His call follows a Soil Association event earlier this month at which Professor Dave Goulson presented new evidence on how 97 per cent of neonicotinoids brought back in pollen to honey bee hives in arable landscapes was from wild flowers.
But von Westenholz said: "The really interesting thing evident in what was a fairly one-sided event was the lack of consistency in all this evidence coming out. Some studies purport to show effects on bumblebees but not honey bees and some show effects on honey bees and not bumblebees, which underlines the fact that the evidence is very poor and very poorly understood.
"That means we should be very careful about taking action or making solid claims based on the existing evidence. Like it or not there needs to be more work done."
Working together with green pressure groups, as suggested at the event by Syngenta's Dr Peter Campbell, is something he wants to do, said von Westenholz, and the chemical industry is "very aware of public concerns about pesticides and that's completely understandable".
He said there is no doubt that the agriculture and pesticides industries are very happy to work with the full range of stakeholders on these issues, but added: "I don't think it helps when Dave Goulson casts aspersions about the efficacy of these products and the fact farmers are pressurised to use them even if they don't work. That's clearly rubbish. Farmers are not idiots and see the value and won't use more than they have to because they are expensive."
Further land-scale studies are good in principle but cost money, he added, while chemical companies already spend huge amounts bringing products to market, mainly testing them for safety, so he questioned how they would be funded.
On glyphosate, which is threatened with losing registration for amenity and home use, von Westenholz said it is his "firm hope" that glyphosate is given the full 15-year re-registration later this month rather than the seven years recommended by the European Parliament environment committee last month.
He said glyphosate is "one of the safest plant-protection products out there" and it would be "extraordinary" if it was not re-registered. Not re-registering "would be the result of a political process rather than the evidence", he added.
As for glyphosate alternatives, he pointed out: "Whether they are better is another question." He added that the weedkiller is cheap, effective, easy to use and safe, so "ticks all the boxes". Foam alternatives are "not inexpensive and only usable in certain areas, has its own risk profile as boiling liquid and requires new equipment".
Meanwhile, the US Environmental Protection Agency has published its official classification of glyphosate as "not likely to be carcinogenic to humans."