Issue No. 21, Article 7/August 12, 2005
Susceptibility of Western Corn Rootworm First-Instar Larvae to Cry3Bb1 Protein
In a very recent article (Journal of Economic Entomology, August 2005), scientists from the University of Nebraska (Blair Siegfried and Terence Spencer) and Monsanto (Ty Vaughn) published very interesting data on the baseline susceptibility of western corn rootworm larvae to the CryBb1 (Bt) protein. They used neonates (newly hatched larvae) obtained from laboratory colonies as well as field populations. The first instars were exposed to increasing levels of the Cry3Bb1 protein (artificial diet), and mortality and growth were assessed after 4 to 7 days had elapsed. Field populations were collected from eight states: Yuma County, Colorado; Scott and Weld counties, Iowa; Iroquois and Warren counties, Illinois; Gove County, Kansas; Hamilton, Clay, and Seward counties, Nebraska; Winona County, Minnesota; Ontario County, New York; and Lancaster County, Pennsylvania.
The authors offered the following comments in their results and discussion section: "The results of these bioassays suggest that western corn rootworm larvae are in general not extremely sensitive to the Cry3Bb1 toxin because both LC50 and EC50 values are several orders of magnitude greater than values reported for lepidopteran-active Cry toxins by using similar bioassay methods. This lack of sensitivity is consistent with the designation of MON 863 as a low-to-medium dose product (EPA 2003)." The authors are to be commended for publishing this very important paper. As they indicate, developing baseline susceptibility data is the first step in a monitoring program designed to pinpoint when levels of susceptibility in a corn rootworm population may begin to change in response to long-term exposure to Bt proteins expressed in transgenic plants.
A review of this paper revealed interesting differences in susceptibility of western corn rootworm neonates from the two field populations collected in Illinois. The EC50 values (concentration of Cry3Bb1 that produces 50% growth inhibition relative to untreated controls) for first-instar western corn rootworm larvae collected from Iroquois and Warren counties were 2.80 and 0.98, respectively. These values refer to micrograms of Cry3Bb1 per square centimeter of artificial diet that has been treated. The LC50 values (micrograms of Cry3Bb1 protein per square centimeter needed to kill 50% of neonates) for Iroquois and Warren counties were 4.22 and 0.74, respectively. In essence, it took nearly three times as much Cry3Bb1 protein to cause a 50% growth inhibition of first-instar larvae collected from Iroquois County compared with Warren County. Likewise, a concentration of Cry3Bb1 protein, nearly six times greater, was required to kill 50% of neonate western corn rootworm larvae from Iroquois County compared with Warren County.
Considerable speculation and discussion have occurred during the past 3 to 4 years regarding a suspected increased vigor of variant western corn rootworms compared with nonvariant populations. Although these data do not necessarily confirm this suspicion, they do suggest that the hypothesis should be more carefully tested. Iroquois County lies in the geographic heart of the variant western corn rootworm population. It is a county with a strong corn and soybean rotation history, believed to be a partial explanation for the evolution of the variant western corn rootworm. Warren County, in northwestern Illinois, has only recently become susceptible to variant western corn rootworm damage in rotated corn. This area of the state has a longer history of continuous corn production practices.
It should be pointed out that among the 12 field populations of western corn rootworm larvae tested, the EC50 values obtained from neonates in Iroquois County were the greatest. Interestingly, the LC50 values for neonates exposed to the Cry3Bb1 protein from three other locations were greater than those obtained from larvae collected in Iroquois County (4.22) and include Weld County, Iowa (9.20); Gove County, Kansas (4.90); and Lancaster County, Pennsylvania (6.61).
We continue to learn more about the biology and ecology of the variant western corn rootworm and the responses of corn rootworms in general to the Cry3Bb1 protein. As new transgenic events are commercialized for this important corn insect pest, there are sure to be surprises. I encourage you to take a look at this paper.--Mike Gray