Assessing the effectiveness of air-assisted and hydraulic sprayers in cotton via leaf bioassay.

Compared with conventional over-the-top sprayers, air-assisted and drop-nozzle sprayers should provide improved pesticide penetration and coverage within the plant canopy where the insects feed. This study compared the effectiveness of the insecticide Tracer, a natural insecticide produced by the actinomycete Saccharopolyspora spinosa, (Dow Agrosciences, Indianapolis, IN) applied by these three sprayers within the canopy of cotton (Gossypium hirsutum L.). Leaves from the top, middle, and bottom of cotton plants were used in a bioassay as an indicator of insecticide deposition within the canopy. The test measured the mortality of beet armyworm (Spodoptera exigua, Hubner) larvae feeding on the sampled leaves. All three sprayers provided adequate coverage for good insect control in the top of the cotton canopy. The air-assisted sprayer provided the best insect mortality throughout the canopy, while the over-the-top sprayer had the poorest insect mortality in the bottom. Air-assisted and drop-nozzle sprayers provided good insect mortality in the middle of plants. The success of the boll weevil, Anthonomus grandis, eradication program in the U.S. Southeast has reduced the amount of conventional insecticides used in cotton. However, lepidopterous insects can still cause substantial losses in cotton across the South, and use of conventional insecticides to control them may be severely curtailed due to environmental concerns and the failure of H.R. Sumner, USDA-ARS-IBPMRL, P.O. Box 748, Tifton, GA 31793; and G.A. Herzog, Dep. of Entomology, Univ. of Georgia Coastal Plain Exp. Stn., Tifton, GA. Received 23 Feb. 2000. *Corresponding author (hsumner@tifton.cpes. peachnet.edu). 80 SUMNER & HERZOG: LEAF BIOASSAY ASSESSES SPRAY EFFECTIVENESS insecticide manufacturers to re-register these products for use in cotton. Lack of registered conventional insecticides and/or development of insecticide resistance could lead to crop loss when outbreaks of lepidopterous pests occur. Industry has developed commercial formulations of several pathogenic organisms for use in insect control. Examples of these are the viruses, Gemstar (Thermo Trilogy Corp., Columbia, MD), the fungi, Naturalis-L (Troy Biosciences), and the bacteria, Dipel DF (Abbott Labs, Chicago, IL) that have promise for use in managing lepidopterous insects. These biological/biorational insecticides could provide alternatives to conventional insecticides for cotton pest management and, theoretically, could reduce lepidopterous pest populations below economic damaging levels while protecting beneficial insect populations. A need exists to develop and evaluate methods to effectively apply these insecticides to cotton. Placing more active ingredient on the target in an effective manner could enhance product performance, prolong the effective activity, and provide greater economic returns. Improved performance would result in wider acceptance of biological/biorational insecticides by growers. Application technology has been developed in recent years to improve pesticide deposition and leaf coverage. Mulrooney and Skjoldager (1997) found that air-assisted application of insecticides significantly enhanced the efficacy of boll weevil and beet armyworm control in cotton. Compared with over-the-top and drop-nozzle sprayers, the airassisted sprayer provided greater canopy penetration and deposit of fluorescent dyes/markers on Mylar sheets and water-sensitive papers in cotton (Womac et al.,1992); plus, it also increased deposition of bifenthrin on leaves and squares within the canopy. Howard et al. (1994) reported that three air-assisted sprayers deposited more bifenthrin on both the upper and under-sides of leaves in the middle of the cotton canopy and had a higher percent coverage than conventional over-the-top hydraulic sprayers. Therefore, the improved canopy penetration and leaf coverage available with air-assisted sprayers should improve the performance of biological/biorational insecticides, which require direct contact or ingestion for effective control. The objective of this study was to compare the application effect of three sprayer methods (airassisted, over-the-top, and drop-nozzle) on the effectiveness of Tracer for mortality of lepidopterous insect pests in cotton. MATERIALS AND METHODS Application of Insecticides Field tests were conducted in plots eight rows wide by 15.2 m long planted to cotton, cv. DPL5415 (Delta and Pineland, Scotts, MS) at the Coastal Plain Experiment Station, Tifton, GA, during 1998 and 1999. Application methods were (Fig. 1): • Air-assisted (Berthoud row crop sprayer that delivered air at 54 m s) equipped with two 15/10 blue spray nozzles per row that have 1.5 mm diam. orifices and operated at 103 kPa to deliver 187 L ha of spray solution (Berthoud Sprayers, South Haven, MI). • Over-the-top with two TX-6 hydraulic nozzles per row that operated at 414 kPa and delivered 78 L ha (Spraying Systems Co., Wheaton, IL). • Hydraulic drop nozzles with one TX-10 hydraulic nozzle on each side (38-cm drops) and one over-the-top of the row that operated at 552 kPa and delivered 136 L ha . These sprayers were operated as recommended by their manufacturer or as generally used by spray applicators in cotton. The sprayers differed in spray droplet size, spray rate, and spray coverage throughout the cotton canopy. The insecticide evaluated was Tracer 4SC, applied at 70.1 g ai ha with each sprayer. The effectiveness of each sprayer in getting Tracer 4SC to leaves in the top, middle, and bottom of plants was determined by a bioassay of leaf samples, described below. An untreated control was included as a check. All plots were over-sprayed in 1998 with a blanket treatment of Karate, Lambda-cyhalothrin (Zeneca, Wilmington, DE) applied at 22.4 g ai ha on 9, 15, and 21 July using the over-the-top sprayer 1 Mention of a proprietary product does not imply an endorsement or a recommendation for its use by USDA or the University of Georgia. 81 JOURNAL OF COTTON SCIENCE, Volume 4, Issue 2, 2000 Fig. 1. Schematics of the configurations of the three types of sprayers used in this study to analyze which gives the best overall penetration of the cotton leaf canopy. treatment. Tracer treatments were applied on 28 July; on 4, 11, 18, 25 August; and 1 September. In 1999 no blanket treatments were applied, and Tracer plots were treated on 20, 27 July; and on 2 and 10 August. Bioassay of Applied Insecticide Residue Leaf bioassays were conducted on 4, 18, and 25 August 1998, and on 20, and 27 July and 2 and 10 August 1999 using foliage from the Tracer-treated and check plots. The experiment design was a randomized complete block with four replications where treatments were arranged in a 3-by-4 factorial to evaluate efficacy by plant location and application method for both years. On the treatment day, after the spray solution had dried on the leaves, five leaves each were sampled randomly from the top, middle, and bottom of the cotton plants (15 leaves per plant) in each plot treated with Tracer and the untreated check plots. Leaves were trimmed to fit into 100 mm diameter sterile petri dishes containing a 75 mm diameter filter paper disk that had been moistened with distilled water to prevent premature leaf desiccation. Ten 5-dold beet armyworm larvae obtained from the USDAARS-IBPMRL insect rearing facility in Tifton, GA were placed on each leaf sample. Five petri dishes were used for each plant location in the plots. Petri dishes were held in an environmental chamber at 24 (C at 50 % RH and 12:12 light-dark photophase. The larvae in each dish were examined 72 h after test initiation, and the number of live larvae recorded. Table 1. Beet armyworm mortality on leaves from three locations within the canopy of Georgia cotton plants treated with Tracer insecticide applied by three sprayer types on three dates in 1998. Sprayer method, location sampled within canopy Beet armyworm mortality 4 August 18 August 25 August Means† % -Air-assisted, top 81 ab‡,y 84 ab,y 54 b,y 73 b,y Air-assisted, middle 92 a,y 91 a,x 87 a,x 90 a,x Air-assisted, bottom 76 b,x 74 b,x 83 a,x 78 ab,x Over-top, top 92 a,y 79 a,y 89 a,x 87 a,y Over-top, middle 75 b,y 64 b,y 63 b,y 68 b,y Over-top, bottom 55 c,y 28 c,y 39 c,y 41 c,y Drops, top 87 a,y 92 a,y 82 a,x 87 a,y Drops, middle 86 a,y 90 a,x 89 a,x 88 a,x Drops, bottom 60 b,xy 48 b,y 53 b,y 54 b,y Untreated, top 12 a,z 2 a,z 1 a,z 5 a,z Untreated, middle 13 a,z 2 a,z 1 a,z 6 a,z Untreated, bottom 19 a,z 2 a,z 1 a,z 8 a,z LSD (sprayer method) 14 15 24 13 LSD (canopy location) 20 24 24 17 † Means are the average of the three sample dates. LSD is the weighted average of the three sampled LSD, Steel and Torrie, 1960. ‡ Mean values in columns with common letters (a,b,c, for leaf location within sprayer method, x,y,z, for sprayer method within leaf location) are not significantly different by Fishers LSD test (P = 0.05). 82 SUMNER & HERZOG: LEAF BIOASSAY ASSESSES SPRAY EFFECTIVENESS Then SAS Proc Mixed procedures (SAS Institute Inc., 1989) were conducted on mortality data for application methods and leaf position. Means were separated by Fishers LSD (P = 0.05).