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Providing evidence to improve practice

Action: Incorporate parasitism rates when setting thresholds for insecticide use Natural Pest Control

Key messages

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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.

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.

Referenced papers

Please cite as:

Wright, H.L., Ashpole, J.E., Dicks, L.V., Hutchison, J., McCormack, C.G. & Sutherland, W.J. (2019) Some Aspects of Enhancing Natural Pest Control. Pages 603-625 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK