When it comes to pesticides, there’s good news and there’s bad news.  The good news is, farmers aren’t using as many tons of insecticides as they did in years past.

The bad news is, they’re switching to more pervasive, longer-lasting chemicals – making U.S. farm fields nearly 50 times more toxic to honeybees now than they were just 25 years ago.

The study that reported these shifts, published in August in PLOS One, examined the period between 1992 and 2014.

During that time, the amount of organophosphorus pesticides used on U.S. croplands each year dropped by about two-thirds, from nearly 35 million kilograms to around 12 million kilograms.  The same window saw the rise of neonicotinamide pesticides (known colloquially as neonics), from zero to around 3 or 4 million kilograms annually.

That represents a drop in the overall weight of pesticides applied to fields each year, but there’s a catch – actually, a few catches.

First, neonics persist much longer in the environment that organophosphates – up to nearly three years from a single application.

Second, they’re applied preventively as a seed coating to crops like corn and soybeans – which together accounted for nearly half the increase in pesticide toxicity in U.S. agriculture overall.  That eliminates the possibility of waiting to see whether, and to what extent, pests appear during the growing season before deciding whether to apply pesticide.

Third, neonics permeate all the tissues of the plants they’re supposed to be protecting, affecting all insects that rely on any part of the plant – including nectar and pollen – for any part of their life cycle..

Fourth, they’re water-soluble, meaning that as little as five percent of the pesticide stays on the plant.  The rest washes off to impact non-target organisms.

The study meaured toxicity by combining three aspects of each insecticide: toxicity to honeybees, persistence in the environment, and the amount used annually.  Using this index, average contact toxicity increased fourfold over the study period.  Average oral toxicity went up even more – by the 50 times mentioned in the headline.

The study only assessed toxicity to honeybees, using them as an indicator species.  This overlooks many other species, but parallels the approach the EPA uses when evaluating pesticides’ potential impacts.  The impacts likely extend to many other beneficial insects, which have been part of the mass declines in insect populations over the same time period.

National Geographic featured the study’s results in an August article.