According to the wikibook Predation and Herbivory:
ecologically, predation is defined as any interaction between two organisms that results in a flow of energy between them. This definition is applicable to both plants and animals. There are four commonly recognized types of predation: (1) carnivory, (2) herbivory, (3) parasitism, and (4) mutualism.
So, this reference, in ecological situations, clearly accepts an animal eating a plant as a "predator" of that plant.
However, in more general non-technical contexts, dictionary.reference.com defines a predator as (depending on which of its sources you use) either:
any organism that exists by preying upon other organisms
any carnivorous animal
The second definition would preclude its usage for eating plants. The first hinges upon the definition of "preying". Using dictionary.reference.com again, prey as a verb is:
to seize and devour prey
where that prey (as a noun) is:
an animal hunted or seized for food
I would therefore suggest that unless you are talking to a technical ecological (or possibly biological?) crowd, predator would not be an appropriate term.
Possible terms (thanks to Hotlicks for the first one, and typing it into thesaurus.com gave the other two):
eater: from the verb to eat: to take into the mouth and swallow for nourishment
devourer: from the verb to devour: to swallow or eat up hungrily, voraciously, or ravenously
consumer: an organism, usually an animal, that feeds on plants or other animals.