Eldana saccharina is an economically important indigenous stem borer species that attacks maize in Africa. (Photo credit: Bugwood.org)

By Josh Lancette

When using Bt crops, a constant concern is preventing insect pests from becoming resistant. According to a paper published in the Journal of Economic Entomology that contains new analysis of previous studies, a common tactic to prevent resistance in Africa might not be working as hoped.

Bt crops are plants that have been genetically modified to express a protein produced by the bacterium Bacillus thuringiensis. This protein is toxic to some insects, so when an insect feeds on the plant, it ingests the protein and dies. These crops are useful because they can provide an alternative to pesticides, which benefits the environment, other beneficial insects, humans that would otherwise come in contact with pesticides, and operators of farms who don’t have the means for large-scale pesticide applications. Furthermore, Bt crops typically are considered safe for consumption by humans and other non-target animals.

Josh Lancette

However, while Bt crops have benefits, one risk is that insect populations can grow resistant to them if effective management strategies are not used.

A strategy to prevent insects from becoming resistant to Bt crops is to plant a “refuge” non-Bt crop around the main Bt crop. The idea is that the refuge crop produces non-resistant insects, which then mate with any resistant insects in the main crop, producing mostly non-resistant offspring. So, the non-resistant insects on the main crop die, and the resistant insects that don’t die produce mostly non-resistant offspring that will die.

In parts of Africa dominated by small crops and farms rather than large industrial agricultural operations, wild plants are commonly used as refuge crops. However, the evidence suggests they aren’t working in regards to lepidopteran stem borer pests of corn.

“Recent studies from East and Southern Africa have … started questioning the contribution that wild host plants could make as reservoirs for stem borer pest infestation,” writes Dr. Johnnie Van den Berg, the author of the paper and a professor at North-West University in South Africa. “While these plants may have the characteristics of refuge crops … the fact that they do not produce sufficient numbers of high-quality moths make them unsuitable as refuge crops. This poor suitability of wild grasses as a refuge for lepidopteran stem borers became evident from field studies conducted throughout Africa over the last decade.”

Van den Berg argues that wild plants have been used, even though they don’t work, because of faulty information given to farmers.

“The use of unstructured refugia and wild host plants as refuges is not approved as an IRM [insect resistance management] strategy of African stem borers, but it is often suggested as a possible solution,” writes Van den Berg. “This continuous erroneous reporting on the importance of wild hosts over the years created the perception that stem borers were present in high numbers in wild host plants, that these plants were abundant, and that they could serve as refugia for stem borers and be part of an IRM strategy for Bt maize.”

Moving forward, Van den Berg thinks that new strategies for managing insect resistance need to be developed.

“Current IRM strategies and reliance on wild host plants as refuge in most of the developing world is not appropriate to small farming systems,” writes Van den Berg. “Previous experience has shown that compliance to requirements of structured refuge approaches will be low, necessitating novel approaches to address this problem. It is therefore necessary to have a new look at integrated pest management strategies that may serve to reduce selection pressure for resistance evolution.”

While IRM strategies need to work from a biological perspective, they also need to work from a sociological perspective, as not all strategies can or would be used by farmers.

“To be accepted by farmers, IRM strategies must be compatible with the existing cropping systems and normal farming practices,” Van den Berg writes. “If other crops are planted as refugia, these must be economically viable, socially acceptable and easy to implement by those making the management decisions at the farm level.”

Read more: “Insect Resistance Management in Bt Maize: Wild Host Plants of Stem Borers Do Not Serve as Refuges in Africa,” Journal of Economic Entomology

Josh Lancette is manager of publications at the Entomological Society of America.