GM Free Cymru

Crucial paper 32:
BT crops: another nail in the coffin for substantial equivalence

After considering, in this long and detailed chapter, just how BT crops work in controlling insect pests, the authors conclude that their mode of operation is so different, in so many ways, that the idea of them being "substantially equivalent" to their isogenic or parent lines is scientifically nonsensical. No surprise there, then............


Eds: Isaac Ishaaya, Subba Reddy Palli and A Rami Horowitz

Chapter 10, p 197-230 Comparative Aspects of Cry Toxin Usage in Insect Control

by András Székács and Béla Darvas


Based on the above (text), Bt -based bioinsecticides and crops cannot be considered by far as equivalent technologies. Their application differs as Bt bioinsecticides allow singular applications, while Bt crops exert a continuous production of the Cry toxin. This results in higher environmental doses of the plant-expressed toxin(s) than in the case of the Bt bioinsecticide. For example a single treatment of Dipel bioinsecticide at the registered dosage (1 kg/ha) contains 4.8–60.2 mg/ha (average 20.6 mg/ha) of bioavailable Cry1Ab toxin, while the amount of bioaccessible amount of Cry1Ab toxin is 0.085–8.16 g/ha. In contrast, the production of plant-expressed Cry1Ab toxin was found to be 147–456 g Cry1Ab toxin/ha, representing 18–56 treatments with Dipel (on the basis of its maximally detected bioaccessible Cry1Ab toxin content, 8.16 g/ha). The level of plant-expressed Cry1Ab toxin can be further elevated by soil fertilization (2.3–6.8-fold) and the use of long maturation maize varieties (2.5–5.8-fold), representing, in worst case scenarios, in 625–1,930 treatments with Dipel. Moreover, it has to be mentioned that stacked genetic events may further elevate toxin production (twofold). These ratios are even higher if lower bioaccessible Cry1Ab protoxin content biopesticides or bioavailable Cry1Ab toxin contents are considered.

Beside toxin ratios, another characteristic difference is that while Bt bioinsecticides are composed of several crystalline toxins, single genetic event Bt crops express only a single toxin molecule. This has severe consequences in resistance development, which may be alleviated, yet not eliminated by the use of "pyramid" Bt event varieties, expressing several Cry toxins acting on the same insect order, as the evolutionary driving force remain the same. The active ingredient of Bt bioinsecticides are bacterial protoxins stabilized in crystalline form and requiring enzymatic activation, while Bt plants (e.g., MON 810 ) express a truncated form of the protoxin, so-called preactivated toxin. This has severe consequences in product registration, as the active ingredient toxin in the Bt crop is not the registered active substance of the corresponding Bt bioinsecticide, and the required toxicology studies have been carried out not with the plant-expressed preactivated toxin, but with the bacterial protoxin or the enzyme-activated active toxin. Moreover, commercial ELISA systems utilizing antibodies against the bacterial protoxin and analytical standards of that protoxin consistently underdetect actual toxin content in Bt plants due to their lower cross-reactivities to the plant-expressed preactivated toxin. As a result, all reported results obtained by protoxin-based ELISAs, including manufacturer documentation, are subject to correction. And fi nally, although Bt crops have been widely advocated to be included in integrated pest management (IPM) practices or even in ecological agriculture, Bt crops cannot fulfill the main ecological principle of IPM that any protection measures should be timed only to the period(s) when pest damage exceeds the critical level, and therefore, regardless how environmentally mild their active ingredient is, do not comply with IPM.

A. Székács and B. Darvas Authors: Department of Ecotoxicology and Environmental Analysis, Plant Protection Institute , Hungarian Academy of Sciences, Budapest, Hungary Contacts: e-mail: ;