Safe Haven for Insects Helps Protect Farmers' Crops

There is something different about Margaret Mulaa’s trial plots at the National Agricultural Research Center in Kitale, Kenya. The trial seems more like a botanical collection or an ornamental garden, featuring Guinea grass, napier grass, Giant Panicum, and Sudan grass, not to mention local and exotic sorghum varieties. Mulaa is happy when she discovers insects, particularly stem borers, devouring her plants. What’s going on?

The Insect Resistant Maize for Africa (IRMA) Project, a collaboration between CIMMYT and KARI, funded by the Syngenta Foundation for Sustainable Agriculture, uses biotechnology and conventional breeding to develop maize resistant to stem borers, one of the most devastating pests in Africa. Mulaa, a KARI entomologist, and her CIMMYT counterpart, entomologist David Bergvinson, seek plants that might serve as refugia in a system for limiting insect resistance to Bt maize, a genetically engineered plant that represents one of the best hopes for controlling stem borers.

Although Bt maize differs from other pesticide technologies because it produces its own pesticide rather than requiring spray applications, it shares a vulnerability common to plant protection measures: the target pest can build up resistance. To prevent this from happening, farmers in developed countries must plant a significant proportion of their fields (e.g., 20%) to varieties that are susceptible to the target pests. These refugia provide a safe haven where insects that would otherwise succumb to the Bt toxin can reproduce. The resulting populations of susceptible insects mate with the few resistant insects that evolve and greatly slow the development of pest populations resistant to the Bt toxin (or any other form of insect resistance controlled by a single gene). Refugia are a central component of a broader insect resistance management strategy, which includes integrated pest management and the combination of multiple sources of insect resistance in the maize plant.

Mulaa and Bergvinson must develop economically viable management strategies suited to small-and large-scale cropping systems in Kenya’s five major maize growing regions.

The most demanding clients of refugia are not farmers but insects. Each borer species has its own characteristics and life cycle. The borers must find the refugia plants attractive for oviposition (egg laying); the plants must then support larval development and provide a favorable environment for the borers to complete their life cycle. Further complicating matters, the stem borers must develop at about the same rate on the refugia plants as in the maize crop, to synchronize their mating.

Given all this complexity, why not ask farmers to plant susceptible maize, as in developed countries? Although this approach might work for large-scale growers, the economics work against resource-poor smallholders, the majority of Kenya’s farmers. For large-scale farmers, identifying and planting alternative refugia could significantly reduce the area needed for susceptible maize, thereby increasing overall yields.

Mulaa multiplied prospective refugia plants at Kitale in 2000. The next year, 30 alternative hosts for stem borers were evaluated in experiment station trials in four of the five maize growing regions. Sorghum, particularly local varieties, had the highest borer damage rating and number of exit holes (which indicate larval survival). Columbus grass and Sudan grass appeared effective as refugia but were not economic. Napier grasses supported oviposition and provided good economic returns but did not excel for larval development.

Laboratory bioassays were also undertaken in 2001, using the most common stem borer species, to determine larval survival and development, as well as fecundity, on a range of hosts. Specific sorghum varieties, maize hybrids, and forage grasses supported stem borer survival and development well. The results are being verified and integrated with experiment station data.

Devising the right refugia for each category of farmers is a challenge. Farmer surveys are being completed in the highland area of Kitale, the lowland tropics (Mtwapa), and the midaltitude dry zone (Katumani). The midaltitude transitional zone (Kakemega) and the midaltitude moist zone (Embu) will be surveyed in 2003. These surveys will provide estimates of r efugia species and area in these zones.

Tale of Two Farmers

Two Kitale-area farmers reflect the extremes that refugia strategies must cover.

Collins Omunga (top photo) has more than 600 hectares in the highlands in Trans-Nzoia District. His primary income is derived from maize and livestock. He has a Certificate of Agriculture and has been building his farm operation for more than 30 years. He already grows napier for livestock feed and erosion control on about 18 hectares and has no doubt about its economic value.

In a bad year, Omunga reckons 20% of his maize crop is lost, and he is ready to try something effective against borers. He does not believe that managing a refugia presents a significant obstacle. “Farmers are eager to adopt new technologies,” he says. “Knowledge is spreading, as you can see by the wide adoption of hybrid maize and fertilizer. But there are some ‘lazy’ farmers out there, ”he concedes, “and they might be more problematic.”

Mulaa takes issue with the characterization of “lazy” farmers. “There are some very small landholders,” she says, “farming half an acre or even less, that are not diversified and only plant maize. For this group, we are considering establishing some kind of rotating communal refugia.”

Not far away, Samson Nyabero (bottom photo) works his six hectares. The diversity found on his farm heartens Mulaa. Aside from some small fruit plots, he grows maize and, most positively, sorghum, finger millet, and napier, all potentially effective refugia. He says in a bad year he loses 30–40% of his maize to stem borers. Nyabero does not like to buy pesticide because it is often “bogus” or sold after its “effective date.” The timing and number of applications also pose constraints. He too saw little problem adjusting his crop practices if it would allow his crop to repel stem borers.

Mulaa is encouraged by what she has seen on the Kitale farms, but she recognizes that more research is needed before firm recommendations can be made. Even so, the development of a well-tailored strategy for managing insect resistance should just be a matter of time.