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

Abstract

By consuming mulberry leaves covered with pollen from nearby genetically engineered, insect-resistant rice lines producing Cry proteins derived from Bacillus thuringiensis (Bt), larvae of the domestic silkworm, Bombyx mori (Linnaeus) (Lepidoptera: Bombyxidae), could be exposed to insecticidal proteins. Laboratory experiments were conducted to assess the potential effects of Cry1C- or Cry2A-producing transgenic rice (T1C-19, T2A-1) pollen on B. mori fitness. In a short-term assay, B. mori larvae were fed mulberry leaves covered with different densities of pollen from Bt rice lines or their corresponding near isoline (control) for the first 3 d and then were fed mulberry leaves without pollen. No effect was detected on any life table parameter, even at 1800 pollen grains/cm2 leaf, which is much higher than the mean natural density of rice pollen on leaves of mulberry trees near paddy fields. In a long-term assay, the larvae were fed Bt and control pollen in the same way but for their entire larval stage (approximately 27 d). Bt pollen densities ≥150 grains/cm2 leaf reduced 14-d larval weight, increased larval development time, and reduced adult eclosion rate. ELISA analyses showed that 72.6% of the Cry protein was still detected in the pollen grains excreted with the feces. The low exposure of silkworm larvae to Cry proteins when feeding Bt rice pollen may be the explanation for the relatively low toxicity detected in the current study. Although the results demonstrate that B. mori larvae are sensitive to Cry1C and Cry2A proteins, the exposure levels that harmed the larvae in the current study are far greater than natural exposure levels. We therefore conclude that consumption of Bt rice pollen will pose a low to negligible risk to B. mori.

The development and status of Bt rice in China

“…For example, recent laboratory experiments were conducted to assess the potential effects of Cry1C‐ or Cry2A‐producing transgenic rice (T1C‐19, T2A‐1) pollen on B. mori fitness. B. mori larvae were not negatively affected when fed mulberry leaves covered with pollen from Bt rice lines for 3 days, even when Bt rice pollen density was at 1800 pollen grains/cm 2 mulberry leaf, which is much higher than the mean natural density of rice pollen on leaves of mulberry trees (93 grains/cm 2 leaf) near paddy fields (Yang et al ., ). In the long‐term assay, the larvae were fed Bt and control pollen for their entire larval stage (approximately 27 days), which was significantly longer than the rice pollen shedding duration, B. mori larvae were negatively affected at Bt pollen densities exceeding 150 grains/cm 2 leaf (Yang et al ., ).…”

“…Previous studies showed significant differences in toxicity of pollen from different Bt rice lines to silkworms (Wang et al ., , ; Yang et al ., ; Yao et al ., , ; Yuan et al ., ). The differing results may be attributed to (i) different types of Bt proteins expressed in pollen; (ii) different expression levels of Bt proteins contained in pollen; and (iii) different levels at which silkworm larvae were exposed to Bt protein in the respective feeding studies (Yang et al ., ; Yao et al ., ). In addition, different silkworm varieties showed significantly different sensitivities to the same Bt protein (Yuan et al ., ).…”

“…However, to further guarantee the safety of the silk industry, Yang et al . () suggested that mulberry leaves on trees that are near paddy fields planted with Bt rice lines should not be used to feed B. mori larvae or should be washed before they are fed to B. mori larvae.…”

“…Thus, silk worms may be subject to indirect risk posed by production of stem borer-resistant Bt rice, especially because silkworms belong to the same order as the stem borers (Romeis et al, 2008. Therefore, the potential effects of Bt rice on silkworms have raised much concern (Wang et al, 2001(Wang et al, , 2002aYang et al, 2014;Yao et al, 2006Yao et al, , 2008Yuan et al, 2006). Previous studies showed significant differences in toxicity of pollen from different Bt rice lines to silkworms (Wang et al, 2001Yang et al, 2014;Yao et al, 2006Yao et al, , 2008Yuan et al, 2006).…”

“…Therefore, the potential effects of Bt rice on silkworms have raised much concern (Wang et al, 2001(Wang et al, , 2002aYang et al, 2014;Yao et al, 2006Yao et al, , 2008Yuan et al, 2006). Previous studies showed significant differences in toxicity of pollen from different Bt rice lines to silkworms (Wang et al, 2001Yang et al, 2014;Yao et al, 2006Yao et al, , 2008Yuan et al, 2006). The differing results may be attributed to (i) different types of Bt proteins expressed in pollen; (ii) different expression levels of Bt proteins contained in pollen; and (iii) different levels at which silkworm larvae were exposed to Bt protein in the respective feeding studies Yao et al, 2008).…”