Chemical polymorphism regulates the attractiveness to nymphs in the bean bug Riptortus pedestris

Abstract

Riptortus pedestris (Fab.) is an important pest on soybean plants in East Asia. Females lay eggs scattered not only on host plants, but also on non-host plants, probably for avoiding detection by natural enemies. Newly hatched nymphs can develop to the second instar without the need of food except water. Second instar nymphs, the first-feeding stage with a strong locomotion, need to find suitable feeding sites on host plants. The mechanism is still not well understood. Here, we found second instars were attracted by both male and female adults, but generally repelled by second and fifth instar nymphs, wherein their responses were little affected by food volatiles (soybean seeds or seedlings). Nymphs distinctly differed from adults in chemical composition of the whole-body extracts. 4-oxo-(E)-2-Hexenal was the major component in nymph’s extracts, but it was not found in adults, whereas some ester compounds, such as (E)-2-hexenyl esters, were exclusively found in adults and accounted for the main part in their extracts. The three esters, (E)-2-hexenyl (Z)-3-hexenoate (E2-6:Z3Hx), (E)-2-hexenyl (E)-2-hexenoate (E2-6:E2Hx) and myristyl isobutyrate, were male specific and had been reported as an aggregation pheromone. Synthetic 4-oxo-(E)-2-hexenal strongly repelled second instars, while (E)-2-hexenyl hexanoate emitted by both males and females attracted them. The main component of the aggregation pheromone, myristyl isobutyrate, also strongly attracted second instars. How the findings may improve the efficiency of chemical traps in fields was discussed.