Susceptibility of Liriomyza sativae Blanchard (Diptera: Agromyzidae) melon populations to abamectin, cyromazine, and cyantraniliprole

Melon is one of the most exported fruits from Brazil. The leafminer fly, Liriomyza sativae (Blanchard) (Diptera: Agromyzidae), is one of the major problems that negatively interfere in the productivity increase in this crop. Chemical control is the most adopted method for this pest; however, producers have reported reduced susceptibility to insecticides. Thus, this study aimed to evaluate the susceptibility of L. sativae larvae to abamectin, cyromazine, and cyantraniliprole from populations collected from two melon crop-producing regions in Northeast Brazil (Juazeiro, BA and Icapuí, CE). The insecticide’s lethal concentration 50 (LC50) was obtained in the laboratory using the leaf dip bioassay method with L. sativae larvae. No difference was observed in the LC50 between the populations for the three insecticides. Both populations showed low variation susceptibility for abamectin, cyromazine, and cyantraniliprole.

Melons are one of the most exported fruits from Brazil, and in 2021, the country achieved U$ 220 million in income (FAO 2021). The leafminer fly, Liriomyza sativae (Blanchard) (Diptera: Agromyzidae), is one of the major problems that negatively interfere in the productivity increase in this crop (Costa-Lima et al. 2015). As the number of larvae per melon leaf increases, the photosynthesis rate, stomatal conductance, and transpiration decrease .
Synthetic insecticides are the most adopted method for L. sativae control by melon producers. However, the indiscriminate use of insecticides leads to select resistant populations and reduces the occurrence of natural enemies www.bioassay.org.br Sociedade Entomológica do Brasil (Guantai et al. 2015, Gao et al. 2017. Resistant populations of the Liriomyza genus were already reported in China to abamectin (Fergunson 2004, Wei et al. 2014) and the United States to pyrethroids, abamectin, cyromazine, and spinosad (Macdonald 1991, Fergunson 2004).
In the melon-producing regions in Brazil, the problematic control of L. sativae with synthetic insecticides has led to studies aiming to search for different control methods, such as botanical insecticides (Oliveira et al. 2020) and biological control (Costa-Lima et al. 2019). Therefore, the present study aimed to evaluate the susceptibility of populations of L. sativae to three major synthetic insecticides used in melonproducing regions in Northeast Brazil.

Material and Methods
Bioassays were performed using two L. sativae populations collected from melon-producing regions in Juazeiro (BA) and Icapuí (CE). In the laboratory, the populations were reared over cowpea plants, according to Costa-Lima et al. (2017).
Three insecticides were evaluated, all registered for melon crops in Brazil to control the larval stage of leafminers. Increasing concentrations were used until a minimum of five points were obtained, with mortality ranging from zero to 100%. Three products were evaluated with the following active ingredients and concentrations: abamectin (30 to 1,920 ppm), cyromazine (75 to 1,200 ppm), and cyantraniliprole (120 to 2,000 ppm).
For the bioassays, cowpea plants [Vigna unguiculata (L.) Walp.] were exposed to L. sativae breeding cages for 24 hours. After the infestation, the plants were moved to a glasshouse. Every day, the plants were observed under stereomicroscopes for the presence of larvae. For the experiments, plants with larvae with less than 24 hours were used. Distilled water was used as the control. A 500 mL solution was prepared using distilled water for each insecticide concentration test. The method used was adapted from Ferguson (2004), with the solution immersion of leaves with L. sativae larvae for five seconds. After that, the plants were maintained in a climatized room, at 25 ± 2°C, RH of 50 ± 20%, and a 12h photophase. After five days, the number of dead larvae was counted using the stereomicroscope (40x) with transmitted light.
The experimental design was completely randomized. Each larva was considered one repetition, with a minimum of 100 repetitions per treatment. To assess the relative toxicity of insecticides against L. sativae populations, the LC 50 lethal concentration and its 95% confidence interval (CI) were estimated. Therefore, the probit analysis was conducted with the Ecotoxicology package (Gama 2015) from R Software. Significant differences in susceptibility were stated between populations when its 95 % confidence interval did not overlap.

Results and Discussion
No difference regarding the susceptibility of L. sativae was detected between the Juazeiro and Icapuí populations for the three insecticides evaluated. However, when comparing the LC 50 between the three insecticides, the susceptibility to abamectin was lower. No difference was detected in the lethal toxicity between cyromazine and cyantraniliprole for both populations (Table 1).
The LC 50 in the present study for abamectin ranged from 434.45 to 603.51 ppm. These are high values compared to results from resistant L. sativae populations in China (Gao et al. 2017, Wei et al. 2014. In these studies, the highest LC 50 for L. sativae was 1.00 ppm, obtained after a 16 selection cycle during 22 generations, from a population recovered from sponge cucumber in China (Wei et al. 2014). For the closely related species, Liriomyza trifolii (Burgess) (Diptera: Agromyzidae), the LC 50 achieved 30.52 ppm from a population obtained in cowpea plants in China (Gao et al. 2017). Abamectin has been registered for use in melon crops since the 1990 decade in Brazil for leafminer control (Agrofit 2021). The end of the patent had, consequently, new products with abamectin and reduced prices (Paiva 2011). Thus, abamectin became one of the major insecticides sprayed in melon crops for leafminer control. This long-term exposure to abamectin can be related to the low susceptibility observed for both L. sativae populations compared to the other two insecticides.
For cyromazine, the LC 50 ranging from 127.17 to 159.17 ppm also was high compared to other studies with L. sativae (Gao et al. 2017, Wei et al. 2014. The highest LC 50 found in these studies was 12.62 ppm, from L. sativae population collected from cowpea plants (Gao et al. 2017). The same authors obtained an LC 50 of 71.81 ppm for an L. trifolii population. Abamectin and cyromazine were the only registered insecticides recommended to control leafminer larvae in melon crops in Brazil for many years (Guimarães et al. 2005). This long-term exposure could also be related to the low susceptibility from both L. sativae populations to cyromazine.
Diamides are one of the most recent insecticides classes with a different mode of action (Teixeira & Andaloro 2013). In Brazil, cyantraniliprole was registered in the 2010 decade to control L. sativae in melon crops (Agrofit 2021). There are no reports in the literature of Liriomyza spp. populations resistant to diamides or LC 50 for leafminers populations.
The low susceptibility observed for both L. sativae populations for the tested insecticides could be related to resistance to the active ingredients and therefore explain the difficulty to control this pest in melon crops in northeast Brazil. Nonetheless, to elucidate these hypotheses, a study with more populations compared to a susceptible population would be necessary.
We conclude that there are no differences in susceptibility to abamectin, cyromazine, and cyantraniliprole by L. sativae populations from Icapuí and Juazeiro. However, abamectin showed lower toxicity for the populations tested than cyromazine and cyantraniliprole. In addition, high LC 50 values were obtained for abamectin and cyromazine compared to previous studies.