Distribution of Heliothine larvae in B.t. and non-B.t. cotton in Texas
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We conducted a study to assess larval survivorship among diverse cotton plant structures in B.t. vs. conventional cotton. This study was motivated by B.t. cotton failures in the Brazos River Bottom in 1996, when bollworm (Helicoverpa zea) larvae were found in flowers, apparently feeding on pollen with low toxin content. In 1997 and 1998, B.t. cotton varieties carrying the Cry1Ac toxin gene and non-B.t. cotton were scouted in field pairs in four locations in the Brazos River Bottom around College Station, TX. Larvae were classified as found on terminals, squares, flowers and bolls. The node position was also recorded. Our data indicate that the majority of the larvae are found in terminals and squares in B.t. cotton, and that the terminal is a critical factor in the mortality of heliothine larvae exposed to B.t. cotton. Third instar larvae found in flowers in B.t. cotton were identified either as H. zea or Spodoptera spp., as expected. The vertical distribution of live larvae is different in Bt cotton than in conventional cotton. In Bt cotton live larvae are found towards the middle of the plant, mainly in squares, the most damaged structures.
