Biocontrol of Cassida vittata Vill. (Coleoptera: Chrysomelidae) in Sugar Beet Crops Using Streptomyces sp. Strains

Cassida vittata Vill. is a major pest of sugar beet crops worldwide. This study evaluated the efficacy of Streptomyces sp. strains E23-2, E23-9, E23-3, and E25-12 in managing this pest under both laboratory bioassays (26 ± 2 °C, 60 ± 10% RH, and 12 h of photoperiod) and field conditions. In the laboratory bioassays, insecticidal and repellent activities were assessed using topical and leaf dip methods. The insecticidal activity test involved five concentrations of bacterial suspensions (102 to 1010 cfu. mL−1 (Colony-Forming Units per milliliter)) against pest larvae and adults. Only E23-2 and E23-9 strains at concentrations of 101⁰ and 10⁸ cfu. mL−1, exhibiting the highest insecticidal activity, were used for the field bioassay. Carbosulfan at 0.25 g/L served as a positive control. Results indicated E23-2’s high efficacy against C. vittata, with the lowest LC50 values: 323.5 (larvae) and 5.1 × 103 (adults) cfu. mL−1 in topical contact, and 1.9 × 103 (larvae) and 3.1 × 104 (adults) cfu. mL−1 in the leaf dip method. LT50 values of 3 days for larvae and adults in the topical contact method supported E23-2’s efficacy. E23-2, at 101⁰ cfu. mL−1, displayed notable repellency against C. vittata adults (RI = 84.9% at 48 h). In field trials, Henderson–Tilton adjusted rates revealed E23-2’s substantial reductions of 88.6% (larvae) and 85.9% (adults), aligning closely with Carbosulfan’s efficacy. Enzymatic analysis underscored the versatile biocontrol attributes of E23-2, E23-3, and E23-9, providing insights for targeted pest management strategies. Field conditions, notably temperature, can influence the establishment and efficacy of EP bacteria. Further field studies are imperative for a comprehensive understanding of these influencing factors.