Mangum told the British Protected Ornamentals Association spring conference in Stratford that there had been a "long line of pressure against Bayer and Monsanto" that "had not met a lot of success so they have gone to the retailers".
The US bedding grower said that led to protest groups holding picket signs in front of Home Depot and Walmart wearing bee costumes with crosses on the eyes, saying "these plants are killing the bees". Friends of the Earth have been prominent in campaigning.
Mangum said neonicotinoids were safe for humans and better than the alternative - organophosphates.
He said protestors had "applied negative PR pressure to big box stores, which is something everybody wants to avoid". This development has led to Bell "going neonicotinoid-free for two years now", and that Home Depot will be "neonicotinoid-free in two years".
He said plants that had been sprayed need a tag.
Mangum added: "That push has gained a lot of traction" but "we don't want to lose neonicotinoids because it's a slippery slope...we're fighting hard to keep them."
The event featured BPOA technical committee chairman Mike Smith talking about US Study tours, including a planned 2017 trip.
Dr Gideon Avigad of Vineland Research Centre in Canada spoke on robotics in horticulture production, while Dr Jill England, Dr Simon Pearson, Dr Tim Pettitt, Jude Bennison, Martin McPherson and Neil Bragg spoke on various research projects.
Kernock Park Plants' Bruce Harnett presented his Nuffield Farming scholarship findings on technology use worldwide in horticulture.
See the next edition of Horticulture Week magazine for a full report.