ARBORICULTURE 



ARBORICULTURE 



365 



3ii. Lepidopterous caterpillar. ( X ^4) 



they feed, based on the fact that one series feed by 

 chewing and consuming the tissue of the trees, while the 

 members of the other class derive their nourishment 

 from the juices or sap of the trees which they suck out 

 by means of needle-Uke sucking mouthpaits. Most 

 insects pass through three, or at least two, preparatory 

 stages, very different from the adult, before they reach 

 maturity. They hatch from eggs deposited by the adult 

 female, as larvae (grubs, caterpillars, maggots, and so on) 

 and in this larval stage feed till they attain their full 

 size. The larval stage is, therefore, the most destructive 

 one. ^^'hen full grown, the larva usually changes to a 

 resting condition or pupal stage (chrysalis), and during 

 this time does not feed. Finally the adult appears and 

 the life-c}-cle is complete. Many insects undergo a single 



generation every j'ear, 

 but a number grow 

 more rapidly, produc- 

 ing several broods 

 each season, while a 

 few require two years 

 or more to mature. 

 Insects provided 

 with chewing mouthparts may feed on the leaves and 

 are then known as defoliators; or they may feed on 

 the bark, cambium, sapwood, heartwood, or roots, in 

 which case thev are known as borers. Caterpillars, 

 beetle grubs and saw-fly larvae are the most important 

 insects of the latter kind. 



Among the insects with piercing mouthparts, some 

 suck the juices of leaves, while others affect the twigs 

 and branches, or even the roots. Plant-lice and scale 

 insects are the most important insects of this kind. 



Methods of destroying tree-pests. 



There are five widely different methods of destroying 

 insects which injure trees, and the selection of an appro- 

 priate method must d^-pend primarily on the feeding- 

 habits of the species to be dealt with. 



(a) All defoliators, such as caterpillars (Fig. 333), saw- 

 fly larvae (Fig. 334), and those beetle larviE that feed 

 e.xtcrnally upon the leaves, are best killed by the appli- 

 cation of an arsenical poison (Figs. 3.3.5, 336). This is 

 appUed preferably by some sort of a spray-pump that 

 throws a fine mist or spray of water in which the arsen- 

 ical has been mixed. Such spraying machines may be 

 obtained in sizes to suit any needs, from hand-pumps 

 holding a quart of liquid to power sprayers equipped 

 for spraying extensive woodlands thoroughly and rap- 

 idly. The most satisfactory poison is arsenate of lead, 

 an insoluble arsenical which can be mixed with water 

 in the proportion of six to ten pounds to each 100 



gallons of water and 

 sprayed upon foliage 

 without danger of dam- 

 age to the leaves. It is 

 sold commercially as a 

 white paste and is easily 

 handled. This sub- 

 stance is far superior 

 to paris green, london purple, and the like. It should 

 be applied only in sufficient amount to show after 

 drying as whitish specks upon the surface of the leaves. 

 (I)j Plant-lice, since they feed by inserting their 

 beaks into the tissues of the leaves, and other soft parts, 

 are not affected by arsenicals and must be destroyed 

 by spraying with what is known as a contact insecticide, 

 one that kills through the application upon the surface 

 of the insect's body. The most generally used, and one 

 of the best contact insecticides is kerosene emulsion. 

 This is prepared by dis.solving half a pound of common 

 laundry soap in one gallon of hot water and then adding 

 two gallons of kerosene while stirring or churning the 

 mixture violently. The creamy mass thus formed 

 thickens on cooling and must be diluted with nine times 

 its volume of water before being sprayed upon the 



334. Sawfly caterpillar. {Natural size.) 



335. Injury by leaf-eating caterpillar of 

 small size. 



plants. Scale insects feed like plant-lice, by inserting 

 their delicate beaks into the tissue of the tree and suck- 

 ing out the sap, but usually occur on the thin bark of 

 the branches and twigs rather than on the leaves. Each 

 insect secretes a scale-like covering beneath which it 

 hvcs while growing, and, even when adult, the female 

 never leaves her position beneath the scale. As the 

 scaly covering is 



of a waxen nature, ^ ^rw^^^ 



these insects are 

 not so easily de- 

 stroyed by con- 

 tact insecticides 

 as are plant-lice, 

 although kerosene 

 emulsion is some- 

 times effective, 

 especially in the 

 case of young insects which have not yet secreted a 

 thick scale. Many older or more resistant scales can- 

 not be destroyed in this way and they may be killed 

 when the trees are in a dormant condition by a spray- 

 ing with lime-sulfur wash prepared as follows: Water, 

 forty gallons; fresh lime, twenty pounds; flowers of 

 sulfur, fourteen pounds. These are boiled together for 

 one hour and then applied as a spray to the branches 

 and twigs. This wash should never be sprayed upon 

 trees when in leaf. 



(c) Bark-beetles. — These are small insects that live 

 during the larval stages beneath the bark of the trunk 

 and branches of trees. The parent insects enter the 

 bark and excavate a small tubular gallery through 

 the cambium or iimer bark. Along the sides of this, 

 the eggs are laid and the developing larvae eat out 

 sinuous burrows through the cambium. They thus 

 interfere with the sap-flow and cause a general weak- 

 ing or even death of the tree. Bark-beetles are liable 

 to attack sickly or dying trees, and the only feasible 

 method of lessening their depredations is to remove and 

 burn such trees or branches as are affected, in addition 

 to improving in all possible ways the conditions under 

 which the tree is growing, bearing in mind that healthy 

 trees are much less hkely to be troubled by these 

 insects than are weak, poorly nourished ones. 



(d) Wood-borers, — The larvae of certain beetles and 

 moths subsist upon the woody tissues of trees, excava- 

 ting galleries through the wood of both living and dead 

 trees. Such larvae are usually whitish, often with brown 

 head and have powerful jaws, by means of which they 

 can cut through the solid wood. They may attack trunk, 

 branches, or twigs, some working in the sapwood, others 

 in pith, while a few feed mainly on the heartwood. In- 

 sects of this kind are the most difficult to combat as they 

 feed where they cannot 

 be reached by means 

 of insecticides duiing 

 the greater part of 

 their life. Due to their 

 concealed position, 

 they are not so readily 

 noticed and may fre- 

 quently cause irrepa- 

 rable damage to trees 

 before their presence 

 is recognized. Besides 

 cutting out the indi- 

 vidual larvae or de- 

 .stroying them in their 

 burrows by means of 

 a piece of wire, no 

 general method of destruction can be recommended. 

 It is important, however, to maintain the trees in as 

 healthy condition as possible, and to remove all dead 

 or dying timber in order to reduce the number of 

 breeding-places for the insects. 



(e) Leaf-miners. — To this class belong some of the 



336. Injury by leaf-eating cater- 

 pillar of large size. 



