You must understand that there is no perfect correspondence for "vaquinha verde amarela" in English. In fact, I know this species quite well and it's very common in a number of crops, not only in potato. It occurs in bean, soybean and other crops. It is definitely a leaf beetle, but the larvae also eat roots and tubers. Distribution is mainly confined to South and possibly Central America. So, no matter what term you use, it is essential to include the species name in italics right after YOUR choice of a common name. My suggestion therefore is:

The Brazilian potato leaf beetle, Diabrotica speciosa.

The main crop attacked by this pest is bean, so you <n>could</n> call it the "Brazilian bean leaf beetle", but since you are referring to potato, then use potato. The most important thing: use the species name in Latin. The genus is well known worldwide, but this species has limited distribution. Using the word "Brazil" defines the bug appropriately.

Just my two cents.

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Note added at 10 hrs (2010-02-25 16:04:13 GMT)
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This bug is also called "patriota" by growers and technicians because the elitra are patterned after the main colors found in the Brazilian flag (green and yellow).