Plant volatiles mediate Aphis gossypii settling but not predator foraging in intercropped cotton

The cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae) is an important pest of cotton and horticultural crops globally. In China, smallholder farmers regularly intercrop cotton with garlic or onion. Aside from higher farm-level revenue, cotton intercrops are typified by lower A. gossypii abundance than monocrops. So far, the mechanistic basis of this lowered pest pressure has not been empirically assessed. Field trials showed that A. gossypii abundance is lower and (relative) abundance of aphid predators higher in early-season cotton intercrops than in monocrops. Cage trials and Y-tube olfactometer tests further indicated that garlic and onion volatiles repel A. gossypii alates. Electrophysiological bioassays and mass spectrometry (GC-MS) identified two physiologically active volatiles i.e., diallyl disulfide and propyl disulfide from garlic and onion respectively. Next, behavioral tests confirmed that both sulfur compounds exert a repellent effect on alate A. gossypii. Garlic and onion volatiles interfere with A. gossypii settling, but do not affect its main (ladybird) predators. Meanwhile, early-season cotton-onion intercrops bear higher numbers of A. gossypii predators and fewer aphids. By thus unveiling the ecological underpinnings of aphid biological control in diversified cropping systems, our work advances non-chemical management of a globally-important crop pest. This article is protected by copyright. All rights reserved.