The invasive Asian cerambycid beetle Callidiellum rufipenne (Motschulsky), informally known as the Japanese cedar longhorned beetle, was first detected in North America in North Carolina in 1997. The beetle has since been detected in neighboring states and is expected to further expand its range. However, delineating the current distribution of C. rufipenne has been hindered by the lack of efficient sampling methods. Here, we present the results of research on the chemistry of volatile pheromones of C. rufipenne. Analyses of headspace odors revealed that males produce (R)-3-hydroxyhexan-2-one, with lesser amounts of (S)-3-hydroxyhexan-2-one, and (R)- and (S)-2-hydroxyhexan-3-one. In field bioassays conducted over several years in Connecticut, where populations of the beetle were well established, no reconstructed blend of these compounds was significantly attractive to beetles of either sex. However, during field trials in Japan that targeted another species, we discovered that adult male and female C. rufipenne were attracted to a blend of racemic 3-hydroxyhexan-2-one and a novel natural product, 1-(1H-pyrrol-2-yl)-1,2-propanedione. Attraction to (R)-3-hydroxyhexan-2-one and the pyrrole subsequently was confirmed in field trials in Connecticut. Although it is unclear why the pyrrole acts as a synergist for a species that apparently does not produce it, the serendipitous discovery that adult C. rufipenne are attracted by the blend of ketone and pyrrole provides a badly needed method for monitoring its ongoing range expansion within North America, and for detecting new introductions in other parts of the world.