PARASITE HOSTS.

Waikato Times, Volume 104, Issue 17589, 19 December 1928, Page 12

PARASITE HOSTS.

HOW THEY ARE CONTROLLED. "The aggregation of man into concentrated communities," says the Lancet, "has produced most of the problems with which modern hygiene has been mainly concerned. In nature hosts and parasites have generally come to some sort of stable balance with 'live and let live' as their motto. Of this we should know more If we understood the parasitology of wild animals better, trypanosomes in game in Africa, and Gaertner's bacillus in rats at home are familiar examples. Similarly, when vegetable crops are grown over wideareas in more or less pure culture, special troubles with parasites generally arise. This is particularly apt to happen when the plant itself has been moved from its natural home; coffee, for example, and rubber and bananas and oranges are now grown all over the world where climatic conditions are appropriate. Along with this transportation of hosts there has been a dispersion of parasites, and out of the troubles that have corne from upsetting the balance natural to any country has arisen an extraordinarily interesting chapter of biology. "The parasites of a plant arc, it seems, normally controlled and kept within bounds by a second scries of parasites, and these again sometimes by a third. In the moth Pyrausta nublhlis, for instance, something of a pest to corn in Southern Europe, and a serious injury in the United. Stales since its introduction in 1910, IS parasites (mostly insects) have been found to make their living on the larva, one on the egg, and two on the chrysalis. If a parasite gains access to a new country without its own parasites it is obviously in a very advantageous position, of which it may make full and foul use, and similarly the multiplication of a host on an unprecedented scale may give parasite? a chance *o increase beyond the controlling powers of the secondary parasites. The plain remedy is to encourage, or introduce the parasites of the parasite, and in a number of instances this plan has been put into operation with signal success. "An Australian ladybird, for instance, has proved a perfect remedy for the cushion scale of oranges and lemons, in California, Portugal, South Africa, Egypt, and elsewhere. A chalcid wasp from South Africa similarly controls the black scale insect in California, and the Italian silk industry lias been 'saved from ruin by importing a parasite of the scale insect which destroys the mulberry trees. Plants which grow 100 profusely have in the same way been brought within bounds by importing insects which feed on the seeds. It is certain that African sleeping sickness will ultimately be controlled only by finding out how

the tsetse fly lives and moves and has its being; by discovering its parasites, and othe:' difficulties, and increasing them; and by ascertaining its favourite food and the conditions of life which it likes, and by destroying them."