Stink bug outlook unpredictable in cotton

Although stink bugs can now be regarded as the primary mid- to late-season pest of Bt cotton in North Carolina, their levels, and thus damage potential, remain very inconsistent from year to year.

At the time, 2004 seemed like the beginning of a rough period for stink bug damage. Likewise, the lighter boll damage from stink bugs in 2006 and 2007 may have given us a false sense of security. In hindsight, 2004 was close to a perfect storm of a mild winter, no late frosts, and plentiful rainfall over most of the state that served to increase both the growth of the many wild and cultivated hosts of green and brown stink bugs as well as the attractiveness and susceptibility of cotton to stink bugs.

Although 2006 and 2007 showed far less boll damage from stink bugs here than in 2004 and seemed to suggest a downward trend in damage, at this point in the growing season we are almost as likely to get another 2004 “bug year” as another 2007.

Presently, brown and green stink bug levels in 2008 seem to be at far higher levels than in 2007. However, dry weather and low cotton yields could translate into lower boll damage from stink bugs unless weather patterns change.

Irrespective of stink bug levels, we are now in a far better position to manage these pests than was the case in 2004, thanks to a Southeastern multi-state effort that shed significant light on how to deal with these emerging pests.

Entomologists from Virginia, North Carolina, South Carolina and Georgia participated in the first Cotton Incorporated-sponsored regional project from 2005 to 2007 entitled: “Identifying Practical Knowledge and Solutions For Managing The Sucking-Bug Complex In Cotton: Research In The Southeast Region”

Some of the practical results of this project follow.

• Species encountered: Green and brown stink bugs caused the most damage in North Carolina and Virginia; a mixture of greens, southern greens and browns in South Carolina, and primarily southern greens and browns in Georgia. For the most part, plant bugs appeared to cause minor boll damage at the many test sites in the four states.

• Stink bug movement: Grass species, particularly wheat, serve as a major nursery crop for brown stink bugs, with corn serving as a “bridge” to late season hosts such as cotton and soybeans. Peanuts can also serve as a nursery for southern green stink bugs into cotton, especially in Georgia, where higher levels of bugs in cotton field edges adjacent to peanut fields have been observed, particularly in high stink bug years like 2005.

• Trapping: Although standard black light traps consistently captured high numbers of green stink bugs (sometimes over 100 per two nights) in a 2-year study in North Carolina, no association was found between the type of crop in which the trap was placed and the number of stink bugs found. Also, stink bug levels found in traps did not correlate well with times of maximum cotton susceptibility to boll damage. Pheromone attractant traps for brown stink bugs provided some information on the timing of flights, though trap counts were often low and the traps were both cumbersome and fairly expensive.

• Stink bug damage to small bolls: Quarter-sized bolls were confirmed as the target size for sampling, providing the best predictive value in correlating with yield loss at a time when stink bugs are likely to still be in the field. However, the relationship between damage to quarter-sized bolls and yield was extremely variable from one year and even from one field or test to the next.

• Stink bugs less damaging than bollworms?: Good news: To date, data seems to indicate that on average, a stink bug damaged boll (stained lint or a detectable wart) causes about one third of amount of yield loss per boll as a bollworm-damaged boll. This is not surprising, as year end stink bug damaged bolls vary from minimal damage symptoms to a complete hard lock and yield loss. With bollworms, boll feeding, and the resulting often sizable boll penetrations, typically result in complete boll loss.

• Spray Timing: Sixteen replicated small plot tests designed to evaluate the relationship between yield loss and the timing of stink bug sprays during the flowering period suggested that weeks 3 to 5 (and perhaps 6) of bloom period represented a window of greatest susceptibility to yield-reducing damage.

Test results showed very little benefit in treating for stink bugs during the first two weeks of bloom — both boll levels and stink bug levels tended to be low during this time period. Conversely, averaged over the tests, the economic returns in treating for stink bug during weeks 3, 4 and 5 of blooming was positive and averaged just under $70 per acre during week 4 of bloom.

And finally, returns following week 5 to 7 of blooming were low to negative, probably the result of a predominance of larger, older bolls (greater than 3.5 weeks old) that were no longer susceptible to stink bug damage.

In addition to using the dynamic threshold to help target times of the bloom period during which large numbers of stink bug-susceptible bolls are present and lower threshold are appropriate, investigations into more effectively managing this pest complex are also under way. These include:

• Landscape impact on stink bug movement: In both North Carolina and in Virginia, studies are under way to better understand how the sequence of cropping systems may help predict the incidence of sting bug damage on late season susceptible crops, such as cotton and soybean.

• In-field distribution of stink bugs: Work in Georgia to more precisely characterize and define field distribution of stink bugs may lead either to a better predictive tool for scouting to determine if further scouting or treatment may be in order based on finding hot spots of stink bug activity in portions of the field, for example, field edges. This in turn could lead to selective treatment within cotton fields.

• Threshold refinement: Research continues into refining the current state stink bug internal boll damage thresholds under a wide range of stink bug levels and cotton plant phenological (e.g., week of bloom) stages. Although the dynamic appears to be logical and is presently recommended in North Carolina, more information is needed to confirm this approach will work under a wide range of conditions.

• Bug resistant cotton plants: In theory, if cotton plants could be transformed to make toxins lethal to stink bugs and plant bugs (similar to the current Bt toxins against caterpillars), this development would have a huge positive impact on the U.S. cotton industry, particularly in the case of plant bugs in the Mid-South. So far this development has eluded both corporate, university and USDA researchers, but significant work is now under way to develop bug-resistant cotton plants.

• External boll damage: Early returns from studies conducted in North Carolina and Virginia in 2006 and 2007 have found a fairly high correlation between external boll damage, internal boll damage and yield loss. If this relationship holds up under a wider range of stink bug levels and cotton crop conditions, the relative ease of checking for external bug damage could lead to scouts being able to evaluate more bolls in a shorter period of time.

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