HARDWOOD RECORD 



27 



The oak pruner shoulci not be confounded with the sawfly, which 

 is likewise a pruner. It severs limbs, after depositing eggs in them ; 

 but the work is differently done. The twig is gnawed off from the 

 outside, not from within as when the oak pruner does it. The pur- 

 pose of the two insects is probably the same. Each is providing a 

 dead branch as a hatching place; bvit the sawfly cuts the branches off 

 for the benefit of its offspring, while the oak pruner does it for its 

 own benefit. 



Another common and very numerous class of insects attacks living 

 oak trees, and the injury is great in the aggregate. They infest 

 other trees also, but they seem to single out oak in preference to all 

 others. These are the insects which cause oak galls. It is scarcely 

 proper to say that the insects make the galls, for the excresences 

 grow; but there is no doubt that the insects cause them. The bark 

 and twig borers described in preceding paragraphs are beetles, when 

 fully matured, but the authors of the galls are of another kind. They 

 never develop into beetles, though most of them pass through the 

 intermediate larval stage, the same as beetles. Galls are the work of 

 flies, creatures with four clear wings. This is not strictly true in all 

 cases, for some galls are caused by other insects; but nearly all the 

 galls which infest oaks are the work of gall flies. These constitute 

 a very numerous family, more than 200 speeits being known. Sixty- 

 three different species of these pernicious flies are known to work on 

 oak. They cause the characteristic growths on leave.s and twigs com- 

 monly called galls. These are known to all persons who are in the 

 least acquainted with oak trees. Some oaks may be wholly free from 

 them, while others are so overloaded that the branches appear de- 

 formed. A single oak has been known to have 20,000 galls on its 

 twigs at one time. 

 Some people suppose that galls never appear on healthy oaks, but 

 confine their attacks to stunted and diseased trees. There is danger 

 of mistaking the cause for the effect in reaching this conclusion. 

 The trees are stunted because the galls are present, and the galls are 

 not there because the trees are in poor health. There is no evidence 

 that gall flies go about searching for diseased trees ou which to make 

 their attacks, but they attack whatever they find that suits their 

 purpose, and in course of time the trees most infesteil become the 

 stunted specimens. The mistaken notion that galls occur only on 

 poor trees, which are of little account anyway, is responsible for the 

 totally inadequate estimate of the damage actually done by the pests. 

 Were it not for the jiresence of the galls many of the trees, now 

 regarded as stunted, would be valuable. Their vitality has been 

 sapped in supplying material to produce the excrescences, and good, 

 healthy wood cannot be built up. 



Gall flies belong to what is known as the hymenoptera order of 

 insects — the order to which wasps and ants belong. They are in the 

 cynips family, and are usually very small. The largest number in the 

 family of 200 species is not much bigger than a robust buft'alo gnat, 

 and the smallest is hardly visible to the naked eye. but when they 

 make a systematic onslaught upon the most vigorous oak they can 

 undermine its strength, and ultimately ruin the tree. 



Some attack the leaf, others the leaf steam, and some single out 

 the tender and succulent twigs as places for assault. The "first step 

 in the operation is to thrust the sting into the tender bark of the twig 

 or leaf, and (according to some authorities) place a drop of poison 

 in the bottom of the wound, where the egg is also deposited. The 

 egg hatches in two or three weeks and becomes a tiny grub. Already 

 the irritation at the bottom of the wound, due to the poison or 

 possibly to the presence of the egg, has upset the healthy functions 

 of the tissues at that point, and the sap accumulates there, forming a 

 liquid bath in which the grub floats, and from which it obtains its 

 food. The gall has begun to grow. The tree is trying to heal the 

 wound, and the swelling, which ultimately becomes the gall, is the 

 result. 



If the gall is on the leaf or the leaf stem it ends when the leaf 

 falls in autumn, and the grub in its center hatches into a fly and 

 the fly gnaws its way out and wings its flight. It may not come out 

 as soon as the leaf falls, but may use the gall as a shelter for 

 months before leaving it. The gall on a twig does not need to com- 

 plete its work in one season, though it usually does. It may hang 



for years, but after the first season it is usually dead and dry, and 

 often the twig is dead, too, because the sap which should have nour- 

 ished it was consumed in making the gall and in feeding the insect 

 within, 



A gall may have a single iuhabitant, or it may have a score of the 

 same kind. It depends on the species. One kind of fly may lay a 

 single egg at a point; and a fly of a different species may plant 

 them in clutches liks eggs in a hen's nest; and the resulting gall will 

 enclose a single grub or a brood of them. It is not unusual for 

 insects to occupy galls which they had no part in making. They are 

 simply intniders. The gall provides a good shelter and they crawl in 

 and make themselvesat home. For that reason, the finding of insects 

 in a gall is no proof that they had anything to do with making it. 

 In one instance, ^a single oak gall contained a mixed congregation of 

 forty-one individuals, five different species, consisting of one fly, one 

 beetle, five moths, two cynips, and one neuropteron. 



The practical question from the wood-user's standpoint is not the 

 number of insects that make or occupy galls, but the effect on the 

 trees. In the aggregate, this damage is great, but there is no means 

 of measuring just how great it is. 



There is another group of insect enemies of the oak. Thev are 

 the borers which penetrate the trunk while it stands in the forest. 

 The injury from that cause is well known, because the work is appar- 

 ent wherever it exists. It is not like the indirect damage due to 

 bark borers, twig pmners, and gall flies, the work of which cannot 

 always be seen. 



Insects which tunnel into the living wood are many, but one well- 

 known species may be taken as a representative of all. It is the 

 larva of the carpenter moth (PrioJioxi/sUis robince), sometimes known 

 as the locust borer, though it is not the insect which for some years 

 has been devouring locust trees in many parts of this country. It is 

 not a beetle, but is a moth; and in the Middle West it is most fre- 

 quently seen flitting about electric street lamps. The largest speci- 

 mens of the moth have a spread of wings from twq to three inches. 

 The moth itself is a characterless creature. It eats nothing from the 

 day it emerges from its cocoon until it dies of starvation at the end 

 of a mischievous career. The mischief which it does consists in pro- 

 viding for the nest generation, and the "next generation" do much 

 of their bad work before they turn into a moth. 



The moth lays 200 or 300 eggs in crevices in the bark of oak and 

 other trees. The eggs hatch and small caterpillars result. It is the 

 habit of most caterpillars to strike for the leaves of trees, but this 

 one turns its gimlet-like mouth inward and begins to bore straight for 

 the tree 's heart. It has been described as timber 's ' ' most cruel and 

 formidable ' ' enemy in the insect world. A noted entomologist de- 

 clared that if one of these caterpillars finds lodgment in the trunk of 

 an oak, the tree is doomed. That is putting it pretty strongly. Of 

 course, the tree will die some time, but many a tree survives long 

 periods of years until it ultimately dies, apparently, from other causes. 

 Doubtless the majority of trees so attacked recover, and live out their 

 allotted years: but it is equally certain that the tnmks of many are 

 ruined. This is particularly true in the South, though the carpenter 

 moth prosecutes its work from Maine to California. 



The caterpillar is small when it begins to gnaw into the wood, 

 and it makes a small hole. The worm grows as it penetrates the 

 wood, and it seems to know that it will want to come out some time, 

 and it goes back and forth at intervals, gnawing the hole larger, so 

 that when it wants to come out it can do so. The caterpillar attains 

 full size about the third year, when it may be three inches long. Its 

 tunnels through the wood are as large as half-inch auger holes, and 

 extend froin the outside to the heart of the tree. The creature spends 

 about three years at its destructive work, and apparently works all the 

 time, except when the weather is too cold. It ruins the inside of the 

 tree, not only by the holes it makes in the wood, but indii'ectly by 

 decay. Water runs in the holes, causing rot to set in. The worm has 

 no use for rotten wood, and it always picks out parts where decay has 

 not yet appeared. The holes are often lined with a smooth, plaster- 

 like substance, and this helps the lumberman to identify the work of 

 this caterpillar, 



.\t the end of three years the caterpillar's race is run. It is then 



