Rainfastening of Bifenthrin to Cotton Leaves with Selected Adjuvants

There are thousands of adjuvants on the market, yet little is known about their effects on the activity of insecticides on plant surfaces. The effects of 11 selected adjuvants on the rainfastness and retention of bifenthrin ([laJa-(2)-(i-)-(2 methyl[l,l'-biphenyl]-3-yl) methyl 3-(2-chloro-3J,3-trifluoro-l-propenyl)-2~-dimethylcyclopropanearhoxylate) on cotton (Gossypium hirsutum L.) leaves were investigated. In addition, the effect of the adjuvant Bond on the efficacy of bifenthrin and a Bacillus thuringiensis (Berliner) formulation wa9 determined. Bifenthrin mixed with each adjuvant was applied to greenhouse grown cotton plants using a spray chamber. Simulated rainfall of 13 mm was then applied to treated cotton plants at 0.25, 1, and 4 h after treatment. Bond and Agimax-3 were the only adjuvant. to significantly increase the rainfastness of hifenthrin on cotton leaves. Agri-Dex, Soy-Dex, and Dyne-Amic significantly decreased the rainfaqtness of bifenthrin. In tests conducted with an immersion cell apparatus, Orchex 7% resulted in twice the retention of Agri-Dex, the nkit highest retained adjuvant. Both petroleum and veget-able oils enhanced retention of bifenthrin on the leaf surface. Bond mixed with bifenthrin m d a B. thuringiensis formulation did not reduce the activity of these insecticides against tobacco hudworm (Heliothis virescens) and soybean looper (Pseudoplusia includens) larvae. Retention on the leaf may he related to spread and to the degree of binding of the insecticide to the surface by properties of the adjuvant. Yet, the properties of Agri-Dex that enhanced the retention of bifenthrin to the cotton leaf decreased its rainfastness. S PRAY adjuvants may enhance pesticide activity and minimize the effects of environmental factors on pesticide application and decomposition. Thousands of adjuvants are now on the market. Foy (1993), in a survey of product labels from 485 formulations of crop protection chemicals for use in 1992, found that 49% of the formulations recommended adjuvant use, 5% recommended no adjuvant, and the remaining 46% did not mention adjuvant use. Adjuvants were recommended with 71% of the herbicide formulations and with 30% of the other classes of crop protection chemicals, while 14% of the other classes of crop protection chemicals did not recommend an adjuvant. The greatest reduction of pesticide efficacy results from environmental factors. While sunlight gradually decomposes pesticides on the leaf surface, rainfall has an immediate effect on the adherence of pesticide residues to plant surfaces, especially if a storm occurs shortly after application. Studies have shown that rainfall amount more greatly affects the washoff of insecticides from cotton plants than does rainfall intensity. However, the effect of elapsed time between insecticide application and rainfall on insecticide washoff from plants varies with insecticide and/or formulation. For example, methyl parathion that is washed from cotton plants decreases exponentially with time after application and linearly with the concentration on the plant (McDowell et a]., 1984). In contrast, the fractions of toxaphene and fenvalerate washed from the plants were relatively constant at 10 and 7%, respectively, regardless of time after application and amount on the plants. Pick et al. (1984), using overhead irrigation to simulate rair~fall, concluded that endosulfan, cypermethrin, and carbaryl became more resistant to washoff with time after application. Even low amounts of rain can have drastic effects on insecticide residues. About 63% of carbaryl on cotton plants 2 h after application was washed off by 25 mm of rain; while an additional 76 mm of rain removed another 35% of the total (Willis et al., 1988). Insecticide formulations vary in rainfastness. Washoff studies showed that insecticide resistance to washoff decreased in this order: toxaphene > fenvalerate = permethrin > EPN > methyl parathion (McDowell et al., 1987). About 35% of the permethrin load on plants 2 h after application was washed off by 25 mm of rain with an additional 76 mm of rain removing only 11 % more of the total (Willis et al., 1986). The greatest portion of insecticides used for cotton insect control in the southern USA is applied during July, August, and September, which are months with frequent high-intensity thunderstorms. Adjuvants that provide some rainfastness of insecticides could be costeffective in areas of the country that receive abundant rainfall, especially when insect populations are at an economic threshold and larvae are exceeding controllable size. 'This research determined the effectiveness of several available adjuvants to rainfasten a pyrethroid insecticide to cotton leaves. Bioassays were conducted to detect any antagonism between rainfastening ability and insecticide activity. In addition, the retention of these insecticide-adjuvant mixtures on the surface of cotton leaves was measured using a recently designed immersion cell apparatus. MATERIALS AND METHODS IJSDA, ARS, Application and Production Technology Rcsearch Unit, Rainfastness Test Stoneville, MS 38776. Received 9 Aug. 1999. * ~ o r r e s ~ o n d i n ~ author ([email protected]). Eleven adjuvants (Table 1) representing spreader-stickers (an adjuvant that has the properties of both a spreader and Published in J . Environ. Qual. 29:1863-1866 (20OO). a sticker), crop oil concentrates. and nonionic surfactants were Table 1. Adjuvant5 used in rainfastness tests of hifenthrin applied to greenhouse cotton at Stoneville, MS. Adjuvant Oh (vlv) Manufacturer I)e\cription Comtituents