The principal sex pheromone component of the whitemarked tussock moth (WMTM), Orgyia leucostigma, was recently identified as (Z,Z)-6,9-heneicosadien-11-one (Z6Z9-11-one-21Hy). However, it is thermally unstable and quickly degrades under field conditions so that baited traps are effective for only one night. We have developed a solution to this problem that combines two techniques: (1) the use of a stable pheromone precursor, (Z,Z)-6,9-heneicosadien-11-one ethylene ketal, which is hydrolyzed to the dienone by an acidic aqueous solution (2% p-toluenesulfonic acid in 35% aqueous sorbitol), and (2) use of a small, off-the-shelf, autonomous pump (the Med-e-Cell Infu-disktrade mark) to deliver the precursor continuously to a suitable substrate where it is converted rapidly into the attractive dienone pheromone component. The pump and hydrolysis substrate fit inside sticky traps and because generation and release of pheromone is continuous, the instability of the pheromone is not an issue. In electroantennogram bioassays, dose-dependent responses were obtained with 1 to 1000 ng of hydrolyzed ketal on filter paper, but no response was obtained to 1000 ng of the ketal itself. In wind tunnel bioassays, males were attracted to lures emitting the dienone pheromone component generated from 0.1 to 100 ng of the hydrolyzed ketal. Field tests in 2004 and 2005 showed that sticky traps fitted with the pump delivering the ketal (0.1-1 microg/microL in heptane) at 10 microL/hr to a cotton pad soaked with the hydrolyzing solution were attractive to male WMTM. No moths were caught in controls or traps baited with (Z)-6-heneicosen-11-one. An average of 0.51 moths per trap night was caught over an 18-night period in 2005. The results represent a first step toward developing a sensitive and practical monitoring tool for the WMTM by using a ketal precursor of its unstable dienone pheromone component.