Efficacy of Experimental Insecticides for Whitefly Control in Cotton, 1997
David L. Kerns, Yuma Agricultural Center
Whitefly populations in this trial were abnormally low relative to previous years experiences. Ni-25 provided excellent whitefly control and was equivalent to the commercial standard (Knack followed by Danitol + Orthene). However, there is some question concerning its adult activity late in the season, when it appeared to be weaker than Danitol + Orthene and Capture + Curacron. At low whitefly populations Thiodan tank-mixed with Knack appeared to extend control over Knack alone.
Sweet potato whitefly (strain - B), Bemisia tabaci (Gennadius), is one of the most serious pests attacking cotton in Arizona. Prior to 1996, growers relied heavily on organophosphate / pyrethroid mixtures for whitefly control. Insecticide resistance and subsequent difficulty controlling whiteflies resulted in much interest in alternative insecticides in Arizona. In 1996 and 1997, a Section 18 emergency use permit was issued in Arizona for the use of pyriproxyfen (Knack) and buprofezin (Applaud). Use of these products appeared to greatly enhance whitefly control over previous years experiences, and set the standard for whitefly control in cotton. However, data from Israel suggests that sweet potato whitefly may quickly develop resistance to these insecticides. Therefore it is important that other alternative insecticides be developed and integrated into a insecticide resistance management program.
The objective of this study was to evaluate acetamiprid (Ni-25) and tank-mixtures including Knack for control of sweet potato whitefly. Ni-25 is being developed by Rhone-Poulenc, and is a chloronicotinyl insecticide similar to imidacloprid (Admire, Provado). Ni-25 is a nerve poison and acts binding to the nicotinic receptor. Knack will be only be applied once, and will be evaluated alone and mixed with either Thiodan or Mustang. Subsequent treatments will include standard non-IGR tank mixes.
Materials and Methods
This trial was conducted at the Yuma Valley Agricultural Center. Cotton, ‘DPL 5461', was planted on 40 in beds on March 19, 1996. The test was a randomized complete block design, with 4 replicates. Plots were 4 rows X 50 ft, bordered on each side by 2 blank rows and on each end by 10 ft alleys. The test was a randomized complete block design, with 4 replicates. Plots were 4 rows X 50 ft, bordered on each side by 2 blank rows and on each end by 10 ft alleys. Applications were made on 18 Jun and 20 Aug. Treatments on 18 Jun were initiated when first colonization began, subsequent applications were made when adult numbers were high. Treatments included an untreated, Ni-25 (acetamiprid) at 0.045, 0.05, 0.075 and 0.10 lbs-ai/ac, Knack (pyriproxyfen) at 8 oz/ac, Knack at 8 oz/ac + Thiodan 3EC (endosulfan) at 0.5 lbs-ai/ac, and Knack at 8 oz/ac + Mustang 1.5EC (zeta-cypermethrin) at 0.0375 lbs-ai/ac. On 20 August, the Ni-25 treatments remained the same, Danitol 2.4EC (fenpropathrin) at 0.2 lbs-ai/ac + Orthene 90S (acephate) at 0.5 lbs-ai/ac was applied instead of Knack alone, Capture 2EC (bifenthrin) at 0.08 lbs-ai/ac + Curacron 8E (profenofos) at 0.5 lbs-ai/ac was applied instead of Knack + Thiodan 3EC, and Thiodan 3EC at 0.75 lbs-ai/ac + Ovasyn (amitraz) at 0.25 lbs-ai/ac was applied instead of Knack + Mustang. All treatments included Kinetic spreader sticker at 0.1% v/v.
Treatments were sprayed using a tractor mounted sprayer. The boom covered 4 rows, with 3 hollow cone D-2 nozzles per row, one centered over each row and two on 18 in drops on each side. The sprayer was calibrated to deliver 20 gallons per acre at 40 psi. The tractor traveled at 3 mph.
Evaluations of the immature whiteflies were made on 16, 24, 30 June, 7, 14, 21 and 28 July, and 4, 11 and 25 August. Evaluation of adults densities were made on 11 and 25 August. Whiteflies immatures were sampled by counting the numbers of eggs, small nymphs, red-eyed nymphs (RENs) and eclosed pupae from 1 cm2 leaf discs. Ten leaves were collected from nodes 3, 4, 5, 6, 7 and 8 from each. The leaf discs were taken from the area between the mid and secondary veins of the cotton leaves. Data among nodes was pooled for analysis.
Adult populations were estimated on 12, 19 and 15 August by counting the number of adult whiteflies from the 5th node from ten plants per plots. Counts were made early in the morning.
Sooty mold / honey dew accumulation was evaluated on ten plants within each plot on at harvest on 27 August. A subjective 1-5 rating scale was used where; 1 = no contamination, 2 = foliar contamination only, 3 = light contamination of lint, 4 = moderate lint contamination and 5 = heavy contamination of the lint.
All data were analyzed using a general linear model, and an F protected (P < 0.05) LSD for means separation.
Results and Discussion
In mid-June large numbers of adult whiteflies moved into these research plots from a post-harvest melon field approximately 0.5 miles south, and began colonization (Table 1). This migration triggered our 1st application. However following this flight, whitefly densities in the area remained extremely low (ca. 95% lower than in the previous three years) until mid-August. Thus the data collected in this trial represents insecticide efficacy at abnormally low whitefly densities.
Six days following application 1 significant differences were detected among treatments only in the number of small nymphs where all the insecticides had fewer small nymphs than the untreated (Table 2). By 12 DAT, significant adulticidal activity was evident based on the number of eggs (Table 3). All rates on Ni-25 and Thiodan + Knack contained fewer eggs than the untreated. The higher rates of Ni-25 and Thiodan + Knack appeared to be slightly more efficacious than the other treatments. At this time, all treatments contained fewer small nymphs and RENs than the untreated. Although we could not detect any significant differences among treatments at 20 DAT (Table 4), at 27 DAT all rates of Ni-25 contained fewer eggs than the untreated, and all insecticide treatments contained fewer small nymphs, RENs and eclosed pupae (Table 5). These data suggest that at mid-season, Ni-25 has greater adulticidal qualities than the other treatments. At 34 DAT, there were no significant differences among treatment in the number of eggs (Table 6). However, Ni-25 and Thiodan + Knack contained fewer small nymphs than the untreated, all insecticides contained fewer RENs than the untreated, and only Knack alone failed to differ from the untreated in eclosed pupae. These data suggest that at low whitefly densities, the addition of Thiodan to Knack, may prove useful in maintaining low whitefly populations for an extended period of time. However, at this effect may not be evident at higher whitefly populations densities.
At > 40 DAT, as expected with low populations, there was a lot of variability within treatments and although some statistical differences were apparent, true commercial differences were not discernable. By 41 DAT, all insecticide treatments contained significantly fewer eggs, small nymphs and RENs than the untreated, but did not differ in eclosed pupae (Table 7). At 48 and 55 DAT, there were no significant differences among treatments for any of the immature stages (Tables 8 and 9).
In mid-August adult whitefly densities began to increase to treatable levels (Table 10). On 20 August a second application was made, which primarily targeted the adult stage. By 5 DAT, Danitol + Orthene and Capture + Curacron appeared to offer the best adult control followed by Thiodan + Ovasyn (Table 11). Although Ni-25 offered adult suppression, it did not appear to be an effective knock-down product for late-season cotton. We could not detect any differences among treatments in the immature stages at 5 DAT (Table 12) or in sooty mold accumulation on 27 August (Table 13). Following the sooty mold evaluations, the cotton was ready for defoliation so the trial was terminated.
This is a part of publication
AZ1006: "Cotton: A College of Agriculture
Report," 1998, College of Agriculture, The
University of Arizona, Tucson, Arizona, 85721. Any
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