Small maize field hosting fall armyworm, African stem borer and Spotted stem borer. A structured refuge cannot be enforced in farms of this size. (Photo courtesy of Johnnie Van den Berg, Ph.D.)

By Johnnie Van den Berg, Ph.D., and Andri Visser, PhD

Johnnie Van den Berg, Ph.D.

Maize is the most important staple crop in Africa, with the livelihoods of more than 300 million people depending on it. In Africa, insect pests are responsible for about 31 percent of the losses in maize if no control is applied.

The introduction of agricultural technologies such as Bt maize could reduce pesticide use and reduce yield loss due to damage by lepidopteran pests. Pest management strategies in smallholder farming plots (less than 2 hectares) must address a complex of pests within mixed cropping systems in different agroecological zones. The lepidopteran pest complex of maize consists largely of stem borers such as Busseola fusca and Chilo partellus and also the fall armyworm (Spodoptera frugiperda), which invaded Africa during 2016.

Andri Visser, Ph.D.

The one big unanswered question regarding Bt maize in smallholder systems is: What is the best strategy to manage resistance evolution? Both B. fusca and S. frugiperda are well known for resistance evolution to Bt traits. Insecticide resistance management (IRM) in Africa cannot be a one-size-fits-all strategy like the high-dose/refuge strategy that the rest of the world with Bt crops adopt and largely comply with. The two available IRM options are structured refuges and seed mixtures. However, the use of seed mixtures is considered inappropriate for pests that move off their natal plants during their larval stages, which is a characteristic of fall armyworm and stem borers.

In our new paper released this week in the open-access Journal of Integrated Pest Management, we used a hypothetical scenario as a model to investigate the potential effects of mixed populations of lepidopteran pests on the design and implementation of IRM strategies for Bt maize on smallholder farms.

Teaching scouting for fall armyworm in maize in Malawi. (Photo courtesy of Johnnie Van den Berg, Ph.D.)

Mixed farming with maize and legumes is typical in smallholder agriculture. (Photo courtesy of Johnnie Van den Berg, Ph.D.)

A smallholder farmer managing weeds. (Photo courtesy of Johnnie Van den Berg, Ph.D.)

A typical smallholder maize area in Mozambique. (Photo courtesy of Johnnie Van den Berg, Ph.D.)

The biological and behavioural characteristics that affect competitiveness of B. fusca, C. partellus, and S. frugiperda were identified and analysed to predict the structure of such mixed populations in different agroecological zones. Factors that affect competitiveness between species include distribution range due to climatic conditions, development time, off-season survival strategies, population density of larvae, and predator behavior. The validity of the assumptions that underlie the high-dose/refuge strategy was compared among the three species. The differences between the species, and the influence thereof on the choice of IRM strategy for a specific environment, were explored through analysis of three hypothetical scenarios.

Our investigation suggests that a seed mixture approach, coupled with an effective IPM strategy, is more practical and sensible, since it could limit the opportunity for a single pest species to dominate the species complex. The dynamic interactions in a multi-species community and domination of the species complex by a single species may influence moth and larval response to maize plants, which could lead to increased infestation of Bt plants and subsequent increased selection pressure for resistance evolution. This hypothetical experiment provides insights into the unique challenges that face the deployment of Bt maize in Africa.

Johnnie Van den Berg, Ph.D. , is a professor and program manager in integrated pest management in the Unit for Environmental Sciences and Management at North-West University (NWU) in Potchefstroom, South Africa. Email: johnnie.vandenberg@nwu.ac.za. Andri Visser, Ph.D., recently received her doctoral degree in environmental sciences and management at NWU. Email: visseran3@gmail.com.