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

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.