Lygus Control Decision Aids for Arizona Cotton

Abstract

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.