Plants are frequently attacked by both above- and belowground arthropod herbivores.
Nevertheless, studies rarely consider root and shoot herbivory in conjunction.
Here we provide evidence that the root-feeding insect Agriotes lineatus reduces the
performance of the foliage feeding insect Spodoptera exigua on cotton plants. In a
bioassay, S. exigua larvae were allowed to feed on either undamaged plants, or on
plants that had previously been exposed to root herbivory, foliar herbivory, or a
combination of both. Previous root herbivory reduced the relative growth rates as
well as the food consumption of S. exigua by more than 50% in comparison to larvae
feeding on the undamaged controls. We found no effects in the opposite direction, as
aboveground herbivory by S. exigua did not affect the relative growth rates of
root-feeding A. lineatus. Remarkably, neither did the treatment with foliar herbivory
affect the food consumption and relative growth rate of S. exigua in the bioassay.
However, this treatment did result in a significant change in the distribution of S.
exigua feeding. Plants that had been pre-exposed to foliar herbivory suffered significantly
less damage on their young terminal leaves. While plant growth and foliar
nitrogen levels were not affected by any of the treatments, we did find significant
differences between treatments with respect to the level and distribution of plant
defensive chemicals (terpenoids). Exposure to root herbivores resulted in an increase
in terpenoid levels in both roots as well as in mature and immature foliage. Foliar
damage, on the other hand, resulted in high terpenoid levels in young, terminal leaves
only. Our results show that root-feeding herbivores may change the level and
distribution of plant defenses aboveground. Our data suggest that the reported
interactions between below- and aboveground insect herbivores are mediated by
induced changes in plant secondary chemistry.
only. Our results show that root-feeding herbivores may change the level and
distribution of plant defenses aboveground. Our data suggest that the reported
interactions between below- and aboveground insect herbivores are mediated by
induced changes in plant secondary chemistry.