Time for a New Look at the Relationship Between Bt Plants and Insect Natural Enemies

Abstract

Insect-resistant Genetically Modified (IRGM) plants have become an important component of integrated pest management (IPM) programs worldwide. The currently available IRGM plants express insecticidal proteins from Bt (Bacillus thuringiensis), however additional insecticidal molecules are being investigated. In 2008, Bt crops were grown on 46 million ha, up from 42.1 million ha in 2007. However, the ecological safety of Bt crops continues to be debated. Much of the debate has focused on non-target organisms, especially predators and parasitoids that help control populations of pest insects in many crops. Thirteen years of commercial use of Bt plants in different parts of the world has documented their effectiveness in controlling target insects and delaying resistance development. The existing “high dose/refuge” insecticide resistance management (IRM) practices certainly contributed to delaying resistance development. However, natural enemies may have played an important role in diminishing the likelihood of resistance development, but this is an area that has not been explored to a great degree. Numerous individual case studies and recent meta-analysis studies of Bt plants and natural enemies indicate that the existing Bt plants have little or no adverse effects on predators, especially compared with conventional insecticides. However, negative effects were found on parasitoids in a few studies but such effects were most likely caused by poor prey/host quality. Using a unique system consisting of a strain of the insect pest, Plutella xylostella (herbivore), resistant to Cry1C that was allowed to feed on Cry1C broccoli plants and then become parasitized by Diadegma insulare, an important endoparasitoid of P. xylostella, our results indicated the parasitoid was exposed to toxic insecticidal protein residues while in the host but was not harmed by such exposure. Parallel studies conducted with several commonly used insecticides indicated they significantly reduced parasitism rates on strains of P. xylostella resistant to these insecticides. These results provide the first clear evidence of the lack of hazard to a parasitoid by Bt plants, compared to traditional insecticides, and help provide new insight on how to rigorously evaluate the ecological risks of Bt plants. Furthermore, these results also suggest that it is time to take a new look at the relationship be tween Bt plants and insect natural enemies. Bt plants can be a friend to natural enemies, rather than a foe. The potential role of natural enemies in regulating insect resistance development to Bt plants is discussed.