Evaluation of the susceptibility to emamectin benzoate and lambda cyhalothrin in European populations of Cydia pomonella (L.) (Lepidoptera: Tortricidae)

Highlights

• The susceptibility to two insecticides was tested in many codling moth populations.

• Small range of responses and no resistant strains were found for emamectin benzoate.

• A wider variability and a resistant population were detected for lambda cyhalothrin.

• 0.021 mg a.i./kg is suggested as discriminant concentration for emamectin benzoate.

• A discriminant concentration of 0.60 mg a.i./kg is proposed for lambda cyhalothrin.

Abstract

The codling moth Cydia pomonella (L.) (Lepidoptera: Tortricidae) is one of the key pests of pome fruit and walnut in almost all growing regions of the world and has developed resistance to several insecticides. In this study, the susceptibilities of 15 codling moth populations to emamectin benzoate and 9 populations to lambda cyhalothrin collected in five European countries were measured by standard diet incorporation bioassays. Variation in susceptibility was observed among populations, with LC₅₀ values ranging from 0.0017 to 0.0119 mg a.i./kg diet for emamectin benzoate and from 0.033 to 0.292 mg a.i./kg diet for lambda cyhalothrin. Our results revealed only small variations in emamectin benzoate susceptibility between populations, indicating no selection of resistance to this active ingredient. Even though a wider range of responses was detected for lambda cyhalothrin, our results also suggest that populations remain susceptible to this insecticide. Based on pooled LC₉₅ evaluations, we propose the use of concentrations 0.02 mg a.i./kg of diet for emamectin benzoate and 0.60 mg a.i./kg of diet for lambda cyhalothrin in order to discriminate between resistant and susceptible individuals.

Introduction

The codling moth Cydia pomonella (L.) (Lepidoptera: Tortricidae) is one of the key pests of pome fruit and walnut in almost all growing regions of the world. Codling moth has a high damage potential and can cause total yield losses (Geier 1964; MacLellan 1976). Although non-chemical control methods such as mating disruption, insect-proof nets and Cydia pomonella granulosis virus (CpGV) are available, codling moth management mostly relies on chemical insecticides. The long-term use of pesticides has selected populations resistant to insecticides with different modes of action, including neurotoxic insecticides, insect growth regulators and granuloviruses (Bosch et al., 2018a).

Selection of resistant populations of codling moth can be dated back to the late 1920s, when resistance to arsenate insecticides was reported in the USA (Hough 1928). Since 1990, there have been reports of codling moth populations resistant to several insecticidal classes, including neonicotinoids, benzoylureas, macrocyclic lactones and older compounds such as pyrethroids or organophosphates (Reyes et al., 2007). Pesticide resistance is a major threat to pest control and causes control failures in numerous cropping systems worldwide, thus proactive resistance management is recommended to maximize the lifespan of pesticides. Monitoring pest susceptibility can provide the basis to understand the status of insecticide resistance and to develop a successful strategy to delay its occurrence (Roditakis et al., 2013).

Emamectin benzoate derives from a fermentation product of the soil microorganism Streptomyces avermitilis (Burg et al., 1979). According to the IRAC Mode of Action (MoA) classification, it is included in group 6 acting as an allosteric activator of glutamate-gated chloride channels in the insect nervous system. Emamectin benzoate shows translaminar movement through the leaf blade and has a residual activity on leaf-chewing lepidopteran species. However, residues on foliage surfaces are rapidly broken-down by sunlight (Feely et al., 1992) and contact activity on beneficial arthropods is limited to short periods (Depalo et al., 2017). This makes the use of emamectin benzoate particularly suited for control of lepidopteran pests in IPM strategies.

Emamectin benzoate was registered in 2000 for codling moth control in Europe and other countries (Ioriatti et al., 2009). To our knowledge no resistance of tortricid moths was previously recorded to emamectin benzoate. However, a few cases have been reported for other lepidoptera such as Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) from Pakistan (Ishtiaq et al., 2014; Ahmad et al., 2018) and China (Che et al., 2015) and Plutella xylostella (L.) (Lepidoptera: Plutellidae) from China (Pu et al., 2010).

Lambda cyhalothrin is a pyrethroid insecticide, i.e. a synthetic chemical analogue of pyrethrins found in chrysanthemums (i.e. Chrysanthemum cinerariaefolium and C. coccineum). Pyrethroids have been widely used since the 1970s on many crops to control insect pests, including Hemiptera, Coleoptera and Lepidoptera. The use of pyrethroid insecticides has increased in recent years, probably due to the withdrawal of the organophosphate chlorpyrifos (He et al., 2008; Hites 2021). Since commercialization, pyrethroids have been largely used all over the world to control codling moth, thus favoring the selection of resistant strains to this class of insecticides (Soleño et al. 2008, 2019). Lambda cyhalothrin has been available for the control of codling moth since the 1980s (Roush et al., 1990) and populations resistant to lambda cyhalothrin have been detected since 2000 in Europe (Bosch et al., 2018b), Northeastern China (Wei et al., 2020) and North America (Mota-Sanchez et al., 2008).

In orchards where chemicals are widely used to decrease pest pressure and crop rotation is not possible, insecticide resistance management (IRM) is crucial (Sparks and Nauen, 2015). Regular monitoring is the core of IRM and is essential to manage insecticide resistance. Therefore, the susceptibility of pests should be regularly assessed when any insecticide is used extensively. When an a.i. is recommended for pests with known problems of resistance, proactive resistance monitoring should be performed even before product commercialization.

In this 4-year survey we examined the susceptibility of 15 codling moth populations to emamectin benzoate and 9 populations to lambda-cyhalothrin across five European countries. The aims of the study were to (1) estimate lethal concentrations (LC₅₀ and LC₉₅) for each population to both insecticides; (2) check the occurrence of a potential shift in the responses of codling moth populations to emamectin benzoate and lambda cyhalothrin; (3) provide an overall European measure of susceptibility to these active ingredients for tracking possible future alterations in the efficacy of the products.