Abstract
Soil treatments with Metarhizium brunneum EAMa 01/58-Su strain conducted in both Northern and Southern Spain reduced the olive fly (Bactrocera oleae) population density emerging from the soil during spring up to 70% in treated plots compared with controls. A model to determine the influence of rainfall on the conidial wash into different soil types was developed, with most of the conidia retained at the first 5 cm, regardless of soil type, with relative percentages of conidia recovered ranging between 56 and 95%. Furthermore, the possible effect of UV-B exposure time on the pathogenicity of this strain against B. oleae adults coming from surviving preimaginals and carrying conidia from the soil at adult emergence was also evaluated. The UV-B irradiance has no significant effect on M. brunneum EAMa 01/58-Su pathogenicity with B. oleae adult mortalities of 93, 90, 79, and 77% after 0, 2, 4, and 6 of UV-B irradiance exposure, respectively. In a next step for the use of these M. brunneum EAMa 01/58-Sun soil treatments within a B. oleae IPM strategy, its possible effect of on the B. oleae cosmopolitan parasitoid Psyttalia concolor, its compatibility with the herbicide oxyfluorfen 24% commonly used in olive orchards and the possible presence of the fungus in the olive oil resulting from olives previously placed in contact with the fungus were investigated. Only the highest conidial concentration (1 × 108 conidia ml−) caused significant P. concolor adult mortality (22%) with enduing mycosis in 13% of the cadavers. There were no fungal propagules in olive oil samples resulting from olives previously contaminated by EAMa 01/58-Su conidia. Finally, the strain was demonstrated to be compatible with herbicide since the soil application of the fungus reduced the B. oleae population density up to 50% even when it was mixed with the herbicide in the same tank. The fungal inoculum reached basal levels 4 months after treatments (1.6 × 103 conidia g soil−1). These results reveal both the efficacy and environmental and food safety of this B. oleae control method, protecting olive groves and improving olive oil quality without negative effects on the natural enemy P. concolor.
Highlights
There is a need to develop effective, economically viable, and environmentally friendly methods for pest control (Nicolopoulou-Stamati et al, 2016), which has become even more critical for those insect pests that have developed insecticide-resistance such as the olive fruit fly Bactrocera oleae Rossi (Diptera: Tehphritidae) (Kakani et al, 2010; Hsu et al, 2015)
The soil applications of M. brunneum in both Northern and Southern Spain reduced the B. oleae population in treated plots compared with controls (Figure 2)
The reduction in the B. oleae adult population emerging from the treated plots compared to the control plots was significant even if the fungus mixed with the herbicide (P < 0.5)
Summary
There is a need to develop effective, economically viable, and environmentally friendly methods for pest control (Nicolopoulou-Stamati et al, 2016), which has become even more critical for those insect pests that have developed insecticide-resistance such as the olive fruit fly Bactrocera oleae Rossi (Diptera: Tehphritidae) (Kakani et al, 2010; Hsu et al, 2015) This monophagous and multivoltine species is the most destructive to the olive crop worldwide (Daane and Johnson, 2010), reducing crop production, but even more important, olive oil quality (Mraicha et al, 2010; Medjkouh et al, 2016; Caleca et al, 2017). Besides entomopathogenic fungi are naturally distributed in a wide range of habitats and the soil is considered their natural reservoir (Quesada-Moraga et al, 2007; Pell et al, 2010; Garrido-Jurado et al, 2015); soil application of entomopathogenic fungi to target soil dwelling stages of insect pests could be a powerful and sustainable pest management strategy (Rogge et al, 2017). Yousef et al (2017) have demonstrated the efficacy of soil treatments under olive tree canopy using the M. brunneum EAMa 01/58-Su strain (hereafter referred to as M. brunneum) for B. oleae control, as well as the compatibility of M. brunneum with commercial herbicides under laboratory conditions (Yousef et al, 2015), while Garrido-Jurado et al (2011a) have demonstrated the lack of negative direct or indirect impact of such treatments on the olive crop soil-dwelling non-target arthropod population
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