Evaluation of Off-Target Effects Due to Basal Bark Treatment for Control of Invasive Fig Trees (Ficus carica)

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Abstract Basal bark treatments involve the application of concentrated herbicide solution on each individual stem of targeted plants. When applied to stands of invasive plants with high stem densities, basal bark treatments may result in the use of large quantities of herbicide in a given area. The effect of basal bark treatments using a solution of 25% triclopyr herbicide and 75% methylated seed oil was tested on research plots located in six different groves of invasive fig, a densely stemmed, problematic invader of riparian forests in California. The experimental treatments resulted in application rates that were equivalent to 28 to 44 kg ae/ha, greatly in excess of the labeled maximum use rate of 9 kg ae/ha (8 lb/ac). At 175 d after application, soils near the fig trunks contained high levels of triclopyr residues (up to 6.6 ppmw), suggesting that the chemical made its way into soils during this period and was not completely degraded. Although the mortality of native plants transplanted into treated fig groves was low (up to 16%), it was significantly greater than the mortality experienced by native plants transplanted into untreated control sites (0%). Although effective in controlling invasive fig trees (> 99% mortality), the high herbicide application rates from basal bark treatment preclude the use of this treatment in large fig groves. These treatments may be appropriate, however, when fig groves are small or isolated enough to prevent overapplication on a per-area basis. In addition, neither limited basal bark applications of triclopyr (< 40% of stems treated) nor foliar spray treatments of 2% glyphosate were effective control measures. Further investigation is needed on ways to control large invasive fig groves.

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