FIELDAND
FORAGE
CROPS
FIELD
The information and recommendations in this newsletter are applicable to
North Carolina
and may not apply in other areas.
Although our hot, parching weather continues to dry out various
hosts for thrips (resulting in higher levels of migrating adult
thrips bound for cotton), a portion of the cotton planted in April
is now beyond the thrips-damaging stage. Visual inspections for
damage, and, if indicated, more intensive
scouting for immature
thrips will provide the best indication of the need to treat the
cotton with insecticides. In our 14-entry insecticide test planted
on April 25, all at-planting treatments were holding immature
thrips to sub-threshold levels at 3.5 weeks. On May 23 (4.0 weeks
after planting), a few of the treatments were almost at threshold,
but many of the plants were approaching the 5 to 6 true leaf stage
(probably safe). When initially evaluating plants for thrips
presence and injury, consider only the uppermost two true leaves.
If they are flat and shiny, these plants are not suffering from
economically-significant thrips damage. Do not consider ragged
cotyledons and lower true leaves (very common at this time). Be
aware that as wheat (and other hosts) continue to dry out, the
potential still exists for some high populations of migrating
thrips.
Just when cotton plants are phenologically past the period of danger for thrips damage, the time has arrived to begin indirectly monitoring cotton for plant bugs. The monitoring of terminal square retention will be the initial, and easiest, step in determining the need for further, more comprehensive sampling for plant bugs. As a rule, if early square retention (check the upper-most easily visible terminal square initially, then a combination of a terminal and an upper first or second position lateral square) remains at or above 80 percent, further checking is not needed.
With square retention less than 80 percent, plants should be swept for the presence of damaging populations of plant bugs. If history is a guide in North Carolina, the vast majority of this year's cotton fields will have early square retention well above 80 percent and sub-economic levels of plant bugs. I urge cotton producers not to overreact to the threat of plant bug damage. Most producers are discarding the philosophies of saving every square and automatically spraying with insecticide. Our licensed consultants did a commendable job of managing plant bugs in 1995. Over 84 percent monitored plants for terminal and/or lateral square retention, and, as a group, advised treatment on 0.24 percent of their collective acreage (275,020 acres). After monitoring a number of fields around the state last year, I only collected two nymph-stage plant bugs on cotton (an indication of successful reproduction), although many nymphs and adults were present on weed hosts around some of the monitored fields.
Monitoring for early square retention or sweeping a subsample of
cotton fields is probably in order. I do not know what the 1996
growing season will bring; however, a handful of fields reached the
threshold for plant bugs in the years 1993 through 1995. For more
details on scouting and thresholds, see pages 145-155 in 1996
Cotton Information or pages 14-16 in the new Cotton Insect Scouting Guide.
As the peanut crop is emerging, closely watch the level of thrips
damage. The weather has been quite variable across the peanut-
producing region which appears to be affecting the effectiveness of
in-furrow, at-plant insecticides. Check newly-opened leaflets for
thrips damage and look at unopened leaflets for the presence of
thrips. Damage on old leaflets means very little. Remember that
additional foliar treatments over the top of the crop will help
control thrips, but will have little if any effect on the level of
tomato spotted wilt virus. Also, remember that the earliest
planted peanuts are the most likely to have serious problems with
thrips. Spend more time scouting these peanuts for thrips than
peanuts planted later. The value of a foliar spray for thrips
becomes questionable seven or more weeks after planting, since the
plants are larger and the severity of the thrips pressure usually
is not as great.
Kathy Kidd of the North Carolina Department of Agriculture's Bio-Control Laboratory is reporting the highest levels of cereal leaf beetle parasites so far this season. The parasite populations often peak after the host population, but now is the time for collection and distribution!
This biological control program is not a quick fix to the problem of cereal leaf beetles in small grains. Also, it may require patience and some restraint on the part of the grower. As these parasites become established and more widespread in an area, their impact may increase. It is hoped that with an integrated management approach, one day we may see below-threshold levels of cereal leaf beetles in parts of the state.
Any county extension agents who know of growers interested in
participating in the program and would like to receive live
parasites should contact Kathy Kidd (telephone: 919-233-8214) or Stephen Bambara (telephone: 919-515-1661).
Budworm infestations are continuing to increase on tobacco in the
Coastal Plain. Growers should be checking fields carefully. The
treatment threshold for budworms is 10 percent. Foliar insecticide
sprays are most effective when applied early in the morning or at
night (when the bud is most open). Use an adequate volume of water
to flood the bud area; most spray should be deposited on the upper
third of the plants. Bacillus thuringiensis baits, either
commercial or homemade, are very effective against budworms. These
baits can be applied more quickly than many growers imagine (1/2
acre or more per man hour).
Adult vegetable weevils have been reported attacking recently-set tobacco in the lower Coastal Plain. These weevils typically move in from field borders and can cause heavy damage to young plants along field margins. The first indication of this pest is usually the appearance of very ragged, irregular-feeding damage on leaves and stems. If vegetable weevils are suspected, dig carefully around damaged plants. The weevils hide during the day under dirt clods and leaves or just below the soil surface. Adult weevils are gray or brown snout beetles and have a V-shaped mark on the rear half of the wing covers. They are about 1/4 inch long.
The treatment threshold for vegetable weevils, and other pests
seriously damaging small plants, is 5 percent of plants killed or
seriously damaged. Keep in mind, however, that a few rows on the
field border can be well above threshold while the field as a whole
is well below threshold. This is a case where spot treatments
(edge treatments, actually) might be considered.
A flurry of telephone calls concerning white-fringed beetles have been received over the last week. As plants begin to wilt and die, growers have become aware of and alarmed by this pest. Most growers will not see this pest, but those that do indeed have some cause for alarm. Damage can be heavy and insecticides applied after transplanting are not effective. Fortunately, this pest is scattered and sporadic and often damages only small sections of individual fields. Calls received so far have been from Craven, Lenoir, Pender, and Pitt Counties in North Carolina. The problem seems to be worst in tobacco planted after soybeans. Whitefringed beetle grubs are present in the soil at the time of transplanting and feed heavily on the stems and roots of young plants. This results in stunting, wilting and death of plants and can seriously reduce yield if the infestation is heavy.
To confirm the presence of this pest, dig around plants beginning to wilt. The whitish, legless grubs should be found in the top one or two inches of the soil near the damaged plant. Over the next few weeks, larvae will be transforming into pupae and then adults, and the amount of new damage will soon begin to decline. Adults feed on the leaves of plants, but the damage is negligible.
Other than keeping the crop growing as well as possible, there is
nothing that can be done now to remedy this situation. This pest
does not fly as an adult and the larvae move slowly through the
soil. Larvae can, however, be moved around in soil on equipment,
so farmers should be careful to clean equipment used in infested
fields. Soybeans and other legumes should not be used in the
rotation for infested fields as these species boost reproduction.
Some soil insecticides suppress populations (none offer effective
control) and should be used in following crops to limit (and
hopefully gradually control) whitefringed beetle populations.
Web page last updated on 27 May 1996 by Stephen J. Toth, Jr..
