The Role of Plant Growth Regulators in a Plant–Aphid–Parasitoid Tritrophic System

Abstract

Plant growth regulators (PGRs) play important roles in regulating plant growth. Application of PGRs may conversely affect the tritrophic plant–herbivore–parasitoid interactions. To manipulate the growth of winter wheat for rearing the English grain aphid, Sitobion avenae, which is used to rear an aphid parasitoid, Aphidius gifuensis, we determined the effects of four commonly used PGRs, indole-3-acetic acid (IAA), naphthalene acetic acid (NAA), gibberellic acid (GA3), and one plant growth retardant paclobutrazol (PBZ), on winter wheat seedlings and their indirect effects on the aphids and the parasitoids. Foliar application of IAA, NAA, and GA3 to wheat seedlings significantly affected the growth of the winter wheat plants, promoted plant height and leaf length, and increased fresh and dry weights. In contrast, PBZ had a negative effect on wheat growth. The PGRs did not affect the development, but had negative effects on fecundity and intrinsic rates of natural increase of S. avenae. The PGRs also exhibited negative effects on the parasitoids by having lower parasitism, emergence rate, and proportion of females when the host (aphids) fed on the PGRs-treated winter wheat seedlings than on untreated wheat plants. In conclusion, the PGRs-mediated effects strongly impact not only plant growth, but also the herbivores and the parasitoids. Our results suggest that the PGRs should be cautiously used to regulate plant growth if aphid parasitoids are dominant natural enemies. The information should be useful for understanding potential consequences related to biological control of aphids via parasitoids when plant growth regulators are used on crops.