Characterization of the probing behaviors of the hibiscus mealybug, Nipaecoccus viridis trialing different tethering materials and AC-DC EPG setting

Hibiscus mealybug, Nipaecoccus viridis (Newstead) (Hemiptera: Pseudococcidae), is a phloem-feeding pest that causes fruit and leaf deformation on citrus. Its cryptic probing behaviors coupled with a waxy body covering limit the effectiveness of contact insecticides. Field assays suggest that systemic insecticides can disrupt N. viridis probing behaviors, but the mechanisms of disruption are poorly understood. The objective of this study was to investigate the probing interactions of second instars on Volkamer lemon trees, Citrus volkameriana, using AC-DC electropenetrography (EPG). Specifically, we sought to identify and quantify phloem salivation (waveform E1). Insect saliva has been shown to induce physical distortions in plants in other systems, thus we hypothesize that systemic salivation may be a critical behavior inducing damage. Preliminary recordings failed to distinguish phloem salivation from phloem ingestion (waveform E2). To enhance the quality of our recordings, we tested the effects of three tethering materials on N. viridis probing: two gold wire diameters (12.5 µm and 25 µm), and a Wollaston platinum wire (2.5 µm diameter). We also tested a combination of three different amplifier sensitivities (input resistances) and substrate voltages: 109 Ω – 250 mV; 1010 Ω – 100 mV; and 1013Ω – 0 mV. We discuss the duration and count of phloem salivation events in relation to feeding damage, as well as waveform fine structures, electrical origins, monitor settings, and tethering materials to recommend the best EPG methods for future studies with mealybugs. Our work will improve efficacy of systemic insecticides against N. viridis, reducing environmental and application costs.