Coming from Central America, the prickly pear ( Opuntia ficus-indica ), like other species of the Opuntia genus, has been cultivated in Spain since the mid-16th century. It was one of the first plants brought from the New World, along with the potato and tobacco.
The prickly pear as a crop
One of the applications of the prickly pear with the greatest demand is the commercial exploitation of its fruit . The prickly pear is a highly appreciated product that fetches high prices on the market, both for direct sale as a fruit, and for the production of derivative products such as jams, preserves and juices, or the oil from its seeds, used in cosmetics, which are attributed dubious properties.
Another important sector of prickly pear cultivation is the use as a substrate and food for the carmine cochineal ( Dactylopius coccus ). This small insect with a characteristic red color is a parasite of plants of the Opuntia genus, and from which a highly appreciated dye in food, cosmetics, textile and pharmaceutical industries is extracted: carmine red . Depending on the substances with which it is mixed, it provides from a bright red color, crimson, to a wide range of violets and purples.
In addition, it is a highly appreciated plant as an ornamental cactus , very tolerant of arid climates, where it is planted both individually and in hedges.
The prickly pear invasion
The prickly pear is widely naturalized in much of the Mediterranean area of the Iberian Peninsula. We find populations in Extremadura, Andalusia, Murcia, the Valencian Community, Catalonia, and in both archipelagos, and even enclaves with populations in Aragón, Castilla – la Mancha and Castilla y León.
In many of these enclaves, the species behaves as an invader . The same is true in Australia, Arabia, Iraq, Syria, Southwest China, Hawaii, California, Arizona, Florida, Texas, and other states in the United States of America.
In places where the climate is dry and warm, the prickly pear competes with great advantage with the native vegetation, displacing it and even inhibiting its growth.
In Spain, prickly pear populations have been observed successfully invading native pine and mastic scrub communities in Doñana . Not only can it displace native vegetation, but it also changes natural succession patterns —the way ecosystems regenerate—altering the dynamics of nutrient cycles or light availability.
Its invasive character made it a candidate to enter the Spanish Catalog of Invasive Exotic Species , where it was included in its first edition in 2013. The inclusion in the Catalog, it is assumed, entails the prohibition of possession, transport, traffic and trade of live specimens. or their propagules —fruits, seeds…—, as well as the prohibition of their introduction into the natural environment. However, it is not difficult to find places to buy fresh prickly pears —from which it is possible to extract seeds for cultivation— and nurseries that sell plants in different stages of development.
A study carried out at the Doñana Biological Station (Seville) in 2019 already highlighted the fact that up to eight invasive species regulated by the Catalog —including the prickly pear— were available for sale in nurseries, and concluded that “ there is a urgent need to enforce current legislation , as well as to raise awareness among the population to avoid trade in these species”.
How to fight the invasion
Among the control methods proposed for prickly pears, three have shown some efficacy, although they are quite dependent on the context and not all are without risk.
On the one hand, mechanical control is proposed, or what is the same, the direct extraction of the specimens and their destruction. It is an effective method, but it requires attention and vigilance; plants of the Opuntia genus have a great capacity for regeneration, and can form a whole new plant from just a small fragment left in the ground. In addition, it is an expensive method, which requires the mobilization of a large number of trained personnel equipped with individual protection equipment to avoid the damage caused by their spines.
On the other hand, chemical control seems to be effective in preventing regrowth. Glyphosate has been used successfully during the summer, applied on the cut parts. However, it does not manage to eradicate the populations, and there is a risk of affecting other plants in the environment.
In some places they have used biological control , especially with the prickly pear mealybug Dactylopius opuntiae , of the same genus as the already mentioned carmine mealybug. When they grow in large numbers, they weaken the plant so much that they can dry it out. These insects have a very high specificity, and only attack cacti of the genus Opuntia and Nopalea . Given that neither of the two genera has species native to the Iberian Peninsula, biological control with mealybugs may be the best way to control the invasion of prickly pears. In fact, since this insect was introduced in Spain —for control purposes—, the prickly pears have suffered a significant setback.
An invader attacking another invader?
There are those who consider that the prickly pear mealybug should be considered an invasive species, as it is an exotic organism that negatively affects prickly pear cultivars. However, the invasive character of the cactus is undeniable , and in the case of mealybugs, they only negatively affect these species, and it is not capable of expanding if there are no Opuntia specimens that can be infected.
Unlike other biological control methods, in this case, the cochineal is an example of an ideal biological control textbook . If the insect is considered an invasive species, it would have minimal or no negative impacts on the environment, and with considerable advantages: it successfully fights another species that is invasive, and its expansion would be totally self-limited ; if the prickly pear disappeared from the Iberian Peninsula, the cochineal would also disappear.
The only problem associated with the expansion of the cochineal is the large amount of organic matter that remains after the death of the plant, which can have negative consequences for the environment: bad smells, proliferation of insects and possible eutrophication of the environment. It is important, therefore, to remove plant debris whenever possible.
Apart from that, there is no solid scientific argument, from the ecological point of view, that invites to actively avoid the effects of mealybugs on prickly pears; On the contrary, it is an effective and safe method of combating the invasion of the cacti, with proven success.
References:
Bayón, Á. et al. 2019. Horizon scanning to identify invasion risk of ornamental plants marketed in Spain. NeoBiota, 52, 47-86. DOI: 10.3897/neobiota.52.38113
BOE. 2013. Royal Decree 630/2013, of August 2, which regulates the Spanish Catalog of invasive alien species. BOE, 185(Sec. I.), 56764-56786.
Rodrigo, E. et al. 2010. Comparative study of the morphology and biology of Dactylopius coccus Costa and D. opuntiae (Cockerell) (Hemiptera: Dactylopiidae), two species present in the Valencian Community. Plant Health Bulletin. Plagues, 36, 23-35.
SanzElorza, Mario. et al. (Eds.). 2004. Atlas of Invasive Alien Plants in Spain. General Directorate for Biodiversity.
