Lethal effects of insecticides on adults of Eriopis connexa (Germar, 1824) (Coleoptera: Coccinellidae)

The predator Eriopis connexa (Germar, 1824) (Coleoptera: Coccinelidae), is an important biological control agent in wheat fields. In this work, we evaluated the lethal effects of fifteen insecticides used in the wheat crop on adults of the predator E. connexa, using the methodology proposed by International Organization for Biological and Integrated Control (IOBC). Sublethal effects have not been evaluated. The insecticides were sprayed on glass plates that were used to set up the exposure arenas. The chitin synthesis inhibitors diflubenzuron SC, diflubenzuron WP, lufenuron, triflumuron, and the pyrethroid beta-cyfluthrin were considered harmless to adults of the predator and should have their use prioritized. On the other hand, methomyl, lambda-cyhalothrin, thiamethoxam + lambda-cyhalothrin A and B were considered harmful to E. connexa and should be avoided when adults of the predator are present in the crop.

Chemical control with broad-spectrum insecticides is the most used method to control these insect pests. Despite the proven efficiency of this method, the use of these products is associated with several negative effects in the agroecosystem, such as selection of resistant individuals and the resurgence of secondary pests. Also, the indiscriminate use of insecticides can impair the biological control of natural enemies (Pasini et al. 2021).
The predator Eriopis connexa (Germar, 1824) (Coleoptera: Coccinellidae) is an important biological control agent in wheat fields, that can feed on a large number of insect pests, especially the aphid complex that occurs in this crop. This ladybug has high biotic potential and polyphagia. Both larvae and adults can feed on aphids, aleyrodids, mealybugs, mites, insect eggs, and larvae of lepidopterans and coleopterans (Santos et al. 2020).
To achieve success in an Integrated Pest Management (IPM) program, it is essential to integrate chemical and biological control through the use of selective pesticides. Thus, our study aimed to evaluate the lethal effects of insecticides registered to control insect pests in the wheat crop on adults of E. connexa, using the standards proposed by the International Organization for Biological and Integrated Control (IOBC).
Adults of E. connexa used in the bioassays came from a colony kept in the Integrated Pest Management Laboratory of the Federal University of Pelotas (LabMIP/UFPel, RS, Brazil). The coccinellid population was collected in wheat fields from the municipality of Rondinha, RS (27°48'S and 52°54'W) and was reared in laboratory conditions (temperature 25 ± 1 °C, RH 70 ± 10%, photophase 14 h). Larvae of E. connexa were reared with Ephestia kuehniella Zeller, 1879 (Lepidoptera: Pyralidae) eggs ad libitum until the adult stage. Adults were separated into couples and kept in disposable plastic pots (9 cm high × 12 cm in diameter) and fed with E. kuehniella eggs and honey ad libitum.
Fifteen insecticides registered for wheat (Brasil 2021) were selected for the bioassays, as follows; insecticide [chemical group/ active ingredient (maximum dosage recorded for wheat in L ha - The selection was based on the widespread use of these products in the crop by farmers. The insecticides were tested in the maximum field recommended concentration (MFRC), adjusted to correspond to a spray volume of 200 L.ha -1 , using a CO 2 pressurized spray, with a uniform flat spray nozzle (Teejet XR110015EVS). The insecticides were sprayed on glass plates. The working pressure used in the spraying was approximately 50 psi, which corresponded to a spray deposit of 2 ± 0.2 mg.cm -2 (Schmuck et al. 2000).
The bioassays were conducted in the laboratory using the methodology proposed by IOBC (Schmuck et al. 2000). Adults (males and females) of E. connexa were exposed to insecticide residues applied on glass plates. The glass plates served as a background and cover to produce the exposure cages. Each exposure cage was Bioassay composed of a methacrylate ring (10 cm in diameter x 3 cm in height), with five holes of 1.3 cm in diameter closed with "voile" type fabric to allow ventilation. One hole was connected to a suction pump for the elimination of toxic gases. In another hole measuring 0.8 cm, we supplied water to the insects. Eggs of E. kuehniella were provided in strips of paper in sufficient quantity to perform the bioassay. We evaluated male and female mortality as well as total mortality and IOBC selectivity classification at 24, 72 and 120 hours after exposure to dry residues of insecticide.
For this purpose, adults previously separated by sex, with approximately one week old, were added to the exposure cages. We used a completely randomized experimental design. Each treatment consisted of four cages each containing five couples, each cage being considered a repetition in the completely randomized design. In addition to the insecticides tested, the control treatment consisted of the absence of insecticides. Accumulated mortality (number of dead insects) of males and females, as well as overall mortality, were assessed at 24, 72 and 120 hours after exposure to the insecticides.
The data were submitted to the Shapiro-Wilk normality test and homogeneity of variances of Bartlett. When these assumptions were not met, Kruskal-Wallis non-parametric analysis of variance (ANOVA) was performed and the means were compared using the Dunn test, with the Bonferroni correction (p<0.05). The statistical software R 4.0.0 (R Development Core Team 2021) was used to perform the analyzes.
The chitin synthesis inhibitors diflubenzuron SC, diflubenzuron WP, lufenuron, and triflumuron showed mortality ranging from 0.00 to 0.50 (Tab. 1) to males and females; the pyrethroids beta-cyfluthrin and zeta-cypermethrin exhibited mortality rates of 0.00 to 2.25 (Tab. 1) of E. connexa. Due to these mortality values obtained, these insecticides can contribute to the maintenance of the population of the predator in wheat fields.
The insecticides diflubenzuron SC, diflubenzuron WP, lufenuron, and triflumuron were considered harmless (class 1) (Tab. 2) to adults of E. connexa. The selectivity of diflubenzuron SC and WP, lufenuron, and triflumuron is possibly associated with the mode of action of chitin biosynthesis inhibitors. Growth regulating insecticides affect immature stages of insects throughout the molting process and, as a result, adults of non-target species, such as predatory insects, are rarely affected (De Armas et al. 2020). However, despite their reduced lethal effect on adult mortality, these insecticides can negatively other life stages of the predator as eggs, pupae (Pasini et al. 2018) and larvae (Pasini et al. 2021).
The carbamate methomyl showed 100% of mortality after 24 hours of exposure (Tab. 1) and was considered harmful (class 4), while the neonicotinoid thiamethoxam had 95% mortality and was classified moderately harmful (class 3) for adults of E. connexa (Tab. 2).
The insecticide imidacloprid + beta-cyfluthrin showed a 95% mortality and was classified as moderately harmful (class 3), while thiamethoxam + lambda-cyhalothrin A and thiamethoxam + lambdacyhalothrin B killed 100% of the ladybird adults and were classified as harmful (class 4) (Tab. 2).
The pyrethroid beta-cyfluthrin with 27.50% of accumulated adult mortality was considered harmless (class 1). Etofenprox and zetacypermethrin were considered slightly harmful (class 2) showing 52.50 and 35% of mortality, respectively. Permethrin and gammacyhalothrin presented, both, adult mortality of 92.50%, and were classified as moderately harmful (class 3). The insecticide lambdacyhalothrin was classified as harmful (class 4) to adults of E. connexa with 100% of accumulated mortality (Tab. 2). The observed survival in beta-cyfluthrin, etofenprox, and zeta-cypermethrin (Tab. 2) is probably due to enzymatic detoxification or lack of sensitivity of the active site present in the organism of the predator because 24 hours after contact with the insecticide part of the insects were still alive. Pyrethroids contain ester bonds that are susceptible to inactivation by enzymes such as esterases and cytochrome P450 monooxygenases (Yu 2004).
of transmission of nerve impulses between the different orders of insects (Amarasekare et al. 2016). These insecticides can affect not only Coccinellids but also other important orders of predators such as Neuroptera (De Armas et al. 2020;Pasini et al. 2021) and Hemiptera (Machado et al. 2019;Morales et al. 2019). Laboratory tests expose natural enemies to the worst-case scenario of insecticide residues. In our study, the growth regulators diflubenzuron SC, diflubenzuron WP, lufenuron, and triflumuron and the pyrethroid beta-cyfluthrin were considered harmless and should whenever possible, have their use prioritized when adults of E. connexa are present in the crop. Insecticides considered to be slightly harmful, moderately harmful, and harmful to adults of E. connexa should be avoided since show high levels of predator mortality. However, more research is needed to understand the lethal and also, sublethal effects of these pesticides mainly under field conditions, given the importance of this coccinellid for IPM in wheat.