May I9U6 LIBRARY STATE PLANT BOARD E-689 United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine SUGGESTIONS CONCERNING THE USE OF DDT DUSTS OH POTATOES IS EASTERN WASHINGTON By B* J. landis and C. W. Getzendaner, Division of Truck Crop and Garden Insect Investigations^ Potato growers in parts of the Pacific Northwest infested with the tuber flea beetle (Bpltrlx tuber! s Centner) and the western pota- to flea beetle (E, subcrinita (Leo*)) find it necessary to apply in- secticides periodically for the control of these insects. For the past several years large numbers of the green peach aphid (Myzus per si cap (Sulz*)) and smaller numbers of the potato aphid (Macro si phum solanifolii (Ashau)) have infested potatoes, but few growers have ap- plied insecticides for control of these insects. The calcium arsen- ate or cryolite dusts applied to control flea beetles on potatoes do not oontrol the aphids and this practice even may encourage the de- velopment of abnormally large numbers of aphids* Damage to potato tubers caused by larvae of potato flea beetles can be identified readily during the growing season and is recognized as a grade defect at harvest* The devitalizing effect of aphids suck- ing the Juices from potato foliage causes a reduction of yield but does not cause grade defects in marketable -size potatoes* The green peach aphid and the potato aphid are of greatest importance as car- riers of potato virus diseases from diseased to healthy plants* Cer- tain virus diseases greatly reduce yields of potatoes and also cause an internal deterioration of the tubers, which may worsen during periods of storage* In I9U5 winged green peach aphids were first observed on small potato plants on May 5 And vere present in most potato fields by May 15* The aphids occur on the under side of the lower leaves of potato plants during the first part of the growing season and may escape casual observation because of their small size and pale green color* They gradually increase in number during the spring months and become most abundant at about the time the plants bloom* Numbers of aphids sufficient to produce a wet, shiny glaze on the leaves may be pro- duced from June 15 to 30 on early potatoes, and from July 25 to August 20 on late potatoes* Varying numbers of winged aphids develop on po- tato plants throughout the season, and these may fly into other fields of potatoes* 1/ In cooperation with the Washington Agricultural Experiment Station* UNIVERSITY OF FLORIDA mi ii mi i 3 1262 09238 7488 Experimental work with DDT dust 8 applied periodically to foliage in fields of potatoes during the last 2 years has shown that aphlds and flea beetles nay he kept at relatively low numbers throughout the period of treatment. In fields so treated until the Tines matted the rows* the amount of flea "beetle injury at harvest was no greater, often less, than that after similar applications of either calcium ar- senate or cryolite dust* Observations as to the effectiveness of single applications of DDT dusts in several fields of potatoes heavily infested with the green peach aphid showed that, although very few aphids were killed during the first 2 days following treatment, their population decreased $2 to 97 percent within the first week. Under conditions in the Yakima Val- ley DDT dusts gradually lose their effectiveness beginning about 10 days after they have been applied to foliage. In this respect DDT dusts are not materially different from e ither calcium arsenate or cryolite dusts, and therefore should be applied repeatedly for best results. The frequency with which ground-type dusters may be used in applying insecticides depends, in many localities, upon the number and duration of periodic irrigations of the crop. Because of the gradual reduction of effectiveness of the DDT dusts following application, it is desirable to adjust the irrigation practice so that dusts may be applied at short intervals. Dusts containing 5 percent of DDT are preferable to those of lower concentration where flea beetles and aphids are present. The most sat- isfactory control was obtained where the first application was made soon after the potato plants came through the ground and additional applica- tions were made at 10-day intervals until the vines matted the rows. The first application may be made at the rate of 15 pounds of the 5- percent DDT dust per acre and the dosage should be increased gradually to 35 pounds for the last dusting. Ground dusting machines should have an 8- to 10-foot canvas drag to prevent excessive drift of the dust. Although experiments with DDT for the control of flea beetles and aphids on potatoes have not been conducted in western Washington , there is no reason to believe that the material will not be effective in that area.