Well for starters you had a case of an individual having a baby with microcephaly born in the US but infected with the Zika virus, although it could be that individual had been to Brazil specifically. (Part of the issue is time to fullly diagnoses cases especially since you really need babies to start getting born after infection to really diagnosis it with the evidence being the virus generally isn't such as big as issue if the mother is already 8 months pregnant at the time for instance.)Simple test. Has Zika and birth defects been linked in any country outside Brazil?
You also had French Polynesia see an increase in microcephaly cases with its outbreak in 2014, but it is fair to note the increase was not specifically linked to the Zika virus at the time. (This may have been a consequence of fewer cases with a smaller infected population, or someone not happening to put two and two together, but I don't know the full story of the French Polynesia stats personally.)
http://www.sciencemag.org/news/2016...s-linked-zika-leads-who-declare-public-health
Another practical question in the Brazil case is if the larvicide in question was used significantly enough in other areas of the country to explain the rise in cases in other regions of Brazil. (If not the case the larvicide clearly does not explain the outbreak on its own, and there is the additional question if the product has been used elsewhere in the past with no increase in birth defects being detected.)