LIBRARY STATE PLANT -BOARD May 1944 B-619 United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine STYRENE DlBHOMIDE: A SUBSTITUTE FOR PYR3THRUM IN INSECTICIDAL OIL USED FOB CONTROL OF EARWORMS IN SWEET CORN By G. W. Barber, Division of Cereal and Forage Insect Investigations, and J. vilcox, Division of Truck Crop and Garden Insect Investigations A widely used method of control .of the corn earworm in sweet corn is the mineral oil treatment (l).i/ As usually recommended, this method consists of applying about 37^ cubic centimeter of highly refined mineral oil, ranging from 100 to 150 seconds Saybolt viscosity at 100° F. and containing 0.2 percent of pyrethrins, to the tips of ear* about one weak after silking, or at a time when the silks have *•: ted or their tips have become brown. Scarcity of pyre thrum for use as an insecticide on cultivated crops, owing to war conditions, has stimulated search for a satisfactory substitute for use in mineral oil. Among many chemicals that have been tested as substitutes for pyrethrum in oil, styrene dibromide ( alpha-be ta-dibromethyl-benzene) has been found to be the most satisfactory. This chemical has been tested in New Jersey and Texas by Barber (2), in Ohio by Davidson (^), and in California by Wilcox.S/ Styrene dibromide is a colorless, crystalline chemical having a mild naphthalenelike odor. It is soluble in and imparts no color to mineral oil. At concentrations of from 0.5 to 1.5 grams per 100 cc. of white oil the mixture kills earworms by contact, but they succomb more slowly than when oil containing 0.2 percent of pyrethrins is used. Half-grown rabbits ate the silks and tips of the cobs of several ears treated with 3/k cc. of oil-styrene dibromide mixture daily without observable effect. The chemical was studied by Barber at concentrations ranging from 0.5 to 1.5 grams per 100 cc. of oil, by Davidson at a concentra- tion of 1 gram per 100 cc. of oil, and by Wilcox at concentrations of 1, 2, and 3 grams per 100 cc. of oil. The average results of apply- ing oil containing 1 percent of styrene dibromide or oil containing 0.2 percent of pyrethrins to ears of sweet corn were, in percentage of pro- tected ears, 64*. 6 and 64.0 percent respectively, in Nev; Jersey in 194l; 78.7 percent for each material in Texas in 1942; 91*7 ani 81.6 percent, respectively, in Ohio in 1943; and 77.1 and 90«8 percent, respectively, in California in 19^3* These results showed that oil containing 1 per- cent of styrene dibromide gave practically the same protection to sweet corn against earworms, on an average, as was obtained by use of oil containing 0.2 percent of pyrethrins. 1/ Underscored numbers in parentheses refer to Literature Cited, p. 3. 2/ Wilcox, J. Unpublished data from experiments in California during 1943. "* - 2 - Although a mild odor of styrene dibromide may "be detected in the silks of treated ears at roasting-ear harvest, tests have shown that no residue flhat can he detected either "by taste or odor occurs on the kernels. The mixture caused no discoloration of any part of the ears and does not cause an increase in the length of the undevel- oped tip "beyond that caused by the oil containing other insecticides, Styrene dihromide is more satisfactory than dichloroethyl ether (which has "been recommended at a concentration of 2 percent in oil) because it does not present the hazard of leaving a detectable residue on the tips of treated ears, such as may he deposited hy oil- dichloroethyl ether when the temperature of the interval between treat- ment and harvest is not sufficiently high to cause evaporation of the dichloroethyl ether from them. Small quantities of styrene dihromide will he available in I9HU. for use in mineral oil for earworm control in commercial plantings or Victory gardens, and this mixture is being prepared for sale by several concerns dealing in insecticides. It should be applied to the tips of ears not earlier than 1 week after the silks have been exposed, at a dosage of l/2 to 3/^ cubic centimeter per ear, as described in Circu- lar 657 » copies of which may be obtained from the U. S. Department of Agri culture . In victory gardens, when force oilers which are used to make accurately measured applications in commercial fields aro not obtainable, the ears of small plantings of sweet corn msy be treated by use of ordinary medicine droppers, the capacity of which usually is 1 cubic centimeter. The level of oil in the medicine dropper should be marked when it contains 20 drops (l/2 cubic centimeter) or JO drops (3/^ cubic centimeter), that is, when it is approximately one-half or three-fourths full. These are the proper amounts of oil containing styrene dibromide for application to small or large sweet-corn ears, respectively. In a sweet-corn ear having a husk extension of about 2 inches at the time of roasting ear harvest, the l/2-cc. dosage will usually result in a barely perceptible increase in the number of undeveloped kernels at the tip of the cob. Larger dosages per ear or applications to ears having a shorter husk extension will often cause a small increase in length of the undeveloped tip of a half inch or more. Although a moderate increase in the undeveloped tip of the cob does not affect the marketability of the crop, this loss may be avoided by choosing a sweet- corn variety that has a uniform husk extension of at least 2 inches, and by not applying more than 1/2 to }/k cubic centimeter of the oil contain- ing styrene dibromide per ear, depending on whether the ears of the variety used are small or large. An ear, of course, should never be oiled more than once. - 3 - The rate of control obtained has been found to depend on several factors, among v/hich a vigorous, regular growth of the plants is the most important. Consequently,- the plants should "be given suffi- cient plant food and frequent watering in order that they nay reach their greatest perfection. In well-watered and fertilized fields when every ear is attacked by several earworms, an average of about 90 percent of the ears have been protected by application of the mineral- oil treatment. In proportion as the fields are less well fertilized or suffer from drought, the rate of control is progressively less. LITERATURE CITED (1) Barber, G. W. 19^2. Mineral-oil treatment of sweet corn for earworm control, TJ. S. Dept. Agr. Circular 657, 15 pp. (2) Barber, G. W. 19^3* Styrene dibromide as a substitute for pyrethrum in oil for corn earworm control. Jour. Econ. Ent. 36: 330-2. (3) Davidson, Balph H. 19^+3* The relative effectiveness of some corn earworm control measures in sweet corn. Jour. Econ. Ent. 36: 938. UNIVERSITY OF FLORIDA Mil 3 1262 09239 1316 - II \