Cotton Insect Newsletter Vol. 21, No. 9, July 16, 2003 Call 1-877-629-1474 for Cotton Insect Hotline

Cotton Situation:  This cotton crop is still a little bit behind schedule.  Robert Graham stated in the July 13 issue of the “South Carolina Crop and Weather Report” that the effects of the cool spring continue to show up in the slow development of the corn, cotton and soybeans.  Their estimate of boll set was 5% for 2003, which was nearly 20% less than the 5-year average.   With the crop developing slowly, one can ill afford to lose much fruit.  Where cotton plants are beginning to set bolls, insect scouting will be vitally important during the next 4-5 weeks.   Bollworms and stink bugs will be the primary insect pests of concern.  Make sure you know how to scout for these pests.   Scouting for bollworm is a little different in Bt cotton.  If no insecticide applications are being made for bollworm control in a cotton field, the risk of economic infestations of stink bugs will be even greater.      Insect Situations:  I just received word from Tommy Walker this morning that bollworm egg counts have gone up in the Savannah Valley.  Some fields had up to 30 eggs/100 plants on Tuesday.  Growers in the Estill-Luray area started spraying some fields on Tuesday afternoon for four to six small worms per 100 plants.  Steve Roach reported that moths were showing up in and around cotton fields in Darlington and Marlboro Counties on Tuesday.  Scouting for bollworms in blooming Bt cotton requires a different technique than in non-Bt cotton.  Egg counts are important in both situation.  Our thresholds for insecticide applications are 20 eggs/100 plants, 3 small worms (less than 1/4"), or 5% damaged squares in non-Bt, and 75 eggs or 30 small worms in Bt cotton.   In Bt cotton, we also utilize an escaped worm concept.  A worm that has achieved the size of 1/4 inch or larger is home free, therefore, an escapee.  In other words, a large worm will probably not be killed or inhibited by further feeding on the toxic plants.  For large worms (1/4" or more), spray for 3 worms/100 Bt cotton plants, or 5% damaged bolls.  In checking Bt plants for escaped worms or their damage, examine a white bloom, a pink bloom, and two of the smallest bolls per plant.  These checks should entail looking inside the blooms for large worms.  Also, look for boll penetration under fresh blooms and dried bloom tags.  Worms that avoid being killed by the toxin often penetrate the very tips of the bolls in a covert fashion that will go undetected by a scout that fails to look inside blooms and under bloom tags.    Brown stink bugs are plentiful this year.  These bugs have been abundant in corn fields and they will be moving into cotton fields as the corn matures.  Green and southern green stink bugs are generally the most important species in cotton, but so far we haven't seen many of either species.  Both species are usually a little slower than browns in invading cotton fields.   Stink bugs possess piercing-sucking mouthparts, which they utilize to obtain nourishment from the developing seeds within small bolls.  When the beak penetrates the carpal wall, it leaves a distinctive mark on the inner surface.  Initially there will be a water-soaked spot marking the entrance of the beak.  After several hours, a wart will form around the spot.  Both the spots and the warts are evidence of stink bug injury.  Exterior spots often form on bolls, but sometime they don't.  Thus exterior spots can not be used as a reliable criterion for boll damage by stink bugs.  As time goes by, the seed will become shriveled and lint will be stained from the actions of microbes that were injected during boll penetration.  The stains will usually be yellowish brown to dark brown in color.  When bolls are penetrated at a very early age, the entire contents may be affected and bolls may be shed.  Tarnished plant bugs can produce injury symptoms within a small boll that is indistinguishable from stink bug injury.    Aphid populations are beginning to crash throughout the state.  The disease showed up first in the Savannah Valley in some collections made by Tommy Walker.  We collected samples of aphids on July 10 in two fields in Marlboro County that were both heavily infected with the fungus disease.  There were lots of dead aphids in those two fields, and with a hand lens you could see that they were covered with a fuzzy growth.  Scouts should be able to monitor the progression of the disease and report it to the farmer.  Just turn over the infected leaves and closely examine the aphids to determine if there are some that have been infected and killed.  Once the infection rate reaches 10-15%, most of the aphids will soon be gone.  The fungus reproduces by forming spores that are forcibly ejected from the aphid cadavers.  Healthy aphids that are nearby may pick up the spores from the leaf surfaces, by direct contact with infected aphids, or the spores may land directly on them.  Eventually, the spores will move on the air currents to other fields where uninfected aphids can become infected.  Boll Weevil Traps:  This has been a tough year for placing pheromone traps.  The Grower Foundation was unable to get intended acres for cotton in four counties: Darlington, Dillon, Florence and Williamsburg.  And with the wet spring, some of the fields growers had intended to plant were too wet so they switched to other fields.  Let's all keep a sharp eye out for those fields that haven't been trapped, and when you find them, report to Randal Lynch or Wyman Taylor at 800/269-9928.   Mitchell Roof  Extension Entomologist

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