bassiana BBPTG4 strain (A) conidia viability in microgranular formulation (MGF). Conidia viability was determined in MGF at time zero (4 d) (1.5 × 10⁷ conidia/mL as active ingredient (AI) with MGF) and up to 91 d storage at 25 °C ± 2 °C or 4 °C ± 1 °C, as explained in the text. (B) Percentages of Ae. aegypti adults attracted to different treatments. Ae. aegypti adults were exposed to MGF without additive, MGF + acetone (added at 1% (w/v) of MGF), MGF + 100 µL/g of lactic acid at 85%, and MGF + 100 µL/g of lactic acid at 85% + 1% acetone/g of MGF and attraction percentages determined, as explained in the text. Data represent mean + SEM of triplicate determinations from three independent experiments. Same letter on each column indicates that treatments are not significantly different (HSD Tukey test; p ˂ 0.05).

Figure 2

Percentage of Ae. aegypti adults attracted and killed by microgranular formulation (MGFs) with ( gray bar) or without ( white bar) Beauveria bassiana as active ingredient (AI). (A) Attraction percentage and (B) mortality percentage after exposure to BBPTG4 on MGF. Data represent mean + SEM of triplicate determinations from three independent experiments. Same letter on each column indicates that treatments are not significantly different (Student t test; p ˂ 0.05).

Figure 3

Ae. aegypti attraction by formulations. (A) Attraction percentage in Ae. aegypti adults of microgranular formulations plus Spirulina. Same letter on each column indicates that treatments were not significantly different (HSD Tukey test, p > 0.05). (B) B. bassiana conidia viability after different days of storage at different temperatures. Dark bars correspond to MGF + Spirulina at 25 °C ± 2 °C and gray bars represent MGF + Spirulina at 4 °C ± 2 °C storage temperature. Same letter on dark or gray column indicates not significantly different treatments (HSD Tukey test, p > 0.05). Data represent mean + SEM of triplicate determinations from three independent experiments. The same letter between columns in the same storage time indicates not significantly different treatments (Student t test; p ˂ 0.05). Treatments were MGF + Spirulina at 25 °C ± 2 °C and 4 °C ± 1 °C.

Figure 4

Beauveria bassiana conidia viability in microgranular formulation (MGF) at (A) 0 d, (B) 15 d, (C) 30 d, and (D) 120 d in colony forming units per gram (CFU/g). MGFs = MGF plus sugar, FS = solid formulation. Data represent mean + SEM of triplicate determinations from three independent experiments. Same letter between columns indicates that treatments are not significantly different (HSD Tukey test; p > 0.05).

Figure 4

Beauveria bassiana conidia viability in microgranular formulation (MGF) at (A) 0 d, (B) 15 d, (C) 30 d, and (D) 120 d in colony forming units per gram (CFU/g). MGFs = MGF plus sugar, FS = solid formulation. Data represent mean + SEM of triplicate determinations from three independent experiments. Same letter between columns indicates that treatments are not significantly different (HSD Tukey test; p > 0.05).

Figure 5

Survival percentage of Aedes aegypti adults exposed to four different formulations of the B. bassiana BBPTG4 strain stored for (A) zero days and (B) 30 d. The four different formulations stored at 4 °C and 25 °C. Data represent mean + SEM of triplicate determinations from three independent experiments. MGF + Sp = microgranular formulation + Spirulina, MGFs = plus sugar, SF = solid formulation, C+ = positive control, C− = negative control.