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Twenty-five aphids of three different species, Brevicoryne brassicae L, Myzus persicae Schulzer, and Rhopalosiphum padi L(Hemiptera: Aphididae) were each allowed to infest leaves of a young plant of their respective host plant species for 4 days, except that the oldest expanded leaf (the ‘systemic’ leaf) was kept free of aphids. Each preinfested plant thus had two types of leaves, local leaves (preinfested with aphids) and one systemic leaf, the oldest true leaf that had been kept free of aphids.In subsequent choice tests, settling preference of aphids was tested between the systemic leaves of these preinfested plants and leaves of uninfested control plants. Aphids significantly preferred leaves of control plants in settling choice tests, thus indicating some resistance in the systemic (uninfested) leaves of the preinfested plants. Plant penetration and feeding was further investigated with the electrical penetration graph (EPG) technique using B. brassicae on broccoli, its host plant. The tests included both the systemic and infested (local) leaves of preinfested plants as well as control plants. Aphid-induced resistance in systemic leaves was confirmed by EPG data. Fewer aphids showed phloem feeding on systemic leaves, only 30% as compared to 100% on control leaves. However, on local leaves 100% of the aphids showed phloem feeding, indicating a strong reduction in systemic resistance induced by aphids in these leaves. Phloem factors are the main cause of induced resistance. The possible roles of different phases of salivary secretion in systemically-induced resistance and its local reduction are discussed. In addition to these preinfestation experiments, EPG tests were also done on aphids on broccoli plants that were exposed to volatiles emitted from aphid-infested broccoli plants to compare probing behavior of volatile-induced resistance with systemic resistance due to preinfestation. Phloem factors also appeared to be involved in volatile-induced resistance, although some behavioral details differed.
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