The Harrisia cactus mealybug (HCM), Hypogeococcus pungens (Hemiptera: Pseucococcidae), is known to be an effective biological control agent for invasive columnar cacti in Australia and South Africa. In the Caribbean, Central and North America, HCM is an alien invasive species that is threatening native cactus. Despite their negative impacts on ecosystems where cacti are native, no publications have described the symptomatology associated with HCM infestation. This article describes the morphology of the galls produced by HCM on seven host cactus species occurring in Puerto Rico.