Golf Course Management

MAY 2019

Golf Course Management magazine is dedicated to advancing the golf course superintendent profession and helping GCSAA members achieve career success.

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05.19 GOLF COURSE MANAGEMENT 71 Cases of imidacloprid breakthrough Five golf courses in New York and Penn - sylvania that reported imidacloprid break- through in the fall of the previous year were included in the spring study. Each course had applied imidacloprid on a calendar basis for more than 10 years, had never applied other preventive white grub products and had ob - served damage in an imidacloprid-treated rough-fairway area in fall. Study areas con - sisted of 400, 1-square-foot plots arranged in a 40-foot × 40-foot square, with equal parts rough and fairway. Turfgrass species and thatch thickness were recorded before de - structively sampling for white grubs. e spatial distribution of white grubs, turfgrasses, and thatch thickness and asso - ciations between spatial patterns (for exam- ple, white grubs:thatch thickness) was char- acterized using Spatial Distances by IndicEs (SADIE) freeware. In addition, soils were removed from the densest grub plots in the fairway and rough and assayed to determine whether microbes prematurely degrade imi - dacloprid. Each soil sample was divided into sterilized (to remove soil microbes) and non- sterilized treatments, and then seeded with perennial ryegrass. Imidacloprid was applied at label rates once plants were mature, and concentrations were measured in the plant up to 56 days after treatment (DAT) using an Enzyme Linked Immunosorbent Assay (ELISA) kit. Results Significant differences in imidacloprid concentrations were detected between steril - ized and non-sterilized treatments at 28 DAT, indicating that microbes contribute to prod - uct degradation. However, despite statistical differences, the numerical differences were small (~5-10 ppb) and unlikely to lead to dif - ferences in control or to be a sole factor in the reduced field efficacy. A concurrent field study aimed at examining residual imidacloprid concentration over time also casts doubt that imidacloprid residual activity is compromised, as we observed relatively stable concentrations in roots for more than 80 DAT. Spatial analyses of affected field sites re - vealed that white grub spatial patterns in imidacloprid-treated areas were significantly associated with those of turfgrass species (Kentucky bluegrass, perennial ryegrass) and thatch thickness at three of four sites (those with enough grubs to perform analyses). White grubs were significantly aggregated at four sites, and thatch was significantly aggre - gated at five. Greater grub densities and thicker thatch levels were observed in the roughs at all sites with significant aggregations. Although theses analyses do not directly assess the influence of thatch on insect abun - dance or distribution, the strong associations between thatch and white grub spatial pat - terns suggest that elevated thatch levels create conditions leading to either greater grub den - sities (for example, preferred beetle oviposi- tion sites) and/or negatively affect insecticide performance. We hope that ongoing studies examining the effect of imidacloprid trans - location throughout the plant with different thatch levels provide a better understanding of the relationship between thatch and insecti - cide performance. Andrew Huling is a graduate student in turfgrass science, and Ben McGraw (bam53@psu.edu) is an associate pro - fessor of turfgrass science in the Department of Plant Sci- ence at Penn State University in University Park, Pa. Raccoons were able to roll up sections of this fairway because white grubs had been feeding on the turfgrass roots. White grubs and thatch are often found together in the golf course rough.

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