The Prickly Pear Pest.
AbstractThis paper deals with the effect of the cochineal insect (Dactylopius coccus) upon the prickly pear. This plant is not indigenous to South Africa, but is becoming a serious problem owing to its abundance and disagreeable spines. A Commission appointed by the Queensland Government, which also has considered this plant a menace, found in Ceylon that prickly pear plants infested with cochineal insects became sickly. When in South Africa, this Commission gave into the charge of the Division of Entomology at Pretoria, for experimental purposes, a piece of stem of this plant infested with these insects. Experiments with these were carried out at the Natal Museum under charge of Dr. C. Akerman.
There are two species of prickly pear common in South Africa, a thin-stemmed long-spined species of a bright green colour, Opuntia monocantha, and a thick-stemmed short-spined species of a pale blue-green colour, O. tuna. In addition to these wild and useless species, certain spineless varieties of O. anacantha and O. monada are grown to a certain extent as cattle food for the dry season. The material for these experiments was received in June 1913, and only the first two species mentioned, were used as food-plants. O. tuna gave negative results, as the insects would not settle, and died after two days wandering; three other attempts to infest it meeting with the same result. On O. monocantha however they settled down and completed their life-history. There is no metamorphosis, and development is simply a matter of growth. Young individuals can crawl some distance-in one case as much as 10 feet-and on emerging from the envelope of the parent are of a dark brown colour, just visible to the naked eye. When full grown the insect may measure 0.25 inch in diameter. The young grow to maturity in about 4 or 5 weeks and usually settle down permanently after about two days. For about 8 weeks the cactus stem showed no other symptoms of ill-health than to become yellow in patches around the cochineal insects, but after about 9 weeks the plants began to shrivel quite suddenly, and dried up in a very short time. The collapse was striking and appeared to indicate a poisonous effect of the Coccid. Plants growing in the Museum garden were next tried, with the same results, though there the growth of the insect was slower than in the laboratory. In August 1913, vigourous plants in two different localities near Pietermaritzburg were infected; the insects took good hold on the plants and portions of these at the time the author wrote (January 1914) exhibited marked symptoms of ill-health. It is considered that these insects may be of use in keeping the prickly pear within bounds; and should they stand the winter and no enemy arise, might become so abundant as to exterminate O. monocantha. In such a case, in districts where prickly pears are grown for cattle, a variety which is spineless and immune to the cochineal insect would be necessary. Probably it is more important to exterminate the long-spined species than to continue to grow that variety for cattle-food, should an immune variety be unobtainable.