Companion Plants Deter Aphids

Which companion plants affect the performance of green peach aphid on host plants? Testing of 12 candidate plants under laboratory conditions
Entomologia Experimentalis et Applicata
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        Want to deter aphids?  Plant some rosemary throughout the garden, African and French marigolds work well too.  A group of French researchers designed an experiment that tested how effective 12 different companion plants were at deterring aphids from infesting a garden.  The researchers set up two rows of pepper plants inside 13  individual growing chambers (there were 12 companion plants treatments and one control treatment with no companion plant), and then placed adult female aphids on each plant.  Then, the various companion plants were set in between the two rows of pepper plants and allowed to grow alongside each other.  Each day, the scientists would count the number of adult females remaining on the pepper plants, and also the number of neonate nymphs (baby aphids) that were produced by the adult female. 
        During the 12 day experiment, the scientists found that the aphids were more likely to flee from the pepper plants that were near companion plants.  The control pepper plants did not lose any aphids, as the bugs were happy in their companion-plant-free environment.  They also found that  rosemary, lavender, geranium, African marigold, and French marigold significantly decreased the numbers of aphids being reproduced on the pepper leaves.  In the end, rosemary outperformed the other companion plants in both deterring aphids, and lowering their reproduction numbers.  The scientists attribute this effect to the VOC, or volatile organic compounds, produced by the plants.  The VOCs signal to the aphids that the vegetation in a certain area near companion plants is not suitable for rearing young, so encourages the bugs to search on for more reproduction friendly fields.  It looks like a flat of rosemary will be on the planting list for this spring. 

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