Susceptibility of Liriomyza sativae Blanchard (Diptera: Agromyzidae) populations to reduced risk insecticides
Introduction
The polyphagous and cosmopolitan serpentine leafminer, Liriomyza sativae Blanchard (Diptera: Agromyzidae), is among the major pests of many agricultural and ornamental crops (Reitz et al., 2013). It is one of the most economically important pests of horticultural crops in the Northeast region of Brazil, where it has inflicted yield losses up to 15% in melon (Cucumis melo L.) in the Rio Grande do Norte state (Araujo et al., 2007). Although cultural and behavior-based practices are used to manage it, chemicals sprays are an integral measure to reduce L. sativae population outbreaks in Brazil (Lima et al., 2012). Cyromazine, abamectin and cartap hydrochloride are the most used molecules to control L. sativae in melon crops (AGROFIT, 2019). This narrow portfolio of modes of action (MoA) has curtailed the proper rotation of active ingredients, increasing the risk of resistance evolution. Despite the frequent use of abamectin and cyromazine for several years against L. sativae in many countries, few resistance cases have been reported so far (Cox et al., 1995; Ferguson, 2004; Ferguson and Pineda, 2010; IRAC, 2019; Reitz et al., 2013). However, previous studies showed that Liriomyza spp. can develop resistance to various insecticides from distinct chemical groups compromising sustainable control in the future (Askari Saryazdi et al., 2014; Ferguson, 2004; Ferguson and Pineda, 2010).
The limited number of insecticides used to control L. sativae associated with its frequent outbreaks are potential factors to increase its tolerance to insecticides, rendering them ineffective in the future (Parrella, 1987). Registration of new active ingredients is important to delay the loss of insecticides already in use, allowing an adequate rotation of MoAs. In this context, a baseline of susceptibility of L. sativae in Brazil is needed for any potential new insecticide, because it is the first step of a successful resistance management program. With that baseline, the natural variability of the species’ response to a given insecticide prior to market release may be determined. Spinetoram (a spinosyn) and cyantraniliprole (an anthranilic diamide) were recently commercialized in Brazil against the larval stage, but no data have been published regarding susceptibility of populations to these two insecticides.
There are no reported instances of L. sativae resistance to insecticides to date in Brazil. However, there is a high risk of resistance evolution in this species in Brazil because of the previous use of spinosad and chlorantraniliprole. Resistance to insecticides is an inherent ability of an organism to survive insecticide doses that would be lethal under normal conditions to other individuals of its species (Croft and van De Baan, 1988; Whalon et al., 2008). Other than cases with abamectin and cyromazine (Ferguson, 2004), Askari Saryazdi et al. (2014) reported resistance in Iranian populations of L. sativae to fenpropathrin after selection pressure in greenhouses. Mechanisms involved in the development of insect resistance can include the reduction of penetration through the insect cuticle, target site insensitivity, and/or increased detoxification by metabolism (Hemingway, 2000). Major detoxification enzyme groups include esterase, glutathione S-transferases (GST) and oxidases, which have been observed at significant high titration in resistant populations (Askari-Saryazdi et al., 2015; Askari Saryazdi et al., 2014; Wei et al., 2015). High levels of activity of esterase and monooxygenase have been reported in populations of L. sativae resistant to fenpropathrin (Askari Saryazdi et al., 2014). Additionally, higher GST activities have been reported in L. sativae populations resistant to abamectin (Wei et al., 2015). Thus, these mechanisms of resistance may hinder the management of the leafminer in the field in view of the scarcity of products available for its control. The objectives of this study were to assess the susceptibility of L. sativae to several insecticides, and further investigate detoxification enzymes among populations for possible correlation with susceptibility.
Section snippets
Collection of L. sativae populations
Insects were collected (at least 500 larvae and pupae per population) in the municipalities of Mossoró area, state of Rio Grande do Norte (RN) and Camocim de São Felix, state of Pernambuco (PE), Brazil (Table 1, Fig. 1). The population of Mossoró 1 was obtained from colony at the Laboratory of Applied Entomology at the Federal Rural University of the Semi-Arid (UFERSA), where it has been maintained for >10 years with no insecticide exposure. The other populations were obtained at larval stages
Results
The recommended doses of abamectin and spinosad were effective for all assessed populations of L. sativae, causing mortality of 100% of all individuals. The concentration-mortality data were adjusted to the Probit model (non-significant χ2, P > 0.05) (Table 2). The LC50 values for abamectin ranged from 2.8 (PB1 population, assumed as reference) to 4.9 (MSR2 population) mg a. i./L. The lethal ratios (RR50) ranged from 1.1 (PB2 population) to 1.9-fold (MSR2 population), showing a very low natural
Discussion
The evolution of resistance to insecticides is a major risk for the sustainability of melon agroecosystems and this phenomenon in L. sativae has not been reported in Brazil, though it has been observed in the USA (Ferguson, 2004) and Vietnam (Johansen et al., 2003). This is the first report of susceptibility of L. sativae to insecticides in Brazil and results underline the risk of resistance evolution. The populations responded homogeneously to abamectin, cyantraniliprole, spinosad and
Authors’ contributions
PAFS: Conceived the work, investigation, analysis and wrote the manuscript; HAAS: Conceived the work, funding, analysis and wrote manuscript; WMS: Investigation and analysis; ELA: Investigation and funding; ABEF: Investigation. All authors reviewed and approved the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
To CAPES for granting the scholarship to first author. To the Laboratory of Applied Entomology of the Federal Rural Semiarid University for the support to the project. To Dr. Valdir Balbino from Genetics Laboratory of the Federal University of Pernambuco for identifying the species at molecular level.
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