Cotton Insect Newsletter Vol. 20, No. 13, August 14, 2002 Call 1-877-629-1474 for Cotton Insect Hotline

Cotton Situation: It's still uncommonly dry, with a chance of showers today and for the next several days.  Some of the later-planted cotton could certainly profit from rain.  Where cotton plants have ceased producing flowers, moisture will be needed to fill out the younger bolls. The “South Carolina Crop and Weather Report” for the week ending August 11, stated that 7% of the bolls were open.  The 5-year average for the same time frame is 4% open bolls.   Crop condition was rated as 54% poor to very poor. Insect Situation: I'm still seeing plenty of bollworm moths in cotton fields with plants that are blooming and setting new bolls.  Egg counts in such cotton fields are typically running from 20 to 80 or more per 100 plants.   I have mentioned in an earlier newsletter that thresholds need to be adjusted upwards as the season progresses.  If you now find 75 eggs/100 plants in a field of Bt  cotton where plants have bolls attached most of the way up the stalk, it's not the same as it would be in the middle of July when growers are advised to treat a field with 75 eggs/100 plants.  There are several reasons for adjusting the thresholds.  First, the bolls that are developing in the tops of the plants will be smaller and they will contribute less to yield.  Also, a lower percentage of eggs laid during the middle of August will end up as worms in bolls.  Most of the bollworms that are successful in penetrating the defensive shield provided by the Bt toxin do so by chewing into the tips of bolls after penetrating blooms and bloom tags located in the middles of the plants.  When the blooms are located primarily in the upper portions of the plants, the 1st-instar larvae will have a much lower success rate.  There is also a tiny wasp, a Trichogramma species, that parasitizes bollworm eggs.  While the incidence of parasitism may be low in July, we have seen as much 40% or more of bollworm eggs parasitized at this time of year.      Beet armyworms continue to be a threat in some fields.  Scouts are finding some fields that are infested with a mixed bag of worms including beets, fall armyworms, cabbage loopers, soybean loopers and bollworms.  In most cases, there aren't enough of any one species to do economic damage, but when you consider their combined injurious effects, they need to be controlled.  Steward, Intrepid and Tracer will each provide fair-to-good control of beets, falls, and loopers.  Although Steward and Tracer are weaker on bollworms than pyrethroids, they should provide sufficient control of  bollworms in Bt fields at rates that will control the armyworms and loopers.  Intrepid will need some pyrethroid assistance for bollworms.  Pyrethroids will not control established infestations of beet armyworms, fall armyworms or soybean loopers; although they will control cabbage loopers (which aren't much of a problem anyway). I have been looking at some Agdia Hel-ID information provided by Candy Roach.   Her egg collections from different locations in the Pee Dee show that tobacco budworms comprised 22% of the 13 collections, and about 50% of the eggs were bollworms (28% were not identified as either species).  The relatively high percentage of budworm eggs is surprising for this time of year.    As I said last week, stink bug numbers seem to be somewhat lower than has been the case in recent years.  I did find one field in Florence County that had about 80% of the quarter-sized bolls injured by stink bugs.  Scouts had no trouble finding stink bugs, with a total of 26 green, southern green and brown stink bugs per 100 pan samples.  I suspect that this is not the only cotton field in South Carolina that has been badly hammered by stink bugs this year.  It just goes to show you, even when the overall populations of these bugs seems to be down, they can thrive under the right set of circumstances.   Finding the stink bugs is usually a difficult task.  You could walk right through that field and never see a stink bug setting on a boll or a leaf.  They are very good at hiding themselves.   For years, in our research and demonstration plots we have used a plastic wash pan (11 inches wide, 13.5 inches long and 5.5 inches deep) to assess populations of beneficials, plant bugs and stink bugs.  A pan sample consists of shaking the center portions of two or three plants into the plastic pan.  Give the plants two or three shakes and quickly move down the row.  Take 25 samples in a row while walking a couple of steps between samples, then move to another spot for 25 more.  A scout should look at a minimum of 100 samples per field.  I would be concerned if I found six or more stink bugs per 100 samples, but your best measure for determining the need to spray or not is still 15% of quarter-sized bolls injured. We are seeing a few fields with whiteflies.  These insects have piercing-sucking mouthparts and they suck juices from the plants while excreting a sugary substance referred to as honeydew.  These insects could cause some problems similar to aphids when the honeydew falls on the lint in open bolls.  A sooty mold that grows on the honeydew will cause reductions in color grades.  Sticky fibers would create big problems for the spinning process.  Fortunately, this is a problem that very rarely occurs in South Carolina.   Boll Weevil Program: We are still growing cotton without boll weevils in South Carolina.  It's also quite possible that some fields may be growing cotton without traps.  If you should see a cotton field with no traps in sight, please give Randy Lynch a call at 1-800-269-9928.    Mitchell Roof  Extension Entomologist

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