Impact of Pymetrozine on Bemisia tabaci (MED Whitefly) and Amblyseius swirskii, 2017

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 (Endeavor), 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 × 91 × 61 cm) for ~8 weeks to prevent contamination prior to experimentation. Plants in seedling trays were then transplanted into 1-gal plastic pots and placed in mesh-screened cage (61 × 61 × 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, Ohio). 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 (Koppert Biological Systems, Inc., Howell, MI) were inoculated with 20 mites/plant (3×) starting 1 week after first whitefly inoculation and prior to insecticide application. Plants in cages were sampled to obtain pretreatment 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 by destructive sampling at weekly intervals for a period of 7 weeks. Five leaves were collected from each cage (replicate) and the number of A. swirskii eggs and motiles (nymph + adult) per leaf, and MED whitefly eggs and nymphs per two leaf-disks (~1 cm²) per leaf was recorded, using a binocular microscope. Whitefly adult counts were made by visual inspection using the leaf-turn method before each leaf was collected. Counts of whitefly and swirskii mite from the different treatments were compared 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 from 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, the formula of Henderson–Tilton was used to calculate corrected mortality.

Weekly samplings showed overlapping generations of A. swirskii on host plants in the combination treatment (A. swirskii + pymetrozine) throughout the study period, indicating that foliar application of pymetrozine at 5 or 10 oz per 100 gal was compatible with A. swirskii. No significant difference in A. swirskii counts between mite treated and the combination treatment was reported except for motiles in week 2 (Table 1). Pymetrozine alone was not effective in suppressing MED whitefly populations, as no significant difference in whitefly numbers was observed between treated and the control cages (Table 2). Amblyseius swirskii releases provided better whitefly suppression than the pymetrozine treatment and were as effective as the combination treatment throughout the study period. Overall whitefly mortality in different treatments ranged between 85 and 97% for A. swirskii, 18 and 58% for pymetrozine, and 95 and 100% for combination treatments. No phytotoxicity was observed for any treatment. This research was supported by the Floriculture and Nursery Research Initiative and USDA Farm Bill.