Long-term population studies and the development of an integrated management programme for control of Opuntia stricta in Kruger National Park, South Africa

Published source details Hoffmann J.H., Moran V.C. & Zeller D.A. (1998) Long-term population studies and the development of an integrated management programme for control of Opuntia stricta in Kruger National Park, South Africa. Journal of Applied Ecology, 35, 156-160.
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Summary

The cactus Opuntia stricta, had invaded almost 16,000 ha and become a major weed in Kruger National Park (KNP), South Africa. A control plan was developed, main objectives being to reduce Opuntia density and prevent seed production by preventing young plants from maturing. Herbicides failed to provide satisfactory control as infestations were replenished from seeds in the soil and from small plants overlooked during spraying; areas need to be retreated every 2-3 years. In an attempt at biological control, a phycitid moth, Cactoblastis cactorum, was released in 1988. Despite successful Opuntia control in Australia, the moth although it established in KNP, failed to reach high enough levels to achieve satisfactory control. Therefore in this study, Opuntia populations were monitored in order to determine any synergistic effect of herbicide and Cactoblastis, and to see if this combination might adequately control O.stricta.

A combination of herbicided treatment and biological control (Cactoblastis cactorum, released in KNP in 1988), was assessed.

Two adjacent 1 ha plots infested with O.stricta were marked out. In August 1991, large plants were stem injected and smaller ones sprayed, with a solution of monosodium methanearsenate (MSMA) mixed in water. In January 1993, surviving cacti in one plot received a follow-up herbicided treatment. The other plot was left untreated.

Five, 100 m long transect lines/plot were used to monitor the cacti. Within a metre of the transect line, plant position was mapped, the number of cladodes, fruits and Cactoblastis colonies on each was recorded in June and December (December 1992-December 1996). The first surveys were completed a month before the second herbicide application.

Cactoblastis cactorum populations: In both 1 ha plots, C.cactorum was by far most abundant during the first recording period (around 410 colonies/ha in the once-herbicide treated plot and 240 in the twice-treated plot). In 1993 there was a massive decline (over 97%) when KNP experienced a severe drought and larval predation by baboons Papio ursinus was evident. The moth populations then steadily built up over the following years, with the single-herbicide plot having consistently higher populations than the twice-treated plot. Larval colonies were associated predominantly with the largest plants in the residual cacti populations in both plots.

Opuntia populations: Between December 1992 to June 1993, O.stricta declined after herbicide treatment, with a 63% decline in the twice-treated (from c. 2,200 to 900 plants/ha), and 40% decline in single-treated plots (from c. 2,800 to 1,500 plants/ha). There was then a steady increase over subsequent years as seedlings developed to about the same density in once-treated plot, but only around 1,000 plants/ha in the twice treated plot by December 1996.

Conclusions: Cactoblastis cactorum is making a substantial contribution to Opuntia control in residual infestations of O.stricta that have been treated with herbicides. Counts of C.cactorum and of O.stricta over 5-years show that, even though the moth has not provided complete O.stricta control in KNP, the moderate levels of larval damage have stunted its growth and have considerably extended the time that the young plants take to reach sexual maturity.

Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.blackwell-synergy.com/doi/abs/10.1046/j.1365-2664.1998.00283.x