| Summary: | The efficacy of two selected <i>Metarhizium rileyi</i> Mexican isolates (T9-21 and L8-22) against <i>Spodoptera frugiperda</i> was evaluated under greenhouse conditions. To this end, a suspension (1 × 10<sup>8</sup> conidia/mL) of these isolates was sprayed on maize plants previously infested with six second-instar larvae. No significant differences were observed between the survival curves of the T9-21 and L8-22 isolates. Cadaver sporulation was significantly higher, and the lethal time was significantly lower with the T9-21 isolate compared with those of the L8-22 isolate (97% and 8 days vs. 70% and 10 days, respectively). Based on these results, a small-scale field trial on maize was performed to evaluate the degree of pest control achieved by the T9-21 isolate and compare it with the insecticide spinetoram, applied at a rate of 1 × 10<sup>13</sup> conidia/ha and 75 mL/ha, respectively. No significant differences were observed in the proportion of larval mortality between the T9-21 isolate (0.49) and spinetoram (0.72). However, spinetoram significantly reduced natural enemies and phytophagous insect populations compared with the fungus and the control. In conclusion, <i>M. rileyi</i> T9-21 isolate could be a promising alternative for the control of <i>S. frugiperda</i> larvae.
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