Below is a graph from a Wells (2001) study on the effectiveness of citronella spray collars at reducing the frequency of barking over a four-week time period, using 2 different types of exposure:

Based on this information, contrary to the belief of many owners, the citronella spray collar should not be placed on their pet on a daily basis. A longer, more consitant period of success is more likely if the collar is worn intermittently. These results also show that the frequency of barking increased over time regardless of the type of exposure (possible habituation to the spray). Based on these findings, the long-term success of the collar is questionable.