FIELDAND
FORAGE
CROPS
FIELD
The information and recommendations in this newsletter are applicable to
North Carolina and may not apply in other areas.
The predicted prolonged bollworm moth flight has picked up in many areas of North Carolina during the past few days. A few examples are evident from insect trap catches from Rowland in Robeson County (from 48 bollworm moths on August 11 to 134 moths on August 13 for a one night catch), Lenoir County (from 13 moths on August 12 to 108 moths on August 14, also for a single night), northern Pitt County (from 21 moths on August 11 to 51 moths on August 13 to 86 moths on August 14, all single night catches), Alliance in Pamlico County (from 180 moths on August 13 to 455 moths on August 14, both single-night counts!), and Windsor in Bertie County (from 55 moths on August 11 to 84 moths on August 13). Along with these high moth catches, a number traps have captured low numbers of moths, even in the southern part of the state. A few of the uppermost northern counties are still reporting such low bollworm moth numbers that the flight has probably not yet started there; in Northampton County, the Potecasi and Conway traps had two-night catches of 5 and 0 moths, respectively, while in the western part of the county trap counts of 22, 17 and 18 moths in the Gumberry, Gaston and Seaboard, respectively, may indicate the beginning the major flight.
It will be interesting to see if these recent increases in moth
catches can continue, or whether the numbers will drop after a few
days as has already happened this year. In the case of generalized
or even local rainfall, moth emergence from the ground will tend to
increase, sometimes sharply.
I am observing a lot of variance in moth levels, egg deposition and bollworm establishment from area to area and even from farm to farm (this has been true from the start this year). Generally speaking, the potential for greater levels of bollworm eggs has increased compared to the last two weeks, in most areas. A complicating factor is that a very high proportion of these eggs are being deposited down in the plant canopy, especially on pink and dried blooms. In a cotton field in Greene County approximately 5 miles north of Snow Hill (in which county agent training was conducted on August 13), 1 percent eggs in terminals, 20 percent eggs on fruit (almost all on dried bloom tags), 2 percent large bollworms and 2.5 percent small bollworms (split between squares and under bloom tags) were recorded. This represents a high level of eggs on lower fruit, and if situations like this and others are left untreated, significant damage can result. This field was treated with insecticide a week prior to August 13 and will hopefully be treated again shortly.
For fields which have not yet been treated with insecticide, the egg threshold for bollworms is still appropriate. Do not be surprised, however, if either the egg threshold is met on fruit (2 percent eggs), or more likely a 3 percent larval threshold is found on squares or bolls. It is essential to include bloom tags in your boll sample, but only in proportion to their contribution to the fruit sample. If bloom tags constitute 25 percent of the overall boll population in a field, then the subsample scouted should include 25 percent bolls with bloom tags and 75 percent without bloom tags (essentially the larger bolls). Include white and pink blooms in your boll sample, also in proportion to their occurrence in the field.
Some cotton fields will undoubtedly continue to experience very
light bollworm pressure and treatment may be avoided, especially
those fields which are heading rapidly toward cutout. Most fields
will be susceptible to bollworms, European corn borers, fall
armyworms and stink bugs for several more weeks, and it appears
that the present moth flight is on the increase in a number of
areas. With scouts getting fewer in number (i.e., returning to
school) and under the present circumstances (generally late crop,
with some fall armyworms in the southern half of the state, and
European corn borers and stink bugs at moderate levels in a number
of fields), producers who are considering insecticide treatment may
want to err on the side of protection.
Beet armyworms have started to appear in a few scattered cotton
fields (reports from Hoke and Nash counties). Unfortunately, there
is no insecticide product labeled that will control beet armyworms.
If beet armyworms are suspected (essentially impossible to tell
ahead of time in North Carolina due to consistently low levels of
beet armyworms), two applications of Dimilin can help significantly
to prevent beet armyworms from becoming established. Historically,
we do not have the levels of beet armyworms to warrant a Section 18
exemption for Pirate or Confirm. Pirate is a very effective
material for beet armyworm management. Fall armyworms can be
suppressed by Curacron, Larvin, Lorsban, Lannate and, to a lesser
extent, Bolstar and can be kept at bay if residues of pyrethroid
insecticides are already on the plants; however, none of these
materials provides acceptable control of beet armyworms. Rainfall,
parasites and some diseases are about our only defenses against
beet armyworms at this time.
To date, the bollworm flight on cotton is moving very slowly. On August 13, I caught 352 newly-emerged bollworm moths and 234 moths the night before at a location close to the Vernon James Center in Plymouth. On August 14 quite a few white eggs in local cotton fields were found and several hundred were collected for research purposes. I believe that there are two causes for the low egg counts in cotton to date. First, the corn earworm population in corn was spread out, was only low to moderate in number in most corn fields, and many moths are in the ground waiting for moisture before emerging. Secondly, the temperatures in recent nights have been abnormally cool and a significant portion of the nights have been too cool for moth flight and egg laying. I expect that we will continue to have low to moderate and protracted egg laying and that egg laying will intensify in some fields (particularly those not drought-stressed). This low and more chronic bollworm colonization of cotton fields makes insecticide treatment decision- making difficult. It has also allowed for sampling variation (due to a random chance sampling effect) and poor scouting to have the maximum consequences.
Some fields that were not sprayed with insecticide last week are now meeting the egg threshold and many of these same fields are showing the most eggs in the interior of the plants and on stems (as opposed to eggs on the terminals). Especially in dry fields, eggs are distributed in the lower plant and mostly on fruit. Also, many fields are at or above the larval threshold in spite of never having egg threshold levels in these fields. In this case, many of the larvae are in bloom tag bolls, probably having resulted from eggs deposited on flowers. In this situation surviving caterpillars can accumulate and have a strong impact on yield. For instance, scouting in the upper plant for caterpillars can miss caterpillars in the lower plant, or under bloom tags, and over a period of two scoutings 2 percent live caterpillars plus 2 percent live caterpillars will add up. Add sampling variation to this, which is at itūs maximum influence under marginal infestations, and the population may actually have been 2 percent plus 4 percent (or more), and now the field has 6 percent live caterpillars (6 caterpillars per 100 fruit). A surviving caterpillar may destroy 3 or more harvestable fruit (if not killed) and in field with potentially 625 pounds per acre of cotton this can represent a yield loss of 113 pounds of lint lost in years of little compensation (6 live caterpillars per 100 fruit times x fruit eaten per worm x 625 pounds per acre yield). At $0.70 per pound of lint, this represents a loss of $79 per acre.
Seasons of low and slow infestation by bollworms often results in
more yield loss than a more severe insect year, when decisions on
when to treat and to re-treat with insecticide are clear. I am
advising county Extension agents, consultants, and growers to be
more aggressive on the first insecticide application in situations
where the producer is hesitating to spray cotton fields due to low
scouting reports. In this situation, we can be caught in our own
rhetoric and standards. Truthfully, when bollworm infestation is
occurring as it is now, it is difficult to measure the population
accurately and to assess the impact of other insects that we
ordinarily control with our typical start-up bollworm spray(s).
Yet, we can justify not treating based upon examination and
threshold procedures that we tell farmers are the correct way of
making insecticide treatment decisions. In fact, the story we have
been telling is basically correct, but in situations of low and
chronic invasion and the poor season that we are having, the system
has a weakness (it often will allow damaging populations of insects
to get through). The egg threshold that we normally use does not
represent a defined, damaging number of insects. Rather, the egg
threshold is a measure of the beginning of bollworm invasion into
cotton fields; it is not known if 10 eggs per 100 terminals or 2
eggs per 100 fruit results in a damaging population. Growers
trying to save money by not spraying at the beginning of a moth
flight has a high risk of failure. In fact, it is likely that
growers may spray more by not starting early, allowing a population
to become established, and then attempting to remove the
infestation. This is especially true in years like 1997 where the
crop has been delayed and the ability for fruit compensation by the
plant (at upper nodes) is practically nil.
Potato leafhoppers have become a widespread problem on peanuts over the past few weeks. Any field that did not receive a rootworm insecticide treatment is open to attack from this pest and this year has been a banner year for leafhoppers. Leafhoppers are not a problem every year, but they seem to be as bad as I have seen in the last five years. Leafhoppers are easy to control with a number of foliar insecticides and low rates of these are often used in a tank mix with a peanut leafspot fungicide. We do not have a good threshold for the treatment of leafhoppers and generally use a level of 50 percent of the leaflets with yellow leafhopper damage as an indicator of the need for treatment with insecticide. This threshold is not based on extensive research, but does keep us from jumping the gun on this pest. There are a lot of fields that have damage far exceeding the 50 percent level. It is important to note that the damage shows up several days after the leafhoppers actually begin feeding. Yellow plants do not necessarily indicate that leafhoppers are still active. Run your hand down the row disturbing the vines and watch for the small leafhopper adults to fly out of the foliage. If no adults are observed it may indicate the leafhoppers are no longer a threat in that field.
When considering the need to treat for leafhoppers, note that in many areas it is dry again (following the rain from Hurricane Danny) and spider mites have made a comeback. Random applications of foliar insecticides would only encourage the mites to become worse. I do not have adequate data on the effectiveness of Danitol on leafhoppers when used against mites, but it does have some insecticidal activity. Danitol provides reasonable control of corn earworms and may be a good choice in fields with mites and corn earworms.
It is the time of year for the presence of corn earworms and fall
armyworms in peanuts. Remember not to treat unless there are at
least 4 caterpillars per row foot. Each year a lot of our fields
have populations that are less than this level and do not need to
be treated. In a dry year with the threat of spider mites, the use
of foliar insecticides can lead to increased populations of mites.
Web page last updated on August 18, 1997 by Stephen J. Toth, Jr..
