Insecticide resistance and its molecular basis in Gill's mealybug

Gill’s mealybug, Ferrisia gilli (Gullan) (Hemiptera: Pseudococcidae) is a major pest of pistachio in California. Insecticide treatment is the primary control method and acetamiprid, one of a few recommended chemicals, is widely used to control this pest. However, there have been numerous reports of control failures for F. gilli after field applications of recommended insecticides in recent years. Baseline susceptibility of F. gilli to acetamiprid was established and susceptibility levels from five locations in the central valley, CA were compared. Lethal concentrations to kill 50% of population (LC₅₀) for 2nd instar nymphs at 48 h ranged from 0.367 to 2.398 µg(AI)ml^-1 of acetamiprid. The F. gilli resistance to acetamiprid was discovered as population collected from Hanford area showed up to 6.5-fold decreased mortality compared to other populations. The resistance identified in this study, although relatively low, indicates that there has been repeated pressure to select for acetamiprid resistance and resistance level can further magnify if effective management steps are not taken. To understand the molecular response, RNA-seq was performed to compare gene expression in susceptible (collected on organic pistachio orchard)and resistance (collected on conventional pistachio orchard) populations. Several detoxification genes including cytochrome P450s were upregulated in resistance F. gilli. Gene expression investigation provided insights to the molecular mechanism for acetamiprid resistance in F. gilli. In the long-term, results of this study will support the resistance management strategy through monitoring the shifts in susceptibilities of F. gilli populations.