Last update: August 18, 2011 04:22:50 PM E-mail Print

 

Karoo caterpillar - some of your questions answered

Where did the Karoo caterpillar originate from?

 

MW Pretorius

 

THE Karoo caterpillar is indigenous to South Africa and is widespread. However, the caterpillars assume pest status only in the Karroid areas.

 

Why is the Karoo caterpillar a pest of economic importance only in the Karoo?

TO answer this question meaningfully, it is necessary to take a brief glimpse into the past.

The well-known botanist P H Acocks states in his publication "Veld types of South Africa" that Karoo veld has undergone tremendous changes over the last 150 years. There has been a gradual increase in numbers of many species of Karoo bushes, such as those already mentioned, resulting in the present composition of Karoo veld. These Karoo bushes serve as fodder for stock and are also host plants of the Karoo caterpillar. The caterpillars feed on the leaves of various species of Karoo bushes, especially Pentzia Thunb., which are the dominant plants over large areas of the present Karoo.

It can be accepted that man, as a result of his agricultural practices contributes to the origin of some pests. Many insects have an exceptional ability to adapt to changes in environmental conditions. In the case of the Karoo caterpillar the increase in the numbers of Karoo bush species benefited the insect tremendously, since there is an abundance of food sometimes suddenly after droughts.

 

When was the Karoo caterpillar first perceived as a pest?

THE first Departmental investigation into caterpillar outbreaks took place at the request of the Aberdeen farming community during 1896, when the pioneer entomologist, C P Lounsbury, addressed the "Zwart Ruggens Boeren Vereniging" in this regard. It has been established that this Farmer's Association existed in the southern parts of the Aberdeen district at the time. Therefore, the Karoo caterpillar was already troublesome almost a century ago.

 

Does the Karoo caterpillar have any natural enemies?

RESEARCH showed that the caterpillar has various indigenous natural enemies, of which 2 species of parasitic wasps and a fungal disease are widespread and abundant in the Karoo. Collectively these natural enemies exert a powerful controlling influence on Karoo caterpillar populations.

Parasites attack the eggs or larvae and kill the caterpillars after the cocoons have been constructed in the soil beneath the host plants. Unfortunately the caterpillars have already done the damage when the natural enemies destroy them.

 

What then is the use of natural enemies if they kill the caterpillars when it is too late?

They lower subsequent populations. This is one of the reasons why severe outbreaks are followed by lighter ones.

 

Can the indigenous natural enemies not be supplemented with parasites from other parts of the world?

THE importation of natural enemies has in the past often been mooted as the obvious solution to the problem.

In fact, large numbers of natural enemies have been imported and released in attempts to establish them. However, research has shown that the influence of the indigenous enemies is so powerful that additional natural enemies, even if they were established, would not contribute to control of the pest.

 

If the indigenous enemies "control" the pest so effectively, why do serious outbreaks still occur?

THE reason for this is apparent when the interaction between natural enemies and host plants, and its influence on the extent of Karoo caterpillar populations, is considered.

Although the natural enemies exert a powerful controlling influence on the Karoo caterpillar, they are unable to compensate for the vast food supply which is available – sometimes quite suddenly after droughts - to the pest populations. The denser the stands of host plants, the more caterpillars can feed and the more moths are produced. It has been calculated that certain areas can yield more than 100 000 moths per hectare. To compensate for this unlimited food supply, the natural enemies must cause mortalities in excess of 97 %, which is an unrealistically high rate of mortality to expect from biological control agents, imported or otherwise.

 

Besides natural enemies, are there any other natural control mechanisms?

MOST definitely. Sometimes the veld becomes overpopulated with caterpillars, that is, the caterpillars are so abundant that the food supply is insufficient. This leads to very high mortalities through starvation and even local extinctions. This is probably the most powerful natural control that occurs in Karoo caterpillar populations.

 

Despite the powerful influence of natural enemies and periodical food shortages resulting from overpopulation, the pest is not constantly kept at a low level. What is the reason for this unpredictability?

THIS is due to the unpredictability of rainfall in the Karoo. During droughts there is little or no food for the caterpillars. After rains, as already mentioned, an unlimited food supply is quite suddenly available and the caterpillars “escape" to assume pest status. Remember, one caterpillar can yield only one moth, but one female moth can lay up to 200 eggs.

 

Why can the Karoo caterpillar not be controlled chemically?

PASTURE scientists and entomologists agree that spraying would be totally impracticable, as vast areas have to be sprayed in less than 7 days to prevent damage by the caterpillars. Moreover, the high costs of insecticide applications, considering the relatively low yield per area unit In the Karoo, would be prohibitive.

 

If the Karoo caterpillar cannot be controlled by any of these methods, can any other methods of control be employed?

ALLEVIATION of the problem must perhaps be sought in the application of modified farming practices. A Working Committee, consisting of agriculturalists of the Karoo Region and the Plant Protection Research Institute, as well as farmers appointed by the NWGA, ECA U and NCA U, has been formed to formulate strategies to circumvent or alleviate the problem.

It is an unfortunate fact that there are no obvious economical and practical control measures for this problem however, the application of certain management practices could hold some promise to circumvent or at least alleviate the Karoo caterpillar problem

The developing of grazing practices that take Karoo caterpillar outbreaks into account, and the utilisation of drought fodder crops are only two of several aspects receiving urgent attention.

According to the Working Committee, various strategies followed by farmers, based on observations and experience gained during previous outbreaks, appear to hold possibilities for the circumvention or alleviation of the problem.

These measures albeit not empirically confirmed, include the following:

 

Make use of various types and kinds of animals with different grazing behaviour and dietary selection, e.g. cattle/woolled sheep, goats or cattle/mutton sheep combinations.

Continuous scrutinising of the veld will be necessary after late summer or early autumn rains, especially after exceptionally dry periods. Scouting should preferably take place at weekly intervals to look for Karoo caterpillar eggs (very small yellow eggs) on the plants. The presence of eggs is a relatively sure indication of a threatening caterpillar outbreak. The following guidelines are then suggested by the Working Committee:

 

The Department of Agriculture and Water Supply accept that the above-mentioned guidelines would not be applicable to all situations, but they are proposed as practical possibilities that can also be monitored in practice.

 

 

Published

Karoo Regional Newsletter 1