The European Union's top watchdog launched an investigation on Tuesday into whether the EU's executive has taken sufficient account of new scientific evidence on the link between certain pesticides and bee deaths.

The United Nations has estimated that a third of all plant-based foods eaten by humans are dependent on bee pollination, but bee numbers in Europe and elsewhere have fallen rapidly in recent years, and scientists have been at a loss to identify the cause.

Some blame the decline on the use of pesticides, and in particular on a widely used class of insecticides called neonicotinoids - nicotine-like chemicals used to protect various crops from locusts, aphids and other pests.

Last month, two scientific studies showed that even low doses of neonicotinoids could harm bumblebees and honeybees, interfering with their homing systems and increasing the chance of individual bees dying while away from the hive.

European Union Ombudsman P. Nikiforos Diamandou said he had opened an investigation after a complaint from the Austrian Ombudsman Board, who said the European Commission had failed to take account of the new evidence on the role of neonicotinoids in bee mortality.

"In its view, the Commission should take new scientific evidence into account and take appropriate measures, such as reviewing the authorisation of relevant substances," said a statement from the EU Ombudsman's office.

Four neonicotinoids are currently authorised for use in the EU, although several countries including Germany have banned their use in treating seeds after accidental releases resulted in substantial bee deaths.

One of the authorised substances - imidacloprid - is among the most widely used insecticides in the world and is manufactured chiefly by Bayer's Cropscience unit.

The ombudsman has asked the Commission to submit an opinion in the investigation by June 30, after which it will issue a report. Recommendations by the ombudsman are non-binding but are usually followed by the EU's institutions.

On Tuesday, the Commission said it had already asked the European Food Safety Agency (EFSA) to carry out a full review of all neonicotinoid insecticides by April 30 and that it would take appropriate measures based on the findings.

The EU executive has been working on the issue of bee health for 20 years, but there could be new elements to consider in terms of the role of neonicotinoids in bee mortality, a spokesman said.

An online petition calling on politicians in the European Union and the United States to ban the use of neonicotinoids has so far received more than a million signatures.