FIELDAND
FORAGE
CROPS
FIELD
The information and recommendations in this newsletter are applicable to
North Carolina and may not apply in other areas.
Light trap counts have mercifully begun to decline in a number of areas in North Carolina, but remain moderate to high in general. Egg levels are still high enough in many fields to warrant extreme caution; some counts from 80 to 300 or more have been confirmed in the past week (about as high as many of us can remember). On the other hand, a few pockets of relatively light pressure have also been reported. In Northampton County, for example, recent light trap counts were in the 2 to 50 range, with a several cotton producers still waiting to make their first insecticide application. Without trying to overgeneralize, for the week of August 12-18, my advice would be to err on the protective side, because of: 1) the very high bollworm egg levels in such a high proportion of the state; 2) the number of eggs now being deposited down in the plants on fruit at this time of year; and 3) the high potential cotton yields in much of the state. With any luck, this generation of bollworms will be declining throughout the coming week. Additionally, the high percentage of cotton fields with heavy boll loads (planted on time) are racing toward cutout and will become increasingly unattractive to bollworm and European corn borer moths.
Because differences between cotton fields in their attractiveness
to bollworm and European corn borer moths are becoming more
important than the moth numbers, an adequate scouting frequency of
4 to 6 days is essential in determining the need to spray
insecticide. Some fields are already past the point of bollworm
and European corn borer establishment, while others (primarily the
late-planted, hail damaged and replanted fields) could be in need
of protection for the remainder of August.
A few pheromone and light traps have caught budworm and European
corn borer adult moths, respectively, in the range of 100 moths (2
to 3 night catch) this past week. Budworms can be more difficult
to control than bollworms. However, every place I have visited
during the past few days, bollworms have been overwhelmingly the
more common of the two species. In North Carolina, we probably
have enough European corn borer adult moths in areas with
significant corn acreage to account for some establishment of
caterpillars in our later, more rank
cotton fields ; these
situations (primarily noticing European corn borer adult moths
within or around cotton fields and an increase in
stem tunneling)
may warrant an extra pyrethroid insecticide treatment, especially
if more than a week has elapsed since the last insecticide
application for bollworms.
Both of the migratory species beet armyworms and fall armyworms have increased in number in North Carolina in the past two weeks, particularly fall armyworms. Hatching fall armyworms are suppressed by bollworm insecticide sprays, although fall armyworms which have been feeding on bolls or in blooms are very difficult to control with any of our presently-labeled insecticides. In a 1993 test in Onslow County, the highest labeled rates of fall armyworm insecticides provided from 18 to 36 percent control of medium to large fall armyworms. The anticipated 1997 labeling of Confirm (Rohm and Haas), Pirate (American Cyanamid) and Tracer (DowElanco) should help immensely in our occasional battles with armyworms. In the meantime, if levels of 2 to 4 percent small 1/4-inch or smaller fall armyworms are found on or in sampled fruit along with bollworms, consider about 0.25 to 0.33 pounds active ingredient per acre of Larvin, Curacron or Lorsban with a pyrethroid insecticide in a tank mix. If few or no bollworms exist along with 5 percent or more fall armyworms, try the highest labeled rate of the above non-pyrethroid insecticides and keep your fingers crossed. Watch for fall armyworms in blooms or between the bracts and boll surfaces. Control may be poor.
Beet armyworms have begun to occur in higher numbers in a few
cotton fields, particularly in the Piedmont North Carolina counties
(Union, Anson, Stanley, etc.). These
caterpillars are even more
difficult to kill with insecticides than fall armyworms. South
Carolina has issued a Section 18 Emergency Exemption for the use of
Confirm and Pirate for beet armyworms. Due to our history of light
beet armyworm infestation, North Carolina did not qualify for a
preapproved Section 18 exemption. If our beet armyworm situation
worsens, we can hopefully apply for and receive a Section 18 Crisis
Exemption for Confirm and Pirate. Both of these insecticides are
expensive ($12 to $15 per acre), but are far more effective than
our presently-labeled insecticides. As of August 9, very few
economic problems with fall armyworms and beet armyworms have been
reported in North Carolina.
The moth flight and egg laying seemed to be held down by rain last week. However, this week both bollworm and budworm moths have been out in force and egg counts have been extremely high in many fields. Consultants have reported egg counts in terminals up to 180 eggs per 100 terminals and I have made counts that have exceeded 250 eggs per 100 terminals. All of our fields are presently susceptible to loss if caterpillar survival after treatment is above threshold numbers. Whereas pyrethroid sprays usually reduce caterpillars well below a damaging level, under severe egg pressure the number of caterpillars surviving may be high enough to still cause problems. This is especially true when insecticide application has been less than fully effective because of weather, aerial application restrictions, weather, etc. Therefore, careful scouting for larva and damage is essential, especially in fields where less than ideal insecticide performance is suspected (like it has recently due to rainy weather).
With regard to damage, in previous years a 3 percent damage
threshold (recent damage with living caterpillars present) was
employed by the Cooperative Extension Service in North Carolina and
this criterion still has utility. Since fruit are examined for
living larva anyway, it is easy to recognize and record the damage
level on the fruit also. Using the damage threshold can have
particular utility in fields where infestations are low and
sustained, as they often are at the late part of the moth flight.
Actually, I visited several fields last year where the scouts found
below-threshold levels of caterpillars for successive weeks and
insecticide was not applied, but damage ultimately rose to about 8
percent. I believe that using the damage threshold may have
prevented this situation. Also, when scouting B.t. cotton,
monitoring fruit damage can be particularly meaningful since egg
counts are useless and larval counts are hard to interpret because
of low survival and slow growth (unless larva have attained some
size).
Currently, bollworms/budworms are capturing most of our attention,
but fall armyworms are showing up in some locales. In Currituck
County, fall armyworms have been found in many cotton fields, and
late-planted corn plots on the Tidewater Research Station in
Plymouth have been devastated by this pest. Cotton scouts need to
be able to recognize these caterpillars when they are small in size
since a different insecticide is required for fall armyworms.
B.t. cotton has received a lot of negative publicity (by farmers
and consultants) on it's inaugural season due to unexpected
survival of bollworms in states south of North Carolina. Also, I
suspect that there has been panic spraying of insecticide; growers
and consultants are not psychologically prepared to wait for a lot
of new caterpillars to die, especially in the face of heavy egg
pressure. The Cooperative Extension Service has done a good job of
making cotton producers very responsive to insect invasion.
Survival of bollworms in B.t. cotton seemed to be a surprise to
most people, so much so that an article titled Pests Overwhelm
B.t. Cotton Crop appeared in Science (July 26 issue). Actually,
survival of bollworms in B.t. cotton is not surprising. In tests
conducted from 1991 to the present (in Chowan, Edgecombe and Martin
Counties), we have documented bollworm survival and damage in B.t.
cotton in each case, without exception. At the 1995 Beltwide
Cotton Conference, we reported: In short, transgenic B.t. cottons
will not stand alone in areas where high populations of bollworm
occur, particularly when heavy egg lay occurs during the latter
part of the season, when bolls remain susceptible. This year we
are experiencing high bollworm invasion and I suspect that B.t.
cotton may need to be treated with insecticide. B.t. cotton is
impressive in its resistance to insect pests, but it is not
perfect. Scouting procedures are listed in the new Cotton Insect Scouting Guide. Do not ignore late season plant bugs and stink
bugs in B.t. cotton as I have observed both of these pests this
week.
Corn harvest has begun and I expect county extension agents to hear
of stink bug damage from growers. We have had very high levels of
stink bugs this year. At this time, stink bug injury will be
displayed as crooked ears with missing grains, mostly on the ear
surface away from the stalk. Stylet (feeding) marks can be seen on
the inside of green shucks and appear as pin-point bruises.
As with cotton, the corn earworm (bollworm) moth flight has invaded soybeans. Already insecticide spraying has begun in some locations in the central coastal areas of North Carolina. Sampling should begin for corn earworms. As a reminder, thresholds for corn earworms and stink bugs in soybeans are provided below:
A. Shake Sheet Thresholds (30 inch to 40 inch rows):
***************************************
AVERAGE CORN EARWORMS
SELLING PRICE PER 6 FOOT SAMPLE
***************************************
$5.00 11
$6.00 9
$7.00 8
$8.00 7
***************************************
B. Rigid Beat Cloth Thresholds (7 inch to 21 inch rows):
****************************************************
AVERAGE CORN EARWORMS PER SAMPLE
*********************************
SELLING PRICE 7" ROWS 14" ROWS 21" ROWS
****************************************************
$5.00 1.3 1.2 1.9
$6.00 1.1 1.0 1.6
$7.00 0.9 0.8 1.4
$8.00 0.8 0.7 1.2
****************************************************
C. Sweep Net Thresholds (7 inch to 21 inch rows):
****************************************************
AVERAGE WORMS PER 15 SWEEP SAMPLE
*********************************
SELLING PRICE 7" ROWS 14" ROWS 21" ROWS
****************************************************
$5.00 3.6 3.5 4.6
$6.00 3.0 2.9 3.8
$7.00 2.6 2.5 3.3
$8.00 2.2 2.2 2.9
****************************************************
A. Shake Sheet Thresholds (30 inch to 40 inch rows):
B. Rigid Beat Cloth Thresholds (7 inch to 21 inch rows):
********************************************
AVERAGE STINK BUGS PER SAMPLE
*****************************
ROW WIDTH GRAIN SEED AND EDIBLE
********************************************
7" 0.4 0.2
14" 0.4 0.2
21" 0.6 0.3
********************************************
C. Sweep Net Thresholds (7 inch to 21 inch rows):
********************************************
AVERAGE STINK BUGS PER SAMPLE
*****************************
ROW WIDTH GRAIN SEED AND EDIBLE
********************************************
7" 2.3 1.2
14" 2.3 1.2
21" 3.5 1.8
********************************************
The weather over the past few weeks has been ideal for southern
corn rootworm egg hatch and survival of the small rootworms. If
you have not applied an insecticide for rootworms in high risk
fields, it may be too late. The only acceptable solution is an
insecticide treatment applied immediately and rain following the
application. Any insecticide treatment applied and not watered in
soon will probably be an investment that has little, if any,
benefit.
Web page last updated on August 12, 1996 by Stephen J. Toth, Jr..