Assessing the invasive status and potential for biological control of Bilberry Cactus, Myrtillocactus geometrizans .

Alien invasive plants are generally a problem in the areas they invade around the world. This is because of the negative impacts they usually have in the ecosystems they invade. Amongst the most problematic alien invasive plants are those in the Cactaceae family. These are found in South Africa where they have a negative impact on indigenous fauna and flora. Biological control of cacti in South Africa has provided solutions to the problem of these alien invasive cacti. One of the currently emerging alien invasive cacti is Myrtillocactus geometrizans (Mart. ex Pfeiff.) Console. This cactus was probably introduced into South Africa through horticultural trade. This cactus species is likely to become another problematic one because it is closely related to some of the most damaging cactus species. Hypogeococcus sp. (Hemiptera: Pseudococcidae) has been used in Australia and South Africa to control alien invasive cacti such as Harissia martinii and Cereus jamacaru . It is reported as having done a good job in clearing vast areas of those cacti. Here we investigated the invasive status of this emerging cactus and determined if the mealybug, Hypogeococcus sp. would be a damaging biological control agent to M. geometrizans . Determining the invasive status of the infestations was done by looking at the population dynamics of each infestation. To do this we found the biggest/oldest plant in each infestation, measured parameters of this plant, i.e. the height, canopy diameter, the number of branches, and whether each plant had reached an age that was reproductively viable by checking if there were any flowers, fruits or flower buds. To determine if Hypogeococcus sp. would be a damaging control agent, M. geometrizans , and four other cactus species, namely, Cereus jamacaru DC. , Harrisia balansae (K.Schum.) N.P.Taylor & Zappi , Harrisia martinii (Labour.) Britton and Hylocereus undata (Haw.) Britton & Rose, was planted plant pots. These were all replicated ten times. On each plant, parameters which were measured are: number of branches, length of each branch, height of the whole plant, the circumference of the trunk base, the circumference of the thickest branch before the top, and the diameter of the canopy of the whole plant. A photograph of the plant, next to a ruler was taken. The biological control agent, Hypogeoccocus sp. , was introduced by making a small cut on the thickest branch of each plant. Then, one Hypogeococcus sp. gall, on each plant, on a sosatie stick was introduced on the branch on the area cut and left there. The results we obtained show that the infestations surveyed in this study are small since they are easy to quantify, these populations are, however, spreading and proliferating at a potentially high rate. This suggests that, similarly, the other populations detected around the country may also be growing and spreading. Myrtillocactus geometrizans is a high-risk species which should be kept under Category 1a of the NEMBA legislation. This species also has a high percentage of viable seeds and is at an advantage since it can spread through vegetative plant parts and could also potentially germinate from dispersed seeds from fruit eaten by birds, other animals and human beings. We were, unfortunately, not able to obtain and quantify any significant results from the biological control agent damage test. We do, however, still view Hypogeococcus sp. as a good potential candidate for the control of M. geometrizans —we suggest further studies on this both at the individual and population levels. There is still a need to come up with an eradication or control strategy for this alien invasive species before it spreads beyond control and becomes a problem.