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Field and Forage Crops |
Our early maturing cotton crop has cut out over most of the state, with the insect season thankfully over for most cotton producers. However, enough bollworm moths and green and brown stink bugs are present to cause further damage in late cotton. With an early cotton crop, the potential for post-defoliation regrowth is probably high, and with it the potential for rebounding aphid populations.
Although in most cases stink bugs are probably more attracted to soybeans at this time of year, they may persist in late-maturing cotton fields and cause considerable damage to bolls less than three weeks old. In these situations, continue to monitor quarter-sized bolls, but also be sure to raise the internal boll damage threshold accordingly. For example, if the safe bolls outnumber susceptible bolls by about 3:1, 75 percent of the bolls are too mature to be damaged and the threshold may be raised from the earlier 10 percent to approximately 40 percent. Except for keeping track of boll size to adjust stink bug thresholds upwards, mature bolls do not to be monitored for damage.
Bollworms have a very difficult time becoming established as new hatchlings unless there are squares and blooms present. However, at this time of year bollworm and budworm moths can still be found in later maturing cotton fields and can become established in damaging numbers. As with stink bugs, be sure to raise the 3 percent live bollworm threshold according to the level of mature, speckled bollworm-safe bolls. In many cotton fields, the live bollworm threshold should be increased 3- to 5-fold.
We have very seldom seen boll-damaging populations of fall armyworms beyond the end of August in North Carolina. Although the feeding of early instar fall armyworms between the inner bract surface and the outer surface of medium and large bolls may be easy to detect in some fields, this damage is seldom an economic concern.
In the next few weeks, rebounding aphid populations are possible in cotton fields that have had very low populations for the past 4 to 6 weeks. On the positive side, defoliation essentially removes both the major food source for aphids (leaves)and therefore most of the cotton aphids. However, aphid population buildups are possible in cases of cotton regrowth following defoliation, perhaps more likely this year with our early crop. High cotton aphid populations in opening cotton can occasionally cause either sticky cotton (a problem for mills) or dark sooty mold (the darker cotton may be discounted) as a result of aphid honeydew. Fortunately, however, economic discounts for this damage are rare in North Carolina.
We will begin our damaged boll survey early next week (August 30). I would expect stink bug damage on both conventional and on Bollgard cotton to be high compared to previous years. Bollworm and tobacco budworm damage also appeared to be high in a number of conventional cotton fields in 2004.
Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Last modified on August 30, 2004 by Stephen J. Toth, Jr.
