A scientist who wants to study the affects of fertilizer on plants sets up an
experiment. Plant A gets no fertilizer, Plant B gets 5 milligrams of fertilizer, and Plant
C gets 10 milligrams of fertilizer. Which plant is the control group in this experiment?

"do a testcross with a homozygous recessive plant."

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.