EARLY SEASON ABUNDANCE AND DISTRIBUTION OF LYGUS BUGS, PREDATORS, AND EXTRAFLORAL NECTAR IN DRIP AND FURROW-IRRIGATED COTTON (PEST MANAGEMENT, AGGREGATION, ARIZONA).

Abstract

Lygus bug (Lygus Hahn) and predator (Chrysopa carnea Stephens, Geocoris, and Nabis) abundance and spatial distribution in production-managed, drop and furrow-irrigated cotton (Gossypium hirsutum L.) in central Arizona were compared during the early seasons of 1980-1983. Sweep net and drop cloth samples revealed lygus bug populations were established and reached pest status in drip-irrigated cotton 2-3 weeks prior to those in furrow. Several factors combined to make drip cotton more favorable than furrow for Lygus: more dense plant populations, taller plants, lower vapor pressure deficits within the plant canopy, and earlier availability of squares. Within furrow-irrigated cotton, lygus bugs and the percent Lygus-damaged squares were greater in areas of tall, dense plantings, primarily in areas distant from the water source. In drip, damage was significantly greater than in furrow, and comparable in all field ares. Predator populations were not consistently greater under either irrigation system. Under furrow, Geocoris and Nabis populations were greatest in areas of Lygus abundance. Morisita's index and Taylor's power law indicated a higher degree of aggregation in the spatial patterns of Lygus and predator populations in furrow-irrigated cotton than in drip. The two indices agreed in magnitude and trend across the insects studied, but Morisita's index was more frequently significant for contagion. Sweep net samples revealed a higher degree of aggregation than drop cloth. Taylor's coefficients were used to determine optimum sweep net and drop cloth sample sizes to evaluate Lygus populations for pest management. The sweep net requires a greater sample size than the drop cloth to ensure a given level of precision, and more samples are required in furrow-irrigated cotton than in drip. Based on percent damage and the relative variability of damage estimates, the number of squares examined for lygus bug damage should be comparable in all drip field areas, but twice as many should be examined in the furrow head-water as in the middle and tailwater. Extrafloral nectar volumes are greatest in leaves of the middle and top plant regions, and in the blossom and young boll stages of fruiting bodies. Irrigation timing and method affect nectar production. The effects of extrafloral nectar production phenology on insect distribution are discussed.