INSECT INVESTIGATIONS.

The investigations to determine what part, if any, insects take in the transmission of the chestnut blight have been continued under the immediate direction of Prof. A. G. Ruggles. A number of interesting facts have been determined, but several important studies were just well under way when the work was suspended.

The relation of insects to blight dissemination comes under three headings; first, insects that carry the spores of the fungus and actually start new infections at the time; second, insects that carry the spores but do not directly start infections; and third, insects that make wounds in which infection readily takes place through spores carried by some other agency.

INSECTS CAUSING DIRECT INFECTION.

To the present time very little definite data have been obtained on this point, but the longer the subject is studied, the more probable it appears that ordinary insects traveling over a tree, although they may carry hundreds of spores on their bodies, do not directly start new infections.

INSECTS CARRYING SPORES BUT CAUSING NO DIRECT INFECTION.

Ants were allowed to run over cankers showing pycnidial pustules or “spore horns,” and also cankers where ascospores were shooting, and then placed in flasks of sterile water and washed from two to twenty-four hours. Plate cultures made from this material showed in many instances the presence of blight spores on the bodies of the ants. In the same way it was determined that other insects to the number of about twenty species also carry the spores of chestnut blight. The number of spores carried in each instance varied from a very few to the enormous number of 336,900. The particular insect, (Leptostylus maculata), carrying the 336,900 spores mentioned, is one of the beetles named in a recent press notice of the U. S. Department of Agriculture, as being very active in eating spores of the blight fungus. Therefore this beetle while destroying spores of the blight is at the same time covering its body with thousands of other chestnut blight spores which it carries from tree to tree, making it probably an injurious insect, instead of a beneficial one in this respect.

INSECTS MAKING WOUNDS IN TREES THUS OPENING THE WAY FOR INFECTION.

This is probably the most serious way in which insects are related to blight dissemination. Among the most serious of wound making insects are the seventeen-year cicadas, tree-hoppers, bark borers, and bast miners. Of these only two have been studied closely,—the cicadas and the bast miners.

CICADA STINGS.

In 1911 there was a brood of seventeen-year cicadas in several counties in the eastern part of Pennsylvania. The relations that these stings bore to blight infection have been studied near Lehighton. Many counts were made on trees and sprouts. While only 4.3 per cent. to 10.4 per cent. of all stings were found to be infected with chestnut blight, from 86 per cent. to 93.8 per cent. of all infections were in stings. This cicada injury was studied where the blight seemed most abundant. In the same tract where blight was less prevalent, other counts were made with less striking results. These observations would seem to show that blight infection is influenced considerably by the number of wounds made, but that infection many times does not take place through a wound although seemingly appropriate openings for catching blight may be present.

THE BAST MINER.

The work of the bast miner was first called to our attention by Mr. S. B. Detwiler. It is believed to be the most important insect causing wounds in the chestnut. Experiments and studies up to the present time make it probable that the bast miner is responsible for much blight infection. To understand thoroughly the relationship of this insect to the blight fungus, the life history has to be known. Much time has been spent upon this subject, but unfortunately to date, the work has not been completed.⁠[8] The injurious period of its life history has been obtained, but the period that would have to do with its suppression, namely the adult period and time of egg laying, has not been discovered.

LARVAL EXIT HOLES AS POINTS OF INFECTION.

Hundreds of sticks of smooth bark trees of chestnut were examined during the past winter and spring to determine the number and nature of the larval exit holes of the bast miner. Every piece a foot long and over two inches in diameter had bast miner burrows present. The lowest number for a linear foot was one burrow while the highest was fifteen. The number of exit holes for a small tree, therefore, would vary from ten to one hundred and fifty. In one acre of chestnut trees the number of these exit holes would be enormous. In the light of what we now know, recent observations show that 50 per cent. of this class of infections originated in bast miner exit holes.⁠[8]

CROTCH INFECTIONS.

Many infections are known to start around crotches, and we speak of them as crotch infections. The eggs of the bast miner are laid near crotches and the newly hatched larvas may make entrance holes sufficiently large to allow spores of blight to enter. Here again the bast miner may be responsible, and if such proves to be a fact, this insect would be the indirect cause of 90 per cent. instead of 50 per cent. of the infection on smooth bark trees. All other insects mentioned as making wounds, with perhaps the exception of the tree hoppers, are local or else the number of wounds is not appreciable; but in the case of the bast miner, the insect is found wherever the chestnut grows.

EXPERIMENTS WITH ANTS.

Ants being found so commonly around blight cankers on chestnut trees, it has been claimed that in some instances they are responsible for as much as 90 per cent. of blight dissemination. To obtain information on this matter, it was decided during the winter to experiment with ants in the greenhouse. Two rooms were set off as an insectary. The inner of these two rooms being thoroughly sterilized, was called the sterile room, and the outer room was called the blighted room. In the latter as much blight material of the kind required as could be obtained was kept and placed on the ant table, where three colonies of ants made their homes. From the table in this room the ants were allowed to run through a glass tube to sterile seedling trees in the sterile room. The ants were of the same species as those suspected of carrying the blight, and were the common mound-builders, (Formica integra), being obtained in the region of Lewisburg, Union County.

The result of the experiment was that with the exception of a few dried leaves on each tree which were chewed or worked on by the ants, the trees in the sterile room are as healthy as when first placed on the table to be run over by the ants. The indication, therefore, is that ants are not responsible for blight infection.

INFECTION IN GALLS.

A more or less cylindrical gall is found on the tips of branches and on sprouts of chestnut, caused by an insect claimed to be a moth. At West Chester and Valley Forge, these galls are very numerous. Out of 161 galls examined by the plant pathologist, forty-five of the 28 per cent. showed the presence of blight, while 49 per cent. showed the presence of another fungus. A gall that shows the presence of chestnut blight in such a large percentage of cases should be given careful study.