The pair of words that would work BEST to fill in the blanks in the sentence is:
vibrant, eradication
The sentence would read: "Butterflies were strongly attracted to the plant's vibrant blooms and therefore spread its pollen far and wide, making eradication of the weed difficult." This makes sense because "vibrant" describes the attractive quality of the blooms, and "eradication" relates to the challenge of removing the weed due to its successful pollination.