Study

Fertilizer addition does not reduce semaphore cactus Opuntia corallicola mortality due to crown rot on the Florida Keys, 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. These planting trials were undertaken to determine whether fertilizing cacti promoted better growth and survival, especially via resistance to crown rot, which had caused high and significant mortality in earlier planting trials (see Cases 537 and 538).

Study site: The semaphore cactus Opuntia corallicola NPK fertilization experiment was undertaken on six Florida Keys: No Name; Big Pine; Little Torch; Cudjoe; Sugarloaf; and Ramrod, and ran from 2000 to 2003.

Experimental design: It was thought that association with prickly pear Opuntia stricta (a cactus moth Cactoblastis cactorum host) made semaphore cactus O.corallicola more susceptible to cactus moth attack, thus following on from results of earlier trials, the effect of Cactoblastis was minimized by planting semaphore cacti 500 m away from prickly pear. Earlier trials also indicated that semaphore cacti also had a tendency to grow slightly better in shade, and hence all cacti were planted in the shade of tropical hammocks survival in light gaps rather than deep shade compared. As crown rot had also been observed to consistently cause high mortality, whether fertilizing cacti promoted better growth and survival, especially via resistance to crown rot, was tested.

At each of the six separate Keys, 40 cacti were divided into four replicates of 10. One of four treatments was applied to each replicate:

1) full shade of a tropical hammock and 250 g of 6-6-6 (NPK) fertilizer added once a year in the spring

2) light gaps with fertilizer

3) light gaps with no fertilizer

4) shade with no fertilizer

Average height of the surviving cacti in each replicate over the 3-year period was analyzed, and death by crown rot and falling branches was compared between treatments.

Semaphore cactus growth & mortality: Mortality in this experiment was again high (see also Cases 537 and 538) with 108 cacti (45%) dying within 3 years. Cactoblastis killed no cacti, most succumbing to crown rot. It was hoped that fertilizer addition might infer some crown rot resistance but there was no significant decrease in the fertilized treatment (42.5% ± 9.1) compared to the control (37.5% ± 9.2). There was also no effect of light gaps on crown rot (shade, 44.2% ± 8.8; light gaps, 35.8% ± 9.3). There were few deaths from falling branches uprooting cacti, but there was a tendency for more death from falling branches in shade (8.3% ± 3.2) than light gaps (1.7% ± 1.1).

For surviving cacti, gain in height during the first 3 years was significantly higher in fertilized treatments (35.7 cm ± 6.1) than in control plots (18.7 cm ± 3.5). Growth was similar in light gaps (28.8 cm ± 6.2) and in the deep shade of hammocks (25.4 cm ± 4.8). Some replicates had 100% cactus death.

'Volunteers' (cactus pads appearing at the base of parent plants) grew at about the same rate as the cacti planted in the cage experiment (Case 538), and by the end of 3 years, growth averaged 7.0 cm ± 0.9. However, mortality appeared to be much lower among volunteers than among planted cacti, and after 4 years only 22.3% ± 4.2 had died, all from crown rot.

Conclusions: Planting semaphore cacti at least 500 m away from prickly pear appeared to negate the potential problem of mortality due to Cactoblastis. NPK soil fertilization greatly stimulated cactus growth but did not affect cactus death from crown rot.


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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