Abstract
Among 28 isolates of Beauveria bassiana tested for virulence against F. occidentalis in laboratory bioassays, we found strain SZ-26 as the most potent, causing 96% mortality in adults at 1×107 mL−1conidia after 4 days. The effect of the strain SZ-26 on survival, longevity and fecundity of the predatory mite Neoseiulus (Amblyseius) barkeri Hughes were studied under laboratory conditions. The bioassay results showed that the corrected mortalities were less than 4 and 8% at 10 days following inoculation of the adult and the larvae of the predator, respectively, with 1×107 conidia mL−1 of SZ-26. Furthermore, no fungal hyphae were found in dead predators. The oviposition and postoviposition durations, longevity, and fecundity displayed no significant differences after inoculation with SZ-26 using first-instar larvae of F. occidentalis as prey in comparison with untreated predator. In contrast, the preoviposition durations were significantly longer. Observations with a scanning electron microscope, revealed that many conidia were attached to the cuticles of F. occidentalis at 2 h after treatment with germ tubes oriented toward cuticle at 24 h, penetration of the insect cuticle at 36 h, and finally, fungal colonization of the whole insect body at 60 h. In contrast, we never observed penetration of the predator's cuticle and conidia were shed gradually from the body, further demonstrating that B. bassiana strain SZ-26 show high toxicity against F. occidentalis but no pathogenicity to predatory mite.
Highlights
Western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is regarded as an important economic pest of a wide range of agricultural and horticultural crops worldwide [1,2,3,4]
Screening fungal isolates Of the 28 strains of B. bassiana (Table 1) tested at 16107 conidia mL21 in the laboratory, the SDLZ-12 strain caused only 43% mortality after 4 days, while strain SZ-26 killed the highest percentages with 96% mortality (F = 11.212, p,0.001) (Fig. 1)
Furtado et al [30] reported that a strain of the fungal pathogen, Neozygites acaricida was pathogenic to a phytoseiid mite, Euseius citrifolius, while other studies have shown other fungal pathogens displayed no pathogenicity to predatory mites [9,18,23]
Summary
Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is regarded as an important economic pest of a wide range of agricultural and horticultural crops worldwide [1,2,3,4]. Because F. occidentalis has developed a high level of resistance to many chemical pesticides [5,6,7], it is essential to adopt a biological control program for this pest. The predatory mite Neoseiulus (Amblyseius) barkeri (Hughes) (Acarina: Phytoseiidae) and the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin have been shown to be potential biological control agents of F. occidentalis [7,8,9,10,11]. The application of the entomopathogenic fungus B.bassiana against F. occidentalis results in high rates of mortality in laboratory screenings and greenhouse conditions [16,17,18]. In order to obtain the highest efficiency in controlling F. occidentalis, it is suggested that B.bassiana should be applied along with the releases of predatory mites under field conditions [9,18]. A better understanding of the factors that minimize undesirable effects of insect pathogens on natural enemies could improve their integrated utilization against pest insects [19]
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