Relative toxicity of insecticides to the crucifer pests Plutella xylostella and Myzus persicae and their natural enemies Farida Anjum * , Denis Wright Department of Life Sciences, Imperial College London, Silwood Park Campus, Ascot, SL5 7PY, UK article info Article history: Received 27 November 2015 Received in revised form 30 May 2016 Accepted 1 June 2016 Keywords: Insecticides Chinese cabbage Plutella Cotesia Myzus Aphidius abstract The widespread and intensive use of conventional pesticides, particularly insecticides, presents a major risk to natural enemies of target pests, as well as to the environment in general. The aim of this study was to investigate the differential intrinsic toxicity of insecticides to two key pests of crucifers, Plutella xylostella and Myzus persicae and their respective hymenopteran parasitoids, Cotesia vestalis and Aphidius colemani. Such knowledge can help inform effective integration of insecticides and biological control in IPM systems. Three insecticides generally regarded as being compatible with natural enemies (aba- mectin, spinosad and indoxacarb) and one compound regarded as harmful to natural enemies (lambda- cyhalothrin) were examined. A comparative measure of the intrinsic toxicity of fresh deposits of in- secticides on Chinese cabbage leaf discs was determined for both pest and parasitoids species after exposure to insecticide for 24 h and 120 h, and after 24 h exposure to insecticide plus 96 h on untreated leaf discs. Differences in the susceptibility of pests and parasitoids to different insecticides were marked for P. xylostella and C. vestalis, LC 50 values being significantly lower for the pest species. Such differences were not observed for M. persicae and A. colemani. There was a direct relationship between dose, exposure time and toxicity for all insecticides tested. All insecticides tested showed lower toxicity to both parasitoids compared with P. xylostella, which suggests that for this pest species side-effects on para- sitoids can be minimised through IPM practices that reduce exposure time to such non-target organisms. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction The diamondback moth Plutella xylostella L. (Lepidoptera: Plu- tellidae) is an oligophagous species, which is the most important cosmopolitan pest of crucifer crops and has become particularly difficult to control because of its ability to develop resistance to pesticides (Furlong et al., 2013; Johnson et al., 2009). The overuse of insecticides against P. xylostella has also resulted in damaging ef- fects on its natural enemies, particularly key species such as the larval endoparasitoid, Cotesia vestalis Kurdjumov (syn. Cotesia plu- tellae) (Hymenoptera: Braconidae) (Li et al., 2007; Haseeb and Amano, 2002). Cotesia vestalis parasitizes all four larval instars of P. xylostella, preferring the second and third instars (Shi et al., 2002). The peach-potato aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) is a highly polyphagous species, which feeds on 40 different families of plants (Blackman and Eastop, 2000). Overuse of pesticides has also resulted in widespread resistance in M. persicae (Foster et al., 2007; Barber et al., 1999) and negative impacts on non-target organisms (Foster et al., 2012). Aphidius colemani (Hy- menoptera: Braconidae) is one of the important parasitoid species for M. persicae and is available commercially for biological control of aphids on horticultural crops (Jones et al., 2003; Fern andez and Nentwig, 1997). Today, many insecticides with a relatively narrow range of ac- tivity are marketed against specific groups of insect pests; these compounds can be far less harmful to the beneficial insects, including natural enemies such as C. vestalis (Cardwell et al., 2005). Abamectin, indoxacarb and spinosad are insecticides generally regarded as reduced risk or compatible with natural enemies (Liu and Zhang, 2012; Zhao et al., 2006) and lambda-cyhalothrin is regarded as harmful to natural enemies (Tillman and Mulrooney, 2000). Knowledge of the potential impact of pesticides on parasitoids and other natural enemies is important for the successful integra- tion of chemical and biological control measures (Blumel, 2004). Adult parasitoids can be directly affected during spray application * Corresponding author. E-mail address: faragric@gmail.com (F. Anjum). Contents lists available at ScienceDirect Crop Protection journal homepage: www.elsevier.com/locate/cropro http://dx.doi.org/10.1016/j.cropro.2016.06.002 0261-2194/© 2016 Elsevier Ltd. All rights reserved. Crop Protection 88 (2016) 131e136