The distribution of the invasive Harrisia cactus mealybug on Puerto Rico, and other biotic and abiotic factors threating native and endemic cacti

Background/Question/Methods

Cacti are a key component of Puerto Rican dry forests ecosystems. Currently, four native columnar cacti, including two endemic species, are threatened by the invasion of Hypogeococcus pungens, the Harrisia cactus mealybug (HCM). HCM infection is characterized by the growth of galls or tumors around the insect feeding site. The majority of research has focused on HCM control and biocontrol. Little research has been published concerning the consequences of HCM in the island’s cacti, and its distribution and intensity of infection still not well understood. We conducted a survey in eleven plots in different parts of the island, where cacti were identified to species and severity of infection was determined using an index from 0-5. Zero are healthy, uninfected cacti; one, light infection with >10 gall >2cm in diameter; two, light infection with 10-20 galls, >6cm in diameter; three, moderate infection, 20-40 galls, >10cm diameter; four, heavy infection, <30 galls, <10cm diameter; and five, heavy infection, <30 galls, <10cm diameter, and sings of necropsy and dead branches. Survival, seedling recruitment and infection evolution was determined by revisiting the plots. 

Results/Conclusions

HCM was found in all plots, but intensity of infection differed greatly between plots. Even plots within the same area differed greatly in terms of severity, indicating limited dispersion of the HCM. Its apparent dispersion limitation also suggests wind as a possible vector. Mortality rate differs by species, with Melocactus intortus suffering higher mortality, and along with Leptocereus quadricostatus appears to be more susceptible to HCM than the other cacti species. Stenocereus fimbriatus and Hylocereus trigonus seem to be more resistant. Recruitment of juvenile was low in a few plots and none in most, which implies a negative effect on reproduction. Also, the occurrence of invasive plants (a vine and several grasses) as well as anthropogenic fire disturbances, could have an impact on the survival of these cacti species. High mortality of cacti was attributed to fires, and cacti were less abundant in the presence the invasive vines and grasses. To our knowledge, this is the first study on the distribution of HCM on the island of Puerto Rico, and in a large invaded landscape, and the first time that data indirectly suggest HCM dispersal by wind, which had been assumed but never proven for this species.