FIELDAND
FORAGE
CROPS
FIELD
The information and recommendations in this newsletter are applicable to
North Carolina and may not apply in other areas.
Two-spotted spider mite levels will likely drop significantly in areas which have received 1 to 2 inches or more of rain from Hurricane Danny. Fortunately, much of the heavy rainfall occurred in peanut-producing counties where mite levels were highest on cotton, not to mention peanuts. If we get additional rainfall, the mite situation can be expected to improve further.
Plant bugs continued to be observed in at least low levels in many cotton fields throughout the state this past week. This coming week, square retention assessments, as an indicator of the need to further sample for plants bugs, becomes less valuable. As the cotton crop begins to put on bolls, square retention will naturally drop. Additionally, square loss itself becomes far less important, as we no longer have enough time for these squares to develop into a harvestable boll, given average weather conditions. This leaves less of a possibility that plant bugs will damage blooms (dirty blooms in which the pollen anthers are blackened which can lead to deformed bolls) and small bolls. Plant bug damage to small bolls looks very much like stink bug damage, but the spots are much smaller. Fortunately, we seldom have the plant bug populations necessary to cause this kind of damage, except for in a few easternmost counties where cotton is grown adjacent to potatoes.
Where plant bugs do occur in higher numbers, we can not afford lost or deformed bolls. When plants bugs or their damage (small blackened squares easily found during inspections or in sweeping) is noted, more careful sweeping is recommended (see the Cotton Insect Scouting Guide), even during the blooming period. After initiation of blooming, 20 plant bugs per 100 sweeps may indicate the need to treat with insecticide. The 30 plant bug per 100 sweep threshold published in the Cotton Information booklet and in the Cotton Insect Scouting Guide has been lowered to reflect our late crop and the associated need to protect fruit during this generally short effective blooming period. The need to treat for plant bugs in the coming weeks, even with this year's higher numbers and orientation toward protection, will likely be minimal.
The major bollworm moth flight has finally begun the southern counties of North Carolina. Unfortunately, information on the progress of this moth flight becomes dated quickly. We have already had two light traps go over 100 for a two-night period, one in Rowland (in southern Robeson County) on July 23 and another in Sampson County on the morning of July 25. Light trap counts have gone up significantly in other counties as well (Duplin, Lenoir, and Scotland counties, to name several). It would appear that this flight will develop in the northern counties of North Carolina in about 7 to 10 days (an estimate). So far this flight is composed of almost all corn earworms. Insecticide spraying for bollworms has begun in several our southern and southeastern counties (i.e., Scotland, Robeson, Duplin, and undoubtedly a few other counties). From July 25 through July 28, perhaps 20 percent of our acreage could be at or approaching the egg threshold.
Although it is too early to judge the size of this flight, it does not appear to be small. Additionally, based upon the lengthy time for corn earworm development on corn, it looks like we may be in for an extended flight, perhaps tailing off over a longer period of time than is often the case, and mixed with more European corn borer moths than usual. Fall armyworms have also begun to show up in a number of fields in the southern counties, so far at low numbers.
With a few exceptions, this cotton crop is probably the most susceptible to bollworms and European corn borers that I have seen. I can not overemphasize the importance of quickly responding to threshold levels of eggs, and not overlooking the possibility of this egg threshold being met on fruit down in the plants. Furthermore, do not overlook larval thresholds being met on fruit, especially in or under pink and dried blooms.
In Bollgard cotton, look for either 6 percent economically-damaged squares or 3 percent one-eighth of an inch-long bollworms on fruit. This threshold may occur on either squares or bolls, especially associated with white, pink, or dried blooms. Most threshold levels on Bollgard cotton were associated with blooms during 1995 and 1996. In extremely high egg pressure (i.e., 10-fold the egg threshold on conventional cotton, or 100 eggs per 100 terminals, or 20 eggs per 100 fruit), treatment with insecticide will likely pay. A scouting frequency of 3 to 5 days is recommended for Bollgard cotton. European corn borers and tobacco budworms will not become established on Bollgard cotton in economic numbers at least for several years or more (until these species develop resistance to the Bollgard endotoxin).
Remember that stink bugs are not controlled in B.t. cotton.
Slicing bolls that appear to have stink bug damage to look for
internal discoloration will tell scouts whether further, more
tedious, assessments for live stink bugs are necessary. See the
Cotton Information booklet or the Cotton Insect Scouting Guide
for details. For twice per week insect updates, call 1-800-662-
7301; the cotton update is No. 11.
Web page last updated on July 28, 1997 by Stephen J. Toth, Jr..