Adjuvants in Sprays for Insect Control on Sweet Corn

Abstract

Everglades sweet corn must be sprayed at regular intervals to control the corn stem weevil (Hyperodes humilis (Gyllenhal)), budworms, and earworms. From eggs embedded in the leaf sheaths or hypocotyl of the young corn plant, corn stem weevil grubs hatch to mine through the stem. Beginning within a day of seedling emergence, about six sprays must be applied twice weekly or every four days to control this pest. The fall armyworm Spodoptera frugiperdca (J. E. Smith), and the corn earworm, Heliothis zea (Boddie), feed within the whorls of the growing plants as budworms and also upon the ears as earworms. Budworm control sprays must be applied once or twice weekly between cessation of corn stem weevil sprays and initiation of earworm control sprays. From the day after the first silks appear until shortly before harvest, earworm control sprays must be applied daily or every other day depending upon population density and weather conditions. DDT emulsions are more effective than DDT wettable powder sprays for sweet corn insect control (Harris 1961, 1961a). However, emulsions frequently damage plants, especially at the high concentrations needed for corn stem weevil control when the plants are young and tender. DDT wettable powder sprays are less likely to damage plants or be incompatible with the fungicide wettable powders that are frequently added to the spray. Therefore, it is desirable that the efficacy of DDT wettable powder sprays be made more nearly equal to that of emulsions by the use of adjuvants or by some other measure. Triton X-100 (a wetting agent made by Rohm and Haas Company) was evaluated as an adjuvant in DDT wettable powder sprays. Under heavy population pressures, even the most effective! of recommended practices may not adequately control sweet corn insects. At one time, 2.5 gallons of white mineral oil was recommended as an additive to 1 gallon of 25% DDT emulsifiable concentrate in 50 gallons of emulsion per acre (Anonymous 1956) to control earworms in Florida. The DDT emulsifiable concentrate was supposed to supply enough emulsifier to emulsify the mineral oil. Frequently poor yields and off-color husks resulted. The recommended amount of white mineral oil was first reduced to 1.75 gallons (Brogdon and Marvel 1959) but later completely withdrawn from recommendations (Brogdon, Marvel, and Mullins 1963). A miscible spray oil2 was substituted for the white mineral oil and evaluated at several dosages to seek a level less likely to affect sweet corn yield or quality adversely and yet increase the efficacy of DDT emulsion against earworms. The miscible spray oil was also evaluated at a single dosage in toxaphene and DDT emulsions for budworm control.