Imágenes de páginas
PDF
EPUB

of the pest from Virginia last year, accompanied with requests for the disease, but from the weather conditions he felt warrauted in replying to these inquiries that the heavy rains then prevalent would probably bring about the death of the chinch bugs without artificial introduction of disease, and subsequent events substantiated this belief.

Mr. Kirkland read the following paper:

A NEW INSECTICIDE.

By A. H. KIRKLAND, Malden, Mass. To no phase of the work of exterminating the gypsy moth has more careful attention been given than to the investigation of insecticides that might be effectively used against this insect. The entomologist, the director, and the chemists connected with the work, together with the several assistants employed in experimentation, have all devoted to this question their best effort, while the committee supervising the work have placed all possible facilities at the disposal of those charged with the investigations with a liberality that has left little to be desired. Spraying with arsenicals being the chief method of combating injurious lepidopterous larvæ, the first efforts of the gypsy-moth commission were directed toward spraying infested trees, pursuant to the recommendations of both Professors Fernald and Riley. That Paris green could not be profitably used to destroy this insect seemed at first incredible, but many careful experiments demonstrated the fact. The subsequent chemical investigations by Mr. Moulton while employed by the gypsy-moth committee, resulting in the discovery of arsenate of lead, are familiar to all present. This insecticide, which has been used exclusively in our spraying operations during the past three years, possesses a feature which is an aid and a hindrance at one and the same time. From its insolubility it may be used in great strengths without injury to the most delicate foliage, yet at the same time its effect upon larvæ, owing to the difficulty with which it decomposes, is not as complete as might be desired. Numerous experiments show that great strengths of arsenate of lead when used against the larger gypsy-moth larvæ give results hardly commensurate with the outlay, although this poison is very effective when used against the smaller caterpillars at the rate of from 10 to 20 pounds to 150 gallons of water.

During the winter of 1895–96 I was enabled to spend a few weeks in the botanical laboratory of the Massachusetts Agricultural College, studying the effects of arsenical compounds upon vegetable tissues with a view to gaining, if possible, further light concerning the cause of the injury to foliage by soluble compounds of arsenic. It was found that these soluble forms of arsenic transfuse by osmosis with the cell contents and plasmolyze the protoplasm, thus producing the death of the cells. This plasmolysis was, as might be expected, in direct ratio to the solubility and quantity of the compound employed. The objective point of these investigations was to find out what cheap arsenical

[ocr errors]

compounds would not injure foliage, and, with this as a basis, later to test the effects of the poison upon the larvæ. The arsenates of the alkalies, from their extreme solubility, were found to be debarred from use as insecticides, but in the group of alkaline earths the behavior of arsenate of barium was found to present features that would warrant a thorough test of its merits when used against caterpillars.

Unfortunately, the same legislative policy of delay that bindered an early resumption of our field operations in the spring of the present year also handicapped to a serious extent our experimental work. We were unable to begin insecticide experiments until about june 1, at which time larvæ were in the second and third molts and the opportunity for experimentation upon the younger caterpillars had passed. However, it seemed fair to infer from past experiences that poisons effective in destroying the partly grown larvæ would be at least equally as effective upon the younger and less hardy caterpillars. Experiments made with barium arsenate upon foliage showed its burning point to be at about the rate of 20 pounds to 150 gallons water. The foliage tests were made upon oak and apple. On hornbeam the burning was more marked, yet not sufficient to cause any great injury. In experimenting with larvæ a definite number of the insects was confined by means of thin cloth bags upon foliage sprayed with a known proportion of the insecticide. In the case of larvæ in the second, third, and fourth molts feeding upon foliage sprayed with 3, 4, and 5 pounds to 100 gallons water, all were dead in five days. With the fifth-molt larvæ from 10 to 20 pounds were necessary to kill all the insects in from six to eight days, showing, as in insecticide experiments of past years, that the resistance to poison increased with the age of the caterpillars. Duplicate experiments, tried after larvæ had reached maturity, gave less satisfactory results, on account of the pupation of a considerable part of the insects involved. About 200 larvæ were used in these experiments, and where any fed for four or five days upon foliage treated with the poison at the rate indicated death ensued in nearly all cases.

From results of this season's experimental work it would appear that arsenate of barium is probably the best arsenical compound yet used against the gypsy moth. A limited number of experiments upon the larvæ of Hyphantria cunea and Datana ministra showed that 5 pounds to 150 gallons were sufficient to destroy the caterpillars in from four to eight days. We simply wish to bring this poison to the notice of the association, so that it may be tested further by those who may so desire, and thus its actual merits will be demonstrated. We feel that more experimentation is absolutely necessary, and particularly the extended use of the poison on a large scale, in actual field operations, before we can get at its true value.

Mr. F. J. Smith, chemist to the committee, informs me that this poison may be prepared by a process similar to that employed in the manufacture of arsenate of lead; i. e., by precipitating a soluble arsenate (arsenate of soda is probably the best from economic reasons) in an alkaline medium with a soluble barium salt. Barium exists in considerable quantities in nature as the carbonate (witherite). This mineral may be treated with hydrochloric acid, and soluble barium chloride results. When mixed with an alkaline solution of a soluble arsenate, barium arsenate is thrown down as a fine, flocculent, white precipitate, possessing plıysical qualities which render it easily suspended in water. When prepared in a wholesale manner, its cost should not greatly exceed that of arsenate of lead. In the reaction between arsenate of soda and barium chloride there is, of course, a by-product of sodium chloride (common salt) left in solution. This does not interfere with the effect of barium arsenate upon either larvæ or foliage, as shown by careful comparison of experiments with salt-free v. saltcontaining barium arsenate.

I am inclined to the belief that where arsenate of lead or arsenate of barium is used in spraying better results will be obtained by dissolv. ing the essential ingredients of either poison and adding them to the contents of the spraying tank, thus allowing the reaction to take place in the tank. In this manner the original precipitate is left in suspension in the tank contents, and will stay suspended longer than when dried and then remixed with water.

The discussion of this paper was deferred until the reading of a series of papers which had been arranged for during the afternoon session, covering the general subject of insecticides and practical operations with them against particular insects.

The session adjourned until 2 p. m.

AFTERNOON SESSION, AUGUST 21, 1896. The auditing committee, Messrs. Smith and Lintner, reported the accounts of the secretary to be correct. The report was accepted and the committee discharged.

The committee appointed to arrange a joint meeting with the Society for the Promotion of Agricultural Science reported that such joint meeting would be held at 4 o'clock in the rooms occupied by the latter society, and that papers by Messrs. Hopkins, Howard, and Duggar would be presented. This report was also accepted and adopted.

As arranged for at the morning session, the following three papers were read, so that these, with the one presented by Mr. Kirkland in the morning, could be discussed together.

Mr. Marlatt read the following paper:

COMPARATIVE TESTS WITH NEW AND OLD ARSENICALS ON

FOLIAGE AND WITH LARVÆ.

By C. L. MARLATT, Washington, D, C. In the last volume of Insect Life, in an article on Paris green, attention was called to the very large amount of this substance now being used annually as an insecticide, amounting to 2,000 tons or more. The same matter was also briefly discussed before this association at its last meeting, at which time attention was called again to the possibility of the use of a simple arsenite of copper rather than the much more expensive aceto-arsenite hitherto employed under the name of Paris green.” The tests made with the simple arsenite of copper, or Scheele's green, last season were very favorable to this new insecticide, but were not carried out with sufficient fullness to justify its being unreservedly recommended. The present summer has been taken advantage of to give this insecticide a much more thorough test, making the trials in conjunction with the ordinary Paris green and a sample of the same which had been somewhat finely pulverized, and also parallel tests with London purple, arsenite of lead,' and a mixture submitted to us for experiment which consisted of a combination of arsenite of lead and simple arsenite of copper in equal proportions. With these six substances many tests were made on foliage, and by feeding larvæ on leaves poisoned at different strengths. Through the cooperation of the Division of Chemistry, also, chemical and physical examinations were also made of the several arsenicals. The results of this work are here summarized:

Foliage tests.-All of the first series of experiments to test the effect of these arsenicals on foliage were made on young and vigorous pear trees, and with freshly mixed poisons, both with and without lime. An entire tree was devoted to each experiment, the application being made most thoroughly. When lime was added the amount was equal in weight to the poison employed. The washes were applied at the following strengths: Paris green, ord. Paris green, pul. at the rate of 1 pound to 160 gallons; 1 to 100; 1 to 80; and 1 to 531. Scheele's green. London purple, arsenite of lead,

and the mixture of Scheele's at the rate of 1 pound to 160 gallons, and 1 to 100. green and arsenite of lead.

No injury or scalding resulted in any case, even with the very strong mixtures of Paris green and Scheele's green. The weather conditious were such as to give the applications a very fair trial.

Subsequently foliage tests were also made on peach, apple, cherry,

Colored by the addition of a fraction of 1 per cent of aniline dye and styled "pink arsenite.”

1

and cotton. The fruit trees in this series and the cotton plants were all young and vigorous. The weather conditions subsequent to the applications were favorable, as in previous experiments, no rain falling for a sufficient time to give a very satisfactory test. As before, all poisons applied were freshly mixed, and in every case lime was added equal in weight to the poisons used. The poisons were Paris green, ordinary; Paris green, pulverized; London purple, and Scheele's green. The first test was at the rate of 1 pound to 200 gallons of water. No signs of injury developed after two weeks, even on the tender foliage of peach and cherry. The applications were renewed on the same plants at the rate of 1 pound to 100 gallons of water. For a week no injury was apparent, even at this unusual strength, but after that period the leaves began to fall from the peach trees treated. The falling leaves were green and unspotted or scalded, but their connection with the stems was much weakened and the least wind caused them to fall to the ground. This falling continued for several days, in most cases resulting in the loss of nearly all the foliage. A few of the remaining leaves afterwards became somewhat scalded. In the case of the apple a scald in brown spots resulted in varying degree with the different washes, and in the cherry the injury was very slight and insignificant. No damage whatever resulted to the cotton.

The following table indicates the amount of damage caused by the different poisons:

Poison.

Peach.

Apple.

Cherry.

Cotton.

Paris green, ordinary.. 1 leaves lost... Very slight spot. No injury....... No injury whatever.

ting. Paris green,pulverized. I leaves lost... All more or less ! Injury slight

Do. spotted.

insigniticant. Scheele's green I leaves lost... do

..lo

Do. London purple..... All leaves lost. Much scalded, in- ..do

cluding edges.

Do.

This application was made at this unusual strength, especially to peach, with the intention of causing injury, so as to be able to determine the relative amounts. The injury resulting from the Scheele's green was not very much greater, if any, than with the Paris green pulverized, and probably resulted from the more finely divided condition of the powder, as a corresponding increase of damage is seen between the ordinary Paris green and the same pulverized. The damage by London purple in the case of peach and apple was much greater than with the other poisons. It may

be noted here that the effect of an arsenical spray on the plant is often long in being manifested. This has been previously noted in the course of experiments with these poisons conducted by the Division of Entomology in different years, and was again shown the present season in the work against the elin leaf-beetle. Trees on the grounds of the United States Department of Agriculture were sprayed repeatedly,

« AnteriorContinuar »