FIELDAND
FORAGE
CROPS
FIELD
The information and recommendations in this newsletter are applicable to
North Carolina and may not apply in other areas.
Cotton aphid levels continue to decline throughout cotton-growing
areas of North Carolina, although enough small populations remain
to hold the potential for later buildups. Once the parasitic
fungus becomes more widely established, aphid colonies will have
difficulty coming back. The mummifying wasp parasite is also able
to withstand mid to late-season pyrethroid insecticide treatments
and to hold aphids to very low levels.
This past week, mite populations appear to have held their own, declining in some areas while increasing in others. If the symptoms for miticide treatment exist (yellow leaf speckling advances to the point where a bronzing or purplish coloration has begun to occur in leaf folds on plants throughout the field, and some defoliation of lower yellow leaves has begun) and spraying is done, complete coverage is essential, especially with Danitol. Be sure to use high pressure (at least 60 pounds per square inch) and a high number of gallons (the Danitol product label calls for up to 50 gallons per acre). Kelthane and Capture have been providing fair to good control. Mite control should not be attempted by aerial application.
Better rainfall will greatly reduce these mite levels and their
associated problems. Also, as July proceeds the fungus should
increasingly reduce mites to low numbers.
Although plants bugs are typically a very minor pest in North Carolina, every year treatable levels occur. As has been the case with thrips, aphids and mites, plant bugs appear to be at higher levels than in previous years. Due to our very late crop, taking square retention counts, as an indicator of whether further plant bug sweeping is warranted, is essential. Under normal conditions, if cotton plants are retaining less than 80 percent their newer squares, further sampling for plant bugs (a possible cause) is indicated. This year, a level of 85 percent might be more appropriate. Likewise, the plant bug thresholds should be adjusted downward slightly (i.e., more protective) to accommodate this late- maturing crop:
First two weeks of squaring - 5 plant bugs (adults and large nymphs) per 100 sweeps (down from 7.5 plant bugs per 100 sweeps)
Third week of squaring until bloom - 10 plant bugs (adults and large nymphs) per 100 sweeps (down from 15 plant bugs per 100 sweeps)
After bloom - 20 plants bug (adults and large nymphs)
If the lower plant bug thresholds are used, we would still
anticipate very few cotton fields will likely require treatment for
plant bugs, though perhaps somewhat more than in recent years. For
the past two years, less than 1 percent of North Carolina's cotton
acreage has been treated for plant bugs.
The tobacco budworm flight appears to have wimped along at a low level again this past week, with only a few cotton fields in need of treatment. Budworm levels in terminals or squares for the most part are still in the 0 to 1 percent range in the southern and middle portions of North Carolina, although a few fields have met the 8 percent terminal and/or the 6 percent larval threshold for these late June to early July budworms. Although most producers can expect little or no terminal or square damage from this budworm generation, fields should nevertheless be inspected regularly for budworms as well as our upcoming bollworm generation.
Another factor to consider is that, after this second generation flight, tobacco budworm levels on cotton also can pick up prior to or as part of the bollworm generation, even if we continue have a light June/early July generation on cotton. This new crop of budworms has occurred in Georgia at moderate to heavy levels, despite their light second generation populations earlier (similar to what we are experiencing now). We must be alert to the possibility of budworms becoming established prior to the bollworm flight. This happened in some areas in North Carolina in 1995. In mid-July in the southern and mid-Coastal Plain counties, for cotton which is 2 weeks or more behind schedule and not blooming, a 3 percent larval threshold (in terminals, or on squares or bolls) should be considered prior to the bollworm moth flight.
If tobacco budworms constitute most of the larval population
(usually the case prior to the major bollworm moth flights) and
treatment is indicated, Tracer at the lower rate of 0.045 pound of
active ingredient per acre should provide budworm on a par with
pyrethroid insecticides, and will have a minimal effect on
beneficial insects.
Although our upcoming major bollworm moth flights will likely be late this year, the cotton crop will certainly be even more delayed than usual relative to the flights across much of the cotton acreage. Depending upon the size and duration of the bollworm and European corn borer moth flights, we could be in for an extended period of crop susceptibility to these pests. On the front end, a 3 percent threshold for bollworms in terminals or in squares at the onset of blooming should be followed. In some cases, the bollworm moth flight may occur prior to bloom (almost unheard of before this year's late crop). Under these circumstances (a cotton crop which is two or three weeks behind), the egg threshold should be the focus. When using the egg threshold (10 percent eggs in terminals or 2 percent on fruit at the onset of the major bollworm moth flight), a very timely response could be particularly important this year.
Conversely, although our cotton crop looks as though it will be late and attractive in the coming weeks, the bollworm moth flight could also be light, so adherence to thresholds could also save money by avoiding unnecessary treatments.
Generally speaking, however, it would appear as though most
responses to insects for the next six weeks or should be on the
side of protection.
Twospotted spider mites continue to infest many fields above the Albemarle Sound and a low percentage of fields have been treated with miticide during the past week. Examination of mite colonies from cotton leaves have shown a low incidence of dead mites infected with fungi. Hopefully this is the beginning of a general epizootic that will bring mite populations to a screeching halt. The incidence of rainfall and high humidity has been increasing, but the first general rainfall event occurred on July 10. The mite fungal parasite needs high populations of mites and high humidity in order to spread rapidly. Those conditions now exist and I anticipate a general mite population crash within 7 to 10 days.
A few fields are still coming up with above-threshold Lygus bug
numbers and sub-par fruit retention. As stated last week, I
believe it is important to carefully manage the fruit load in order
to have a chance at a good yield. Ordinarily cotton farmers in our
area depend on nodes 9 through 12 or 13 for the bulk of the lint
yield (from first and second-position bolls). Nodes 14 to 16 will
have a few first-position bolls, up to maybe node 18, but the
percentage of yield contribution is usually small (ca. 2 or 3
percent). First square in 1997 seems to range from nodes 5 or 6 up
to 7 or 8, depending upon the field and amount of early thrips,
etc. I believe that our best chance to attain a full yield in 1997
will be to try to extend the fruiting range downward since the
short season will likely reduce harvestable bolls at the plant's
top. Therefore, more attention to all factors which preserve lower
bolls (now) and all bolls that set early enough to mature (during
worm season) will be very important.
Thrips, potato leafhoppers, and spider mites are present in peanuts
everywhere. So fields that are still infested with thrips may
appear to need a foliar insecticide treatment. This decision
should be made in the context that spider mites are present and the
use of foliar insecticides generally increase the likelihood of
spider mite outbreaks. The use of rootworm insecticides appears to
have the same impact, but application of rootworm insecticides will
reduce the leafhopper damage seen in some peanut fields. Remember
that two products, Comite and Danitol, are labeled for spider mite
control in peanuts. This year is setting itself up to be a very
bad year for mites and the use of insecticides should be considered
as to their potential effect on mites if those fields are already
infested.
Web page last updated on July 14, 1997 by Stephen J. Toth, Jr..
