FIELDAND
FORAGE
CROPS
FIELD
The information and recommendations in this newsletter are applicable to
North Carolina and may not apply in other areas.
This week the cotton in North Carolina has experienced high to very low thrips populations and slow seedling growth, resulting from our unseasonably cool weather. Very little of our cotton crop is past the thrips-susceptible stage. With our very late cotton crop (for the most part), decisions on whether to spray insecticide should err on the side of protecting the crop. Further delays in cotton maturity can not be tolerated for this late crop.
Scouting for thrips and responding quickly to their presence will be essential for the next 2 to 3 weeks. Scouting for immature thrips and their damage on a 5 to 7-day schedule is recommended. (See 1997 Cotton Information or the Cotton Insect Scouting Guide for specifics on scouting.) Most of the recommended foliar insecticides for thrips should provide 6 or 7 days of protection.
Remember also that in many areas of North Carolina, thrips
populations are very low, and that cotton plants will not show
enough damage or harbor enough live thrips to justify treatment
with insecticide. However, most of our cotton is presently very
late and slow-growing and if the thrips levels are even moderate,
scouting-based spraying may be indicated.
Tobacco budworms have been late and light so far this season (this
observation based mostly on observations of tobacco throughout the
southeastern U. S.). The earliest budworm infestations on cotton
in North Carolina have appeared in the Onslow/Jones/Craven County
area and in the Scotland/Robeson/Hoke County area around June 8-10.
As a rough estimate, at present it looks like budworms could start
showing up around June 15-25 in our most southern counties, about
June 20-30 in our central counties, and generally after June 30 in
our most northern counties. Budworm scouting should begin about 1
week prior to the earliest date of these ranges (for example, June
8 in the southern counties). The earliest-planted cotton in most
cases will be most attractive to budworms. More information about
budworms will follow in the next three issues of the North
Carolina Pest News.
Although cotton aphids can survive and multiply well in cool, wet
weather, it is still a bit early for their population buildup.
Expect increases to occur just prior to or following initiation of
cotton bloom. Biocontrol has worked well in North Carolina for the
past 7 years. I will be reporting on the status of aphid
population buildups and their predators, parasites and pathogens in
about 4 weeks.
Indirect sampling for plant bugs should be getting underway as
squaring begins. One small (1/8 inch total length) terminal plus
a lateral square (when they begin to show up) per plant should
serve as the basis for square retention counts. The taking of a
100-square sample (and counting missing positions and brown to
black tiny squares) from throughout each cotton field should
suffice. These small squares are the most susceptible to plant bug
feeding. A square retention count of over 85 percent (85 percent
green and healthy-looking squares versus missing positions and
discolored squares) indicates that plant bug damage is minimal, and
that further direct plant bug sampling is not needed.
Historically, only a small number of fields have required this
additional sampling, and less than one percent of our cotton
acreage has required treatment. More information on plant bug
scouting will be discussed in upcoming issues of the North
Carolina Pest News. In the interim, please refer to the
Cotton Insect Scouting Guide.
Thrips
About two weeks ago and since, we have experienced a heavy adult thrips migration from early spring hosts into cotton fields. This has been accompanied by very cool temperatures which has prevented cotton seedling growth. Additionally, in many cases the at-planting insecticide used has reached the time limit on effectiveness. As a result, many cotton fields are now showing unacceptable thrips injury and have need to be treated; many fields have already been sprayed with insecticide for thrips. As an example of thrips infestation, one of my field tests in Washington County was sampled on May 27 (22 days after planting) and showed deformed new leaves and heavily-blackened bud tissue in the untreated control and wash-out samples averaged ca. 3 adult and 2 larval thrips per plant. Some of the insecticide treatments at this early time were showing only about 50 percent control of adults and larvae.
When making a treatment decision for thrips, one can consider many bits of evidence. Some evidence leads to an automatic decision on whether to treat or not, whereas other information may only suggest that treatment with insecticide may or may not be needed. When attempting to identify fields requiring treatment for thrips, the following categories should be examined and can be used to arrive at a decision (I have assigned a point score for each category so that a threshold-type determination can be made). The categories are:
1a. On average, the newly-expanding seedling leaves and bud tissue show
some deformation, but lack dark necrotic tissue ("blackened buds").
3 POINTS
1b. On average, newly-expanding seedling leaves and/or bud tissue show
significant deformation and tissue blackening (especially in the bud) and
thrips are present (this is usually interpreted as heavy thrips injury).
10 POINTS
2a. Thrips adults and larvae are present on most seedlings.
3 POINTS
2b. Wash samples, if taken, show two or more larvae per plant.
10 POINTS
3. Expected active period of at-planting insecticide is at, or past,
it's end. Estimates of longevity include: Gaucho - 3 weeks; Disyston
and Thimet - 4 weeks; Temik at 3 pounds - 4 weeks; Temik at 5 pounds
- 5 to 6 weeks; Payload - 4 to 5 weeks; and Orthene SP (in-furrow
liquid) - 3 to 4 weeks. Conditions, especially soil moisture, will
affect initial and residual activity.
3 POINTS
4. Cotton remains small and slow-growing (e.g., in cool weather) so
that thrips protection is needed into the short-term future.
3 POINTS
A total of 10 or more points are needed to indicate the need for foliar treatment. Needless to say, when considering all categories a score of about 10 is likely to be borderline, whereas a high point accumulation probably shows a significant problem, one that well justifies the treatment.
Aphids and Mites
Our thrips wash samples from Chowan, Gates, Perquimans, and Washington counties during the last 6 days have shown not only high thrips counts, in many instances, but also greater than average counts of aphids and two-spotted spider mites (I have also seen some small caterpillars). The mites came mostly from sandy soil fields in Chowan County, where rainfall has been sparse, but the aphids have been found everywhere. The recent weather has been near ideal for aphids development. Both of these pests are noted for being rebound pests and spraying insecticide for thrips may stimulate populations of aphids and mites. Sprayed fields need to be followed in case aphids and mites become abundant. Ordinarily, we try to hold off on spraying insecticide to let biological organisms control aphids and mites; however, in some instances spraying for these pests can be a necessity.
Black Flea Beetles
On the unusual side, I have had reports of cotton being killed in Hyde County by black flea beetles. This occurred on rows where the Temik tube had stopped up; however, the beetles were on all rows to some extent. Orthene apparently did not affect them. The beetles were defoliating the seedlings. Watch for these pests.
Cutworms
Some fields have been treated for black cutworms, mostly no-tillage
fields. In some cases, the cutworms alone did not justify
treatment but low seedling populations and other seedling-feeding
insects, along with the cutworm, seemed to justify treatment to
keep the stand from falling too low. The other insects included
salt marsh caterpillars, black crickets, grasshoppers, and flea
beetles.
Thrips are very common in many peanut fields. The threat of thrips injury to the eventual health and yield of the peanut crop is further enhanced by the poor growing conditions. Under good growing conditions, the peanut plants can outgrow the feeding and still get off to a good start. Under cool conditions, the plants wait for warmer conditions, while the thrips impatiently continue to feed on the leaflets. This is a very difficult situation because the growth and development of the plant has already been slowed by the weather and the thrips are only making the situation worse. Surveys this week indicate many fields have between 75 to 100 percent of the newly-opening leaflets with extensive thrips injury. However, there are some fields with very little thrips injury and very few thrips in the unopened leaflets. Once again, scouting is very important.
Many fields that were treated at-planting with an in-furrow
insecticide also have quite a bit of damage. This is most likely
the result of the poor growing conditions making it difficult for
the plant to soak up the insecticide from the soil and move it
systemically through the plant. In other words, under these
conditions there may not be a toxic dose present in the plant. One
question that always emerges is whether the insecticide will begin
to work once the temperature warms up. If it has been 6 weeks or
longer since planting, then there is a limited likelihood that the
insecticide will provide good control of thrips once the
temperature increases. Even if the insecticide is not being taken
up by the plant, it is still being broken down by microbes, etc. in
the soil, although perhaps at a slower rate due to the cool
temperatures. However, the original amount of insecticide placed
in the seed furrow at planting is certainly not present after 6
weeks even if the plants did not absorb it very well. Scout all
peanuts, even those treated at planting. If at least 25 percent of
the newly-opened leaflets are severely damaged by thrips and thrips
are still present in the folded leaflets, an insecticide spray will
probably provide an economic return to help jump start the plants
once temperatures approach normal.
Web page last updated on June 9, 1997 by Stephen J. Toth, Jr..