Abstract: 【Objective】 Bemisia tabaci, an important invasion insect pest in agriculture, has displayed a high resistance to many conventional pesticides. The objective of this study is to evaluate the synergistic effect of joint application of entomopathogenic fungi and chemical pesticides against B. tabaci in laboratory and field, and to provide a new approach for effective control of B. tabaci. 【Method】 Based on the previous study that a highly virulent of Metarhizium flavoviride strain (Mf96) was screened for control of B. tabaci Q biotype, three gradient emulsifiable formulation of Mf96 (1.0×10⁸, 1.0×10⁷, 1.0×10⁶ conidia/mL), which were combined with five low doses of abamectin EC (0, 15, 30, 45 and 60 µg·mL^-1) were separately sprayed to the body surface of 2nd instar nymphs of B. tabaci Q biotype in laboratory, and the mortalities were tested. The number of conidia deposited on the unit surface was counted under stereomicroscope. Single agent and their equal volume mixture of the suspension emulsion of Mf96 strain (1.0×10⁸ conidia/mL) and 1.8% abamectin WP (60 µg·mL^-1) were separately sprayed to control B. tabaci Q biotype on tobacco in the field, and the control efficacies were evaluated. 【Result】 LC₅₀ of Mf96 against 2nd instar nymphs of B. tabaci Q biotype decreased from 1 376 to 183 conidia/mm² during 4th to 8th day. After 7 days, LC₅₀ of the fungus combined with abamectin (60 µg·mL^-1) decreased from 378 to 46 conidia/mm². Abamectin had no significant effect on the fungal outgrowths and infection to 2nd instar nymphs of B. tabaci Q biotype at the low spray rates. The mixture suspension of different concentrations of fungus (low, medial and high) with abamectin (15, 30, 45 and 60 µg·mL^-1) resulted in different rates of mycotized cadavers, and high concentration of fungal mixture suspension with 30 µg·mL^-1 of abamectin caused 86.8% mortality of B. tabaci. No cadavers were observed in untreated control and single abamectin spray. Fungal conidia suspension emulsion, abamectin and their mixture were separately sprayed in the field, and the decline rates in nymphae of B. tabaci Q biotype population was the highest (53.6% and 85.7%) when the mixture was sprayed after 5 d and 10 d. Meanwhile, the changing trends of corrected control efficacies and population decline rate of B. tabaci Q biotype were similar at 5 random checks, the corrected control efficacy of the mixture was the highest (88.9%) after 25 d. The population decline rate of B. tabaci Q biotype in untreated control was negative value.【Conclusion】The combined of M. flavoviride with abamectin showed synergistic interaction, with additive effects on Q biotype B. tabaci. Therefore, it is an effective approach to control B. tabaci by combining M. flavoviride with abamectin at low spray rate.

Key words: Bemisia tabaci Q biotype, Metarhizium flavoviride, abamectin, synergistic effects, field control efficacy