Effect of Light Intensity On the Efficacy of Systemic Insecticides In Suppressing Populations of the Citrus Mealybug, Planococcus citri, and Sweet Potato Whitefly B-Biotype, Bemisia tabaci

Systemic insecticides are used to prevent outbreaks of populations of phloem-feeding insects such as aphids, whiteflies, and mealybugs in greenhouse-grown crops. However, no information exists on the impact of light intensity on the movement of systemic insecticides in plants and how this may affect efficacy. Therefore, three greenhouse experiments were conducted to determine the movement and activity of two systemic insecticide active ingredients that vary in water solubility; imidacloprid and dinotefuran. Each insecticide was applied at two rates: its designated label rate and the label rate of the other insecticide. Each experiment included one plant species; yellow sage, Lantana camera (experiment one) or poinsettia, Euphorbia pulcherrima (experiments two and three). Plants were artificially inoculated with either sweet potato whitefly B-biotype (SPWF), Bemisia tabaci (experiments one and two), or citrus mealybug (CMB), Planococcus citri (experiment three), treated with the insecticides at the designated rates, and placed into polyvinyl chloride (PVC)-framed cages modified to simulate light conditions of either ambient or shade. For each experiment, evaluations including light intensity in each cage, plant parameters (e.g., stomatal conductance) and percent mortality of each insect pest were measured weekly over a four-week period. In addition, the concentration of active ingredient in plant leaves was determined using an enzyme-linked immunosorbent assay (ELISA). In general, light intensity (ambient or shade) had minimal effect on the movement of the active ingredients in both plant species for all three experiments. In experiment one, dinotefuran moved into the leaves of yellow sage faster than imidacloprid, resulting in higher mortality of SPWF nymps (>90%) than imidacloprid (<90%). Additionally, the dinotefuran treated plants had fewer SPWF pupae (<1.2) on the final evaluation period (week four) than the imidacloprid treatments (19 to 33) and controls (129 to 171). This was similar in experiment two, in which dinotefuran was present at higher concentrations in the leaves of poinsettia and provided quicker and higher mortality of SPWF nymphs than imidacloprid across all four-weeks. However, efficacy was less for CMB in experiment three where mortality values were <70% among all treatments across the four weeks although dinotefuran moved into poinsettia leaves much faster and at higher concentrations (324 to 1271 ng/leaf disk) compared to imidacloprid (32 to 414 ng/leaf disk). The two rates of the insecticides resulted in minimal differences on the two insect pests regardless of the rates used.