The entomopathogenic fungi (EPF) used in this study, at a dosage of 108 spores/ml, were pathogenic to T. absoluta larvae in all conducted bioassays. In all treatments, mortality rates increased with the increase in time (days). They were very low in the first 3 days and started to be significantly different among the treatments (p < 0.01) from the fourth day after application (Fig. 1). Various studies also reported that T. absoluta control could be achieved by using M. anisopliae (Shalaby et al. 2013; Contreras et al. 2014; Shiberu and Getu 2017) and B. bassiana (Qazzaz et al. 2015; Youssef 2015; El-Kichaoui et al. 2016). The results obtained form the basis for further studies on these EPF in a view to find the appropriate ways of using them under field conditions.
The formulation based on M. anisopliae (Metatech® WP) was insignificantly different from B. bassiana (Beauvitech®) in all bioassays on days 5 and 6, except on day 5 of bioassay one. In most cases, Metatech® WP recorded higher mortality rates than Botanigard®ES (B. bassiana, Strain GHA) and imidacloprid in all bioassays. The highest mortality rates observed were 82.8, 60.8, 48.8, and 33.5% for Metatech® WP, Beauvitech® WP, Botanigard® ES and imidacloprid (control), respectively (Fig. 1). Higher virulence of M. anisopliae compared to B. bassiana was also reported by Murerwa et al. (2014) against the aphids Rhopalosiphum padi and Metopolophium dirhodum.
Conflicting results were obtained by Moawad et al. (2017), who reported that B. bassiana was more effective than M. anisopliae in all treated larval instars of Stomphastis thraustica (Meyrick) (Lepidoptera: Gracillariidae), a leaf miner of Jatropha curcas. Similarly, Youssef (2015) observed a high-mortality rate of T. absoluta larvae inside the galleries with B. bassiana (86.7%) than with M. anisopliae (76.7%), using a dosage of 108 spores/ml. This could be explained by the fact that the pathogenicity of a particular entomopathogen depends on strain/isolate and environment, among others (Borisade and Magan 2014). Thus, screening different EPF species and strains against a particular target host is crucial in development of any control program (Georgis et al. 2006).
At the dosage of 108 spores/ml, the highest mortality rates recorded in the present study were 82.8 and 60.8% for M. anisopliae and B. bassiana, respectively. Nevertheless, other studies could obtain higher mortality levels of T. absoluta larvae inside the leaf mines with higher dosages. For instance, Youssef (2015) observed a mortality rate of 90% in T. absoluta larvae inside the galleries with B. bassiana at a dosage of 1010 spores/ml; while El-Kichaoui et al. (2016) obtained up to 95% mortality of T. absoluta larvae, using B. bassiana at a dosage of 2.5 × 107 spores/ml. This concurs with the findings of Shalaby et al. (2013) who reported a linear relationship between mortality rate of T. absoluta and concentrations of B. bassiana and M. anisopliae.
In all bioassays, the mortality observed in tomato leaflets treated with imidacloprid, the synthetic insecticide mostly used by farmers, was the least than EPF application (Fig. 1). The limited efficiency of imidacloprid could be due to the ability of T. absoluta to develop resistant strains to the frequently used synthetic insecticides (Yalçin et al. 2015; Biondi et al. 2018). Likewise, the resistance of T. absoluta to indoxacarb and chlorantraniliprole was detected by Roditakis et al. (2013) in three laboratories belonging to the three different countries, Greece, Italy, and Spain. In Turkey, the resistance of T. absoluta to five commonly used insecticides, spinosad, indoxacarb, metaflumizone, and chlorantraniliprole, was also recorded by Yalçin et al. (2015).
In the all three bioassays, the lowest LT50 values were recorded by Metatech® WP, followed by Beauvitech® WP, Botanigard® ES and imidacloprid, respectively (Fig. 2). The general linear model (GLM) analysis of the pooled means for three bioassays indicated that LT50 values were 3.9, 5.2, 6.6, and 14.9 days for Metatech® WP, Beauvitech® WP, Botanigard® ES, and imidacloprid, respectively. Metatech® WP gave the lowest LT50 values; therefore, it is more pathogenic than the other evaluated EPF.