Interactions of a biopesticidal formulation of steam distilled shoot extract of Mexican marigold, Tagetes minuta, and entomopathogenic fungi were evaluated for management of the sugarbeet root maggot, Tetanops myopaeformis (Röder). Shoot extract plus surfactant (E–Z Mulse) (=T. minuta oil) was used in a 65:35 ratio to test the hypothesis that this fungicidal and nematocidal biopesticide causes dose-dependent mortality and developmental arrest of T. myopaeformis but does not interfere with the action of entomopathogenic fungi when applied together. A soil-petri dish bioassay system was developed to test the hypothesis. For diapausing, nonfeeding but active 12-mo-old third-instar larvae, 0.5% T. minuta oil treatment (=0.325% active ingredient [AI]) was sufficient to prevent pupation without mortality, but 0.75% T. minuta oil treatment (=0.458% AI) was lethal for 93% of the test insects. The effect of T. minuta oil on fungal efficacy under simultaneous use was studied using a model system of two entomopathogenic fungi, Beauveria bassiana (Bals.) Vuillemin. TM28 and Metarhizium anisopliae variety anisopliae (Metsch.) Sorokin MA 1200, in a soil-based bioassay with larval sugarbeet root maggots. No adverse effects of T. minuta oil on action of entomopathogenic fungi and no synergy were found; an additive effect of the T. minuta oil and each fungal isolate separately was found.