Research team finds that natural enemies help delay insecticide resistance, protect Bt crops

Despite the controversy over them, transgenic crops have helped growers fend off some of the most destructive pests. The higher yield that results has provided consumers more affordable food. When used properly as a part of an integrated pest management program, transgenic crops can be an effective and economical way to manage certain pests, diseases and weeds. Unfortunately, as the failures of Roundup Ready crops have shown, transgenic crops are not sustainable when used as the sole pest management tactic.

When Bt technology came out, the Environmental Protection Agency instituted a requirement to have a “refuge” plot of crops not protected with Bt to maintain a population of pests that were not exposed to the Bt toxin. The refuge could be treated with an insecticide; it just couldn’t be treated with Bt. The theory behind the refuge was that having a plot of the crop that was treated with a different insecticide would give the pest species a non-Bt alternative, so that those individuals would still be susceptible to Bt, mate with individuals who had been exposed to Bt and pass on their Bt-susceptible genes to the next generation, therefore delaying resistance.

Bt is toxic to lepidopteron pests, and although there have been questions about its effect on some of the pest’s natural enemies including lady beetles, research experiments have shown that the Cry protein in the Bt toxin does not seem to have the same negative effects in many non-lepidopteron insects. However, because Bt seemed so much more effective than other chemicals or management techniques, some farmers took the risk of planting a much smaller refuge plot or failing to plant a refuge at all. As a result, a few pest species have begun to develop resistance to Bt.

To test whether alternate pest management strategies such as biological control would help supplement Bt crops and protect the refuge as well, researchers from China, Australia and the U.S. tested larvae and adults of diamondback moth (Plutella xylostella) for resistance to Bt in the presence and absence of the spotted lady beetle (Coleomegilla maculata), a common predator. Previous research has shown that spotted lady beetle is not susceptible to Bt. Their results were published in PLOS One in 2014.

Researchers tested for resistance to Bt broccoli. Moths were observed over six generations in terms of their numbers. Treatments included Bt only, Bt plus untreated refuge, and Bt plus a refuge treated with spinosad. Testing was performed in a laboratory, rather than in field conditions.

By the second generation, moth numbers in the Bt-only treatment averaged 7 per plant, while the other two treatments had only 1 moth per plant. By the 3rd generation Bt-only control had failed and plants were completely defoliated.

Lady beetle populations were higher on the refuge plants than on the Bt plants, possibly because the pest populations were higher in the refuge plants, researchers speculate. Populations of lady beetles were also higher in the non-treated refuge, probably because spinosad is toxic to lady beetles.

Over the life of the project, resistant moth populations reached 75 percent in the Bt-only treatment, after only 3 generations. By the 4th generation, 56 percent of the moths in the Bt and spinosad-treated refuge had survived both treatments. Only 4 to 7 percent of moths survived treatment in the Bt plus untreated refuge option. By the 6th generation, 39.5 percent of moths survived the Bt-only treatment, 72.8 percent survived the Bt plus spinosad-treated refuge treatment (because of the spinosad toxicity to lady beetles) and 6.0 percent—the lowest percentage—survived the Bt plus untreated refuge.

“Our present results indicate that the predator, C. maculate, combined with non-Bt and unsprayed refuge plants, delayed resistance in the P. xylostella population to Bt broccoli plants, when compared to treatments without the predator,” conclude the authors.

Overall, authors conclude that while non-Bt refuges are necessary to delay resistance to Bt, treating the refuges with insecticides could encourage resistance by reducing numbers of natural enemies.

Source: Liu, X., Chen, M., Collins, H.L., Onstad, D.W., Roush, R.T., Zhang, Q., Earle, E.D., and Shelton, A.M. (2014) Natural enemies delay insect resistance to Bt crops. PLoS ONE 9(3): e90366. doi:10.1371/journal.pone.0090366.

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