Lygus Control Decision Aids for Arizona Cotton

Changes in insecticide use, available pest control technologies, and local crop ecology together with severely depressed cotton prices place a renewed premium on Lygus control decision aids for Arizona cotton. As part of an on-going program to develop research-based Lygus management recommendations, we investigated the impact of various timings of chemical controls on Lygus population dynamics, number of sprays, costs of control, and net revenue as well as cotton heights, trash, lint turnouts, and yields. Once there were at least 15 total Lygus per 100 sweeps, sprays were made according to the number of nymphs in the sample (0, 1, 4, 8 or 16 per 100 sweeps). Up to 7 sprays were required (15/0 regime) to meet the needs of the target threshold. Lygus adult densities were largely unresponsive to the treatment regimes or individual sprays made. Three generations of nymphs, however, were affected by the treatments with the ‘15/4’ regime harboring the fewest nymphs through July. This ‘moderate’ regime required 4 sprays and had the shortest plants, cleanest harvest, and highest lint turnouts. In addition, this regime out-yielded all other treatment regimes including the 6(15/ 1) and 7(15/0) spray regimes. Regression analyses of the data suggest that adult Lygus are less related to yield loss than nymphs and that large nymphs are best correlated with yield loss. Thus, spraying based on adults only would appear illadvised. Returns were highest ($747/A) for the 15/4 regime with over $100 more than the more protective regimes. Thus, there is no economic advantage in advancing chemical control when nymph levels are low. Maximum economic gain was achieved by waiting for the 4 nymphs per 100 level (with 15 total Lygus/100; 15/4) before spraying. However, waiting too long (beyond the 8 nymphs / 100 level; 15/8) resulted in significant reductions in yield and revenue. Our recommendations, therefore, are to apply insecticides against Lygus when there are at least 15 total Lygus, including at least 4 nymphs, per 100 sweeps. These recommendations are stable over a wide variety of economic conditions (market prices & insecticide costs). Continued work is necessary to verify these findings over a wider range of cotton developmental stages, varieties, and other environmental conditions. Introduction Lygus control frequency and costs have increased in recent years in Arizona due to a complex of factors (Table 1). As new selective pest control technologies are adopted, the potential window in which Lygus can be an apparent economic pest has grown. Broad spectrum insecticides that were once used against pink bollworm and whiteflies have now been replaced on the majority of acres by the highly effective Bt cottons and insect growth regulators, respectively. These two sets of pest control technologies have helped to reduce the number of foliar insecticides for insect control since 1996 (Table 2) and contributed to a decade-low spray requirement and costs in 1999. The decision for This is part of the 2000 Arizona Cotton Report, The University of Arizona College of Agriculture, index at http://ag.arizona.edu/pubs/crops/az1170/ 270 use of in-season insecticides primarily depends on the presence of Lygus, Arizona’s third ‘key’ pest. Changes in local crop ecology (i.e., the availability of alternate hosts) together with severely depressed cotton prices place a renewed premium on Lygus control decision aids. Information on the relative performance of and timing in insecticide use is necessary for growers to maximize profitability. Insecticide Efficacy ‘Effective chemical use’ is one of the three keys to cotton IPM in Arizona (Ellsworth 1999b). Part of this key depends on selection of the proper insecticide. Our studies in 1999 re-enforced our current recommendations for chemical control: 1) use one of four possible insecticides (acephate [e.g., Orthene], endosulfan [e.g., Thiodan], Monitor, Vydate) at appropriate rates, and 2) avoid the use of pyrethroids which remain ineffective or combinations which have not been proven to be the most profitable option (Diehl et al. 1998; Ellsworth & Diehl 1998; Ellsworth et al. 1998a; Ellsworth 1999a). Another part of this key is resistance management (Ellsworth 1998a). Ample evidence exists to suggest that Lygus can and do become locally resistant to overused insecticides (Dennehy & Russell 1996; Dennehy et al. 1998). Thus in Arizona cotton, we suggest limiting the use of each active ingredient, no matter the insect target, to no more than two uses per season and rotating among classes of chemistry or modes of action (Ellsworth et al. 1996). This is very difficult, if not impossible, to do for Lygus in areas of repeated need for chemical control (e.g., near Lygus sources), especially given that the four recommended insecticides may represent as few as two functional modes of action (i.e., organophosphates / carbamate & cyclodiene). The new class neonicotinoid, which has shown some promise on the eastern species, Lygus lineolaris, so far has not proven effective against our species, Lygus hesperus. Only one new insecticide with pending registration shows consistent promise for the control of Lygus in Arizona (Ellsworth et al. 1998a; ACGA 1999; Ellsworth 1999a; Ellsworth et al. 1999). Fipronil (Regent by Aventis) is a new class of insecticide with registrations in rice and corn in the U.S. and cotton in Mexico. Thresholds Timing of insecticide use (i.e., thresholds) may be the most important part of ‘effective chemical use’ and managing Lygus profitably. In addition to the recent shift in cotton prices and in use of selective pest control technologies, cotton varieties and cultural practices have shifted towards a more determinant, compact fruiting cycle. These changes demand a re-evaluation of our guidelines for Lygus control thresholds. This re-evaluation began in 1997 (Ellsworth 1998b; Ellsworth et al. 1998a) and was continued in 1999 with the replicated timing trial reported here. Methods A set of 5 candidate action thresholds and an untreated check (Table 3) were evaluated for their impact on Lygus population dynamics, number of sprays, costs of control, plant heights, lint turnouts and yield, and net revenues. The test was planted to NuCOTN 33b (13 April) and oversprayed with one application each of Knack (10 August) and Applaud (1 September). This strategy eliminated the confounding effects of pink bollworm (and other lepidopterans) and whiteflies, the two other ‘key’ pests of Arizona cotton. Pix (1 pt/A) was oversprayed on the entire test once (16 July). The Lygus insecticides were rotated among Orthene (1 lb ai/A), Vydate (1 lb ai/A), and Regent (0.05 lb ai/ A) (Fig. 1). Endosulfan was not selected because of its additional impact on whitefly (esp. adults) control. This test system provided for a rare opportunity in field designs to isolate the impact of a single, yield-limiting pest without the confounding effects of other pests or other pesticides. The thresholds were defined by the number of Lygus per 100 sweeps. Prior studies had shown that adult counts were relatively refractory to insecticide sprays (Ellsworth et al. 1998a; Pacheco 1998; Ellsworth 1999a), so thresholds were defined primarily by the presence or number of nymphs in this study. For each threshold, at least 15 total Lygus per 100 sweeps had to be present no sooner than 6 days after the last spray. T1–T5 were then distinguished as having 0, 1, 4, 8 or 16 nymphs per 100 sweeps, respectively (Table 3). All sprays were made within 24 hrs of the indicating sample, rain and irrigation water permitting. All sprays were made by ground, broadcast, two nozzles per row (TeeJet Twinjet 8003EVS) at 20 GPA with a John Deere modified Hi-Cycle 600A. In order to better understand the relative yield differences observed, linear regressions were needed to further explore Lygus density:yield relationships. But because sprays were made according to candidate action thresholds, Lygus