“An aggressive approach to this flight in most areas of the state should help insure the high fruit retention required with North Carolina's relatively short growing season, especially with this season's generally late start.”
“In a few northern counties and western Piedmont counties, a 3 percent bollworm threshold may be appropriate under their historically low pressure. — This approach requires at least twice per week scouting and involves some risk. A 3 percent larval threshold is also appropriate if it is reached prior to the egg threshold after bloom. In these situations, tobacco budworms often ‘sneak in’ prior to the major bollworm moth flights.”
North Carolina's cotton producers have a number of insect control advantages over their colleagues in most other states. However, their main pest, the cotton bollworm (corn earworm), can be as severe here as anywhere in the Cotton Belt.
Additionally, bollworm tolerance to pyrethroids also is higher in the Carolinas and in Virginia than elsewhere.
Field corn, is both a blessing and a curse to our state's cotton producers. The good news is that the first two generations of corn earworm moths are highly attracted to corn and tend to avoid field crops such as soybeans, peanuts and cotton.
Corn earworm feeding causes little economic damage to either whorl or ear-stage corn.
The down-side is that the subsequent large moth flights resulting from the ear-feeding generation find cotton considerably more attractive for egg-laying than a field of dry corn.
From mid-July to early August, cotton is often very vulnerable to the resulting fruit-feeding bollworms. At our northern latitude, cotton usually has little time to compensate for late July to mid-August boll damage.
As a result, in some years cotton can be overwhelmed with such high levels of eggs and hatching bollworms that action thresholds on conventional cotton must be very protective.
Bollworm threshold tests conducted here from 1984 to 1989, and reinforced in 1989-1999, established the value of using egg thresholds for this bollworm generation in most areas of the state.
Although a bloom threshold showed the best economic return in the most recent tests, in seven of eight tests conducted in the mid to late 1980s, the egg threshold proved more profitable.
The 10 total threshold tests were conducted under various levels of bollworm feeding. These tests and producer experiences have shown the value of a quick response at the outset of our major bollworm moth flights.
An aggressive approach to this flight in most areas of the state should help insure the high fruit retention required with North Carolina's relatively short growing season, especially with this season's generally late start.
An additional factor which points toward employing an egg threshold is the finding of increased bollworm (corn earworm) tolerance to pyrethroids here, both in adult vial testing and in the apparent decreased control of medium-sized earworms on soybeans with pyrethroids.
Also, pyrethroid tolerance (a word used to describe the early stages of resistance) added to bollworm control difficulties in 2000, as confirmed by 20 to 30 percent moth survival in adult vial testing at the high 10 microgram pyrethroid (cypermethrin) level.
Controlling bollworms as second or third stage larvae is especially difficult if the population has begun to increase its tolerance to pyrethroids.
Depending upon historical and expected bollworm pressure, the risk the producer and consultant are willing to accept, the producer's ability to treat large cotton acreage in a short period of time, and other circumstances, one of several responses to bollworm eggs and/or larvae may be appropriate.
Egg threshold: 10 percent eggs in terminals, or 2 percent on fruit — Somewhat higher pyrethroid rates are now recommended. Two initial pyrethroid applications, either both at a medium rate, or the first at the industry standard and the second at a high rate (1.6 times the industry rate, where the label permits), are made five days apart.
This latter approach (industry followed by the high rate) has looked promising in Virginia and will be their standard recommendation for 2001. Either option should result in a dependable strong initial response to bollworms, and will control both eggs and small worms at a time when they are least able to tolerate or resist pyrethroids.
Scout two times per week prior to initial treatment, then utilize a 3.5 to five day scouting interval after the initial two applications. Subsequent applications are based on 3 percent worms on fruit, especially small to medium-sized bolls.
Same threshold and two-application approach as above, but with high pyrethroid rates: This strategy provides greater residual control, may lengthen the scouting and spray intervals, and is especially well suited for producers with ground equipment who take six or more days to treat their acreage.
Use the same 3.5 to five day scouting interval prior to the initial application and six or seven day scouting interval following high rate treatments.
In some years, a single application at the high rate may suffice in some low bollworm pressure situations.
Same threshold as above, but Steward or Tracer used: Using these new products to manage the major ‘bollworm generation’ may be useful if it is composed of a significant portion of tobacco budworms. Both products are kind to beneficial insects, and bollworms and budworms have not yet shown resistance to either product.
However, both are expensive, and somewhat less effective against pyrethroid-susceptible bollworms. At this point, we would still recommend that growers evaluate Steward and Tracer on limited acreage.
Larval threshold: In a few northern counties and western Piedmont counties, a three percent bollworm threshold may be appropriate under their historically low pressure. — This approach requires at least twice per week scouting and involves some risk. A 3 percent larval threshold is also appropriate if it is reached prior to the egg threshold after bloom. In these situations, tobacco budworms often ‘sneak in’ prior to the major bollworm moth flights.
General observations: As is the case with Bollgard cotton, scouts are urged not to over-look eggs and larvae deep within the canopy on blooms, bloom tags and bolls, especially if the weather is dry in July and August.
Because even small numbers of bollworms can extract moderate late-season boll damage, it is imperative to get off to an economical, clean start in managing this year's major bollworm generations.
A strict adherence to the egg threshold will provide that start, and a quick response to subsequent threshold levels of small bollworms will maximize the yield potential of this year's cotton crop.