A food-web approach to economic thresholds: A sequence of pests/predaceous arthropods on California cotton

Abstract

The value of an adequate food base for predaceous arthropods as provided byTetranychus is relevant for the development of economic thresholds for this group and also for pests that occur later in the season. A higher predaceous potential was reflected in greater levels ofGeocoris and ofOrius on untreated plants with abundant food, as compared with lower predator numbers on insecticide-treated plants having lower food levels. The higher numbers ofGeocoris andOrius were significant because they persisted through July–August. In the San Joaquin Valley, an increase in predators during this period is highly beneficial, becauseLygus and lepidopterous worms may damage cotton at this time. Policy guidelines on effects from chemicals on arthropods need re-evaluation. The assessment of disruptive effects of chemicals must include their impact on the arthropods that constitute the principal sources of food for predaceous arthropods. The indirect effect of chemicals against beneficial-predaceous insects via disruption of their food chain is highly significant because a considerably longer time-interval is required to replenish the food supply, than is needed for beneficial-predaceous adults to reinvade a field. Results from this study support our contention that at least four components must be considered seasonally in developing economic thresholds for pest management programs: 1) the plants. 2) the complex of key pests, 3) the complex of beneficial arthropods, and 4) sources of food in the form of minor «pests» required to support significant numbers of predaceous arthropods in agricultural crops. Our findings indicate that guidelines for the assessment of pests on cotton should consider also their essential role as primary food sources at all population levels, as well as their potential for yield reduction. The value of an adequate food base for predaceous arthropods as provided byTetranychus is relevant for the development of economic thresholds not only for this group, but also for pests that may occur later in the season. A higher predaceous potential was reflected in the greater levels ofGeocoris and ofOrius found on untreated plants with abundant food, as compared with a much lower (O+K treatment) or higher (MP treatment) food (mite) level on insecticide-treated plants. The increased potential for regulation of pest numbers through higher numbers of predators was especially significant, because the higher numbers were present over an extended period, including July–August. In the San Joaquin Valley, an increase in predation potential during this period is highly beneficial, becauseLygus and lepidopterous worms also occur on cotton at this time. We strongly urge that policy guidelines on effects from chemicals on arthropods be re-evaluated. The long-standing concern over disruptive effects of chemicals must be extended to include their impact on the arthropods that constitute the principal sources of food for predaceous arthropods. It is not sufficient to know the direct mortality impact of chemicals against beneficial-predaceous insects. The indirect effect against them via disruption of their food chain is more significant because a considerably longer timeinterval is required to replenish the food supply than is needed for beneficial-predaceous adults to reinvade a field. Results from this study support our contention that at least four components must be considered seasonally in developing economic thresholds for pest management programs: 1) the plants, 2) the complex of key pests, 3) the complex of beneficial arthropods, and 4) sources of food in the form of minor «pests» required to support significant numbers of predaceous arthropods in agricultural crops.