Use of (E)-2-octenal, a major alarm pheromone of Leptocorisa chinensis, to manage conspecifics in paddy fields

(E)-2-Octenal (E-2-OAL) is a major component of alarm pheromones in the rice-ear bug, Leptocorisa chinensis. Cage experiments revealed that 1 % emulsified E-2-OAL effectively induced escape behavior in adult L. chinensis. In the commercial paddy fields, a treated plot (5 × 5 m) was established with 16 rods (1 m high, spaced 1.5 m apart in 4 × 4 formations), each topped with a glass Petri dish containing a 1 % E-2-OAL solution. A control plot of similar size was established 30 m from the treated plot in the same field. Experiments were carried out in six paddy fields during the milk-ripe stage of rice plants. We observed a significant reduction in pecky rice grains likely caused by L. chinensis in the E-2-OAL-treated plots compared with that in the control plots, successfully maintaining damage levels below the economic injury level.

Methods
Cage bioassay

E-2-OAL was purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). The compound was dissolved in methanol (10 % v/v) and subsequently diluted with distilled water to obtain 1, 0.1, and 0.01 % emulsified solutions.

To determine the effective concentration of emulsified E-2-OAL solution, we conducted a nylon mesh cage bioassay in the fields. Five undisturbed female L. chinensis were placed in a mesh cage (30 × 30 × 50 cm, covered with a 2 mm nylon mesh). A small Petri dish (diameter: 10 cm, height: 3 cm) containing 50 mL of 1, 0.1, or 0.01 % solution was positioned 30 cm upwind from the cage. We observed the behavior of females (standing still, grooming, wandering, and flying downwind) for 5 min. The experiments were repeated six times for the 0.01 and 0.1 % solutions and 14 times for the 1 % solution over three experimental days.

Paddy field experiments

We used emulsified E-2-OAL solution-filled Petri dishes of the same size as the cage bioassay. Each Petri dish containing 50 mL of 1% E-2-OAL solution was placed on a plastic board (26 × 26 cm) with a roof of similar dimensions to protect it from rain and direct sunlight. The board was placed on a 1-meter-long plastic rod called a dispenser.

Experiments were conducted in three commercial paddy fields in the Sayoh (var. Koshihikari) and Aogaki (var. Hinohikari), Hyogo Prefecture, Japan. The experiments were initiated before the rice plants reached the milk-ripe stage. In this study, L. chinensis was the predominant bug species in paddy fields. Pecky rice grains infested with L. chinensis had distinctive brownish margins surrounding the damaged site.

A treated plot (5 × 5 m) was established in each paddy field with 16 dispensers arranged in a 4 × 4 square formation spaced approximately 1.5 m apart. A control plot of similar size, without dispensers, was established 30 m from the treated plot. Approximately 49 rice bunches were planted in each plot. The E-2-OAL solution in the Petri dishes was replaced daily. No insecticides were used in the field until harvest. Following harvest, we counted the pecky rice grains exhibiting characteristic brownish margin damage, likely caused by L. chinensis. In the first, second, and third trials, 16, 12, and 21 rice bunches were selected, respectively. For statistical analysis, since the arcsine transformation of the data did not achieve homoscedasticity, a t-test was conducted after applying the Box-Cox transformation, with the total grain count used as weighting in JMP version 14.2.0 (SAS Institute, 2018).