An American entomologist has concluded that insecticidal seed treatments do not reduce aphid numbers or increase soybean yields.
Jonathan Lundgren, who works for the U.S. Department of Agriculture in Brookings, South Dakota, said two years of field trials have shown that soybean growers are worse off if they plant seed treated with insecticide because the insecticide kills the natural enemies of soybean aphids.
“We found no yield benefits resulting from seed-applied insecticides. The beans with insecticide performed similarly to untreated seeds,” Lundgren said.
“At $12 to $15 per acre (for soybeans), it turns out that this seed treatment is costing growers a lot of money unnecessarily.”
Lundgren and colleague Michael Seagraves conducted replicated field trials in 2009 and 2010 that compared soybean seeds treated with insecticide to untreated soybeans.
The researchers treated the soybeans with imidacloprid, a Bayer product sold as Gaucho, or thiamethoxam, a Syngenta product sold as CruiserMaxx.
“We purchased purified chemistries and put them on ourselves at the label rate,” Lundgren said.
“We used the exact same bean variety for all three treatments, so we controlled for this factor. No fungicide treatment (was used).”
The yields from treated and untreated soybeans were equivalent: 55 bushels per acre for both cases in 2009 and 44 bu. per acre in 2010.
The scientists also concluded that seeds treated with insecticide had no effect on the soybean aphid population.
“(Aphid numbers) reached economic injury levels on the same date in all treatments in 2009, and were equivalent in 2010 as well,” said Lundgren, who in 2010 was named the top young scientist within the USDA’s Agricultural Research Service.
While the insecticides were ineffective on aphids, Lundgren found the chemicals killed off pirate bugs, lady beetles and other predators that feed on soybean aphids.
“So not only do insecticidal seed treatments on soybeans unnecessarily cost farmers money for a non-existent service, but it appears that they may actually cost producers money by reducing natural resistance to aphid establishment,” he said.
The South Dakota Soybean Research and Promotion Council sponsored Lundgren’s research because growers had questions about the efficacy of insecticidal seed treatments, said council chair Dave Iverson.
“We wanted unbiased data compared to the companies’ (data).”
Iverson didn’t know what percentage of South Dakota soybean growers use insecticidal seed treatments, but said the product is widely used.
Manitoba Pulse Growers Association president Andrew Saramaga said it’s hard to know how many growers plant seed treated with insecticide in his province, but a number of them wonder if the potential benefits are worth the $10 per acre cost.
Lundgren said insecticidal seed treatments have become commonplace for almost all field crops in the last decade, but little research has been done to validate the products’ efficacy or economic value.
“These seed treatments are being strongly advocated for most field crops these days, with little thought as to whether they are truly needed and whether they truly save the farmer money,” he said.
For instance, Lundgren said the Bt gene in corn controls the crop’s primary pests, but that leads to the question, what are the seed treatments controlling?
He also said it makes little economic or ecological sense to kill every insect in a field, especially beneficial insects that feed on pests such as soybean aphids.
Agriculture Canada entomologist Julie Soroka said killing insects that prey on aphids may be a problem in soybeans, but insecticidal treatments on canola seed don’t threaten natural flea beetle control.
“Flea beetles don’t have many natural enemies. There is a parasitic wasp…. It can parasitize flea beetles at rates of up to 80 percent,” said Soroka. “But, in general, there are so many flea beetles that the typical parasitism rate is approximately five percent.”
As a result, she said killing off most of the wasps with insecticidal seed treatments wouldn’t lose a substantial amount of natural flea beetle control.
Soroka estimated that 90 percent of canola fields in Western Canada have seed treated with insecticide.
“In any year, in any field, we don’t necessarily need that level of protection, but it’s easier to put it on, just in case we do.”
Lundgren said more research is needed on the economic value of insecticidal seed treatments for all crops.
Iverson said Lundgren’s work is a good start, but he would like to see more data before he makes up his mind.
He expects most South Dakota soybean growers will continue to use insecticidal seed treatments.
“When soybeans are $13 a bushel … it doesn’t take much of a yield difference to pay back $10 an acre (for seed treatment,” he said. “It’s like insurance. Hopefully I’ll get my money back.”
•Cost of insecticidal seed treatment on soybeans: $8-$10 per acre
•Number of acres with insecticidal treated seed: 30 percent
•Canada and U.S. soybean acres: 82 million (2010)
•Value of insecticidal seed treatment industry: $200-$250 million