Study

Planting at distances greater than 500 m from prickly pear Opuntia stricta effectively prevents semaphore cactus Opuntia corallicola mortality due to Cactoblastis on Little Torch Key, Florida, USA

  • Published source details Stiling P., Moon D. & Gordon D. (2004) Endangered cactus restoration: mitigating the non-target effects of a biological control agent (Cactoblastis cactorum) in Florida. Restoration Ecology, 12, 605-610

Summary

The natural distribution of the cactus moth Cactoblastis cactorum is Argentina, Paraguay, Uruguay and southern Brazil. It was released as a biological control in the 1950s and 1960s to several Caribbean islands to control Opuntia spp. cacti. The first US record of cactus moth came from Florida in 1989. It was thought that the moth had either dispersed from Cuba or had been brought in on cacti imported from Haiti. In the USA it now threatens native cacti and concern is greatest for the endangered semaphore cactus, Opuntia corallicola, of which only two wild populations exist. Planting trials were therefore undertaken in an attempt to bolster semaphore cacti numbers and to test the effectiveness of different planting treatments to protect it from cactus moth. These planting trials were undertaken to determine whether plantings distant from other cacti and the use of moth exclusion cages would improve semaphore cactus survival.

Study site: It was thought that association with prickly pear Opuntia stricta (a cactus mothCactoblastis cactorum host) would make semaphore cactus O.corallicola more susceptible to cactus moth attack, thus these planting trials were undertaken to determine whether plantings distant from other cacti, and the use of moth exclusion cages, together would improve semaphore cactus survival. These trials were undertaken on Little Torch Key, Florida, southeast USA.

Experimental design: The 'Cactoblastis Exclusion Experiment' ran from 1996 to 2003. In 1996, 96 small (< 30cm tall) plants were planted at least 500 m from known prickly pears. A 2 × 2 factorial design was used to test the effectiveness of cages at protecting semaphore cacti from Cactoblastis with planting in the shade or in full sunlight as an additional treatment. Cages comprised 60 × 60 × 60 cm wooden frames overlain with mosquito netting, with hinged lids to allow access. For the cage treatment, cacti were planted in groups of four, one in each cage corner, and for the no-cage treatment one cactus was placed at each corner of a 60 × 60 cm plot on similar substrate. In total there were 12 replicates of caged cacti, 12 of uncaged cacti, and in each of these treatments six were placed in the sun and six in the shade.

Cacti were visited every 3 months for 7 years, until 2003, and height and causes of death were recorded. After 3 years, average height of the surviving cacti in each treatment was compared.

Semaphore cactus growth & survival: Cages did not affect cactus growth and semaphore cacti also did not grow significantly more in the shade (6.1 cm ± 7.4) than in the sun (2.5 cm ± 1.4). There was no mortality by Cactoblastis in any of the treatments. However, some cacti were knocked over and uprooted by animals, probably key deer Odocoileus virginianus clavium. The mosquito-net cages, designed to prevent attack by Cactoblastis, unfortunately attracted the attention of some larger mammals, such as key deer which rubbed against and broke them, and sometimes trampled cacti to death. This resulted in a significant difference in trampling death between caged (21% of plants) and uncaged (4%) treatments. Cage breakage, and the resultant trampling of cacti, killed more cacti in sunny areas, where animals could access all cages unhindered, compared to those in shaded conditions where access was more restricted by branches. Storm-induced branch breakage crushed and uprooted some cacti, but such mortality was minimal as most fallen branches were small (4% in uncaged plots, 0% in cages as these effectively protected the cacti). In this study the most important source of mortality was crown rot, though crown rot was not significantly affected by treatments (65% mortality in cages, 73% uncaged). In this experiment, 80 of 96 cacti died over 7 years (83.3% mortality).

Conclusions: Attack from Cactoblastis was successfully avoided (regardless of whether using protective cages or not) for semaphore cacti planted at least 500 m from prickly pear plants. Cages attracted the attention of deer, which sometimes destroyed the cages and trampled the cacti inside to death. Crown rot caused high mortality in this planting. These trial results suggests that it would be pertinent in future planting of semaphore cacti to ensure that individuals are at least 500 m away from prickly pear plants (which may act as a reservoir of Cactoblastis). Replicated plantings near to and far away from prickly pear are recommended by the authors to fully test this.


Note: If using or referring to this published study, please read and quote the original paper. Please do not quote as a conservationevidence.com case as this is for previously unpublished work only. The original paper can be viewed at: http://www.blackwell-synergy.com/journal.asp?ref=1061-2971

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