From 2004 through 2008, a series of replicated trials was carried out in North Carolina, South Carolina, and Georgia to determine: Relative pest status of plant bugs vs. stink 1. bugs in the Southeast.
The major stink bug species in the region. 2. Times of maximum and minimum boll susceptibility to feeding injury caused by stink bug damage during the blooming period. If a dynamic threshold could be developed 4. to reflect predictable periods of susceptibility and tolerance to stink bug damage.
As indicated in the introduction, stink bugs accounted for the predominant complex of boll-feeding bugs important on cotton in the southeastern states. High square-retention, low levels of dirty blooms, and subeconomic action thresholds of plant bugs (Lygus spp.) were typical in this area of the country (Figures 7, 8, 9), indicating that losses due to bug damage were primarily from stink bugs.
Replicated trials from 2004 to 2007 – designed to define periods of maximum and minimum susceptibility of bolls to stink bug damage – indicated that yield was preserved when cotton was protected from stink bugs with insecticide during weeks three to five of bloom.
On average, yields were not increased when insecticide was used during the first or second week of bloom or after the fifth week of bloom. Additionally, the results show that low stink bug levels early in the season and greater levels of large “bug-safe” bolls later in the season suggested less protection was needed in the early and late parts of the bloom period.
As indicate, external stink bug damage to bolls older than approximately 25 days does not translate into internal damage to lint. Based on these data, a dynamic boll-damage threshold was developed where allowable levels of boll damage change with the week of bloom.
Selected treatment thresholds were evaluated for stink bugs in 49 replicated trials during 2005–2008 in North Carolina, South Carolina, and Georgia. About 50 percent of the trials included the following core treatments and/or action thresholds:
- Weekly spray
- Non treated check
- 10 percent internal boll damage (one or more internal warts or stained lint)
- 20 percent internal boll damage
- 30 percent internal boll damage
- Dynamic (threshold changed by week of bloom, as indicated in Table 3)
The 20 percent internal boll-damage threshold has been the primary threshold used in the Southeast for almost a decade. Therefore, the economic returns (lint value minus the cost of control) of the 20 percent threshold were compared with those from the dynamic threshold and other treatments.
To find out how the 20 percent and the dynamic threshold performed under a range of stink bug pressure, the tests were divided into three pressure categories:
- Low – the 20 percent threshold was not reached
- Low-moderate – the 20 percent threshold was reached one time
- Moderate-high – the 20 percent threshold was reached two or more times
Results showed that the dynamic threshold provided higher net returns than the other treatments, including the 20 percent threshold. The net return advantage of the dynamic threshold above the 20 percent threshold was greatest under moderate-high pressure ($33.78 per acre), slightly less under low-moderate pressure ($29.19 per acre), and far less under low pressure ($7.49 per acre).
The static boll-damage threshold (20 percent) has served as the standard threshold for management of stink bugs in the Southeast for almost a decade. These data and additional complementary research information indicated that when cotton was more aggressively protected during the first, second, and third weeks of bloom, the economic returns were greater than those provided by the static 20 percent threshold.
The importance of controlling stink bugs during this critical portion of the blooming period should be stressed to producers and consultants. Several factors help explain why a dynamic threshold should provide increased protection from stink bugs.
The thresh old should change as the crop matures and the number and percentage of stink-bug-susceptible bolls changes. Also, stink bug numbers typically increase during weeks of rapid blooming. A dynamic threshold is effective because the impact of stink bugs early and late in the season is inherently lower.
In most fields, the first two weeks of bloom are relatively unimportant in terms of protection from stink bugs because bolls are not available in large quantities (Figure 21) and populations of stink bugs are generally low.
Late in the bloom period (seventh and eighth weeks), stink bugs also cause less damage in most cases because susceptible bolls are declining in number. The sixth week of bloom can be important, depending on the level of pressure from stink bugs and crop status, but the third, fourth, and fifth weeks of bloom are clearly the most susceptible to stink bug damage and associated yield losses.
By following the dynamic threshold (or a close variation), cotton producers should be able to protect bolls from stink bug damage during critical periods of the bloom period and avoid unnecessary treatments during times of low risk.
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