Ecological studies carried out at the World Vegetable Center (AVRDC) in Taiwan identified the parasitoid Apanteles taragamae as the most promising for controlling the legume pod borer Maruca vitrata in Africa. To test its effectiveness, our researchers in Benin imported A. taragamae under standard quarantine protocols and carried out experimental releases in Benin, Ghana, and Nigeria in 2007 on patches of wild vegetation including plants known to host the pod borer such as Lonchocarpus sericeus, Pterocarpus santalinoides, Lonchocarpus cyanescens, and Tephrosia spp.
Prior to these releases, we had studied the host searching capacity of A. taragamae using a 4 arm-olfactometer, and flowers of three different host plants: cowpea, Pueraria phaseoloides and the three Lonchocarpus sericeus. These studies revealed that A. taragamae uses kairomone-mediated host recognition at the short to medium range.
From as early as six months after the first releases and up until 2009, we conducted a series of surveys to monitor establishment of the parasitoid. Although we were not able to successfully recover the released parasitoid, we got indirect evidence of its establishment in the environment. We ruled out that interspecific competition with indigenous parasitoids exploiting M. vitrata larvae of the same age and on the same host plant could be the cause for this lack of evidence because we had conducted, just before the releases, 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 Benin e.g. Phanerotoma sp. and Dolichogenidaea sp.
In Taiwan, however, A. taragamae is found prevalently on the cover crop Sesbania cannabina, which is difficult to grow in West Africa because of foliage beetles, particularly Mesoplatys sp. that completely defoliates the plant. We recently intensified our studies on African indigenous species of Sesbania that suffer less beetle damage, but so far there have been no signs of direct establishment. This is despite screenhouse experiments confirming the suitability of Sesbania species as feeding substrate for the pod borers and also as host for foraging parasitoids.
From 2007 onwards, we also started testing the newly-discovered Maruca vitrata Multi-Nucleopolyhedrosis Virus (MaviMNPV) found in Taiwan through collaborative studies with AVRDC. After a series of laboratory tests which confirmed the Taiwan results, we carried out host range studies to ascertain its specificity. Of the seven lepidopteran species tested (four Pyralids, two Noctuids, and one Crambid), none got infected by MaviMNPV applied on artificial diet. We then tested the virus in semi-natural condition using field cages with artificial infestations of M. vitrata larvae. Results showed a very high mortality of pod borer larvae (>95%) using standard concentrations comparable to those found in commercial formulation of entomopathogenic viruses (e.g. against the cotton bollworm Helicoverpa armigera).
In the Mono region of Benin, we discovered a few pod borer larvae with apparent signs of MaviMNPV close to the release sites of the parasitoids. This observation was important since we did not carry out open field experiments nor has MaviMNPV been found in West Africa prior to its introduction in 2007. We hypothesized that the parasitoid A. taragamae could have transmitted MaviMNPV to pod borer larvae.
To verify this, we deliberately infected pod borer larvae using three methods: ovipositor only, whole body without ovipositor, and through artificial diet. The parasitoid was able to transmit the virus to the larvae through all of the infection methods.
This finding was significant as the parasitoid could spread the virus without further intervention. This is also indirect evidence that A. taragamae is present in the environment, albeit in low levels, which cannot be detected by current sampling methods, or on yet unknown secondary host plants for M. vitrata. We are currently conducting collaborative studies in our virology lab in Ibadan to identify and ascertain the mechanisms of transmission, and duration of virus retention and transfer.