Effects of belowground herbivores on plant-pollinator interactions in cucumber (Cucumis sativus)

Background/Question/Methods

Herbivores can affect plants through both the direct effects of tissue loss and the indirect effect of altering interactions between plants and other organisms.  In particular, by changing plant interactions with mutualists through influences on plant traits, herbivores may enhance or reduce their negative effects.  Because most terrestrial plants interact simultaneously with both above- and belowground communities, root herbivores have the potential to influence plant-pollinator interactions.  Research in two different plant systems has surprisingly found positive effects of root herbivory on pollinator attraction, opposite the more widespread negative effects of leaf herbivory.  Indirect effects of root herbivory may be particularly important in agricultural systems that depend on pollinator services but where belowground herbivores and their impacts are often cryptic.  We manipulated root herbivory in cucumber (Cucumis sativus) using larvae of the specialist striped cucumber beetle (Acalymma vitattum).  We added beetle eggs at four different quantities (0, 25, 50, and 75 eggs/plant) and measured plant growth, reproduction, and interactions with pollinators.

Results/Conclusions

Root herbivores had weak and limited effects on plant growth and flower production, although plants with medium levels of herbivores produced more flowers.  Plants with low levels of root herbivores attracted more total pollinator visits than other levels of herbivory, and this pattern was driven by honey bees, the most abundant pollinator.  Solitary bee visitation was unaffected by herbivory treatments, but butterflies preferred undamaged plants.  The average length of time spent probing individual flowers declined with root herbivore additions for both honey bees and solitary bees.  Given that there were no changes in flower size in response to root herbivory treatments, belowground damage may affect pollinator attraction through changes in nectar quantity or quality, or through effects on floral volatiles.  Despite the demonstrated effects on pollinator attraction and behavior, root herbivore treatments had little influence on fruit production, suggesting that these indirect effects on pollinators are not as strong as other variables influencing plant growth and reproduction such as spatial variation in abiotic factors.