BOLLGARD COTTON SCOUTING DIFFERENCES NOTED

Jack S. Bacheler

North Carolina Extension Entomologist

Although Bollgard and conventional cotton share a number of scouting similarities, the status of North Carolina's major and minor pest species is often very different in Bollgard cotton, requiring a significant change in scouting emphasis. Tobacco budworms and European corn borers, the former a headache on conventional cotton when it encroaches into our major bollworm generation and the latter an economic problem second only to the bollworm, will be of little or no economic consequence on Bollgard cotton. Scouting for these cotton pests will not be required in Bollgard cotton, at least for the foreseeable future. Plant bugs should be somewhat more a late season problem in the less-treated Bollgard cotton, but primarily due to our generally light plant bug levels, only minimal extra scouting will likely be required. No reported late-season treatments have required for plant bugs on Bollgard cotton to this point. Cotton aphids appear to be equally susceptible to predators, parasites and a fungus in both conventional and in Bollgard cotton. Insecticides are not generally recommended controlling cotton aphids in North Carolina.

The elevation in the status of stink bugs and the approach to scouting bollworms on Bollgard cotton are the most noticeable and economically important changes from conventional cotton. In a survey of 260 Bollgard and 260 conventional cotton fields conducted in here in1996-1997, stink bug levels were approximately 4-fold higher in the 0.52 times treated Bollgard cotton than in the 2.5 times treated conventional cotton. With stink bugs averaging approximately 0.67% in conventional cotton over the past 10 years, very little time or effort is spent scouting for stink bugs. Because stink bugs have averaged approximately 3% damaged bolls in 1996-1997 in Bollgard cotton (generally light `stink bug years' across the board in North Carolina), stink bugs must be regarded as a potentially serious pest in Bt cotton.

During bollworm scouting, if more than 2% stink bug externally-damaged bolls are detected (suspected stink bug-damaged bolls must be cut open with a pocket knife to confirm the internal component of this damage), then one of the three recommended scouting procedures and its associated threshold should be utilized, as outlined in the 1998 Cotton Information booklet or in the North Carolina Cotton Insect Scouting Guide.

Although bollworm-damaged bolls in Bollgard cotton averaged only 25% to 50% as much as in conventional cotton in 1996-1997, some level of bollworms became established in Bollgard cotton in most fields. In replicated field trials, the elimination of seemingly low levels of bollworms and their damage in Bollgard cotton often returns moderate to large yield dividends. Research conducted here suggests that our `protective' Bollgard thresholds may be in need of further downward revision. The following bollworm thresholds for Bollgard cotton should provide good protection under most circumstances- pay particular attention to eggs and small bollworms associated with blooms and dried bloom tags:

3% bollworms, 1/8" or larger, on squares, blooms, bloom-tags, or bolls.
or
(under very high egg pressure)- 100 eggs/100 terminals, or 20 eggs per 100 fruit.

As of this writing, the high observed late June to early July second generation tobacco budworm moth flights should present little challenge to Bollgard fields. However, early indicators point also toward moderate to high levels of bollworms and stink bugs. Careful scouting and a quick response to threshold levels of both of bollworms and stink bugs will provide producers their best odds of managing this new technology profitably.