Distribution of Heliothine Larvae in Bt and Non-Bt Cotton in Texas
Feb 14, 2019

Crop Protection WCRC WCRC2

Knowledge of population dynamics in heliothines in transgenic cotton is needed to develop resistance management strategy and scouting guidelines. The spatial distribution of heliothine larvae within and among transgenic cotton plants is likely to be influenced by variation in toxin expression within individual plants and throughout the growing season. A study was conducted to assess larval survival among diverse cotton plant structures to determine whether variation in larval survival was a function of the proportion of resistant individuals in the population or due to sub-lethal levels of toxin expression in various plant tissues. The study was motivated by Bt cotton failure in Brazos River Bottom in 1996 when Helicoverpa zea larvae were found in flowers, apparently feeding on pollen with low toxin content. In 1997, Bt cotton cultivars carrying the Cry1Ac toxin gene and non-Bt cotton were scouted in pairs in four locations in the Brazos River Bottom around College Station, Texas. The total effort was 79.5 man-hours for non-Bt cotton and 64.75 man-hours for Bt cotton. A total of 2875 plants were scouted in non Bt cotton and 2825 in Bt cotton. On each scouting day, 100 plants were selected randomly and scouted thoroughly. Larvae were classified as found on terminals, squares, flowers and bolls. The node position was also recorded. The data indicate that the majority of the larvae are found in terminals and squares in Bt cotton and that the terminal is a critical factor in the mortality of heliothine larvae exposed to Bt cotton. Data for 1998 show a similar trend. Third instar larvae found in flowers in Bt cotton were identified as H. zea or Spodoptera spp., as expected. The project was funded by a grant from Cotton Inc. to the primary author

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