STATE PLANT BOARL
May 1946 E-691
United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine
THE EFFECT OF DDT ON THE SWEETPOTATO WEEVIL
K. L. Cockerham, P. K. Harrison, and 0. T. Deen, Dvision
of Truck Crop and Garden Insect Investigationsl
The most satisfactory control for the sweetpotato weevil .(Clas
formicarlus elegantulus (Summers)) has been the use of sanitary measures.
The adults the only stage of this insect that may be readily affected by insecticides. Although arsenical and fluorine insecticides are toxic to adults, these materials have not proved satisfactory when applied in
the fields; and no insecticides, except fumigants, have been effective in
storage. In tests during 1944 and 1945 at Baton Rouge, La., with new
insecticides against the sweetpotato weevil, DIDT gave promising results.
The results of these tests are reported for the information of others
interested in the control of this insect.
In the laboratory tosts sprays and dusts were applied with small,
bulb-type applicators to wire-screen cages (fig. 1) approximately 10
inches in diameter and 9 inches high. These cages, four to each treatment, were placed on soil benches either in the screened insectary or
in the greenhouse. Ten adult weevils were then introduced into each
cage and examined daily for 7 days, and the total mortality was recorded.
The field experiments were conducted in plots 4 rows wide and 50
S feet long. The insecticides were applied with rotary hand. dusters.
There were six replicates in a latin-square arrangement in 194 and five
in randomized blocks in 1945.
The DDTI in all formulations tested was the technical grade. The
dust mixture, containing 10 percent of DDT, was prepared by the manufacturer by grinding technical DDT with pyrophyllite. When desired, further
dilutions were made with pyrophyllite in the laboratory. The 10-percent
DDT dust mixture was also used in the preparation of some of the baits
and the spray suspensions. A proprietary emulsion of unknown composition was used in the laboratory tests; the concentrate contained 20 percent of DDT before dilution with water. Benzene and kerosene solutions were also
prepared from undiluted technical DDT.
In 'cooperation with the Louisiana Agricultural Experiment Station and
Louisiana State Department of Agriculture and Immigration.
A kerosene emulsion was used in the storage tests. A 5-percent DDT emulsion concentrate was prepared by dissolving 1 pound of technical DDI! in 21 gallons of kerosene, and adding about 2 ounces of a proprietary emulsifier consisting chiefly of a condensation product of ethylene oxide and an alkylated oresol. This concentrate was diluted with 20 to 50 gallons of water to make an emulsion containing fromn 0.24 to 0.55 percent of DDT and 5 to 11 percent of kerosene. The emulions were applied to storage houses at a pressure of 250 pounds per square inch by means of a 100-gallon power sprayer equipped with an agitator and an adjustable nozzle.
In preliminary tests with the 10-percent d du .et mixture, adult weevils were readily killed when put in cages heavily dusted with the insecticide. The weevils died regardless of the presence or absence of dusted food, and most of them were dead within 2 daye. When this mixture was applied to sweetpotato roots and plants in cages before the introduction of 40 adult weevils, 39 ase 3 adute, respectively, were killed in two tests, as compared with 40 and 39 adults in two tests with potassium fluosilicate and with 32 adults in one test with calcium arsenate.
There was little difference in the mortality of adults placed on potted sweetpotato plants after the application: of dust mixtures containing 10, 5, and 2.5 percent of DDT and of water suspensions cOntaining
1 and 0.5 percent of DDT. A suspension containing only 0.1 percent of DDT was less effective. A few tests were made with the proprietary DDT emulsion in strengths as low as 0.5 percent of DDT, The weevils were killed by the emulsion, but the potted plants were severely.injured.
Five poisoned-bait tests were conducted, each consisting of 4 to
5 replications in which 10 adult weevils were used in each replication. The various baits consisted of 0.25 percent of DDT derived from 10percent DY? dust mixture, 3.0 percent of technical DDT in dry form, and 1.4, 2.8, and 3.0 percent of DDT in benzene solution, all mixed with freshly ground sweetpotatoes. The mortalities produced by these baits, in the order listed, were 42.5, 62, 80, 87.5 and 74 percent. Baits prepared by mixing 1 part of paris green with 40 parts of ground sweetpotato, and tested as the standard for comparison of effectiveness, produced from 92.5 to 100 percent mortality. n these bait tests DDT was more effective in benezene solution than it was in dry form.
Nine cages were sprayed with different DDT preparations on June 22 so as to give thorough surface coverage without run-off, and tested over a period of more than 6 months to determine Ie length of time the DDT residues on the cages remained toxic to the adult weevils. Ten weevils were confined in each cage without food for a week, after which they were removed and the mortality was recorded. Fresh weevils were introduced each week until the weevils were no lorGer killed by the residues.
The nine spray preparations included 1, 3, and 5 percent of DDT in each of three fornulations--kerosene solution, benezene solution, and the proprietary emulsion previously mentioned. The results as given in table 1 show that all sprays caused high mortality for several weeks. The duration of effectiveness increased with the percentage of DDT applied. At comparable strengths there was very little difference in effectiveness between the kerosene solution and the emulsion. The benzene solution at the 5-peroent strength remained effective longer than the other sprays and vas causing 90 percent mortality of the weevils after 23 weeks.
Similar tests were begun September 8 in cages 12 by 15 by 18 inches constructed of rough pine lumber, in rder to simulate more closely conditions found in potato storage. One cage was sprayed once with the proprietary emulsion diluted to contain 3 percent of DT, and another cag was duste with the 10-percent DDI! mixture. Each week 20 weevils were placed in each of these cages and in an untreated cage. All weevils put in the treated ages for 25 weeks after treatment were killed, and at the end of this time the cases were discontinued. Weevils in the check cap Were normal during the observations, only an occasional sp Clmen dying.
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In 1944 five applications of 10-percent DDTI dust were made to small plots of sweetpotatoes at the rate of 10 pounds of the dust per acre per application. The first application was mae on June 6, one week after the plants were set in the field. The weevil inff station in the tubers at harvest was about the same as in the check plots. The vines prior to harvest appeared greener and more vigorous tha those in the check plots, and there was no evidence of injury due to the ineeoticideo In another treatment 10 applications of calcium arsenate at 10 pounds per acre per application reduced the infestation 72 percent, but injured the foliage.
In 1945 a dust mixture containing 5 percent of DDTY and 1 percent of petroleum oil in pyrophyllite was tested. The infestation was too low to warrant any conclusions regarding the effectiveness of this mixture, but the vines in dusted plots appeared more yellow and mature than those in undusted plots. It is possible that the oil was responsible for this injury. As much as 100 pounds of the mixture was applied per acre during the season.
Tests in Storage Houses
During the summer of 1945 four storage houses near Sunset, La., were sprayed with IDT-kerosene emulsions prepared and applied as described under "Methods." House A, approximately 5,000 cubic feet in .size, contained about 30 bushels of dry sweetpotatoes heavily infested with the sweetpotato weevil. On July 4, 20 gallons of a kerosene emulsion continuing 0.6 pound of DDT was applied to the walls and floor and to the old potatoes and empty crates in the house. One-half bushel of the infested potatoes was placed in a large cage and sprayed, and for a check one-half bushel was placed in a cage and removed from the house before spraying. From 75 to 90 percent knock-down of weevils was observed in one-half hour. On July 9 a few "nervous," or agitated, adults were scattered over the house, and only a few active adults were on the potatoes; the remainder were dead. No live weevils were found in the treated cage during observations made at intervals from July 18 to September 8. The adults in the check cage appeared normal,
House B, approximately 700 cubic feet in size, was sprayed on
July 19 with 8 gallons of kerosene emulsion containing 0.15 pound of DDT. After 9 days live weevils were very numerous; so the house was resprayed with kerosene emulsion, 0.5 pound of DDT being used. On July 30 "nervous" and dead adults were numerous on the floor. Observations were repeated at intervals through September 8, and no live adults were found on August 25 or thereafter.
ouse C was approximately 2500 cubic feet in size and contained
about 10 bushel of heavily infested sweetpotatoes and a dense population of adult weevils. The walls and floor were sprayed on July 26 with 10 gallons of kerosene emulsion containing 0.5 pound of DDT. On July 30 dead and affected weevils were numerous on the floor and walls, and emergence from the old potatoes was heavy. By August 17 the emergence was so heavy that the owner cleaned out the house and burned the infested potatoes, but many living adults remained in the house. Although many dead weevils were present up to this time, there were so many live specimern_ that it was hard to estimate the results of the treatment. On August 25 living weevils could not be found, and sweetpotatoes were provided as traps for any that may have been present. A single living adult was found on the traps on August 31. None were observed September 8 either on the traps or in the house.
House D, approximately 2500 cubic feet in size, contained a few
bushels of swee potatoes and numerous adult weevils. The walls, floor, and sweetpotatos were sprayed on August 6 with 25 gallons of kerosene emnulsion contain 1 pound of DDT. One bushel of uninfested potatoes was sprayed and placed in the house ae a trap for adults. The owner cleaned u: the house and destroyed a2.1 old potatoes during the first week after treetnt. Adult weevils could not be found 8 days after treatment and no fmern: observations were made
On August 3, seven crates, each containing a bushel of sweetpotatoes heavily infested with weevils, were thoroughly dusted with 10-percent DDT mixture and stacked in a corner of a room. A similar untreated orate was placed in another room. Daily observations were made for 1 month. As weevils emerged they had to crawl over the treated potatoes and crates, thus coming in contact with the DDT. Weevils were active for a short time after emergence, -but soon became "nervous" and died. A few weevils were found on their sides and kicking, several feet from the crates, but none of these recovered and none were observed to escape. A month after treatment dead adults were so numerous under and around the crates of potatoes that no attempt was made to count them, but it was estimated that between 3,000 and 4,000 adults were killed. All the potatoes in the treated cratea\were dissected, and no specimens remained in them. Emergence and activities of adults in the untreated crate were normal during the entire month.
On November 9, two crates of heavily infested sweetpotatoes and two uninfested crates were dusted with L-percent DDT mixture and placed in the greenhouse for observation. The crates were set side by side, the infested alternated with the uninfested, so that the uninfested sweet-. potatoes would have equal and close exposure to emerging weevils. The
temperature was approximately 800 F., and emergence was heavy. On
December 17, 5 weeks after treatment, between 2,000 and 3,000 dead weevils werc under and around the infested orates. There were also a number of live adults on their sides and wiggling their legs, and a few appeared normal. Emergence was not ooulete, as a few living pupae and adults were found within the sweetpotatoes.
The previously uninfested sweetpotatoes were also examined on Deoember 17. No eggs could be found and only a few tubers had been fed upon, and these only to a slight extent. Some feeding was expected., since the adults move about for short distances after coming in contact with DDT before they die.
Experiments in the laboratory and in storags with DDT in dust
mixtures, emulsions, solutions, and spray suspensions showed that this material was very toxic to sweetpotato weevil adults crawling over siurfaces treated with it. DDT in residual applications will greatly reduce the sweetpotato weevil populations in storage houses, banks, amd on potatoes, particularly crop remnants and cull material, killing the weevils before they migrate to other potatoes or fields and before much oviposition occurs. DDT is poisonous to man and higher animals, however, and cannot be recommended at this time for use on sweetpotatoes that are to be marketed or consumed as food. The most practical formulations have not been determined, but satisfactory results were obtained with kerosene emulsion containing 0.24 to 0.5 percent of DDT and 5 to 10 percent of kerosene. In emulsions about 1 pound of DDT is applied per 3,000 to 4.,000 cubic feet of storage space.
UNIVERSITY OF FLORIDA 3 7496