IntroductionTop

The members of the Trichogrammatidae family rank among the most important biotic agents employed around the world (Gallego et al., 2019; Araujo et al., 2020; Schäfer & Herz, 2020). The Trichogrammatidae family consists of parasitoids of insect eggs; they are amenable to mass production (Jalali, 2013) and highly efficient for controlling Lepidoptera pests (Feltrin-Campos et al., 2019). Among these pests, the European pepper moth, Duponchelia fovealis Zeller (Lepidoptera: Crambidae), is widely distributed and causes crops losses in Europe, Asia, Africa, and North America (CABI, 2020). Duponchelia fovealis is considered a key-pest for strawberry in European countries (Bonsignore & Vacante, 2010) and was also detected in South America attacking strawberry fields in Brazil (Zawadneak et al., 2017).

In Brazil, registered chemical pesticides to control European pepper moth on strawberry (Ministério da Agricultura, Pecuária e Abastecimento, 2020) are lacking, which has led farmers, agricultural technicians and researchers to seek for alternatives, including biological control agents such as parasitoids of the Trichogrammatidae family (Rodrigues et al., 2017). Since the Trichogramma genus consists of cosmopolitan parasitoids, understanding host interactions is an essential step for developing a biological control strategy, as different species exhibit variations in host preference (Paes et al., 2018; Tabebordbar et al., 2020). Trichogramma galoi Zucchi and Trichogramma pretiosum Riley proved promissory to be used in the biological control of D. fovealis (Paes et al., 2018). However, non-selective pesticides in agricultural production systems pose a limitation to the successful implementation of biological control programs employing Trichogrammatidae due to their sensitivity to insecticide active ingredients (AIs) (Gallego et al., 2019). Furthermore, the effect of pesticides used in strawberry crops in Brazil on the fitness of Trichogramma species for D. fovealis eggs has been scarcely studied (Rodrigues et al., 2017). In this context, the current work aimed to assess the toxicity and sublethal effects of insecticides registered and currently used in strawberry cultivation in Brazil on T. pretiosum by applying them on D. fovealis eggs in pre-parasitism non-choice tests.

Material and methodsTo

Insects

Duponchelia fovealis were obtained from a laboratory colony (Federal University of Paraná, Curitiba, Paraná, Brazil) established from wild locally-collected insects and reared at 25 ± 2 °C, 70 ± 10 % RH (relative humidity), and a 14:10 h light:dark photoperiod as described by Zawadneak et al. (2017). Adults were kept in plastic cages (17 × 15 cm) and fed on a nutritional solution developed by Zawadneak et al. (2017). The walls of these cages were wrapped with a paper towel for egg deposition. Eggs adhered to the paper towel were transferred on the same paper towel to Petri dishes (90 × 15 mm; ten eggs per dish) until larval emergence, and then transferred to glass test tubes (2.8 × 8.5 cm) covered with cotton wrapped in a piece of voile. Adults of D. fovealis were fed using cotton moistened into a solution containing beer, honey and water.

A colony of T. pretiosum was purchased from Promip® (Engenheiro Coelho, São Paulo, Brazil) and reared in the laboratory into a climatic chamber (25 ± 2 °C, 70 ± 10 % RH, and a 14:10 h light:dark photoperiod). The wasps were reared on UVL (ultraviolet light)-sterilized eggs of Zeller (Lepidoptera: Pyralidae) from a laboratory colony. The eggs were glued with arabic gum (30 %) onto blue paper cards (4 × 1 cm) and exposed to adult T. pretiosum in glass vials (10 × 1.5 cm). After 24 h of exposure, the cards were transferred to new glass vials, where they were held until adult emergence. Adult T. pretiosum were provided with honey as droplets smeared on the inside wall of the glass vials.

Insecticides

Seven commercial formulations, with different insecticide AIs were used (IRAC, 2020), as listed in Table S1 [suppl]. These compounds were selected because of their current and main use in the chemical management of pests in strawberry crops in Brazil. The doses tested were the maximum authorized or recommended by the manufacturer. Application rates of the insecticide formulations were prepared by diluting the products in distilled water according to the manufacturer’s instructions for the maximum field dose allowed.

Bioassays with the adult stage of Trichogramma pretiosum

Previously non-parasitized D. fovealis eggs were treated by dipping the cards (20 eggs each) into the insecticide dilutions or control solution for 5 seconds according to de Paiva et al. (2018). Then, the cards were air-dried for 1 hour and then transferred to glass Petri dishes (9 × 1 cm) with filter paper to remove moisture excess at ambient temperature. Each card was offered to a T. pretiosum female (24 h old) separately in a glass vial (5 × 1 cm). After 24 h of exposure, the cards were removed and transferred to a clean glass vial until parasitoid emergence. The female was fed on honey droplets. The side-effects of the insecticides were quantified by measuring the mortality of T. pretiosum females in 24 h, longevity of the females after the exposure to the insecticides, parasitism rate (by counting the number of dark eggs), emergence rate (eggs with exit holes/parasitized eggs) and offspring sex ratio (number of females/total number of insects).

After emergence, female offspring were isolated in glass vials and a card with 20 untreated D. fovealis eggs was offered to each female for parasitism during 24 h. The estimated parameters, observed on a daily basis, were progenitor mortality rate, number of parasitized eggs, emergence rate, sex ratio of the offspring and longevity (survival time until death).

In both bioassays, distilled water was used as a control. The bioassays were conducted under controlled conditions (25 ± 2 °C, 70 ± 10 % RH, and a 14:10 h light:dark photoperiod). In the case of the bioassay on the parental generation of parasitoids, every treatment was repeated four times and there were 10 replicates, corresponding to 10 host egg cards (20 eggs each), dipped into one of the seven insecticide solutions or water (n = 40 per treat-ment). In the case of the bioassay on the first generation, the number of cards varied between 9 and 46 depending on the parasitoid availability, which was affected by the previous bioassay.

Statistical analysis

The experiments followed a completely randomized design. Data did not follow the normality and homocedas-ticity assumptions and, consequently, were analyzed by the non-parametric Kruskal-Wallis test, and means were separated using the Dunn’s test at 5% significance level. Data analyses were performed with R 3.6.2 environment (R Core Team, 2019).

Subsequently, the percentage reduction in emergence from parasitized eggs, adult longevity and percentage of parasitism relative to the control was evaluated using this equation: E(%) = [1 – (Q/q) × 100)], where E is the per-centage of reduction of the capacity of a given biological feature, Q is the average value of the analyzed parameter for a given insecticide, and q represents the mean value of the parameter obtained in the control. Based on the re-sults, each insecticide was classified according to the In-ternational Organization for Biological Control (IOBC) criteria for laboratory tests: class 1 = harmless (E < 30% reduction of emergence, longevity, or fecundity), class 2 = slightly toxic (30% ≤ E ≤ 79% reduction), class 3 = moderately toxic (80% ≤ E ≤ 99% reduction), and class 4 = toxic (E > 99% reduction) (Amano & Haseeb, 2001).l