Last update: April 5, 2012 08:58:47 AM E-mail Print

 

HARMFUL INSECTS ON SPINELESS CACTUS

H van Ark

 

IN the Karoo, where drought conditions are very frequent, extensive use is made of drought-resistant fodder crops such as spineless cactus, saltbush and agave. Up to now few insect pests have been found on the last-mentioned two.

In the case of spineless cactus, however, the position is different. Here we find two insect species, which are capable of causing serious damage, namely, the cochineal and the prickly pear moth.

The prickly pear moth (Cactoblastis cactorum) is probably the more harmful of the two insects. Like the cochineal, this species was originally imported from overseas to control prickly pears, which are especially prevalent in the coastal regions. Unfortunately both species also started attacking spineless cactus and thus became noxious pests.

 

PRICKLY PEAR MOTH

The prickly pear moth lays her eggs (about 100 in number), one on top of the other, so that small white thorn-like "sticks" are formed on the leaves. The egg laying period extends from September to October and, for the second generation, from February to March. Even in summer the eggs require a very long time to hatch, namely five to six weeks under favourable conditions. The small larvae, which subsequently emerge, penetrate into the leaves as quickly as possible. Here an almost complete destruction of the leaves takes place, with the result that most of the leaves are completely destroyed and subsequently drop from the trees. After eight to nine weeks of intensive damage the reddish yellow and black and white larvae leave the leaf and form a white cocoon under stones, fallen leaves, etc. Under favourable conditions this pupal stage lasts from 3½ to 4 weeks, after which the adult moths, which are of a light grey colour, appear once more in order to build up the population still further and to spread the infestation.

The appearance of larvae in the leaves in winter is owing to the fact that the second generation of larvae escapes the unfavourable conditions by wintering in the leaves.

Sometimes large numbers of larvae and pupae die of a certain disease (Nasema sp.). However, this disease does not succeed in keeping the pest in check, although it can wreak havoc among larvae, particularly when the infestation is severe.

The removal or destruction of the small egg-covered sticks and affected leaves entails a considerable amount of labour and trouble, but is, nevertheless, the only efficient method of control. As the larvae are found in the leaves, spraying with contact poisons will not give results.

 

THE COCHINEAL

The other pest, which may also assume serious proportions in some cases, is the cochineal (Dactylopius sp.). This insect is closely related to the common mealy bug and also has the characteristic wax layer. The insect itself lives under this wax layer and a deep red colour is observed when this layer is crushed.

The female's eggs remain in her body and hatch out there, with the result that live young are born. A few days after birth the larvae which are then in the crawling stage, usually move to the eyes of the prickly pear leaves or to other small cracks and push their mandibles or mouth parts into the leaves. The female scale usually feeds in the same spot for the rest of her life. After a few weeks have elapsed the larva of the male spins a white cocoon.

The adult male cochineal which emerges from this cocoon has wings and two long white protrusions on the hind part of his body. These males are incapable of ingesting food, as their mouthparts are undeveloped.

The cochineal populations grow at a phenomenal rate during the favourable conditions in summer, and cactus leaves may be entirely covered by these insects. Such leaves turn a lighter green to yellow and frequently drop from the plants.

Wind, currents of air, animals and birds are responsible for spreading the young larvae from one plant to another.

Today numerous plantations are already infested with this pest and the damage may assume serious proportions.

This pest can be controlled by planting only resistant varieties of spineless cactus, such as Monterey, Nudusa, Robusta, and Chico.

Monterey is the variety most resistant to cochineal, but unfortunately stock does not like it as much as the other varieties.

Chico is also regarded as being resistant, but cases have occurred where cochineal was found in fairly large numbers on this variety.

At present the following control measures are recommended for cochineal:

 

Spray for a second time after three to four weeks, if necessary.

This spraying should be carried out under a pressure of at least 80 lb per square inch, otherwise the poison will not penetrate the wax layer of the cochineal and the spraying will be useless.

Parathion is very poisonous to humans and animals, and for this reason protective clothing, gas masks, gloves, etc., should be worn when handling the poison. Always remember the golden rule: One can never be too careful when it comes to handling poisons.

Parathion has a fairly prolonged residual effect. To prevent poisoning of animals, they must be kept away from treated plants for at least three weeks.

Black as well as spotted ladybirds sometimes feed on cochineal. If these beetles are present, chemical control should preferably not be applied, so that these beetles can have an opportunity to multiply and in this way help control the pest.

 

Published

Farming in South Africa 38 (10)