Tri-trophic interactions between parasitoids,Lepidopteran stem borers and BT maize

Abstract

One of the major constraints to maize production in East Africa is attack by lepidopteran stem borers, including Chilo partellus and Sesamia calamistis. Stem borer control by conventional methods has had variable success. The use of transgenic (Bt) maize is being attempted as a novel stem borer control option but the biosafety issues surrounding its use need to be addressed. Studies were conducted at the Kenya Agricultural Research Institute (KARl), Biotechnology Centre, to investigate the effects ofBt maize (Event 216) on these two important stem borers in East Africa, their oviposition preference and development as well as the impact of this GM maize on host finding by and biology of selected stem borer parasitoids (Cotesia jlavipes, Cotesia sesamiae and Xanthopimpla stemmator). The isogenic non-Bt maize (CML 216) was used as a control in all the experiments. The oviposition preference experiments, exposing stem borer pupae or moths to Bt and non-Bt maize under both choice (only one plant line) and non-choice (Bt + non-Bt plants) situations, indicated no significant effects on oviposition behaviour, moth fertility or fecundity. Different stem borer instars were also exposed to Bt maize for 5 days. Earlier instars were more susceptible to Bt maize than later ones. The effect of transient exposure to Bt maize on stem borer development was studied by . rearing third, fourth and fifth larval instars on Bt maize stems for 24h; before and after this exposure, they were reared on non-Bt maize. Bt maize prolonged stem borer development time at the instar during which exposure took place but had no significant effect on subsequent developmental stages. The effects of C. partellus and S. calamistis partially exposed to Bt maize on host finding by C. flavipes, C. sesamiae and X stemmator was determined by exposing female parasitoids to Bt vs. non Bt infested and uninfested plants in a Y-tube olfactometer and information obtained in the choices made. Bt maize had no significant effect in host finding behaviour of C. flavipes and C. sesamiae but was deterrent to X stemmator. Effects of stem borer hosts partially fed on Bt maize on development of C.jlavipes, C. sesamiae and X stemmator was evaluated by obtaining information on parasitoid life table parameters and fluctuating asymmetry. The response to the Bt maize varied between host and parasitoid species. The results alongside previous studies indicate that the existing database on parasitoid exposure to Bt-intoxicated hosts was insufficient to predict the reaction of these important parasitoids to two African stem borers. This underlines the importance of enhancing the empirical database. The generally used "test species approach" that is taken over from pesticide testing regimes, is only a first, imperfect step, to be followed by selecting locally or regionally important natural enemy species for biosafety tests. These should also be combined with at least semi-field studies before we can form a reasonable prediction about possible consequences of a profound technological innovation such as growing transgenic crops in African countries.