The introduction of Harrisia cactus mealybug (HCM), Hypogeococcus pimgens, in Puerto Rico causes concern due to its damaging effects to the structure of cacti communities, eliminating species, and severely compromising plant growth and reproduction of susceptible native species. HCM is a polyphagous soft scaly insect considered an aggressive pest outside of its native range of South America. In Puerto Rico, three native species of cacti have been observed to be heavily infested with this invasive HCM, the natives Pilosocereus royenii, Melocactus intortus, and the endangered Leptocereus quadricostatus. Recent studies have shown that HCM affects the growth and survival of P. royenii, but limited information is available about the other affected species. To understand more about the threat of HCM, greenhouse experiments were designed to evaluate the pest colonization and to describe the development of galls on the columnar cactus L. quadricostatus. The experimental design consisted of two groups of E quadricostatus, infested and non-infested, the infested treatment received twenty crawlers and six females of HCM. The first signs of successful infestation were observed at 27 days after the initial pest transfer, while the first signs of gall formation were observed at 97 days. The initial infestation process was best explained by an exponential growth model (R2=0.93, F=125.4, df 1.9, p-value < 0.05). This stage can be identified by the formation of a powdery wax-like white spot on the areole of the cactus. Also four types of gall structure were observed. To our knowledge this is the first time that the full infestation process and gall development, by HCM has been replicated under controlled conditions. These results provide a better understanding of the interaction between HCM and the endangered host plant, and will help to develop more effective management strategies.