Frass adjuvants improve the efficacy of Steinernema feltiae against vine mealybug, Planococcus ficus (Hemiptera: Pseudococcidae) under laboratory conditions

Vine mealybug, Planococcus ficus, is a major vineyard pest, complicating management through cryptic behavior, patchy distributions, and transmission of grapevine leafroll-associated viruses. Although the entomopathogenic nematode (Steinernema feltiae) shows promise for biological control, its efficacy is limited by desiccation under field conditions. This study evaluated insect-derived frass and chitin as sustainable adjuvants to improve nematode survival and performance. Frass collected from Hermetia illucens larvae reared on three feedstocks of varying fiber content, as well as isolated pupal exuviae, was assessed for water-holding capacity, effects on nematode activity, and impacts on mealybug mortality. High- and intermediate-fiber frass retained significantly more water than exuviae and extended nematode activity by up to three days. When paired with high-fiber frass, nematode applications caused 94% mealybug mortality compared to 76% with nematodes alone. Logistic regression further revealed a 125-fold increase in adult mortality when treated with nematodes plus frass relative to controls. Frass-based adjuvants also increased nematode penetration and load in adult females by up to 3.5-fold. These findings highlight the dual physical and biological benefits of frass, supporting its potential as a cost-effective and scalable adjuvant for insect pest control, including mealybug biocontrol.