Issue No. 11, Article 4/June 9, 2006
Western Corn Rootworm Feeding Behavior on a Transgenic Hybrid Described in Detail
In a recent journal article (Journal of Economic Entomology, June 2006), researchers from the University of Nebraska and Monsanto Company described the feeding behavior of western corn rootworms on a transgenic corn rootworm hybrid (MON 863, Cry3Bb1) and its nontransgenic isoline. The scientists utilized a novel transparent growth medium placed in test tubes in which seedling plants were grown and through which corn rootworm larvae could be observed feeding on root tissue.
Eight days after seedlings were transplanted into the test tubes containing the growth medium, 20 newly hatched larvae were carefully placed into each tube. Three treatments were utilized for the experiment: infested transgenic maize, infested isoline, and uninfested isoline. Each treatment was replicated six times. Observations and collection of data (head-capsule widths, larval weights, and wet-root weights) were made 3, 6, 9, and 12 days following the introduction of larvae into each test tube.
The researchers made the following observations: "Larvae on the roots of the susceptible treatment were actively feeding in one concentrated area, causing severe root injury. Larvae in the resistant treatment exhibited two types of behavior. The first type of behavior was illustrated by larvae with little body movement and visibly reduced abdominal segments (giving them a flattened appearance). The midgut of these lethargic larvae contained visible root tissue, indicating ingestion. Larvae exhibiting the second observed behavior moved actively up and down the root system and were observed to sample maize root hairs with their mandibles or take small bites of the root tissue, never concentrating on one feeding location. These larvae had visibly empty midgets and died, presumably from starvation."
Corn rootworm emergence from fields planted to transgenic corn rootworm hybrids (MON 863, Cry3Bb1) has been reported many times previously. The authors of this paper correctly point out that it is "not entirely clear" how this occurs. Yet some larvae are surviving in fields planted to transgenic rootworm hybrids and emerging as adults in sufficient numbers to warrant scouting of fields for silk clipping in some instances. Much speculation has been used to explain this observation and includes the following: (1) low to moderate dose of MON 863 hybrids; (2) declining titre of Cry3Bb1 in root tissue of some MON 863 hybrids during the growing season; (3) late corn rootworm larval hatch and, thereby, potential exposure to lower concentrations of Cry3Bb1 in root tissue; and (4) corn rootworm larval feeding on alternate grass hosts helping to bridge eventual movement (and survivorship) of later instars to corn roots.
This recent paper by University of Nebraska and Monsanto Company scientists sheds additional light on an intriguing topic and suggests that some western corn rootworm larvae may be able to detect small concentrations of Cry3Bb1 in root tissue, stop feeding, and survive to the next instar. Later instars of corn rootworm larvae are less likely to succumb to the Cry3Bb1 protein expressed in transgenic root tissue. Ultimately, under this scenario, we have adult emergence into producers' fields.
Based on observations in this paper, some first instars consumed enough root tissue to result in death. Are western corn rootworm adults emerging from fields planted to transgenic rootworm hybrids (MON 863) primarily those that exhibit this nonpreference (antixenosis) feeding behavior? Will some larvae also be able to detect Bt proteins expressed in root tissue of other transgenic corn rootworm hybrids (Cry34Ab1/Cry35Ab1, DAS-59122-7), stop feeding, and survive? These and many other questions related to transgenic corn rootworm hybrids will provide fertile research areas for years to come. If you're interested in taking a look at the article, go to the following Entomological Society of America Web site.
Clark, P.L., T.T. Vaughn, L.J. Meinke, J. Molina-Ochoa, and J.E. Foster. 2006. Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) larval feeding behavior on transgenic maize (MON 863) and its isoline. Journal of Economic Entomology 99(3): 722-727.