Efficacy of Soil-Applied Systemic Insecticides Against Nipaecoccus viridis Instars, 2022

Nipaecoccus viridis is an emerging pest in Florida citrus that has historically been difficult to manage with contact insecticides due to its cryptic feeding habits and protective wax. Our objective was to determine if selected soil-applied systemic insecticides applied to citrus trees could induce mortality of N. viridis instars.

We evaluated six systemic insecticides and compared them to a water control to determine their impact on N. viridis second–third instar mortality (Table 1). Trials took place in Lake Placid, FL. A randomized complete block design was used, with five blocks consisting of plots of five young Valencia orange trees each. Each plot of five trees was considered a replicate. Insecticides were applied to the soil around the base of the orange trees at maximum label rates. All insecticides except Verimark were applied in the first week of Mar 2022, while Verimark was applied 2 wk later. Temik was applied to the ground and raked into the soil, while all other insecticides were applied using a handgun applicator (Workhorse Sprayers 25 Gallon Deluxe Spot Sprayer, LG25DSS, Tractor Supply Co., Haines City, FL). Approximately 20 soft fully expanded leaves were collected from each treatment replicate and returned to the lab in a cooler on ice on the date applications were made, and 2, 4, and 6 wk after treatment.

One leaf from each treatment replicate was selected and had its petiole placed into a microcentrifuge tube (1.5-ml polypropylene microcentrifuge tubes, Millipore Sigma, Darmstadt, Germany) filled with DI water and sealed with parafilm (Prafilm, Bemis Company, Inc., Sheboygan Falls, WI). In total, 5–15 second–third N. viridis instars were placed on each leaf using a fine-tipped paintbrush (size 2 camel hair, Torrington Brush Works, Torrington, CT) and allowed to feed. Nipaecoccus viridis were obtained from a lab colony reared on Volkamer lemon trees kept at ~27°C.

Leaves with N. viridis instars were placed into individual 150 mm × 15 mm polystyrene Petri dishes (Thermo Fisher Scientific, Hampton, NH), sealed with parafilm, and kept in an incubator (Percival Scientific, Perry, IA) at 28 ± 2°C. On days 1, 2, 3, 5, 7, and 10 after instars were introduced to leaves, N. viridis instar mortality was assessed by gently probing instars with a fine-tipped brush. Instars that moved in response were considered alive, and those that did not were considered dead. The number of dead instars each day was divided by the total number of instars to obtain the proportion of dead instars, which was used for analyses. The proportion of dead N. viridis did not meet assumptions of normality, and nonparametric Kruskal–Wallis tests were used to determine the effect of treatment on the number of dead instars. Dunn tests were used to determine means separation between treatments. All analyses were conducted in R.

For leaves collected prior to application, there was no significant effect of treatment until day 10 (χ² = 12.557, df = 5, P = 0.0279), when Sivanto led to significantly higher mortality than Platinum (Table 2). For leaves from week 2, there was a significant effect of treatment on day 3 (χ² = 14.076, df = 6, P = 0.0288), day 5 (χ² = 14.076, df = 6, P = 0.02946), day 7 (χ² = 14.659, df = 6, P = 0.02308), and day 10 (χ² = 15.166, df = 6, P = 0.01901), where Temik led to significantly higher mortality than Sivanto (Table 3). For leaves collected 4 wk after treatments, there was no significant effect of treatment on any days (Table 4). For leaves collected 6 wk after treatment, Temik led to significantly higher mortality than Sivanto and the control on day 5 (χ² = 18.784, df = 6, P = 0.004544), day 7 (χ² = 17.772, df = 6, P = 0.006827), and day 10 (χ² = 14.129, df = 6, P = 0.02823) (Table 5).¹

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