INSECT PEST-INDUCED LOSSES IN ALFALFA - PATTERNS IN MARYLAND AND IMPLICATIONS FOR MANAGEMENT

Alfalfa (Medicago sativa L.) crop losses induced by alfalfa weevil, Hypera postica (Gyllenhal), and potato leafhopper, Empoasca fabae (Harris), were assessed from scout-generated data in Maryland, 1983-1988. Weevil-induced expected losses averaged from a minimum of $4.25/ha in 1985 to a maximum of $23.80/ha in 1988, whereas leafhopper-induced losses averaged from a minimum of $32.11/ha in 1988 to a maximum of $66.12/ha in 1987. By simulating three pest management programs, we show that a responsive insecticide program (i.e., insecticide used in response to scouting reports) cost an average of $3.11/ha less than a no-action program (i.e., no insecticide applied), and that both of these programs were $20.91 and $17.80/ha less costly, respectively, than the routine prophylactic use of insecticide. In addition, risk, as measured by the variation of loss among fields, was similar in the responsive and prophylactic programs but was greater in the no-action program than in the other two programs. The effect of integrating other control practices on leafhopper management programs was also simulated; these simulations indicated that expected losses are minimized in the no-action program for all years if leafhopper density is reduced by 50%, or if crop tolerance increased 2-fold. However, minimizing risk requires a leafhopper density reduction of almost-equal-to 95%, or 20-fold increase in crop tolerance. Optimal selection of the insecticide-based pest management strategy depends on a prediction of the extent of pest abundance during the year, information on the effect of pest or crop management practices on pest impact, and whether the primary economic criterion for decision-making is loss or risk.