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An Enemy of the Hickory 



Editor's Note 

 The samples of hickory referred to in the aocomi.aD.vjns; article were cut in Marvland and were sent to Hahd- 

 WOOD Recobd with a request for information as to the cause of the injury and the best means of treatment. 



The abnormal growths on the branches of the samples of pignut 

 hickory are caused by the attacks of a very small fly which seems to 

 be a cynipidean insect of the order hymenoptera. As far as could 

 be discovered by a somewhat minute examination under the glass, 

 not a single insect, dead or alive, remaips in the sphaeroblasts, or 

 hard galls, on the branches, and it was not easy to determine the 

 exact species that did the injury. The old saying, "by their fruits 

 ye shall know them," has to apply in this ease, in the absence of 

 the creatures themselves. There is little doubt that a gall fly was 

 responsible for the work. This insect belongs 

 to the same order as wasps and ants. It has 

 four bright, thin wings. There are about 

 7,500 species of hymenoptera in this country, 

 from which fact it is not to be wondered at 

 that, in this case, the exact species is hard to 

 determine, particularly since the guilty little 

 fellow was out and gone probably two years 

 ago, at least, and only his dry, fungus-eaten 

 house remains to give a hint of his identity. 



A good many interesting facts, or supposed 

 facts, may be given regarding the cynips re- 

 sponsible for the excrescences in question. In 

 the first place, the enormous disproportion 

 between the size of the insect and its house is 

 astonishing. Though the creature itself was 

 not seen, the hole in the sphacroblast through 

 which it erterged was carefully measured un- 

 der the microscope, and it was found that the 

 fly must have been of such smallness that 

 more than 200,000 could occupy a single cuVjic 

 inch. That was a small thing to raise such a 

 wart on a hickory limb, and it is an inter- 

 estiug story how it does it. The little wasp- 

 like creature thrusts its sting tlirougli the 

 outer bark of the twig and penetrates the 

 cambium layer, or inner bark, and there de- 

 posits an egg. In a few weeks it hatches and 

 becomes a minute grub. Up to that time 

 nothing in particular happens to the twig, and 

 it would be next to impossible to detect any 

 wound or injury, because the hole made by 

 the sting is so small that a microscope is 

 needed to reveal it. 



When the egg hatches, it is not long before 

 something begins to happen to the hickory. 

 The larva, or grub, begins to absorb the juice 

 from the cambium layer. It has a ravenous 

 appetite, but it has no mouth, consequently it 

 cannot rat common food. It must have thin 

 juice, and it absorbs it through its skin. It 

 must have a perpetual and bounteous juice 



bath all the time, and it is the grub's business to take measures to get 

 it. It does it by irritating the tissues of the inner bark. It may 

 do this in two ways; one by exuding from the pores of its skin a 

 poisonous substance, or it may irritate by simply wriggling and 

 flouncing about in its juice bath. The so-called "jumping bean of 

 Mexico" is a well-known example of the activities of larva while 

 going about its ordinary business. It can be readily understood that 

 a grub with sufficient strength and activity to cause a "bean" to 

 liop all over a table top would set up considerable irritation if con- 

 fined in the soft tissues under a tree's bark — particularly if supple- 

 mented by a blistering poison. 



The vital forces of the tree rally to throw off or isolate the irri- 

 tating influence. The grub will not he thrown off and the tree 

 isolates it by forming the excrescence. ,. 

 —24— 



ATTACK OX PlGXt'T HICKORY BY CYNIFS 



By that time the grub has its growth and has no further need of a 

 juice bath. 



The larva may lie, apparently asleep, for months before it hatches 

 and decides to bore its way out to try the realities of the wide world. 

 It is believed that some remain two years before they try to escape. 

 They seem to know something about the world which they have never 

 seen, for it is said they will not emerge at any time of year except 

 in spring while plants and leaves are tender and juicy, and are just 

 right for sucking and stinging. How they can tell when to come out, 

 no man knows. Perhaps it is the same kind 

 of knowledge which prompts the groundhog to 

 come out of his hole on "groundhog day." 



So much for the growth and flight of cynips. 

 The vacant house left behind is an interesting 

 object. One of the hickory excrescences was 

 broken open and examined both with and with- 

 out a microscope. When no glass is used it is 

 found to be a hard mass of bark and wood — 

 mostly bark. Detached nodules of wood are 

 scattered through the mass. Irregular cavi- 

 ties, some about the size of a small bean, are 

 found here and there, arid a tunnel about as 

 large as a cambric needle leads from the cham- 

 ber to the outer world. This is evidently the 

 passageway bored by the insect when it wants 

 to escape. The outer opening of this passage- 

 way, if everything is favorable, may barely 

 be discerned by the naked eye, but under a 

 magnifying glass it is seen to be as round as 

 a gun barrel. In fact it suggests a miniature 

 crater leading from a subterranean volcano. 

 In the bottom of each chamber lies a small, 

 white mass, twice as large as a pinhcad. It 

 looks like mold, but wlieu highly magnified it 

 is found to be a web, woven in sheets resem- 

 bling tennis nets, and probably used by the 

 larva as blankets in which to wrap its body 

 when it takes its long sleep. Some of the web 

 is not made into sheets, but lies in long 

 strands, resembling flax tow, or frazzled silk. 

 The question may naturally be asked 

 whether injury to the tree is likely to result 

 from attack by cynips of that kind. The tree 

 from which the sjiecimens were taken was un- 

 doubtedly injured. The excrescences killed 

 the branches beyond the points of attack, 

 though they do not do so if the excrescences 

 remain small. The cambium layer is destroyed 

 — it seems to dry up — beneath the growth. 

 Probably all its juice goes into the excres- 

 cence and is there drunk by the larva or dis- 

 sipated through the abnormal tissue. A cross section through one of 

 the sphaeroblasts — which seemed to be a typical one — revealed the 

 branch's struggle to overcome the injury. For two years, and prob- 

 ably longer, growth continued on one side of the twig after the other 

 side was dead, but the whole circumference finally yielded and the 

 branch died. In every one of the specimens examined, the outer end 

 of the limb was dead. If the tree is attacked in numerous places, 

 death to the whole trunk will doubtless follow. 



Such an attack might be successfully combatted, but it is doubtful. 

 In the case of the hickory from which the samples were cut, the attack 

 was evidently made by a swarm of insects. Ordinarily, such attack 

 would not be discovered until the excrescences begin to enlarge. The 

 larvae are by that time hatched and are entirely concealed beneath 

 the bark and it is difficult to see how aHV kind of spraying could 



