Effect of Foliar Application of Pymetrozine on Bemisia tabaci (MED Whitefly) and Amblyseius swirskii, 2016*

With the overall goal to integrate predatory mite Amblyseius swirskii in the management program of MED whitefly, the specific objective of this study was to evaluate pymetrozine, a pyridine azomethine derivative for whitefly control, and assess its compatibility with swirskii mite. The trial was conducted on an ornamental host, Salvia nemorosa L. (New dimension blue) under greenhouse condition at U.S. Horticultural Research Laboratory (USHRL). Salvia plants were grown from seed in Premier Pro-mix General Purpose Growing Medium in seedling trays, and placed inside plexiglass screened cages (61  cm × 91  cm × 61  cm) for ∼8  wk to prevent contamination prior to experimentation. Plants in seedling trays were then transplanted into one-gal plastic pots and placed in mesh screened cage (61  cm × 61  cm × 61  cm). Potted plants were irrigated as needed and fertilized with 50  ml/pot of Peters Professional^® 20-10-20 (325  ppm) (Scotts Co., Marysville, OH). Four treatments — (1) control, (2) pymetrozine, (3) swirskii, and (4) swirskii + pymetrozine were arranged in a randomized complete block design with six replicates, where each replicate (cage) consisted of four plants per cage. Salvia plants were inoculated with MED whitefly by releasing 100 adults per cage (25 per plant) three times at weekly intervals. Treatment cages with swirskii mite were inoculated with 20 mites/plant (3×) starting one week after first whitefly inoculation and prior to insecticide application. Plants in cages were sampled to obtain pre-treatment counts and to confirm whitefly biotype. Foliar applications of pymetrozine were made on week 0 and week 4 at the rate of 5  oz/100  gal and 10  oz/100  gal, respectively. Treatment evaluation was made at weekly intervals for a period of 7  wk by destructive sampling five leaves per replicate (cage) and recording the number of A. swirskii eggs and motiles (nymph + adult) per leaf, and MED whitefly eggs and nymphs on two discs (∼1  cm²) per leaf, using a binocular microscope. Whitefly adult count was taken per leaf sample by leaf-turn method (visual counting of adults before removing leaves from the plant). Counts of whitefly and swirskii mite observed in different treatments were analyzed independently using a generalized linear mixed model with the SAS (SAS Institute, Cary, NC) procedure GLIMMIX. The model was used to determine the effect of insecticide treatments, sampling period and their interaction on the arthropod counts. Since the response variable was count data with no upper bound, in model statement distribution was specified as Poisson. The autoregressive correlation structure was applied to take care of the correlation between observations collected on the same experimental units over time. Differences among treatment means were separated using Fisher’s LSD test (α = 0.05) in the repeated measures model. As the number of whitefly life stages in the efficacy trial was not uniform, Henderson–Tilton’s formula was used to calculate corrected mortality.

Weekly samplings showed overlapping generations of A. swirskii on host plants in combination treatment (A. swirskii + pymetrozine) throughout the study period indicating foliar application of pymetrozine at the applied rates was compatible with A. swirskii. No significant difference in A. swirskii eggs and motiles between mite treated and combination treatment were reported on any of the sampling dates (Table 1). Pymetrozine alone was not consistent in suppressing MED whitefly life-stages. A significantly lower number of whitefly eggs, nymphs and adults compared to untreated control were recorded on week 3 and 7, week 2, 3 and 6, and on week 3, 4 and 6, respectively (Table 2). A. swirskii provided higher suppression in whitefly life-stages than the pymetrozine treatment, and were as effective as combination treatment throughout the study period. Overall whitefly mortality in different treatments ranged between 84 and 97% for A. swirskii, 26–64% for pymetrozine, and 66–100% for combination treatments. No phytotoxicity was observed for any treatment.

Author notes