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The response of plant cultivars to infestation by single aphid species has been widely reported, whereas the response of plants to multiple aphid species, and the mechanisms by which various aphids benefit from the induced changes, has been largely ignored. Here we analyzed the symptoms and physiological responses of sweet orange, Citrus sinensis (L.) Osbeck (Rutaceae), as affected by Aphis spiraecola Patch and Aphis (Toxoptera) citricidus (Kirkaldy) (Hemiptera: Aphididae). We further compared the feeding behavior, population abundance, and proportion of winged morph of the two aphid species when separately feeding on sweet orange. Results showed that feeding by A. spiraecola, but not A. citricidus, caused leaf curling, reduced shoot length, shoot fresh weight, and chlorophyll concentration. In addition, A. spiraecola feeding increased the concentration of essential amino acids in leaves when compared with A. citricidus and uninfested control plants. Electrical penetration graph-studies showed that A. citricidus gained faster access and acceptance of phloem, and spent more time ingesting phloem sap, than A. spiraecola. Furthermore, after 2 weeks of infestation, A. spiraecola produced more winged morphs, whereas A. citricidus achieved higher population abundance. Thus, our results indicate that the two citrus aphids utilize divergent strategies to manipulate host metabolism, and gain fitness benefits in population abundance or winged form production. The results could improve our understanding of varied adaptive strategies of co-existing insects and extend our knowledge of aphid-plant interactions.

Indirect interactions between herbivorous insects that share the same host have been focused on insects feeding on herbaceous plants, while few studies investigate similar interactions on woody plants. We investigated performance and feeding behavior of two citrus aphids, Aphis spiraecola Patch and Toxoptera citricida Kirkaldy, on sweet orange as affected by prior infestation of conspecifics and heterospecifics. Results showed that pre-infestation-induced interactions between A. spiraecola and T. citricida were asymmetric, with A. spiraecola gaining more fitness. In detail, pre-infestation by A. spiraecola decreased adult weight, enhanced survival rate and accelerated phloem sap acceptance of conspecifics. However, A. spiraecola pre-infestation did not affect performance or feeding behavior of T. citricida. In another infestation sequence, the pre-infestation of T. citricida did not affect conspecifics, but positively affected heterospecifics, indicated as a decreased pre-reproductive period, enhanced survival rate, adult weight, fecundity, and feeding efficiency, i.e., faster access and acceptance of phloem sap, and longer phloem sap ingestion duration. Furthermore, we found A. spiraecola pre-infestation enhanced amino acid concentration, amino acid to sugar ratio, activated salicylic acid and jasmonic acid marker gene expression, while T. citricida pre-infestation only depressed jasmonic acid marker gene expression. Changes in nutrient and phytohormone-dependent defense probably underlie the asymmetric effect.

The diamondback moth (DBM) is a destructive pest of crucifer crops. In this study, DBM larvae shown to herbivore induced plant volatiles (HIPVs) that were attractive to adult females exposed in a Y-tube olfactometer. Our results showed that olfactory responses of adult females to HIPVs induced by third instar larvae feeding on Barbarea vulgaris were significantly higher (20.40 ± 1.78; mean moths (%) ± SD) than those induced by first instar larvae (14.80 ± 1.86; mean moths (%) ± SD). Meanwhile, a significant concentration of Sulphur-containing isothiocyanate, 3-methylsulfinylpropyl isothiocyanate, and 4-methylsulfinyl-3-butenyl isothiocyanate were detected in HIPVs released by third instar larvae compared to those released by first instar larvae while feeding on B. vulgaris. When the DBM females were exposed to synthetic chemicals, singly and in blend form, a similar response was observed as to natural HIPVs. Our study demonstrated that the relationship between isothiocyanates acting as plant defense compounds, host plant cues emission and regulation of the DBM adult female behavior due to key volatile triggered by the DBM larvae feeding on B. vulgaris.