Action: Incorporate parasitism rates when setting thresholds for insecticide use
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Pest damage: One controlled study from New Zealand found using parasitism rates to inform spraying decisions resulted in acceptable levels of crop damage from pests. Effects on natural enemy populations were not monitored.
The crop studied was tomato.
This involves monitoring parasitism rates of pests by natural enemies and adjusting thresholds for insecticide use accordingly. Conventional threshold-based insecticide use (such as in integrated management) monitors pest populations or crop damage to schedule insecticide applications, but may not consider the action of natural enemies. Parasitoids can reduce pest populations, but there may be a lag between pest population increase and parasitoid population increase. If pests are killed by insecticides during this lag period, parasitoids may also be killed, preventing the parasitoid population from increasing and limiting the ecosystem service they provide. Monitoring parasitism rates to decide whether or not and when to spray is intended to avoid this and reduce unnecessary use of insecticides.
Supporting evidence from individual studies
A controlled study in 2000-2002 in Hawke’s Bay, New Zealand (Walker et al. 2010) found that tomato Solanum lycopersicum damage from cotton bollworm Helicoverpa armigera larvae did not exceed the commercially acceptable level of 5% on 16 of 17 occasions when treatment decisions were based on parasitism-adjusted pest thresholds. Only 1.2-5.5% of tomatoes were damaged in 11 fields where decisions to not spray crops used thresholds accounting for parasitism (damage exceeded the acceptable 5% level in only one field), and 3.0-3.4% were damaged in two sprayed fields where conventional thresholds (using pest but not parasitism levels) were used. Tomato damage averaged 3.9-7.1% in three unsprayed fields where cotton bollworm numbers exceeded parasitism-adjusted threshold levels. Treatment decisions were made for 22 fields which met or exceeded a conventional threshold of one cotton bollworm larvae/plant, suggesting spraying was necessary. However, in 16 fields and one half-field, crops were only sprayed if bollworm numbers exceeded thresholds adjusted for site-specific parasitism rates (ranging 1-8.3 larvae/plant). Controls included two fields sprayed when only the conventional pest threshold was exceeded, and three fields and one half-field left unsprayed despite exceeding all thresholds. Insecticides included spinosad and Bacillus thuringiensis pesticidal bacteria. Fruit damage was assessed for 40 random plants/field.