Downward dispersion of liquid termiticide in gravel was determined by measuring active ingredient (a.i.) residues of two frequently applied liquid pyrethroid termiticides during one year following surface applications at lowest label rates (Up-Cyde Pro® 2 EC-cypermethrin; BaseLine® EC-bifenthrin) over the two most extensively used commercial building construction gravel foundation fills in Oklahoma. Residues were determined within partitioned depths in the two fills at 24.0 hours post-application, then after 6.0 and 12.0 months. Half the test plots were compacted (12-15% weight increase per unit volume) and half were not compacted. Downward dispersion of termiticide was significantly influenced by gravel type, regardless of whether compacted or not compacted. Dispersion of both termiticides in compacted plots compared with not-compacted plots was similar for Class A-#57 Crushed Rock, as residues were near evenly distributed throughout the entire 0.0- to 10.2-cm depth. However, for ODOT (Oklahoma Department of Transportation) Base Type-A Gravel, residues were greater in the top partition for both compacted and not-compacted plots. Residue differences were less pronounced when comparing within the same depth partitions only. Generally, residues within ODOT Base Type-A Gravel were retained in significantly greater amounts within the top partition depth, surface 0.0- to 5.1-cm deep, for both termiticides (cypermethrin: 115-118 ppm; bifenthrin: 43-44 ppm) compared with Class A-#57 Crushed Rock (cypermethrin: 27-33 ppm; bifenthrin: 8-21 ppm) at 24.0 hours after applications. These differences were less pronounced within the 5.1- to 10.2-cm partition depth. Total a.i. recovered accounted for ≈83-95% of amounts applied to ODOT Base Type-A Gravel, whereas residues recovered from Class A-#57 Crushed Rock ranged from ≈38-65% of applied amounts. This reduced recovery percentage is due to some of the termiticides dispersing downward completely through the Class A-#57 Crushed Rock and into the mineral soil beneath. Therefore, termiticide residues in the underlying soil were not recovered during sampling of the gravel fills. For ODOT Base Type-A Gravel, most of the termiticide residues that were recovered (≈59-81%) were retained in the top 5.1-cm-deep partition. Almost no residues of either termiticide (0.0 to <1.0 ppm) applied over both compacted and not-compacted ODOT Base Type-A Gravel dispersed downward deeply enough to reach underlying mineral soil.

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