Tier‐1 assays for assessing the toxicity of insecticidal proteins produced by genetically engineered plants to non‐target arthropods

Abstract

In assessing an insect-resistant genetically engineered (IRGE) crop before its commercialization, researchers normally use so-called "Tier-1 assays" as the initial step to determine the effects of the crop on non-target organisms. In these tests, the insecticidal proteins (IPs) produced by the IRGEs are added to the diets of test organisms in the laboratory. Test organisms in such assays can be directly exposed to much higher concentrations of the test IPs than they would encounter in the field. The results of Tier-1 assays are thus more conservative than those generated in studies in which the organisms are exposed to the IPs by feeding on IRGE plant tissue or in the case of predators or parasites, by feeding on invertebrate prey or hosts that have fed on IRGE plant tissue. In this report, we consider three important factors that must be considered in Tier-1 assays: (i) methods for delivery of the IP to the test organisms; (ii) the need for and selection of compounds used as positive controls; and (iii) methods for monitoring the concentration, stability and bioactivity of the IP during the assay. We also analyze the existing data from Tier-1 assays regarding the toxicity of Bt Cry proteins to non-target arthropod species. The data indicate that the widely used Bt proteins have no direct toxicity to non-target organisms.

Does Bt maize expressing Cry1Ac protein have adverse effects on the parasitoid Macrocentrus cingulum (Hymenoptera: Braconidae)?

“…When researchers measure the hazard of an insecticidal compound to a non‐target organism in a dietary exposure assay, they must confirm the concentration, stability and bioactivity of the test compound in the food source, and also confirm the uptake of the compound by the test organism (Romeis et al ., ; Raybould et al ., ; Romeis et al ., ; Li et al ., ). In our study, ELISA measurements showed that the Cry1Ac toxin concentrations in an artificial diet were relatively stable, with 29.6% degradation occurring during the 2‐day feeding exposure.…”

Consumption of Bt Rice Pollen Containing Cry1C or Cry2A Protein Poses a Low to Negligible Risk to the Silkworm Bombyx mori (Lepidoptera: Bombyxidae)

“…In sensitive-insect bioassays, neonate larvae of C. suppressalis were fed for 7 d with artificial diet containing a range of Cry protein concentrations. The EC 50 (toxin concentration resulting in 50% weight reduction compared to the control) was 18 ng/mL diet for Cry1C [25] and 1310 ng/mL diet for Cry2A [12]. Previous research has demonstrated that different Cry proteins can have significantly different insecticidal spectra even if they have high homology.…”

Use of a dietary exposure system for screening of insecticidal compounds for their toxicity to the planthopper Laodelphax striatellus

“…To identify the potential toxicity of insecticidal proteins produced by novel insecticidal genes or by the insect‐resistant GE plants to target or non‐target insects, dietary exposure systems are necessary. Ideally, artificial diets should be available in which the insecticidal compound can be directly provided to the test species (Romeis et al ., ; Li et al ., , ). In summary, the diet should meet the following requirements: (i) it should be readily accepted by the test species, and efficient for sustaining their normal survival and development; (ii) the test compounds can be readily and uniformly incorporated into the diet; and (iii) the bioactivity of test substance should be maintained during the feeding exposure duration (Li et al ., , ).…”