Alternative use of a decision support model to control thrips in leek
Abstract: Onion thrips (Thrips tabaci Lind.) is considered the most important insect pest of leek crops in Southern and Western Europe. Damage consists of silvery mottling – sometimes in longitudinal stripes – resulting from air filled plant cells after puncturing and sucking the sap by thrips. Damage is predominantly cosmetical but high population pressure may additionally hinder plant development, especially at early development stages or in stressful circumstances. Given the high reproduction rates and high mobility of the polyphagous onion thrips, combined with the year round leek cultivation in the Netherlands, population pressure is always considerable, as winter shelters are abundant. In this situation much attention is given to adequate thrips control, mainly based on regular insecticide sprays. Since leek crops are mostly transplanted in the Netherlands instead of direct sown, the crop protection effect of a possible insecticide seed treatment is limited in the field.In the past spray schedules relied heavily on contact insecticides and/or based on the systemic insecticide methiocarb. However limitations both in the number of registered insecticides and in the number of applications per crop have shifted the focus towards alternative thrips control strategies. The growers’ struggle to optimise the use of the three registered active ingredients – deltamethrin, abamectin and spinsosad – within the label restrictions, has led to practical experiments with the Dacom Thrips Prediction Model, a decision support system (DSS). This temperature based thrips prediction model forecasts the occurrence of thrips based on the threshold development temperature and the actual temperature, indicating the optimal spraying moment. Growers are fairly satisfied with the DSS, although one practical drawback appeared to be its inability to anticipate on the onsite thrips population situation.Based on the DSS a new spray strategy was tested in three consecutive field trials in 2009-2011. In a crop planted in early June, thrips control was postponed until the second half of August. Subsequently three or four spray applications alternating spinosad and abamectin at 7-10 day intervals sufficed to control leek damage resulting in the same product quality as with the practical 9-11 spray passes strategy. The tested strategy may open new possibilities to integrated pest management, lowering the total insecticide input and giving more room to non-chemical control options.