August, 1909.] 



141 



Plant Sanitation. 



the position in our methods of eradicat- 

 ing pests that their services deserve, and 

 will not be extolled as a cure for every ill 

 connected with horticulture. 



Any observant person can follow the 

 rise of insect life in its native land under 

 natural surroundings. For example : A 

 tree becomes so badly infested with scale 

 insects that they form a regular incrust- 

 ation on the stem and twigs, then, as 

 with one of our commonest eucalptus 

 scale insects, Eriococcus coriaceous, we 

 soon find a host of hungry insects taking 

 it in hand. Several of the black lady- 

 bird beetles (Rhizobius) devour the scale 

 both in the beetle and larval state ; then, 

 too, the eggs of the pretty little moth 

 Thalpocharis coccophaga are placed 

 among the scale by the moth, and the 

 resultant larvae not only devour the 

 scale but use up portions of their outer 

 skin to construct the stout cocoons under 

 which they are well protected. Other 

 parasites also are attracted by the 

 abundant food supply, until the twigs 

 are only frigned with ragged bits of 

 scales, and before the season is over the 

 tree is apparently clean and quite 

 recovers its former vitality. If, however, 

 you visit the same bush the following 

 season you will be almost sure to find it 

 more or less infested with the same 

 scale from the survivors of the last year ; 

 it was only the superabundance of scale 

 that had been destroyed ; and the in- 

 festation will increase until it again 

 attracted insects looking for food, which 

 again breed np. 



It is a question of cause and effect 5 

 the pest must appear before the parasite, 

 or there is no food, and in the forest and 

 uncultivated land this works out its own 

 salvation; but under the different con- 

 ditions of the growth of cultivated plants 

 and trees we cannot afford to wait until 

 they are badly infested. 



And another important factor in the 

 sole control of pests with useful insects 

 is that the latter cannot eat up all their 

 food supply, or else they in turn would 

 die out, while if they leave even a small 

 percentage in the orchard (in particular) 

 their value is very much discounted. 



Changes of climate make an immense 

 difference to insects ; and thousands of 

 parasites have been forwarded from 

 temperate climates to semi-tropical coun- 

 tries, with the result that, though 

 surrounded with food when liberated, 

 they have wandered away and died. 

 This was the case with large quantities 

 of ladybird beetles that we sent to India 

 and Ceylon some years ago ; there was 

 plenty of food for them, but they never 

 became acclimatised, so the experiment 

 was dropped, and other means were 

 taken by the tea and coffee planters. 



It has also been the habit to credit the 

 introduced insect with all the dead scale 

 upon the infested plant, whereas we 

 always find on a badly infested tree a 

 large percentage of dead scales that have 

 died or remained undeveloped from many 

 other causes. The native useful iusects, 

 perhaps more numerous than the in- 

 troduced ones, and often quite as active, 

 are ignored in the glowing and usually 

 exaggerated accounts given ,r by the 

 parasite introducer. 



There is another great factor in 

 the increase of insect pests that under 

 their natural surroundings were per- 

 fectly harmless to cultivated plants and 

 crops ; we cut down and burn up the 

 forests and plough up the grass lands, 

 and thus destroy the food supplies of 

 the insects that existed there. Many of 

 the more delicate perish, while the more 

 robust, or those that are fortunate 

 enough to find plants alied to those des- 

 troyed in the newly-planted trees or field 

 crops suitable for food, turn their at- 

 tention to the cultivated things, and 

 adapting themselves to the altered con- 

 ditions and with a bountiful supply of 

 food, they often increase to such swarms 

 as to prove the very worst kind of pest. 



The question is often asked "Why 

 do the grasshopper, locust, and cut- 

 worm plagues only occur every few 

 years in a very acute state, though we 

 always have a few about?" There are 

 several reasons : — First, climatic condi- 

 tions, such as a very dry or very wet 

 season, check or increase the develop- 

 ment of the eggs ; next, we find that 

 many of these recurrent plagues gradu- 

 ally increase in intensity for several 

 years until they have reached their 

 limit ; then parasites increase in pro- 

 poration, or fungus diseases, which are 

 spread by the immense number of 

 insects contaminating the feeding 

 grounds, ill them off in millions. 



Before we can go into the question of 

 pests and parasites, it is only reasonable 

 that we should first know something 

 about the habits and life histories of 

 the insects of both pests and parasites 

 before we attempt to alter the balance 

 of nature, and set " bug to fight bug" 

 — a pupolar saying in the United States. 

 Yet we are often told, in the news- 

 papers and elsewhere, that it is not 

 necessary to be an entomologist to un- 

 dertake the collection and introduction 

 of foreign parasites ; that it is a dis- 

 advantage, in fact, for one may be too 

 good an entomologist to be a practical 

 man. The danger of a practical man 

 who is not a naturalist introducing nox- 

 ious insects or the parasites of useful 

 insects is very much greater than if the 



