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.

Designer (cowpea) plants

Christian Fatokun,

Plants can be designed to order. Science has long found a way to combine good and useful characteristics in a plant by studying the genes for such traits, and putting them together in a process called “genetic engineering.”

Caterpillar boring into a cowpea pod. Photo by S. Muranaka
Caterpillar boring into a cowpea pod. Photo by S. Muranaka, IITA

Cowpea is grown mainly for its protein-rich grains and quality fodder for livestock. At present, biological control and conventional breeding methods are proving inadequate in developing cowpea varieties resistant to destructive pests, such as the legume pod borer Maruca vitrata.

M. vitrata is the most widespread cowpea pest. The adult moth lays eggs on the plant. The larvae that emerge from the eggs damage plants in the field, particularly during the reproductive stage, through feeding on young succulent shoots, flowers, pods, and seeds. This pest can cause significant grain yield reduction, between 20% and 80% if not controlled with insecticides.

Farmers usually spray insecticides to protect the cowpea crop from Maruca and other pests. Purchasing chemicals, however, adds to the production cost, thus reducing the farmers’ profit. Also, farmers are not well equipped to protect themselves when using such toxic chemicals. In some farming communities, adulterated chemicals that do not control the pests are sold to farmers. The development of cowpea varieties with resistance to Maruca and other insect pests would benefit the most resource-poor African farmers who grow the crop.

Cowpea is grown extensively in the savanna region of sub-Saharan Africa (SSA). At least one major insect pest attacks cowpea at every stage in the life cycle, including seeds in storage. These pests are significantly responsible for the low grain yield in farmers’ fields.

Through conventional breeding, some varieties have been developed that show resistance to some of the pests, such as aphids and flower thrips, and low levels of resistance to the storage weevil. However, not much progress has been made in host plant resistance, especially M. vitrata.

Efforts continue to identify parasites and predators that could be used as biocontrol agents. When deployed, such agents would greatly reduce the population of the Maruca larvae in the field, giving the cowpea plant some respite for the production of flowers and pods containing whole and well-formed seeds.

Using conventional breeding, several hundreds of accessions of cultivated cowpea and its wild relatives have also been screened for resistance to this pest. Accessions belonging to Vigna vexillata were found to be resistant to M. vitrata. These accessions were found to be closest to cowpea in a phylogenetic study of diversity in the Vigna species. The study was based on data obtained after DNA genotyping. Efforts were made to cross cowpea with V. vexillata but without success.

This strong cross-incompatibility makes gene exchange between the two species impossible. This is where biotechnology comes to the rescue. Two major steps are needed to develop genetically modified cowpea with resistance to M. vitrata. First is developing a transformation system and the second is identifying the transgene that would be effective against the pest when introduced into cowpea. Since Maruca is a Lepidopteran, some of the genes from Bacillus thuringiensis (Bt) should be effective against the insect’s larvae. IITA screened several Bt protoxins on Maruca by incorporating different concentrations in the diet fed to the larvae. The protoxin of Bt gene ”Cry1ab” was found to be most effective even at very low concentrations in the artificial diet. This Bt gene (Cry1ab) was therefore selected as the candidate gene for designing Maruca-resistant cowpea.

Scientists at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia developed the transformation system using an IITA-developed breeding line ”IT86D-1010” derived from a cross between ”TVx4659-03E” and ”IT82E-60”. CSIRO scientists have now obtained cowpea lines containing the Bt gene. Monsanto donated the Bt gene used to transform cowpea by licensing it to the African Agricultural Technology Foundation (AATF) for use in Africa. The Rockefeller Foundation and USAID funded this cowpea transformation project.

Farmers transporting cowpea harvest. Photo by S. Muranaka
Farmers transporting cowpea harvest. Photo by S. Muranaka, IITA

The Bt cowpea had been tested in the CSIRO laboratory in Australia and found to be effective against the larvae of another Lepidoptera, Helicoverpa armigera. The Bt gene in cowpea is expected to be effective against M. vitrata, the cowpea pest, but needs to be tested in an environment where Maruca thrives. Apart from Burkina Faso, none of the African countries where cowpea is an important crop has a biosafety law in place. A few lines of the Bt cowpea were, therefore, taken to Puerto Rico for field testing. The field trial was carried out in late 2008. If the Bt gene in the cowpea lines is found to be effective against Maruca, the next step would be to transfer the Bt gene into popularly grown cowpea varieties selected from interested countries. The line presently containing the Bt gene is not high yielding, and farmers are not likely to accept it readily.

Under the international biosafety protocol (Cartagena protocol on biosafety) it is necessary to carry out risk assessment on the Bt cowpea before it is introduced to another country. The data obtained from risk assessment form part of the dossier that accompanies applications requesting for importation to any country. Risk assessment would entail studies on gene flow, the effect of the transgene on nontarget organisms, food safety, and resistance management strategies.

A meeting of experts in these various fields is planned in March 2009 at the Donald Danforth Plant Science Center, St. Louis, Missouri, USA. The experts would design studies to address the different questions that may arise from biosafety regulators in the countries where the Bt cowpea is meant to be grown. Many of the proposed studies are necessary, because cowpea is an indigenous food crop in SSA where cross-compatible wild relatives are found growing in agroecologies similar to farmers’ fields. Biosafety reviews in the African countries would, therefore, be rigorous.