Actinobacteria, particularly Streptomyces, have demonstrated their important role in the agricultural sector by controlling plant pests through various direct and indirect mechanisms. They also have fertilizer and growth-promoting properties that improve and increase crop yield (Ebrahimi-Zarandi et al. 2022; Kaari et al. 2023). In this study, the bioinsecticide based on S. griseoflavus strain PAL114 was investigated for its control effect against black aphids Aphis fabae as well as its effect on faba bean plants.
Among the three strains screened for insecticidal activity, PAL114 was the only one that showed a highly significant effect compared to the control. It exhibited good activity against black aphids, achieving a mortality rate of (91%) after 58 hours. Therefore, this strain was selected for formulation and subsequent trials. In our screening test, the actinobacteria were applied directly in the form of spores to elucidate their development and antagonistic capacity against insects. This method shares the same principle of application as that of Xu and Feng (2002), who studied the effect of fungus Pandora delphacis spores against Myzus persicae. Most of the trials carried out on the Streptomyces genus for insecticidal activity have tested the activity of metabolic extracts against various insect species of agricultural and medical importance (Kekuda et al. 2010; Amelia-Yap et al. 2023; Diab et al. 2023). Some others, such as Samri et al. (2017), have used bacteria directly in broth culture.
Basically, the modes of action of microbial agents against insects can be elucidated through the biochemical composition of their exoskeleton and the ability of microorganisms to degrade one or more of its compounds. The insect exoskeleton that forms a protective barrier is mainly composed of proteins and chitin. The latter are therefore the target of chitinase and protease enzymes possessed by microorganisms. The activities of these enzymes have been widely studied in studies on entomopathogenic fungi and bacteria (Bahar et al. 2012; Bensaci et al. 2015). Indeed, actinobacteria in general and Streptomyces in particular are known to produce a multitude of lytic enzymes, including chitinase and protease (Rifaat et al. 2006; Ara et al. 2012; Rashad et al. 2017; Allali et al. 2019). In our study, we also showed that the S. griseoflavus strain PAL114 produces both enzymes, which can explain its insecticidal activity against black aphids. This same species has previously been found to produce protease in the study of Hosseini et al. (2016) and chitinase in the study of Tang-um and Niamsup (2012).
After formulation, the viable spore density calculated for the bioinsecticide produced at ≈ 106 spore ml− 1 remained ≈ 1.12×106 CFU g− 1 while that produced at ≈ 108 spore ml− 1 decreased to ≈ 8.06×105 CFU g− 1. In fact, the phenomenon of a decrease in viable spore numbers is known in the bioformulation processes of biocontrol agents and fertilizers, whether for talc or other carriers (Wong et al. 2019; Meftah Kadmiri et al. 2021; Fatmawati et al. 2023). Our results corroborate those of numerous comparative studies on different bioformulation carriers. Talc powder was found to be a very good carrier with more stable viability and higher efficacy for the Streptomyces genus (Zamoum et al. 2017, 2022; Fatmawati et al. 2023), and for other bacterial genera (Singh et al. 2020; Novinscak and Filion 2020).
The bioinsecticides formulated with PAL114 spores showed very highly significant activity compared with the control. They induced mortality rates very close to those given by the spore suspension first tested in screening. Insofar as the bioinsecticides were produced from this same suspension, we confirm that talc did not affect PAL114’s efficacy, hence the interest in using it as an actinobacterial spore carrier. The results also show that there was no significant difference between the effect of both bioinsecticides formulated at ≈ 106 and ≈ 108 spore ml− 1 density. This can be explained by the viable spore density results after formulation, although the bioinsecticides were produced by two different densities and even had different densities after the process.
In comparison with other trials for microbial control of black aphids, the bioinsecticide based on PAL114 spores showed interesting activity. It gave a very high mortality compared with that given by liquid bioinsecticide based on Cladosporum oxysporum despite being applied in a similar way as described by Bensaci et al. (2015). At the 106th hour of treatment, Cladosporium oxysporum gave a rate between (30% and 50%), whereas the rate given by PAL114 was between (93% and 96%). In the same context, another study of the effect of entomopathogenic fungi Akanthomyces lecanii and Akanthomyces muscarius showed a very low mortality rate (15% on the 7th day), even when they were used in combination (Soltani et al. 2022). These large differences in mortality indicate that the S. griseoflavus strain PAL114 has greater insecticidal power against black aphids than entomopathogenic fungi. The strain also induced a mortality rate of (70%) after 24 h, which is somewhat closer to that of the chemical insecticide Fevantrale (82%) (Purhematy et al. 2013).
Bioformulations intended for insect biocontrol are usually produced on the basis of bacterial or fungal broth cultures, as in (Karthiba et al. 2010), or on the basis of fungal spores, as in (Bensaci et al. 2015, 2022; Mohammed 2018). However, in our trial, the formulations were based on actinobacterial spores, as this type of formulation has already proven its effectiveness in the biocontrol of plant diseases and in growth promotion (Zamoum et al. 2017, 2022; Allali et al. 2022). Furthermore, their production is easier, low-cost, and dose not requires many conditions or large quantities of growth media if large-scale manufacturing is considered.
Regarding its in vivo effect on fava beans, the bioinsecticide based on PAL114 spores had no negative influence on the plant in either mode of application. There was no decrease in any of the parameters tested. In contrast, growth was promoted by an increase in germination rate and plant and root length, especially when delivered as Spray + soil amendment. Our results concur with those of other studies on the fertilizing effect of S.griseoflavus. This species has displayed properties of nitrogen fixation, nodulation, increased nutrient uptake and yield in plants of the Fabaceae family when coinoculated with other bacteria (Soe and Yamakawa 2013b, a; Htwe et al. 2018, 2019).
The application of bioinsecticide by Spray + soil amendment had a more marked effect than that by Spray. The latter is known to be effective in controlling foliar diseases (Meena et al. 2014; Udhayakumar et al. 2019; Bora et al. 2021), while soil amendment is effective mainly for fertilization (Raj et al. 2003; Parveen et al. 2012; El-Komy et al. 2020). This shows why the combination of the two that we used was more efficient in terms of growth promotion.