Pigeonpea (Cajanus cajan) is cultivated as an annual or semi-perennial legume crop, usually in mixed cropping systems. Traditional cultivars or landraces are medium-to-long-duration and are harvested 6 to 12 months after sowing. More recently short- and extra-short-duration genotypes have been developed that mature in 90 days. In contrast to the longer-duration traditional genotypes, short-duration pigeonpea is usually not intercropped.

Pigeonpea plants and seeds attract over 200 species of insects that damage their roots, shoots, flowers, and seeds. Most of these insect species are sporadic in their distribution and therefore may not all be regarded as pests. Pigeonpea plant can compensate for most insect damage during its vegetative phase. However, the reproductive parts of the plant are most attractive to pests and recovery from damage at the reproductive phase is slow and dependent on plant type, soil moisture, and climatic conditions. Thus, most of the economically important insect pests attack pigeonpea at the reproductive phase and in storage, when they damage flower buds, flowers, pods, and seeds.

Broadly, the key insect pests of pigeonpea at the reproductive phase are grouped into three. The flower- and pod-feeding Lepidoptera larvae (mainly Helicoverpa armigera, Maruca vitrata, Etiella zinkenella), the pod sucking Hemiptera (mainly Clavigralla spp.), and seed-feeding Diptera (Melanagromyza sp.). Most of the studies on pigeonpea pests including their distribution, biology, ecology, population dynamics, and management have concentrated on species found in Asia. Limited information is available on pigeonpea pests and their management in southern and eastern Africa.

A series of surveys were conducted in farmers’ fields during 1992 in Kenya, and 1995, 1996 in Kenya, Malawi, Tanzania, and Uganda to determine the incidence, distribution, and damage levels due to insect pests on pigeonpea. Key insect pests were pod sucking Hemiptera bugs (dominated by Clavigralla tomentosicollis Stal), flower- and pod-feeding Lepidoptera larvae (mainly Helicoverpa armigera, Maruca vitrata, Etiella zinkenella), and seed-feeding Diptera- pod fly (Melanagromyza chalcosoma Spencer). Seed damage was 22%, 15%, 14%, and 16% respectively in Kenya, Malawi, Tanzania, and Uganda. Damage levels indicated that pod sucking bugs were more damaging in Malawi (they caused 69% of total seed damage) and in Kenya (43%), while pod borers caused more damage in Tanzania (50%) and Uganda (54%). Pod fly caused more damage in Kenya (19%) than in the other countries.

Pod borer damage was high in early maturing crops, pod fly in late maturing crops, and pod sucking bug damage was high regardless of crop maturity period. Greater variations in seed damage were observed between locations in Kenya, Malawi, and Tanzania than in Uganda. Warm and dry locations had less seed damage than warm and humid, cool and dry, or cool and humid locations in Kenya, Malawi, and Tanzania.

None of the farmers visited in Malawi, Tanzania, and Uganda had used conventional chemical pesticides on pigeonpea in the field. Over 80% of these farmers use traditional methods in storage pest management. In contrast, 35% and 53% of farmers visited in July and August 1995 respectively, in Kenya had used conventional chemical pesticides on long-duration pigeonpea genotypes in their fields. Some farmers in Kenya have adopted the short-duration genotypes and chemical pesticide use could be more intensive than on the landraces. Short-duration genotypes are known to be more susceptible to insect pest damage due to their bunch type flowering or podding at the end of branches, soft pods, and large soft seeds.

Future: Insect pests are a major constraint to pigeonpea production in the region, yet there has been relatively little research investment into the biology, ecology, and management of pigeonpea pests and their natural enemies. Research has concentrated almost exclusively on H. armigera, with little attention given to other pests. Knowledge of the impact, dynamics, and ecology of the pests and natural enemies is essential before effective control strategies can be developed. These studies must focus on the cropping system, as pigeonpea is frequently one component of a complex farming system. Other tropical legumes are particularly important because they share a number of pests and natural enemies with pigeonpea.

Pigeonpea farmers in parts of Africa have rapidly adopted the use of conventional chemical pesticides as a primary means of pest management. To avoid the "pesticide treadmill" which has been experienced elsewhere in developing countries, pigeonpea farmers need effective alternative pest management practices. There is need to develop improved genotypes that combine high yield and insect-resistance into backgrounds with consumer-preferred agronomic characters. Furthermore, there is need to focus research efforts on ways to enhance natural control processes, either by the introduction of exotic natural enemy species or by enhancing the effectiveness of endemic species.