Assessing the Invasive Status and Potential for Biological Control of Bilberry Cactus, Myrtillocactus Geometrizans
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Assessing the invasive status and potential for biological control of Bilberry Cactus, Myrtillocactus geometrizans. Ekhona Zozo1*, Iain Paterson1, Kanyisa Jama2 1Centre for Biological Control, Department of Zoology and Entomology, Rhodes University, Grahamstown, South Africa 2South African National Biodiversity Institute (SANBI), East London, South Africa *[email protected] ABSTRACT 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. Keywords: Emerging, mealybug, weeds, alien invasive, population dynamics. INTRODUCTION Invasive cacti in South Africa Various cacti species have been documented as invasive in South Africa; these include many species from the Opuntia genus (Paterson et al. 2011). These species came into the country in different ways, some were introduced unintentionally, whilst others were introduced intentionally through horticultural trade and for commercialization of, for example, their fruit (Novoa et al. 2015b; Novoa et al. 2017). Many of these alien cacti have subsequently become very problematic in South Africa because they cause negative change in the areas they invade, a process referred to as biological invasion, which poses a threat to the conservation of natural resources and biodiversity (Simberloff et al. 2013). Cactaceae are considered as amongst some of the most damaging invasive alien plants in South Africa (Kaplan et al. 2017). Some of their negative impacts include reducing the carrying capacity of land which could be used for grazing for livestock and wildlife; they also reduce indigenous plant diversity (van Wilgen et al. 2004). Animals which encounter cactus infestations are usually harmed and/or killed—leading to the compromising of the quality of natural resources such as wool and a reduction in native animal biodiversity (van Wilgen et al. 2004). Though it is true that some alien invasive cacti are found in very limited populations, they are likely to become problematic in the future because they have similar characteristics to those cacti which are currently problematic. Biological control is a method of control which has been used for alien invasive cactus control in South Africa. The first intentional biological control program was done in 1913 when Dactylopius ceylonicus (Green) (Hemiptera: Dactylopiidae) was used to control Opuntia monacantha Haw. (Cactaceae) (Paterson et al. 2011). Since then, the work of biological control has continued in the country on various cacti using different agents. Many biological control programs have been very successful (Paterson et al. 2011; Moran et al. 2013). In some cases, there are biological control agents which control multiple species, e.g. Hypogeococcus sp. controls Cereus jamacaru, Harrisia balansae and H. martini (Paterson et al. 2011; Moran & Zimmermann 1991). There is a need to assess cactus species with limited distributions, so that it can be determined if they are likely to become problematic and whether their eradication or control is possible. It is a requirement of the National Environmental Management Biodiversity (NEMBA) Act 10 of 2004 that a control strategy be developed for each alien cactus and to determine whether eradication is feasible, or chemical/mechanical or biological control should be used instead. Myrtillocactus geometrizans The Bilberry Cactus, Myrtillocactus geometrizans (Mart. ex Pfeiff.) Console, is a cactus species that obtains a height up to 4 or 5 metres (Walters et al. 2011). It has a short trunk with numerous, branches that curve upwards, these branches have a blue-green appearance and are 6 to 10 cm in diameter (Walters et al. 2011). Branches have an average of 6 ribs and areoles are separated by between 5 to 30 cm (Walters et al. 2011). Each areole has one central, dagger-like spine, measuring 1 to 7 cm that is black in colour (Walters et al. 2011). The radial spines vary in number from five to nine and are 2 to 10 mm long (Walters et al. 2011). During the flowering season, the tree produces creamy or greenish white flowers, that develop into dark purple or red, globose fruit which are about 2 cm in diameter (Walters et al. 2011). This cactus species is native in Mexico, a country characterised by a semi-arid to arid climate (Hernandez-Lopez et al. 2008; Novoa et al. 2017). Like many cactus species which have been introduced into countries around the world, this species is likely to have been introduced into South Africa through the horticultural trade as an ornamental plant (Novoa et al. 2015b). Myrtillocactus geometrizans is considered as an emerging weed in the country. Emerging weeds are alien invasive plants that are in the early stages of their invasion (Olckers 2004). The species is currently listed, in Category 1a, for control under the National Environment Management: Biodiversity Act, Alien and Invasive Species Regulation (NEM:BA) (Novoa et al. 2015a). Nine areas in South Africa have been reported as invaded by M. geometrizans and some of these areas are very densely infested. The cactus can spread within a site and to new sites from, seeds dispersed after the fruit has been eaten by animals; as well as from detached stems (Chuk 2010). It is likely that M. geometrizans will become problematic if it is not controlled. This assumption is based on other well-studied alien invasive cacti which have been shown to have negative impacts on native fauna biodiversity by, for example, reducing the carrying capacity of land which could be used for grazing of livestock and wildlife (van Wilgen et al. 2004). It is therefore important to either eradicate or implement the control of M. geometrizans before it becomes a problematic alien species in the South Africa. The other cacti Cereus jamacaru Cereus jamacaru DC is native to Brazil (Braun et al. 2017). It grows on sandy to stony soil and on rocks in various ecoregions such as caatinga and agreste which are desert ecoregions (Braun et al. 2017). Its major threat in the native region is habitat loss—its habitat is being converted for use in agriculture and cattle ranching (Braun et al. 2017).The species is used in its native country to feed cattle, its fruit is edible, it is used as a hedge and was, in the past, used as timber for construction (Braun et al. 2017). Cereus jamacaru is an alien invasive in South Africa.