Cotton Insect Newsletter Vol. 19, No. 7, June 27, 2001 Call 1-877-629-1474 for Cotton Insect Hotline

Cotton Situation:  There were probably a few fields with blooms last week, but this week it's easy to find blooms in the oldest cotton.  At the PDREC, a fairly high percentage of the cotton was planted in April, so this would be the wrong place to make judgments about the state of the state's cotton crop.  For example, last week in a mini-tour of the Pee Dee area, many fields of cotton were observed that hadn't formed the first square. The first few weeks of squaring are a critical period in the life of a cotton plant.  The squaring rate should increase about 11/2 to 21/2 fold through the fourth week of squaring.  By the fifth or sixth week, the rate should begin to level off, and it will drop sharply between the seventh and ninth week. Insect Situation:  More bollworm and budworm moths were trapped this week.  Egg counts have also been running somewhat higher than last week.  Late-planted cotton fields are still not very attractive places for moths to lay eggs, so most of the eggs are being deposited in cotton fields where plants have formed several squares.  A preponderance of eggs are often found on the terminal leaves that are about the size of a half-dollar coin.  Look for “dirty terminals” as evidence that small larvae have begun feeding there.  These terminals will have puncture wounds in buds and small leaves and squares, surrounded by some dark-brown colored fecal pellets.  You are much more apt to see dirty terminals in conventional cotton varieties.  Most of the worms feeding in terminals at this stage of the game will be budworms.  You can count on nearly 100% of budworms being killed by the toxin in Bt-cotton before they have had time to create dirty terminals.  After feeding for a short time in the terminals of non-Bt cotton plants, the small larvae will begin feeding in match-head sized squares, then larger squares, and eventually they will migrate down the plant, where they end up feeding on bolls. Around 10 to 30% of the larvae (heliothines) attacking cotton in June and early July will usually be bollworms.  By mid-July, you can expect 75 to 90% of larvae to be bollworms.   The Bt-toxin in Bollgard cotton varieties will be much less effective against bollworms, killing only about 80% on the average.  The 20% that avoid death usually end up in the lower reaches of the cotton plants; such as in blooms, dried bloom tags, and small bolls.  These worms often originate from eggs laid on mature leaves, stems and bloom tags.  Lower concentrations of Bt-toxin in pollen, blooms and bloom tags may allow some bollworms to complete development in the absence of chemical intervention.  For this reason, we suggest that scouts examine a pink bloom, a white bloom and two small bolls per plant in the search for escaped bollworms.  Always examine the tips of small bolls beneath bloom tags to look for  escaped larvae.  Pyrethroid Resistance Management:  All farmers should now be aware of the importance of staying away from pyrethroids in June.  We have found pyrethroid-resistant bollworms and budworms in South Carolina on a number of occasions since 1996.  When you have trouble killing bollworms or budworms in the field with pyrethroids and you continue to spray with pyrethroids, it's somewhat like throwing gasoline on a fire.  Although you may be killing the majority of the worms, you would be removing more and more of the susceptible insects, leaving a higher percentage of resistant individuals.  With susceptible bollworms out of the picture, the few resistant female moths would have a good chance of pairing with resistant male moths thereby producing resistant offspring.  If, however, a grower sees early on that a pyrethroid application or two has been largely ineffective, it would be advisable to switch to an insecticide with different chemical properties.  For example, if the field was then treated with Tracer instead of another pyrethroid, more pyrethroid-susceptible moths would be around to mate with the few resistant ones, and the offspring would have a higher probability of being susceptible.  This is assuming that resistance is a recessive character and when resistant and susceptible moths mate, the offspring is also susceptible.  This is an oversimplification, but it should help to explain why we can minimize selection pressure for resistance by avoiding unnecessary applications of pyrethroids.  Resistance Management Tools:  Our resistance-management tools are the insecticides that have active ingredients that are chemically different than the pyrethroids.  Tracer has been around for a couple of years and Steward is now labeled for use against bollworms and budworms in cotton.  Larvin has been around much longer, but it should still be a useful management tool in areas where tolerance or resistance to pyrethroids has become apparent.  New Publication:  We have a new Clemson Extension publication entitled Pyrethroid Resistance Management that should be out by the end of July.  This booklet will give a little more background information about resistance and the methods we suggest using to control bollworm/budworm in areas where resistance appears to be developing.    Mitchell Roof  Extension Entomologist

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