Impacts of Bacillus thuringiensis toxins on parasitoids (Hymenoptera: Braconidae) of Spodoptera litura and Helicoverpa armigera (Lepidoptera: Noctuidae)

Abstract

Spodoptera litura (F.) and Helicoverpa armigera (Hübner) are potential non-target pests in Bacillus thuringiensis (Bt)-transformed potato and brassica crops that are currently being investigated as candidates for field release in New Zealand. The comparative susceptibility of these pests and two of their larval parasitoids to Bt toxins was examined using diets amended with commercial preparations of Bt sprays and Cry1Ac toxin preparations. Using concentrations that allowed survival of some of the parasitoids and their hosts, development and survival characteristics were compared at two rates of Dipel 2X, six rates of Cry1Ac, and two reduced-nutrition diets. For all these treatments, larval development of both parasitoids, Cotesia kazak (Telenga) and Meteorus pulchricornis (Wesmael), was delayed only when there was a significant impact on the host Lepidoptera. Survival of M. pulchricornis was unaffected even when the survival of S. litura larvae was mildly, but significantly, reduced. In H. armigera, Bt diets that caused severely reduced survival of the host were accompanied by reductions in survival of M. pulchricornis. Similar or increased levels of survival of C. kazak were noted in H. armigera that were fed less toxic Bt-amended diets. Poor survival of parasitoids on reduced-nutrition diets suggested that reduced nutrition had a comparatively greater effect on parasitoid survival than Cry1Ac toxins. Overall, when Bt was ingested at concentrations that had minor effects on development or survival of host larvae, there was no impact on either parasitoid species. Further experiments with specific Bt cultivars proposed for release are required to confirm that in any surviving larvae, the impacts of Bt on these parasitoids are likely to be fewer than the impacts on their non-target hosts.

Introduction

The development of Bacillus thuringiensis Berliner (Bt) transgenic crops provides prospects for widespread control of lepidopteran pests on field crops, but it is also likely to have impacts on non-target Lepidoptera feeding on these crops, and effects on the species that interact with Lepidoptera (Glare and O’Callaghan, 2000). Environmental assessments of Bt crops have included studies of non-target insects, including evaluations of the impacts of these crops on non-target pests and natural enemies (Schuler et al., 2001). Prospective insect resistant Bt crops in New Zealand include potatoes (Davidson et al., 2002) and vegetable brassicas (Christey et al., 2006). Target pests in these crops are the potato tuber moth, Phthorimaea operculella (Zeller) on potatoes, and the diamondback moth, Plutella xylostella (L.) on vegetable brassicas. The interactions of P. operculella and its parasitoid Apanteles subandinus Blanchard (Davidson et al., 2006), and P. xylostella and its parasitoids (Chilcutt and Tabashnik, 1997, Schuler et al., 1999) in Bt-transgenic crops have been investigated previously.

Two polyphagous Lepidopteran species: tomato fruitworm, Helicoverpa armigera (Hübner) and tropical armyworm, Spodoptera litura (F) are occasional pests on potatoes and vegetable brassicas in New Zealand, but they are not usually the subject of control operations and can thus be classified as non-target pests that may be affected by the deployment of Bt-transgenic crops. Helicoverpa armigera and S. litura are attacked by several larval parasitoids, some of which provide significant control in other crops. For example, the braconid Cotesia kazak (Telenga), which is specific to H. armigera, causes 60–80% mortality of small larvae in processing tomatoes and significantly reduces damage in this crop (Walker and Cameron, 1989). This parasitoid also attacks H. armigera in a number of other crops including brassicas (Walker et al., 2005) and reduction of its effectiveness by transgenic Bt is likely to be unfavorable in horticultural environments. A second braconid, Meteorus pulchricornis (Wesmael), similar to C. kazak in being a solitary endoparasitoid species, attacks both H. armigera and S. litura but is widely polyphagous (Berry and Walker, 2004). These two Lepidoptera and their parasitoids are appropriate model systems for examining the possible direct and indirect impacts of Bt crops on non-target Lepidoptera and their parasitoids in New Zealand.

Lepidopteran parasitoids may be exposed directly to Bt-transgenic toxins when adult parasitoids feed on Bt plant material or when larval parasitoids feed within hosts that have ingested toxins, either as single Cry toxins from Bt plants or as Cry toxin mixtures from Bt spray residues. Direct exposure of adults is unlikely as nectar does not contain Bt toxins (Groot and Dicke, 2003). Parasitoids of Lepidoptera are unlikely to be directly affected because Bt toxins bind to receptors in midgut epithelium of host larvae and lose their toxicity to natural enemies. However, immature parasitoids may be influenced indirectly through lethal or sublethal impacts on the health and development of hosts (Bernal et al., 2002). Lepidopteran-active Cry proteins are generally considered to lack direct toxicity to predators and parasitoids (Glare and O’Callaghan, 2000). For example, lack of direct effects of Bt toxins on natural enemies has been established by direct feeding and choice experiments for adult parasitoids including Cotesia plutellae (Kurdjumov) (Chilcutt and Tabashnik, 1999, Schuler et al., 1999). Assessment of indirect impacts that may influence immature parasitoids through multi-trophic food chains is more complex.

Previous studies of the impacts of Bt plants on non-target parasitoids have used commercial Bt insecticides on leaf disks (Monnerat and Bordat, 1998, Chilcutt and Tabashnik, 1999), Bt toxins in artificial diet (Hafez et al., 1995, Salama et al., 1996), laboratory studies with Bt-transformed plants (Schuler et al., 1999, Bernal et al., 2002, Baur and Boethel, 2003), and population studies on Bt plants (Schuler et al., 2001). Field studies surveying the relative abundance of natural enemies in Bt crops have emphasized predators rather than parasitoids (Reed et al., 2001, Conner et al., 2003).

The objective of this study was to examine the potential direct effects of Bt toxins on larval parasitoids within their hosts, and the indirect host-mediated effects on parasitoids using the Lepidoptera-parasitoid systems described above. The methods were based on the use of concentrations of Bt toxin that would allow survival of sufficient hosts and parasitoids to compare both their survival and development. For this we used laboratory assays with host Lepidoptera that were reared on artificial diets, to which commercial Bt preparations or Cry1Ac toxins were added. Significant survival of hosts may occur especially when control of target pests in Bt-transgenic crops is not complete, or when non-target pests are tolerant of transgenic cultivars (Bernal et al., 2002) or the Bt toxins in foliar-applied sprays. Overall, our study aimed to determine whether non-target species on transgenic plants could be potentially subjected to concentrations of Bt toxin that would differentially favor the population dynamics of either Lepidopteran species or their parasitoids. The work also contributes to an overall ecological risk assessment for the potential release of transgenic potatoes into the environment as described by Conner et al. (2003).