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Observations on the Karoo caterpillar

 

C.H. Donaldson

Pasture Research

Grootfontein College of Agriculture

Private Bag X529

Middelburg C.P. 5900

 

The Karoo caterpillar, Loxostege frustalis Zeller (Lepidoptera: Pyralidae) is an indigenous insect that has been known to cause damage to Pentzia dominated veld in South Africa since early years (Lounsbury, 1897). Severe outbreaks of this insect during 1985 were again responsible for heavy defoliation of large tracts of Pentzia-dominated Karoo veld.

Although much is known about the life cycles (Mohr, 1982) and the native enemies of the Karoo caterpillar (Mohr & Pretorius, 1984) there is still no instant practical and economical solution to this problem. Studies by Mohr and Pretorius (1984) revealed a strong relationship between the Karoo caterpillar moths and the density of the Karoo caterpillar's host plants (mainly Pentzia spp). A further common observation made by many farmers is that well-grazed veld is generally less prone to Karoo caterpillar infestations than ungrazed and rested veld.

A preliminary study was undertaken to determine the different degrees of damage caused by a Karoo caterpillar population to Pentzia incana plants in rested, grazed and mechanically slashed veld.

 

PROCEDURE

The study was undertaken on 21 November 1985 at the Grootfontein College of Agriculture in veld dominated by Pentzia incana, a common host plant of the Karoo caterpillar. The three adjoining study sites selected for the trial during an outbreak of the Karoo caterpillar were located as follows:

 

Site 1: Pentzia veld rested and not grazed for at least 15 years.

Site 2: Pentzia veld periodically grazed by sheep – last grazed during August 1985

Site 3: Pentzia veld excluded from grazing for at least 15 years.

 

Site 1 and site 2 were approximately 20 m apart. Site 3 occupied a position 50 m from sites 1 and 2. Two specific study areas were available at site 3, namely three plots (each 20 m x 5 m) which were slashed to a height of 5 cm above ground level with a tractor-drawn rotary slasher on 18 December 1984 and three control (unslashed) plots (each 20 m x 5m).

Field sampling was done on 21 November 1985 when most of the Karoo caterpillars were almost fully grown (approximately 2 cm long). Ten randomly selected Pentzia incana plants were harvested at ground level from each one of the four study areas. The leaves and stems of each plant were hand separated and dried in an oven at 80 0 C for 24 hours and mass-measured. The number of caterpillars on the harvested plants was counted immediately before the plants were cut. The number of caterpillars on the ground and within the canopy spread of the plant was also included in this count.

 

RESULTS AND DISCUSSION

The mean leaf and stem dry matter yields of Pentzia incana plants from the four study areas and the mean number of caterpillars per plant are given in Table 1.

 

 

A feature of the data in Table 1 is the relatively high number of caterpillars recorded on the ungrazed and unslashed Pentzia plants. Most of the leaf material of these plants was eaten by caterpillars. The percentage leaves of the ungrazed and unslashed Pentzia plants was less than 1 % of the total dry mass of these plants as compared to a leaf percentage of more than 30% for the grazed and slashed plants (Table 1). These findings together with the highly significant correlation (r = 0,93) between caterpillar numbers and the size (total standing phytomass) of the Pentzia plants strongly suggests that the moths lay most of their eggs in areas with the largest Pentzia plants.

Because of the relative frailty of the moths, it is also possible that bigger plants (spared veld) would provide better shelter for them against wind and rain, and therefore more eggs could be laid on these plants. It is clear from the data (Table 1) that the relatively smaller and grazed plants and the short young regrowth of the slashed plants were generally less utilised than the bigger plants.

Since the density (numbers) of Pentzia plants did not differ very much between most of the study areas, it would appear that the size of the host plant is at least as important as the density of the host plants (Mohr & Pretorius, 1984) in determining the distribution and intensity of infestation of the Karoo caterpillar.

In conclusion it can be said that the observations of this preliminary trial substantiate the farmers' general observations that the density of the Karoo caterpillar and damage caused to the host plant are generally higher in spared veld than on well-grazed veld. These findings therefore suggest that the treatment and condition of Karoo veld before and during the egg laying period play an important role in the presence and population dynamics of the Karoo caterpillar. A reduction in the density and/or size of the host plants through the application of either long-term veld management practices or drastic short-term veld utilisation treatments hold some promise to at least relieve this problem.

 

ACKNOWLEDGEMENTS

The author wishes to express his sincere thanks to Mr M. W. Pretorius (Plant Protection Research Institute, Middelburg, Cape) for useful suggestions offered on the manuscript.

 

REFERENCES

LOUNSBURY, C.P., 1897. Report of the Government Entomologist for the year 1896. Cape of Good Hope. Department of Agriculture. Govt. Printer. Cape Town.

MöHR, J.D., 1982. The Karoo caterpillar Loxostege frustalis Zeller in relation to its host plants and natural enemies. Ph.D. thesis, Univ. Rhodes, Grahamstown.

MöHR, J.D. & PRETORIUS, M.W., 1984. Controlling the Karoo caterpillar, a pest of Karoo veld. Wool Production C.1.13/1984. Fmg S. Afr. Govt. Printer, Pretoria.

 

Published

Karoo Agric 3 (8), 17-18