Chlorpyrifos, temephos, fenthion, and propoxur were applied to naturally occurring treeholes at the rate of 1 gm AI liter of treehole water. Inspections for reinfestation and bioassays of water samples from the treeholes were performed at intervals of approximately 1 year. Field infestations of Ae. sierrensis occurred during the 2nd year in trees treated with propoxur. Temephos and fenthion treated trees were infested during the 4th year. Larvae of Ae. sierrensis were found in treeholes treated with chlorpyrifos during the 6th year. Fenthion, temephos, and propoxur began to fail in bioassay at 2 years and chlorpyrifos at 5 years.