Widely Used Pesticides Found to Impair Bee Reproduction

By Janet Raloff, for Science News' Science & the Public Blog

Pesticides are agents designed to rid targeted portions of the human environment of undesirable critters – such as boll weevils, roaches or carpenter ants. They’re not supposed to harm beneficials. Like bees. Yet a new study from China finds that two widely used pyrethroid pesticides – chemicals that are rather “green” as bug killers go – can significantly impair the pollinators’ reproduction.

Both chemicals are widely used in North America and elsewhere, including China. And, the researchers point out, the concentration of each pesticide that produced adverse effects in the experiments was at or below those that bees could encounter while pollinating treated crop fields.

In recent years, there’s been a big move by U.S. farmers to turn away from broad-spectrum potent bug killers to the more targeted and environmentally friendly pyrethroids. These synthetic chemicals have been fashioned after the natural pyrethrin bug deterrent in chrysanthemums.

The authors of the new study don’t argue that pyrethroids are a cause of colony collapse disorder, the mysterious die-offs affecting honeybees throughout North America. But they do argue that their findings suggest further investigation is warranted to confirm whether these immensely popular crop-protection chemicals might prove a previously unrecognized threat to pollinators. The source of a double-whammy, if you will, for already hammered bees.

Ping-Li Dai of the Chinese Academy of Agricultural Science and the Ministry of Agriculture led a team of researchers at those Beijing institutions together with a physiologist from the Second Military Medical University in Shanghai. The team investigated sublethal effects of bifenthrin and deltamethrin. Bifenthrin is used to kill everything from termites around homes to fire ants, corn pests and the mites that attack fruit trees. Deltamethrin is targeted at aphids, mealy bugs, whitefly, fruit moths, caterpillars on field crops, roaches, horseflies, mosquitoes and fleas.

After first establishing the dose that would kill no more than five percent of exposed bees, the researchers laced sugar water near bee hives with either of the pyrethroids at that tolerable dose. Worker bees had access for 20 days to the pseudo-nectar in each of three successive years. Queens in each colony were dosed every five days over each treatment period. Studied bees had no access to outside nectar during the trial periods.

Compared to queens receiving clean sugar water, those in the pyrethroid groups were substantially less fecund. For instance, clean queens in 2006 laid a little more than 1,200 eggs each day, compared to not quite 900 a day in the bifenthrin group and roughly 600 per day in the deltamethrin group. In general, the weight of eggs laid was higher in the pyrethroid-treated hives, but the hatch rate of pyrethroid-exposed eggs was significantly depressed. It varied by year, but in 2008, for instance, 88 percent of eggs in the control hives hatched versus 71.4 percent of those in the bifenthrin-treated hives and 80.5 percent of the deltamethrin-treated bees.

The success rate of hatchlings, that is the share that reached adulthood, varied from 75 to 95 percent in the control hive – making it between 20 and 40 percentage points higher than in hives where bees had been exposed to a pyrethroid. Dai and colleagues report their findings in the March Environmental Toxicology & Chemistry.

The bottom line, Dai’s team concludes: “The impact of pesticides on the colony may be severe.”

And the researchers concede that they can only guess at how severe because their paper focused on easily quantifiable, gross effects. Both pyrethroids are neurotoxic, typically causing paralysis and worse in target pests. The Chinese scientists didn’t investigate whether in-egg or juvenile exposures to the pesticides might have resulted in behavioral impacts during adulthood. Perhaps diminishing the bees’ ability to learn tasks or remember where good nectar sources were.

As I pointed out in a story four years back, pyrethroids may be relatively green – but they’re not totally benign to non-target organisms. That story was about little aquatic midges and other sediment dwellers. Essentially the food for fish and other critters people really care about.

Now we see threats to bees. And that should give all of us pause – because these unsung heroes of the farm make much of today’s bountiful harvests possible.

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