Joining hands to fight the legume pod borer

Manuele Tamò,

Maruca vitrata larva affected by the entomopathogenic virus MaviMNPV. Photo by S. Srinivasan, AVRDC.
Maruca vitrata larva affected by the entomopathogenic virus MaviMNPV. Photo by S. Srinivasan, AVRDC.

A new collaborative project has been launched to develop novel approaches against an old problem affecting cultivated legumes—the pod borer Maruca vitrata.

This is one of the major pests of cowpea in West Africa, where, if left uncontrolled, it can lead to 80% yield losses.

Under this new project, funded by the German Federal Ministry for Economic Cooperation (BMZ), IITA and partners, the World Vegetable Center (AVRDC), and the International Center for Insect Physiology and Ecology (icipe), will test a range of new natural enemies against the legume pod borer. In close collaboration with national agricultural research systems (NARS) and scientists and colleagues in the Plant Protection and Quarantine Services, the project will choose the most promising natural enemies adapted to West and East African conditions.

One of the major outcomes of this project will be to quantify the impact of selected biocontrol agents on the population ecology of the pod borer and on cowpea yield in the field. At the same time, detailed molecular analysis of pod borer populations from different parts of the tropics, Africa, South America, and Asia, in collaboration with the BMZ project and a Dry Grain Pulses Collaborative Research Support Program (DGP-CRSP) project with the University of Illinois, will permit the identification of scoreable polymorphisms for determining the genetic similarity and differences between pod borer populations at distant locations. This will enable project staff to answer questions in relation to differential responses to synthetic pheromones, the diversity of biocontrol agents, and the development of an insect resistance management plan in preparation for the deployment of Bacillus thuringiensis (bt) cowpea in the region.

Experimental release of Apanteles taragamae using caged Sesbania cannabina. Photo by M. Tamo, IITA.
Experimental release of Apanteles taragamae using caged Sesbania cannabina. Photo by M. Tamo, IITA.

Prior to this new project, AVRDC and IITA have already collaborated, both formally and informally, on research on pod borer control. Biodiversity studies carried out at AVRDC in Taiwan had identified the exotic parasitoid Apanteles taragamae as the most promising candidate. This was subsequently introduced into the laboratories of IITA Bénin station. After a series of pre-release tests, experimental inoculative releases of A. taragamae were carried out between February and June 2007 in Bénin, Ghana, and Nigeria. The sites were patches of wild vegetation including plants known to host the pod borer, such as the legume trees Lonchocarpus sericeus, Pterocarpus santalinoides, and the shrubs Lonchocarpus cyanescens and Tephrosia spp.

As early as 6 months after the first releases IITA started a series of surveys to monitor the establishment of the parasitoid in the neighborhood of the releases. The monitoring continued until 2009, during which time we were not able to recover the parasitoid. However, we found indirect evidence of establishment in the environment (see below). We ruled out the theory that interspecific competition with indigenous parasitoids exploiting M. vitrata larvae of the same age and on the same host plant was the cause for this lack of evidence. We had conducted, just before the releases, quite elaborate competition studies which did not reveal any problems. Also, in its area of origin in Taiwan, A. taragamae coexists with similar parasitoid species found in Bénin, e.g., Phanerotoma sp. and Dolichogenidaea sp.

In Taiwan, however, A. taragamae is found prevalently on the cover crop Sesbania cannabina. This has been difficult to grow in West Africa because of foliage beetles (particularly Mesoplatys sp.) that completely defoliate the plant. We also intensified our studies on African indigenous species of Sesbania which suffer less beetle damage. So far, there have been no signs of direct establishment, although screenhouse experiments have confirmed the suitability of Sesbania spp. both as a feeding substrate for the pod borers and as a host for foraging parasitoids.

More recently, with funds from DGP-CRSP, we have developed a new release system using caged S. cannabina, infested artificially with eggs of M. vitrata, and subsequently inoculated with adult A. taragamae. Preliminary results indicate that such a cage can produce up to 300 cocoons of the parasitoid. At this stage, the cage can be removed and the parasitoids can emerge from the cocoons and disperse in the surrounding natural habitat. This deployment system is currently under testing in Bénin.

Adult female of Maruca vitrata. Photo by G. Goergen, IITA.
Adult female of Maruca vitrata. Photo by G. Goergen, IITA.

Another important beneficial organism which was identified by AVRDC in Taiwan is the Maruca vitrata Multi-Nucleopolyhedrosis Virus (MaviMNPV). This was imported to IITA-Bénin for further assessment. Again, after a series of laboratory tests which confirmed the results obtained in Taiwan and ascertained the specificity of MaviMNPV to the target host, IITA proceeded to test the virus in seminatural conditions. For this, we used field cages with artificial infestations of M. vitrata larvae. These experiments were also replicated in the screenhouse in Kano, Nigeria. Both tests indicated a very high mortality of pod borer larvae (>95%) using standard concentrations comparable to those found in commercial formulations of entomopathogenic viruses (e.g., against the cotton bollworm Helicoverpa armigera).

In Bénin, we did not carry out any open field experiments, so we were puzzled to discover a few pod borer larvae collected in the Mono region, close to release sites of the parasitoids, with apparent signs of the virus (Note: MaviMNPV had never been found in Bénin nor anywhere else in West Africa prior to the introduction in 2007, as confirmed by surveys of Dr A. Cherry in collaboration with the Natural Resources Institute).

Based on this discovery, and also aided by literature support, we attempted to verify the hypothesis that the parasitoid A. taragamae could have transmitted the virus MaviMNPV to pod borer larvae. We used three different infection methods (ovipositor only, whole body without ovipositor, and indirectly through artificial diet) to test the hypothesis. Results confirmed that the parasitoid was able to transmit the virus to the larvae through any of the infection methods. This discovery is quite significant: the parasitoid may be able to spread the virus in the environment without any further intervention.

This is also indirect evidence that A. taragamae is present in the environment, maybe at low levels, that cannot be detected by current sampling methods, or on secondary host plants for M. vitrata whose identity is still unknown. Further studies indicated that A. taragamae females can pass on the virus up to the third generation.

At present, we are looking for low-cost and efficient ways of producing the parasitoid and the virus so that the technology can be implemented by NARS colleagues and cottage industries at the community level, with financial support from DGP-CRSP. Also, training and demonstration videos of the major cowpea pests, their natural enemies, and detailed rearing methodologies are being prepared.