Linda Benedict | 8/22/2008 1:13:10 AM
P. L. Bommireddy and B. Rogers Leonard
Transgenic plants that express insecticidal proteins from the soil bacterium Bacillus thuringiensis (Bt) have become standard for managing many caterpillar pests in cotton. Three Bt cotton technologies currently available in commercial cotton cultivars are Bollgard, Bollgard II and WideStrike. These plants express single or multiple crystal – or Cry – proteins produced by Bt. When insects eat these, Cry proteins become toxic to the target pests. Bt is environmentally friendly and is practically nontoxic to vertebrate organisms including birds, fish, and humans.
The addition of a second Cry protein in Bollgard II and WideStrike plants has improved overall efficacy and also expanded the target pest spectrum that is now successfully managed with Bollgard.
The current commercial Bt cotton cultivars have been widely accepted by producers and are planted on more than 90 percent of Louisiana’s cotton acreage. However, there are concerns about the similarity in structure and activity of these specific Cry proteins and the potential for target pests to develop resistance to Bt.
Syngenta Crop Protection is developing genetically engineered cotton plants that express a novel vegetative insecticidal protein, Vip3A, from Bt. Commercial lines that express Vip3A will also contain an additional Cry (1Ab) protein. This unique combination of a Vip and a Cry protein in cotton plants has been labeled as VipCot technology. VipCot has the potential to be another effective tool for integrated pest management in cotton with the additional benefit of Bt resistance management. Before VipCot can be fully integrated into a cotton pest management system, however, the consistency of performance against primary target pests must be documented.
LSU AgCenter researchers conducted a series of field studies at the LSU AgCenter’s Macon Ridge Research Station near Winnsboro, La., during 2005-2007 to evaluate the performance of transgenic cotton lines expressing the Vip3A insecticidal protein against two primary caterpillar pests – the bollworm and the tobacco budworm. A non-Bt cotton line, Coker 312, and a VipCot cotton line expressing the Vip3A + Cry1Ab proteins were sampled throughout each season for larval injury to reproductive structures – flower buds (or squares), flowers and bolls – and surviving larvae. In addition, isolated Coker 312 and VipCot plants in field plots were artificially inoculated with bollworm or tobacco budworm caterpillars. The infested plants were visually inspected every two to three days after infestation for damage to the reproductive structures and for the presence of surviving larvae until larvae were no longer detected.
In the field trial against native infestations, significantly fewer damaged reproductive structures and surviving larvae were found on the VipCot plants compared with the Coker 312 plants (Table 1). On plants inoculated with insects, bollworm larvae injured a cumulative total of 44 reproductive structures on Coker 312 plants compared with six on VipCot plants (Figure 1). Tobacco budworm larvae injured 51 reproductive structures on Coker 312 plants and eight reproductive structures on VipCot plants. In addition, fewer surviving larvae of both target pests were recorded on VipCot plants compared with Coker 312 plants (Figure 2). Within seven days after inoculation, no bollworm or tobacco budworm larvae were recorded on VipCot plants, but more than 50 larvae of each pest were recorded on Coker 312 plants.
These results suggest that both target pests were susceptible to the combination of proteins in the VipCot technology. None of the larvae for either species produced significant injury to reproductive structures in the field trials. In addition, none of the larvae were capable of completing larval development on the VipCot line. The results from these trials support the continued development of the VipCot technology as an additional tool for managing caterpillar pests in Louisiana cotton fields.
(This article was published in the summer 2008 issue of Louisiana Agriculture.)