The legume pod borer Maruca vitrata
The legume pod borer, Maruca vitrata (Lep., Crambidae) remains the single most important insect pest attacking cowpea (Vigna unguiculata) in Africa. The amount of damage caused by M. vitrata caterpillars feeding on flowers and pods of cowpea is estimated at 20–80%, depending on agroecological zone and climatic factors.
For a long time, this pest has been labelled as ‘indigenous’ in Africa, and hence most control approaches were based on pesticide applications and in improving host plant resistance. However, as recently confirmed by phylogenetic studies comparing worldwide populations of M. vitrata, it appears that this insect originated in Southeast Asia. This hypothesis is strengthened by the fact that none of the natural enemies observed in West Africa are specific to M. vitrata, whereas, in Southeast Asia, detailed biodiversity studies targeting hymenopteran parasitoids of M. vitrata have revealed two interesting braconid species, Phanerotoma syleptae (an egg–larval parasitoid) and Therophilus javanus (a larval parasitoid), with field parasitism rates of up to 60%. Both parasitoids were introduced from the World Vegetable Center (WorldVeg) to our rearing labs at IITA-Benin for confined testing and eventually experimental releases in Benin and Burkina Faso.
Releases were carried out with the active participation of local communities and were preceded by a sensitization campaign explaining in simple terms the concepts of biological control. In Benin, only a few months after the initial releases in early 2016, P. syleptae was recovered from parasitized pod borer larvae on the target wild host plants, particularly Lonchocarpus sericeus; T. javanus was recovered later in the season, mostly from cowpea and Tephrosia spp. Both parasitoids were also recovered from cowpea and from wild host plants in Burkina Faso during the 2016 cropping season. Furthermore, surveys carried out in Benin (February–April 2017) nearly one year after initial experimental releases indicated unambiguously with certitude that both species had successfully survived the long dry season (particularly harsh that year) on alternative host plants in the absence of cowpea.
Although it is too early to be able to give a proper quantitative assessment of the impact of the released parasitoids on M. vitrata populations it is noteworthy that during the recent post-dry season surveys we were able to recover parasitized M. vitrata larvae from very low pod borer populations, indicating a good ecological adaptation of both parasitoids, and maybe also an early sign of parasitoid efficacy.
Manu Tamò: Insect ecologist, IITA Representative in Benin, M.Tamo@cgiar.org