Evaluating the Induced-Odour Emission of a Bt Maize and its Attractiveness to Parasitic Wasps

Abstract

The current discussion on the safety of transgenic crops includes their effects on beneficial insects, such as parasitoids and predators of pest insects. One important plant trait to consider in this context is the emission of volatiles in response to herbivory. Natural enemies use the odours that result from these emissions as cues to locate their herbivorous prey and any significant change in these plant-provided signals may disrupt their search efficiency. There is a need for practical and reliable methods to evaluate transgenic crops for this and other important plant traits. Moreover, it is imperative that such evaluations are done in the context of variability for these traits among conventional genotypes of a crop. For maize and the induction of volatile emissions by caterpillar feeding this variability is known and realistic comparisons can therefore be made. Here we used a six-arm olfactometer that permits the simultaneous collection of volatiles emitted by multiple plants and testing of their attractiveness to insects. With this apparatus we measured the induced odour emissions of Bt maize (Bt11, N4640Bt) and its near-isogenic line (N4640) and the attractiveness of these odours to Cotesia marginiventris and Microplitis rufiventris, two important larval parasitoids of common lepidopteran pests. Both parasitoid species were strongly attracted to induced maize odour and neither wasp distinguished between the odours of the transgenic and the isogenic line. Also wasps that had previously experienced one of the odours during a successful oviposition divided their choices equally between the two odours. However, chemical analyses of collected odours revealed significant quantitative differences. The same 11 compounds dominated the blends of both genotypes, but the isogenic line released a larger amount of most of these. These differences may be due to altered resource allocation in the transgenic line, but it had no measurable effect on the wasps' behaviour. All compounds identified here had been previously reported for maize and the differential quantities in which they were released fall well within the range of variability observed for other maize genotypes.

Transgenic Pesticidal Crops and the Environment: The Case of Bt Maize and Natural Enemies

“…Similar findings have been reported elsewhere (Ngi-Song and Potting et al, 1997). Also, Cotesia marginiventris Cresson (Hymenoptera: Braconidae) and Microplitis rufiventris Kokujev (Hymenoptera: Braconidae), which are important larval parasitoids, were not able to distinguish between the odours of a Bt maize event and its near-isogenic line (Turlings et al, 2005). This indicates that growing Bt maize is not likely to affect host finding by stem borer larval parasitoids.…”

“…Obonyo (2009), however, showed that there were no significant differences in host acceptance ratio between Bt exposed and non-Bt reared larvae. Turlings et al (2005) observed that braconid parasitoids did not distinguish between odours of Bt and non-Bt maize plants in olfactometer experiments. Although a number of studies have found no significant differences in oviposition choice between hosts fed on transgenic and non-transgenic diets (Bell et al, 1999, Schoenmaker et al, 2001Schuler et al, 1999b), there are cases where parasitoids seem to distinguish by host quality (reviewed in Steidle and van Loon, 2003;Overholt et al, 1994;Sallam et al, 1999).…”

“…More recently, Obonyo et al (2008a) observed that C. partellus and S. calamistis moths did not discriminate between Bt and non-Bt maize plants for egg laying. This non-discriminatory oviposition behaviour could be due to the fact that the ratios of caterpillar-induced odour emissions of Bt maize plants are identical to those of non-Bt plants (Turlings et al, 2005) since genetic modification does not alter the volatile profile of undamaged maize plants (Dean & De Moraes, 2006). These results have important implications for pest resistance management and monitoring.…”

“…One important factor is volatiles emitted by the host plant. There are significant quantitative (Turlings et al, 2005) and qualitative (Dean and De Moraes, 2006) differences in volatile emissions between Bt and non-Bt plants. Both the quantity and composition of emitted volatiles influence host finding by Cotesia species (Steinberg et al, 1993).…”

“…Furthermore, plant volatiles may act as cues for host location by pupal parasitoids (Obonyo 2009). Chemical analyses of collected odours between Bt and non-Bt maize revealed significant quantitative differences (Turlings et al, 2005); this could possibly affect host location by pupal parasitoids such as X. stemmator. Indeed, Obonyo (2009) found that X. stemmator parasitoids preferred host plant odours compared to odours from a blank control.…”