**4. Discussion**

The number of studies investigating the e ffect of *M. anisopliae* on *M. melolontha* larvae is low, although Horaczek and Vierstein (2004) mention that *Beauveria bassiana* and *M. anisopliae* have significance in controlling soil-inhabiting pests of various genera, including *Melolontha* [37]. A still earlier study reported that while a single application of *M. anisopliae* before planting was found to have promising results against *D. balteata*, the e ffects on *Melolontha* larvae, however, was not significant [24]. Our study showed that under open field conditions, with and without compost and di fferent soil cover (agro-foil or textile), damage by *Melolontha* larvae was lower when sweet potato was treated with *M. anisopliae* strain NCAIM 362 in WP formulation and covered by textile (Figure 2, Table 2). A similar e ffect was detected by other authors on the larvae of *Polyphylla fullo* (Coleoptera: Scarabaeidae) when the highest mortality rates of young and older larvae caused by a *M. anisopliae* product were 74.1 and 67.6% for the granular formulation [38]. By comparing the e ffect of *M. anisopliae* with soil insecticide α-cypermethrin, a significantly lower number of survived larvae were detected with α-cypermethrin, and no di fferences were detected between *Metarhizium* treatment and control (Figure 3B). Even when *Metarhizium* concentrations were tripled (1400 g *M. anisopliae* to 12.6 l. of water) compared to the commercially suggested dosage, only 50% of the larvae died on average by the end of the experiment in M<sup>+</sup> treatments. Also, the number of dead larvae were higher with α-cypermethrin, and again no di fferences were detected between *Metarhizium* and control (Figure 3B). Altogether, this suggests that *Metarhizium* strain NCAIM 362 in WP formulation (recommended and commercialized against coleopteran larvae) is less e ffective than α-cypermethrin against *Melolontha* larvae, making its future application in sweet potato control uncertain. This can also be supported by the fact that only a low amount of *Melolontha* larvae were observed to have signs of fungal infection (an average of one infected larvae in 10 *Metarhizium* treated pots) at the end of the experiment in M<sup>+</sup> treated pots (Figure 3B). The damage rate of sweet potato tubers had a strong connection with larval survived rate. Significantly lower damages were observed with α-cypermethrin, and no di fferences were detected between *Metarhizium* and control detected. Recent studies also reported the e ffect of *Beauveria brongniartii* (three isolates) and *M. anisopliae* (three isolates) on *M. melolontha, Amphimallon solstitiale,* and *Anoxia villosa* under laboratory conditions. The highest mortality rates were caused by *B. brongniartii* isolates (100%) on *M. melolontha* larvae, and 60% mortality on *A. villosa*. In comparison, *A. villosa* was found the most susceptible to *M. anisopliae* (35.5% mortality rate), and the fungus had little to no significant e ffect on *A. solstitiale* or on *M. melolontha* [39]. In our experiment, no di fferences were detected in the chemical composition of the soil, in the microbial community, and biological activity of the soil either between treatments and control. The fungal e ffects on *Melolontha* larvae were completed under very similar conditions, ye<sup>t</sup> no e ffects on larval mortality and thereby on tuber damage were detected, suggesting that *M. anisopliae* strain NCAIM 362 cannot e ffectively control *M. melolontha* in sweet potato. This can be explained in di fferent ways. The apparent resistance of *Melolontha* larvae against *M. anisopliae* is hard to be explained without further research. One possible reason can be the fact that a long evolutionary interrelation exists between this soil inhabiting larva and the fungus, meaning that a genetic resistance may have evolved. The e ffects of formulation may also result in di fferent characteristics of the fungus including conidial growth, viability, potential to cause mortality to target organisms, persistence, and resistance to certain environmental factors. One may also notice that while the impact of formulation on fungal performance was intensively researched in the 1990s, the number of research pieces conducted in this area has been lower ever since. One inevitable challenge formulation faces when trying to enhance the e fficiency of the fungal entomopathogen is the presence of ultra-violet light among unprotected field conditions that has a significant negative e ffect on *M. flavoviride* germination [40]. The frequency of bacterial genera were similar for both treatment and control. Most bacterial taxons identified as dominant (Proteobacteria, Planctomycetes, Acidobacteria, Bacteroidetes, Patescibacteria, Chloroflexi), are known to have a significant role in litter biodegradation and mineralization processes [31]. While no high variation in bacterial community were detected between control and treatment, means that no soil inhabiting microorganisms with inhibitory e ffect on *M. flavoviride* were detected. These further demonstrate that, *M. flavoviride's* effects were tested under ideal conditions.

These environmental effects on *M. anisopliae* need further, more detailed tests. When looking for a successful pest control species, other species of the Metarhizium genus may have more potential against Melolontha larvae. In 2015, a genetic characterization studies performed on fungal isolates obtained from fungus-infected larvae of the Coleopteran *Amphimallon solstitiale* collected from roots of various plants in north eastern Turkey revealed that the hosts were infected by *M. flavoviride* [41]. Finally, a series of experiments, including this present one indicate that the effect of *M. anisopliae* on *Melolontha* larvae is non-significant, therefore the effect of other *Metarhizium* species (e.g., *M. flavoviride*) as an effective control agen<sup>t</sup> in sustainable managemen<sup>t</sup> needs to be investigated.
