Protective Bollworm Thresholds Still Urged for Conventional Cotton in North Carolina

Jack S. Bacheler
North Carolina Extension Entomologist

Major Bollworm Generation- North Carolina's cotton producers have a number of insect control advantages over their colleagues in most other states. However, some pests, like thrips, stink bugs and bollworms can be as severe here as anywhere in the cotton belt. On the positive side, the major bollworm moth flight should be later compared with both 2003 and 2004, based on lagging worm development in our late corn. Unfortunately, so far this year's cotton crop is at least two weeks behind in most areas of the state. Both continued high heat units coupled with timely (but not excessive) rainfall will be required to get this crop back on track.

Threshold Tests- In a confirmation of bollworm threshold tests carried out here in the mid-1980's, additional 1998 and 1999 bollworm threshold evaluations showed similar results, as one can see in Table 1.

* The Value in $/acre was determined by taking the lbs. of lint/acre multiplied price of lint minus the treatment cost, with a pyrethroid at $4.50/acre and application cost at $3.50/acre.

Although the bloom threshold showed the best economic return in the above two tests, in 5 of 6 tests conducted in the mid-1980's, the egg threshold proved more profitable. Also, the bollworm "pressure" in Onslow County is higher than in most areas of North Carolina. The 8 total threshold tests were conducted under various levels of bollworms, but averaged on the moderate to high side. One can readily see, however, the value of a quick response at the onset of our major bollworm moth flights. An aggressive approach to this flight should help insure the high boll retention required with North Carolina's relatively short growing season, especially with this season's late start.

Stink Bug Considerations - An additional factor which points toward employing an egg threshold is the coincidental control of stink bugs. In most years, the timing of these bollworm treatments coincides with a time of high crop vulnerability to stink bugs - 2 to 3 weeks after the onset of blooming and beyond. Fortunately, our most common boll-damaging stink bug species, the green stink bug, is controlled with pyrethroids. However, brown stink bugs can sometimes create headaches, as they did in a big way in North Carolina in 2004, arriving both early and in large numbers. In this case, it was important for consultants and producers to have recognized these early, high infestations of "browns" and respond with an alternative product such as Bidrin, most often used in a pyrethroid tank mix. For the first 8 years of Bollgard availability, stink bug damage to bolls in conventional cotton treated an average of 3 times was approximately 0.7%; in 2004, this figure was a startling 7.1%. Let's hope that a trend is not underway.

Recommendations for 2005 - With the exception of 2004, bollworms have been far and away the predominant pest on conventional cotton for the past 20 years. So while stink bug damage to bolls must be taken into account when making bollworm treatment decisions on conventional cotton, bollworm management will likely retain its primary status. Listed below are some bollworm management options:

1. Egg threshold (10% eggs in terminals, or 2% on fruit): Two initial, standard rate pyrethroid treatments 5 days apart. This has proven to be dependable strong initial response to bollworms, and controls both eggs and small worms at a time when they are least able to resist pyrethroids. Scout 2 times per week prior to initial treatment, then 3.5 to 5-days per week (or weekly scouting with twice weekly scouting on selected "target" fields) after any subsequent applications- based on 3% worms (or 2% or 1% bollworms if these lower numbers are found on consecutive scouting trips).

2. Same threshold as above, but with high pyrethroid rates. This bollworm strategy provides greater residual control, may lengthen the scouting and spray intervals, and is especially well suited for producers with ground equipment who take 6 or more days to treat their acreage. Same 3.5 to 5-day scouting for initial application, and 6 or 7-day scouting interval following high rate treatments.

3. Same threshold as above, but Tracer or Steward used. Using Tracer or Steward to manage the major bollworm generation may be useful if it is composed of a significant portion of tobacco budworms. This will likely need to be confirmed with an egg identification kit. Tracer and Steward are very kind to beneficial insects, and bollworms and budworms have not yet shown resistance to these products. However, these new products are expensive, and somewhat less effective against larger and pyrethroid-susceptible bollworms.

4. Larval threshold: In a few northern counties and western Piedmont counties, a 3% bollworm threshold may be appropriate under historically low pressure. However, this approach requires at least twice per week scouting and does involve some risk. A 3% larval threshold is also appropriate if it is reached after bloom but prior to the egg threshold. This threshold is sometimes reached if budworms constitute most budworm/bollworm population early in (or prior to) the major bollworm moth flights.