Cotton expressing endotoxin protein of Bacillus thuringiensis was available for field testing in small plots in 1989. During the 1990's, considerable research was focused on developing these insecticidal plants for use in commercial cotton production. After a few years of research, commercial varieties expressing CryIA(c) proteins of B. thuringiensis were available to cotton producers. More than two million acres of these insecticidal cotton plants were grown in the U.S. in 1997 with grower adoption rates as high as 70% of the commercial cotton in some regions. A major concern for long term utility of this technology is the threat of resistance to endotoxin proteins in targeted pest populations.
The Southern Region IPM Program funded a research project in 1993 to investigate the potential effects of insect behavior on utility of a seed mixture approach to delay resistance. The seed mixture strategy was a popular concept because it provided a within field refuge for susceptible insects that could be easily implemented by blending transgenic and non-transgenic seed prior to distribution to growers. Refugia or production of susceptible insects to mate with rare resistant insects is a critical component in resistance management. The seed mixture approach seemed to be a simple approach that could be effectively implemented. Unfortunately, some simulation studies indicated that the value of the seed mixture approach was greatly reduced if insect movement between plants would create a mechanism for increased survival of rare resistant insects exposed to endotoxin. Thus, knowledge about the amount of movement of targeted insects between plants was needed to choose the most promising resistance management approach. This Southern Region IPM Project generated timely information on oviposition patterns and interplant movement of tobacco budworm, Heliothis virescens, larvae in mixed plant stands of non-transgenic and transgenic cotton expressing endotoxin protein. The tobacco budworm is, perhaps, the most important target of these insecticidal plants. Results of the project had a major impact on resistance management policy.
Several laboratory, cage, and field studies were conducted to determine
if tobacco budworm moths differentiate between the transgenic and non-transgenic
cotton and if larvae moved between plants in mixed stands of the transgenic
and non-transgenic cottons.
No differences were detected in oviposition behavior. Tobacco budworm
oviposition was the same on both types of cotton. In laboratory studies
with paired-cotton terminals as a test arena, neonate larvae moved between
plants. Results indicated that larvae tended to move more when placed on
the transgenic cotton than when placed on non-transgenic cotton. Laboratory
studies were also conducted to measure survivorship of larvae exposed to
the transgenic insecticidal cotton for time intervals less than 48 hours.
Survivorship of one and four-day-old larvae was reduced as compared to
that of larvae exposed to non-transgenic plants. Seven-day-old larvae exhibited
no significant reduction in survivorship. This was an important finding
that indicated that older larvae may survive on the transgenic plants if
they develop on non-transgenic plants during the more susceptible periods
of early larval development.
In field studies conducted with mixtures of transgenic and non-transgenic cotton, the potential for interplant movement of larvae was followed throughout larval development. When neonate larvae were placed on non-transgenic plants, interplant movement of larvae was not observed until larvae were three days of age. Larvae placed on transgenic plants began to move earlier than those placed on non-transgenic plants. More than 80% of the larvae placed on transgenic plants moved to an adjacent plant by the time they were 10 days old. Collectively, the data indicated that tobacco budworm larvae move between plants and that movement increases with larval age. More movement was observed for larvae placed on the transgenic plants. Variability in the amount of time before larval movement is an important factor affecting the value of the seed mixture strategy, especially since susceptibility decreases with age. Knowing that larvae move is important, but knowing when they move and what their expected survivorship is after movement is more important.
The potential influence of age-related movement and age-related susceptibility was investigated in a simulation model. Results confirmed that the seed mixture strategy is risky because of interplant movement of tobacco budworm larvae. Under some circumstances, however, the use of seed mixtures will not speed the rate of resistance. These circumstances require that an external refuge be present.
Data from this research influenced the recommendations that transgenic cotton be grown with a mandatory external refuge. The seed mixture strategy was abandoned. Results were used by industry, regulatory, and extension groups to formulate strategies for the deployment of transgenic cotton expressing endotoxin proteins. The project was conducted as a M.S. student thesis project. In addition to the M.S. thesis, several professional presentations and conference reports were a direct result of the research. Three scientific manuscripts resulted from this research and are currently being considered for publication.