Entomopathogenic fungi as biopesticides against the Egyptian cotton leaf worm, Spodoptera littoralis: between biocontrol-promise and immune-limitation
, Ahmed, A. M. and El-Katatny, M. H. . 2007
In this study, a preliminary virulence test on three concentrations (1 × 106, 1 × 107 and 1 × 108 conidial spore ml-1) of the aqueous conidial suspension of the four entomopathogenic fungi isolates, Beauveria bassiana IMI 382302, Beauveria bassiana IMI 386701, Trichoderma harzianum T24 and Aspergillus flavus Link was carried out against both larval and pupal stages of Spodoptera littoralis within 5 days post-treatment. T. harzianum T24 showed 80% larval mortality only when applied at its highest conidial concentration (1 × 108/ conidial spore ml-1), while A. flavus showed 100% pupal mortality only at all of its conidial concentrations. However, B. bassiana IMI 382302 showed relatively high dose-dependant larval and pupal mortalities. While, strain IMI 386701 of B. bassiana showed a very weak mortality against pupae at its higher concentrations but no virulence against larvae was recorded. Enzymatic and antibiosis bioassays of the four fungal isolates showed relatively high activities against the sensitive fungus, Fusarium oxysporum, for most of the tested isolates. The humoral antifungal response of insect host relatively high compared to the anti-bacterial one. Injection of larvae with the immune sensitive bacteria, Micrococcus luteus, (5 × 103 bacteria/larva) showed a detectable humoral response by 2h, peaked around 12h and become hardly detectable by 24h post-injection. Injection of larvae with conidial suspension (5 × 103 conidia/larva) from each of fungal isolates showed humoral antifungal activity against Beauveria bassiana IMI 386701and Aspergillus flavus only. This activity was detectable by 12h, peaked around 24h and become hardly detectable by 36h post injection. Although the humoral antifungal response was started slowly compared to the antibacterial one, it lasted for longer and enabled larvae to withstand the infection with these immune-sensitive fungal strains. However, no humoral activity was detected against Beauveria bassiana IMI 382302, although however, weak activity was detected against T. harzianum T24 only at the low conidial concentration but not at the higher one (1 × 108ml-1). Thus, this study concludes that larvae of S. littoralis showed immune-dependant sensitivity to T. harzianum T24 and B. bassiana IMI 382302. Therefore, this study may recommend these two fungal isolates as mycoinsecticides in the battle against cotton leaf worm in Egypt. And hence, they have been selected for future comprehensive bioassays in the laboratory under conditions similar to that in the field. This, in fact, may help developing effective mycoinsecticides against this pest.
