Do organic silicon and imidacloprid synergistically induce toxicity to the new invasive mealybug Phenacoccus solenopsis Tinsley on Portulaca grandiflora plants?

Authors: JUN HUANG, JUAN ZHANG, DAOZE WANG, LILI ZHANG, YIJUAN XU, MINGJIANG LI

Abstract: In this study, the effects of organic silicon (Si) on the toxicity of imidacloprid to a new invasive mealybug, Phenacoccus solenopsis (Hemiptera: Pseudococcidae), infesting Portulaca grandiflora (Caryophyllales: Portulacaceae) plants were evaluated using spotting and dipping methods under laboratory and field conditions to improve management strategies for solenopsis mealybug. Results showed significant synergistic effects of 0.06% organic Si with imidacloprid solution against third-instar P. solenopsis nymphs and adult females. However, the percentage mortalities increased remarkably after P. solenopsis was exposed to 100, 50, and 25 mg a.i./L of imidacloprid combined with organic Si. For the third-instar nymphs, these values increased to 100%, 100%, and 92.5%, respectively; for the adult females, these values increased to 100%, 100%, and 82.4%, respectively. After 5 days, the percentage mortalities reached 84.7%, 56.1%, and 39.7% when the third-instar nymphs were exposed to 100, 50, and 25 mg a.i./L of imidacloprid solution, respectively. The percentage mortalities reached 57.5%, 38.9%, and 26.2% when adult females were exposed to the same imidacloprid solutions, respectively. Organic Si alone elicited toxicity to P. solenopsis. The percentage mortalities of the first-, second-, and third-instar nymphs and the adult females were 76.9%, 47.5%, 39.2%, and 18.9%, respectively. Field data showed that imidacloprid combined with organic Si caused sustainable control of P. solenopsis. The percentage mortalities of P. solenopsis reached maximum values (approximately 99.9%) after 21 days. Thus, the combined treatment with imidacloprid and organic Si is synergistically more effective than the individual use of these substances against third-instar P. solenopsis nymphs and adult females.

Keywords: Phenacoccus solenopsis, imidacloprid, organic silicon, chemical control, synergistic effect

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