Opuntia species (prickly pear) were deliberately introduced to many countries around the world for fruit, cochineal dye production, living fencing or as ornamentals. They are now some of the world’s most significant weeds, particularly in regions with warm and or dry climates, as they pose threats to economic and environmental assets. In addition, they can cause considerable health issues for humans and animals. Opuntia spp. have prolific reproduction abilities, being able to reproduce both vegetatively and by seed. They have generalist pollination and dispersal requirements, which promotes their establishment and spread. Opuntia stricta, O. monacantha and O. ficus-indica are the most globally widespread of the Opuntia spp. In many countries, biological control agents, particularly the cactus moth (Cactoblastis cactorum) and various cochineal insects from the Dactylopius genus, have successfully reduced landscape scale populations. On a smaller scale, controlling these weeds by either injecting or spraying the cladodes with herbicides can provide effective control. Care must be taken during herbicide treatments as any untreated areas will regenerate. While biological control is the most cost and time effective control method for landscape-scale infestations, further research into the combined efficacy of herbicides, fire, grubbing and pre-burial techniques would be beneficial for land managers to control small-scale and establishing populations. It would also be useful to have greater knowledge of the potential seedbank longevity and seed ecology of these species so that integrated management strategies can be developed to not only deal with initial populations but also the subsequent seedling regrowth.
The combined factors of prolific fruiting, strong vegetative reproduction and haz- ardous physical traits, with dense growth forms, spines and glochids, make O. stricta, O. ficus-indica and O. monacantha globally significant weeds. The findings of this review demon- strate that biological control agents provide the most effective landscape-scale control of invasive Opuntia spp., especially in degraded, nonarable areas, with dense populations quickly decimated when effectively applied. As Dactylopius spp. are species specific, they pose little threat to cultivated varieties or native vegetation. Due to the prolific vegetative re- production of these species, after the implementation of biological control agents, it would be advised that any surviving cladodes be removed or treated with herbicide to prevent reestablishment. Further seedling emergence should be monitored for up to 20 years due to the persistent seedbank as a result of the seeds having mechanical dormancy due to the thick seed coats. In areas where biological control agents are not suitable, due to either cooler climates, high rainfall zones or limited funding, fire has been observed to successfully remove dense populations when implemented after summer, when these weeds are at their driest. It would be recommended to also treat cladodes that were not completely killed by the fire.
Monitoring seedbank emergence would be critical as fire treatment may increase germination rate and uniformity, as fire is known to scarify hard-coated seeds. Once these Opuntia spp. are treated and removed, it would be beneficial to establish competitive and diverse native plant cover to outcompete seedlings for long-term control. It is recommended that further research investigates the longevity of the seedbank of these three Opuntia spp. and ways to promote uniform germination to flush out the seedbank and target the emerging plants while they are in the juvenile form, as small plants are easier to kill than adults. Biological control should be further investigated for non-American countries that have not yet implemented successful introduction programs. Community awareness initiatives have been effective in reducing infestations of Opuntia spp. (as seen in Portugal and South Africa), and a focus to develop similar programs in other countries could also be highly beneficial.