Sticking Together

IITA scientists are not generally in the business of breaking up partnerships, but they make an exception when the partnership is between two insects that combine to destroy essential food crops.

The African root and tuber scale is a small subterranean insect indigenous to Central Africa. It causes substantial crop yield losses by attacking the roots of host plants. Over the last 30 years, this insect has evolved into a major pest threat in the Congo Basin for crops such as yam, cocoyam, and particularly cassava. IITA initiated a research program to investigate the situation and the work has shed light on what turns out to be a sticky problem.

The brown ant is a catalyst for cassava destruction.

IITA researchers uncovered several reasons for the rise in scale infestations. These included changes in land use such as the shift to short fallow periods and the way farmers handled crop residues after harvest.
But the main culprit is an ant. The nests of this so-called "brown crazy ant" are regularly found alongside the scale. An extremely close partnership has evolved between two, and this is the basis of the problem.

Their interaction is simple but wondrous. The scale produces large amounts of a sweet substance called honeydew in order to rid itself of waste fluids, so much it risks drowning in its own secretions. Ants come to the rescue. The ants consume the honeydew as food, thus

sustaining the life of both The brown ant is a catalyst for insects. But, there is more! The cassava production. ants actually move young scale insects from plant to plant-to increase supplies and make sure they have enough to eat. This leads to rapid infestations of crop fields.

"Ultimately, the scale needs the ant to survive," says Rachid Hanna, an IITA scientist. "Understanding the intricacies of this partnership helps us to develop sustainable tactics for reducing scale infestations and protecting farmers' harvests."

Now that the enemies are better known, researchers are working in Cameroon and the Democratic Republic of the Congo to develop management strategies to control the scale. They want to develop a greater understanding of the behavior of both insects, while searching for natural enemies of the scale and possibly the ant, and identifying and developing resistant crop varieties and better cultural practices for farmers.