Spider mites were more fit when the ate plant tissues with intact rhizosphere mutualists. Females laid the most eggs when feeding on +AMF/+NFB plants, closely followed by -AMF/+NFB and +AMF/-NFB treatments. Quite clearly, -AMF/-NFB treatments yielded the least fit spider mites, producing the fewest number of eggs. This spider mite fitness experiment mirrored the mite host plant choice experiment, with mites having a much stronger preference for plant tissues that grew with both mutualists intact.

This new research changes the way we look at mutualisms within the rhizosphere. Without a doubt, AMF and NFB enhance plant growth, but because plants with these intact mutualists attract more herbivores, it begs the question, what does symbiosis actually entail? If fungal and bacterial mutualists entice more herbivores to feed on that specific plant, and if those herbivores can reproduce more efficiently on these rhizosphere rich plants, would these plants actually do better without these mutualists? This study indicates that plant fitness may be indirectly reduced by its own mutualists. Soil ecology for reasons like this are so extremely difficult to untangle. One thing is for sure though. Belowground species interactions alter aboveground species interactions. Whether or not these mutualisms enhance or reduce plant fitness is a whole other question that will take some careful, long-term, studies across a soil nutrient gradients.

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