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GARDENERS' CHRONICLE 



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I A Lesson on Plant Parasites and Pests I 



I And some of the Principles underlying their Prevention and Control. | 



Being One of a Scnes of Lessons of a Home Study Cuiiisc un Gardtnini;, Apjieanng 



Under the Direction of ARTHUR SMITH 



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111 The Gakdexers' Chkonicle 



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BOTH in the animal and in the vegetable kingdoms, disease, 

 death and decay, are (leaving out of consideration the results 

 o£ physical force) manifestations of other forms uf life and 

 are generally brought about by what are known as parasites, 

 which are organisms having no existence except in connection 

 with a host, feeding entirely upon it, and which also in many 

 cases live and multiply upon some other organism. 



Some parasites are more or less essential to the health and 

 well-being of the host they inhabit, such as for example, in 

 the case of plants, the micro-organisms occupying the nodules 

 upon the roots of clover and other legumes; there are also 

 some parasites which are beneficial lo horticulture by reason 

 of the fact that they use as hosts other parasites which are 

 harmful, as the examples of the organism destroying the San 

 Jose scale, and the Ichneumon hies which lay their eggs in other 

 insect larvae, and the larvae resulting fiom such eggs, living 

 in the interior of their larval host, prevent tlie latter from 

 pupating and carrying on its existence. 



Harmful parasites are divided into three groups, insects, fungi 

 and bacteria, all being embraced under the term "pests," there 

 being also other pests lielonging to the insect class which are 

 not strictly parasites by reason of the fact that while they feed 

 upon plants, their life history is carried on separately. 



It is sometimes asked why "all these pests which attack crops 

 are so abundant," the implication being that in bygone days 

 no such unpleasantness had to be faced, or, if at all, only in a 

 minor degree. While it is undoubtedly true that there has been 

 an apparent increase in the number of enemies attacking orna- 

 mental plants and crops, yet there is no doubt that crops had 

 their enemies in ancient days ; this may be inferred by the 

 allusions to the canker worm and the palmer worm in Biblical 

 history, and in the writings of the ancient Greeks and Romans 

 frequent mention is made of smuts, mildews, rusts, etc. At any 

 rate we may be sure that pests e.x-isted before the era of culti- 

 vated crops. Nevertheless it is obvious that not only have the 

 number of enemies found in our gardens and farms increased, 

 but many kinds are now found in greater numbers. There may- 

 be several explanations of this. The most important is probably 

 the great increase in the amount of land under crops, and the 

 extensive and intensive crop-cultivation of modern times, with 

 the consequent increase in the number of hosts which each pest 

 prefers. Their food is therefore more abundant and more easily 

 accessible, consequently they are able to increase and multiply 

 with ease and rapidity. Practically every pest can be found 

 upon the \v]](\ congeners of its favorite cultivated host, and, 

 further, epidemics may be frequently observed among weeds 

 and wild flowers, to say nothing of wild trees and shrubs. In 

 this connection the clearing up of roadsides, fence rows and 

 many other waste places would do much to reduce parasitic 

 troubles by removing the breeding places of the parasites. 



Whatever the causes, the fact remains that pests have greatly 

 increased in virulence the past few decades, and that, were no 

 control measures put into force, a more or less extensive de- 

 struction of plants would ensue and crops would be reduced to 

 a minimum. 



Up to now, chemical methods, especially spraying, have been 

 the most efficient. They are doubtless the most direct in 

 action, and usually give the quickest results, especially in con- 

 nection with, insect pests. But one must rernember that the 

 whole question of pest control is really in its infancy : and 

 because spraying is the best means available at present, it by 

 no means follows that no other methods will not subsequently 

 prove to be of greater use. 



Already much interesting work has been done on the parasites 

 of economic insects, and the success which has been obtained 

 in this direction indicates other and possibly more far-reaching 

 means of control. .At present, however, spraying undoubtedly 

 holds the field and few would go so far as to prophesy that it 

 will ever be found possible to abandon it : it may be well there- 

 fore to consider the question from as broad a point of view 

 as possible. 



Speaking generally, those having a direct interest in plant 

 parasites may he divided into three classes: (1) Those who 

 seem to think that spraying is a necessary thing in itself. They 

 appear to regard it in much the same way as many people regard 

 patent medicines, wilh an eye of faith, and they with faith 

 spray without knowing whether their plants happen to be in- 

 flicted with the pests they spray against or not, and in this 

 way much blind and unprofitable spraying is done. (This re- 

 mark is disconnected with the necessity, to be enlarged upon 



later, of using spraying as an insurance against fungus diseases.) 

 (2) Those who know more or less about the life history of the 

 pests they desire to control, and who, among other things, re- 

 alize the difference between actually killing the pest itself and 

 preventing the possibility of its existence; and (3) those who 

 never trouble themselves about the matter at all. 



There are several essentials necessary for successful spraying, 

 two of the fundamental ones being, to know what parasite the 

 plant is sufifering from, and a knowledge of its life history. By 

 way of illustrating the importance of this knowledge, the life 

 history of one of tne common apple pests, Af'lits sorbi, or . the 

 rosy apple-aphis, may be briefly given. In the \\'inter it exists 

 on the apple tree as little black shining eggs, laid chiefly on 

 or near the leaf-scars of the fruit-spurs. Toward the end oi 

 -\pril, earlier or later according to climate, the eggs hatch and 

 purplish-green larvae come out which immediately begin to feed 

 upon the under side of the leaves. At first it does nothing but 

 teed, moult its skin and grow, but after a time it becomes fully 

 grown and is then purplish, mealy and rather globular in form, 

 and at this stage :s known as a queen-mother. After reaching 

 this condition it begins to "bud-oiif" little pinkish larvae which 

 immediately insert their probosces into the under side of the 

 leaves, causing extensive curling. This production of young 

 by the process known as budding takes place without the inter- 

 vention of a male. The btidded-ofif young, when fully grown, 

 repeat the process, and so it goes on through several genera- 

 tions. About the end of June, winged individuals appear, and 

 these no longer feed upon the apple leaves, but fly to another 

 host plant, so that about the end of July the apple tree is free 

 from this insect. In September, winged immigrants from the 

 ■ ecoiul host-plant fly liack to the apple and produce a generation 

 of winged males and wingless females. These females soon 

 uegiii to lay their egys on the apple twigs, and their life historj 

 is thus completed. 



In considering this history it will be apparent that from a 

 spraying point of view one may divide it into four periods : 

 (1) The egg stage; (2) The young larval stage, before there 

 IS much or any leaf-curling and before the queen-mothers have 

 commenced to bud-ofif young ; ( 3 ) The viparious stage, or when 

 active reproduction of perfect insects is going on and when the 

 leaves are curled ; (4) The Aulumn stage when males and females 

 are present but no leaf-curling is caused. 



Stages (Ij and (3) do not lend themselves to treatment. It 

 is not practically possible to destroy aphis' eggs b\' a wash, so 

 that spraying in Winter against this pest has so far failed. The 

 viparious stage is equally difficult, owing to the leaf-curl pre- 

 venting the spray from coming into contact with the insect. 

 This leaves the two other stages in which the aphis is com- 

 paratively vulnerable, but (4) presents the difficulty of exces- 

 sive foliage, and therefore it is plain that so far as this particular 

 pest is concerned, the best time to spray for it is undoubtedly 

 immediately after the eggs hatch out in the spring, at which time 

 we can attack it with the greatest prospect of success. .\ 

 knowledge of an insect's life history along these lines is thus 

 an essential for success in its control. 



In addition to the identification of the insect and a knowledge 

 of its life history, it is important to know how the insect feeds 

 so as to know the kind of spray-fluid to employ. All members 

 of the Aphis family and other sucking insects feed upon the 

 juices they suck from the interior of the leaf or plant-stem, 

 they therefore cannot be killed by a stomach poison applied 

 to the surface of the plant but must be destroyed with some- 

 thing which kills by actual contact with the insect. On the 

 other hand contact poisons, unless they are strong enough to 

 kill the plant are of no avail against a pest like the potato beetle, 

 wdiich has to be controlled by stomach poisons. 



Therefore insect pests naturally divide themselves into two 

 classes: Those which feed by sucking plant juices, and those 

 which feed by actually eating the leaf or other part of a plant. 

 This division brings about the necessity of having two kinds 

 of available insectides, contact washes and stomach poisons. 



The principle underlying the action of stomach poisons is 

 simple. The material is sprayed upon the plant, and any insect 

 feeding upon it absorbs a certain amount of poison and dies. 

 It is necessary for the poison to be insoluble in water, as some 

 soluble poisons are nearly always harmful to the plant, -■^t the 

 same time they must be of such a nature that they will be 

 acted upon by the insect's digestive juices and brought into a 

 soluble form. Today, lead arsenate is the great stand-by for 

 this purpose owing to its freedom from harming the plant: its 



