Ovicidal activity of selected insecticides against the brown stink bug, Euschistus heros, and their toxicity to its egg parasitoid, Telenomus podisi, 2023 Open Access

A laboratory test was conducted to determine the differential impact of insecticides on eggs of the brown stink bug, Euschistus heros (F.), and its parasitoid, Telenomus podisi Ashmead, in its immature phases inside the host egg, aiming to select those insecticides most toxic against the pest and harmless to the natural enemy. The bioassay was conducted at the Laboratory of Integrated Pest Management, Luiz de Queiroz College of Agriculture, University of São Paulo, Piracicaba, SP, Brazil, from Jan to Aug 2023. The E. heros and T. podisi colonies, obtained initially from soybean crops (Piracicada, SP, Brazil, 22°50ʹ03ʹS, 48°01ʹ40ʹW), in Dec 2020, were reared in laboratory conditions at 70 ± 10% relative humidity, 25 ± 1 °C, and 14:10 L:D h cycle.

The ovicidal effect bioassay consisted of gluing 48-h-old E. heros eggs in rectangular cards (1.0 × 5.0 cm). Five cards with 60 ± 20 E. heros eggs per treatment were used. For the insecticide side-effect bioassay, couples of T. podisi were initially established and maintained for 24–36 h in glass vials (2.4 cm diameter × 8.0 cm height) containing pure honey as food for mating. Subsequently, 2 females (mated, fed, and without foraging experience with the host eggs) were transferred to a glass vial of the same size containing a drop of pure honey as food and an 18-h-old egg mass of E. heros (cards with 60 ± 20 E. heros eggs). The parasitism was allowed for 24 h. Two females for each card with 60 ± 20 E. heros eggs were used, totalizing 5 cards with 60 ± 20 E. heros eggs per treatment.

Eleven commercial insecticides were selected as treatments for both bioassays (ovicidal effect on E. heros and side effect on T. podisi). The design of this experiment was completely randomized with 11 treatments and 5 replications. The nonparasitized 48-h-old E. heros eggs and parasitized E. heros eggs at 72 h (“larval phase” of parasitoid inside the host) and 216 h (“pupal phase” of parasitoid inside the host) were treated with equivalent field rates of each insecticide adjusted to a volume of 200 L/ha. Thus, cards with eggs in the mentioned different times were completely immersed in the insecticide solution for 5 s. After the immersion, the treated eggs were placed onto paper towels and allowed to dry.

The insecticide effects on E. heros and T. podisi mortalities were assessed by the counting of E. heros-hatched nymphs and T. podisi-emerged adults from insecticide-treated eggs and compared among treatments with Kruskal–Wallis nonparametric analysis of variance using Dunn–Bonferroni post hoc test (P < 0.05). The mortality rates were also corrected for control using the Henderson and Tilton equation. Furthermore, a degree of differential toxicity was obtained by the ratio between the percentage of T. podisi emerged adults and the percentage of E. heros hatched nymphs. As an interpretation of these results, the higher value obtained means that the insecticide is concomitantly more toxic against the pest (E. heros) and more harmless to the natural enemy (T. podisi).

Results showed that insecticides Bold (fenpropathrin + acetamiprid), Maxsan (dinotefuran + pyriproxyfen), and Expedition (lambda-cyhalothrin + sulfoxaflor) provided the most significant ovicidal effects on brown stink bug, reducing the number of hatched nymphs by ≥85% (Tables 1 and 2). Except for Orthene (acephate) and Dimilin (diflubenzuron; “positive control”), all insecticides significantly reduced the egg parasitoid adults’ emergence when larvae and pupae inside the host were exposed to insecticides (Table 1). The less toxic effects for the larval phase of the egg parasitoid were achieved by Orthene, Pirephos (fenitrothion + esfenvalerate), Bold, and Expedition, whereas effects for the pupal phase were achieved by Orthene, Maxsan, Pirephos, and Bold, with mortality rates ≤35% (Table 2). According to the differential toxicity degree obtained (Table 2), the top 3 more toxic insecticides against the brown stink bug eggs and more harmless to the egg parasitoid were Bold, Maxsan, and Expedition.

Conflict of interest

This research was part of the routine pesticide screening work of the Laboratory of Integrated Pest Management (ESALQ-USP). No special research funds were allocated for this work.