Cotton insect scouting is the regular and systematic inspection of cotton for insects and their damage. Its purpose is to obtain an accurate estimate of the types and numbers of important insects and their damage in the field by checking a limited number of plants or plant parts. Judgments are made about managing insects in the whole field(s) from these small insect and/or plant samples. Therefore, following prescribed scouting procedures and taking an adequate sample size are important. Because insect pests can be very damaging to cotton, scouting is a must. Not scouting usually costs the grower substantially, either through yield losses or unnecessary insecticide applications, neither of which is acceptable. By knowing the kind, number, and location of insects and their damage within a field, the producer or consultant can make sound decisions about insect management and can often save several times the cost of scouting.
Thrips.Thrips are a major cotton pest in the Carolinas and Virginia. They invade cotton from outside drying hosts and can cause moderate to high damage to seedlings if not controlled. These tiny insects can cause stunting, maturity delays, and yield losses. This is especially true if the thrips are present in high numbers and are reproducing on the plants (there are thrips larvae present). Thrips are usually more of a problem either under dry conditions when plants take up little of an at-planting insecticide, or under cool, wet conditions when thrips feed repeatedly on the same tissue in slow-growing cotton. Producers can reduce the chance of early infestations by using an in-furrow, at-planting insecticide or treated seed. At recommended rates, this approach is usually effective (though somewhat less so with the seed treatment, which often require a supplemental foliar application), and its impact on beneficial insects is minimal in most situations. The initial thrips and seedling damage check should coincide with a stand count and weed survey just after stand emergence, with weekly scouting thereafter. Examinations of expanded cotyledons, newly unfurling leaves, and the bud area can be carried out quickly.
When scouting for thrips, examine the terminals (or expanded cotyledons at the initial check) of approximately 25 individual plants throughout the field. Carefully examine the terminal and its newly expanded leaves for 1) crinkled or "possum-eared" leaves, 2) gnarled, darkened bud tissue, and 3) small silvery areas in the expanding leaf tissue. Pay particular attention to the expanded cotyledon up to the first or second true leaf stage, when cotton plants are most vulnerable to thrips injury. Older damaged leaves remain crinkled and thus reveal little, if any, recent information.
If thrips damage is found, inspect a portion of these seedlings for live thrips. A sample of at least 10 plants selected randomly from throughout the field is suggested for finding live thrips, focusing on the damaged areas. Both adult (larger, with stalk-like hairy wings, and sometimes varied in color) and tiny, immature thrips (pale yellow) may be present, with the adult:immature ratio higher earlier in the season. Immature thrips should be present before treating with a foliar insecticide. Magnification may be required to detect the very small immature thrips. A 10X to 20X hand lens is ideal for this inspection. Alternatively, several cotton seedlings may be carefully picked or cut off at the base with a sharp knife and firmly beaten against a flat light or dark object, such as a large index card. Then count immature thrips. A treatment "threshold" of approximately 25 percent or more of the plants showing significant injury and an average of 2 or more immature thrips per plant is suggested for 2- to 3-leaf cotton, or an average of 1 thrips per plant at the cotyledon or first true leaf stage.
Although often needed, particularly following seed treatments, foliar sprays should be used with caution, as these treatments can increase levels of cotton aphids, second-generation budworms (though not on Bt cottons), and sometimes spider mites.
Early Tobacco Budworms. The overwintering generation of tobacco budworm moths usually emerges from the pupal stage in the soil in early to mid May. Budworms occur primarily on wild hosts and tobacco. Moths from the second generation of budworms fly to cotton to lay eggs, primarily during June. In some years, terminal and/or square feeding by these second-generation budworms can be damaging to non-Bt cotton, particularly in the southern part of the state. In general, taller, earlier planted cotton is most attractive to the egg-laying female moths.
Weekly scouting is recommended for early tobacco budworms (and, less commonly, corn earworms or bollworms at this time of year) in early to mid June in the southern counties and in late June in the northern counties and western piedmont. The focus of these scouting trips is to find small larvae in terminals and squares. Plant examinations for budworm eggs at this time of year are primarily used to gauge the timing and intensity of the moth flight. A scout should examine 100 squares and 25 to 50 terminals (a terminal is the upper 4-6 inches of the cotton plant and typically includes 4-6 partially to mostly expanded leaves). A sample of 50 terminals is suggested for fields in excess of 10 acres. Avoid sampling yellowish, flared squares in order not to bias the sample results, since these squares show unusually high, older damage and are not representative of the current overall field damage.
It is imperative that the presence of live larvae be confirmed because high levels of terminal feeding symptoms often go hand in hand with substantial mortality due to predators and parasites at this time of year. Predation can be particularly high in the absence of earlier foliar insecticide applications for other insects such as thrips or plant bugs. Afternoon or evening scouting at this time of year is suggested because morning and midday predation may result in lower egg or caterpillar numbers. Thus, scouting late in the day may forestall or avoid unnecessary treatment. It is essential that the higher pre-bloom caterpillar thresholds (see Thresholds section) be followed at this time of year, that treatments for this generation be held to a minimum, and that pyrethroids be avoided. Bollgard and Widestrike cotton are presently so resistant to budworms that scouting for this species is unnecessary.
Bollworms.The cotton bollworm, or corn earworm, is the primary boll-damaging pest of conventional cotton in North Carolina. This species emerges from the soil as a moth in early to mid May and completes two generations, primarily on field corn, before flying to blooming cotton in mid July (southern counties) to late July or early August (northern counties). Regular, systematic scouting for bollworm eggs and bollworm larvae is essential, particularly when the major moth flights are under way. Plant compensation for boll damage at this time of year is minimal, and caterpillar feeding, especially on bolls, can dramatically reduce yields.
When scouting a cotton field for bollworm eggs and small larvae, cover all major areas in the field. Inspect 25 to 100 randomly selected terminals and 100 squares, and 100 bolls from throughout the field.
In conventional cotton, bollworm eggs are the primary focus of the scout until the initial insecticide application(s), although the scout should be alert for and record small larvae. At times, tobacco budworms may occur in cotton before the major bollworm moth flight. In these situations, a larval threshold for budworms may be reached before the bollworm egg threshold (see Thresholds section). Inspect both the terminals and the fruiting forms for eggs and small larvae. Scouts should also report eggs on leaf petioles and other stems. Under average to moist conditions, bollworm moths sometimes lay most of their eggs on terminals during the first week or two of the major flight. However, in many instances, they may deposit their eggs almost entirely on squares, bolls, and stems, especially under dry conditions or later in the flight period.
When the egg threshold has been met or exceeded and treatment(s) made (see Thresholds section), the primary scouting focus shifts toward finding small bollworms feeding on squares and bolls, including those in yellow and pink flowers or under bloom tags (dried flowers stuck to small bolls). Inspect squares from all areas of the plant. Do not sample obviously damaged or flared squares. The boll sample should consist of bloom tags, small bolls, and large bolls in the proportion that they occur in the field. It may be easier to keep track of this ratio by sampling and retaining 10 squares and 10 bolls while walking, then stopping within the field to evaluate the fruit. This process would be repeated 10 times for a total of 100 squares and 100 bolls. A cloth nail pouch with two sides (one side for squares and the other for bolls) makes a handy container for carrying fruit between evaluation stops. These are available at most hardware stores.
Once-a-week scouting leading up to the major bollworm moth flight is usually adequate. When the major flight begins, as confirmed by a significant increase in Blacklight or pheromone trap captures and/or the presence of freshly emerged bollworm moths in the field or around field edges, twice-a-week scouting is advised until the egg threshold is met. As a general rule, a 4- to 6-day scouting schedule is recommended for the remainder of the moth flight, although this schedule may vary according to the moth pressure, the susceptibility of the crop, the insecticide rate used, and the damage risk the producer is willing to take.
Given the varietal, planting date, soil, and fertility differences among fields, both the attractiveness and susceptibility of a field to insects and the period during which the field remains vulnerable to late-season insects may vary greatly. Generally, late-planted, lush, rank fields are both more attractive and vulnerable to late-season insect damage and may require a more extended period of scouting and protection.
When lateral squares have become difficult to find (a few squares will sometimes remain in the terminals, even when cotton has "cut out" and is no longer susceptible to bollworms) and blooming has stopped, scouting for bollworms can be stopped. Likewise, when cotton plants have an average of 3 nodes or fewer remaining above the uppermost first position white bloom or when the upper bolls that will be harvested have become difficult to cut with a pocket knife (approximately 3 to 4 weeks after bloom), they are normally also impervious to bollworm attack. Spot scouting for late bollworms, fall armyworms, and European corn borers may continue through early to mid September in fields of late-maturing cotton or in green areas of the field, if these areas make up a significant portion of the field. Bollworm (and other late season insects) thresholds should be raised as the boll population matures through the season (see Stink Bug section).
European Corn Borer. On conventional cotton (Bollgard and Widestrike cotton are largely resistant to European corn borer [ECB] damage), an early, tunneling type of ECB damage may occur within leaf petioles and stems, usually in mid-July through mid August. Although this damage looks serious, with wilting and eventual death of the tissue above the feeding site, it usually causes little economic loss.
The third-generation (or second field-generation) ECB major moth flight, which is the most damaging to conventional cotton, often occurs about the same time as or somewhat later than the bollworm flight. The female moths lay egg masses that each contain 15 to 75 eggs. These small, flat, scale-like masses are deposited on the underside of cotton leaves deep within the canopy and are difficult to find. At first, small worms feed on the leaf tissue at the egg mass site, but they can begin to enter and feed on medium and large bolls within 48 to 72 hours, particularly after the first week in August. Larvae of the ECB primarily damage cotton by feeding on large bolls from mid August through late September. In rank or late-maturing cotton, this damage can be significant.
Although the caterpillars of this species generally do not feed as extensively within the bolls as do bollworms, most infested bolls are destroyed either directly by the larvae or indirectly from associated bollrot organisms.
Because finding the egg masses is virtually impossible and live caterpillars are spotted too late to achieve effective control, no control threshold has been developed for the ECB. Fortunately, because egg-laying of the ECB and the corn earworm (from their major flights) usually occurs relatively close in time (peak ECB egg-laying is often 1 to 2 weeks later than that of bollworm moths), treating for bollworms usually also controls ECB if these treatments extend into the ECB infestation period (sometimes 3 to 4 applications are needed). In most cases, however, the 2 to 3 applications typically used for bollworm control, or low ECB populations, translate into minimal damage. When treating for ECB, a compound should be selected that is effective against both bollworm and ECB. Pay particular attention to late, rank fields that have not had a recent insecticide application for bollworms.
Scouting for ECB threats or damage in conventional cotton is difficult. The flat egg masses are almost impossible to find, even by trained scouts searching a moderately to heavily infested field. By the time the larvae are found feeding on or within bolls, insecticidal treatments are usually ineffective. Thus, scouting and treating for this pest are not as effective as desired. However, fields should be scouted to detect the caterpillars, since the presence of small larvae on or within bolls may indicate an ongoing flight or egg mass deposition that may lead to additional infestation. Also, newly hatching larvae may be killed with insecticide. When moving through the field, be alert for ECB moths being flushed from the plant foliage and for lush, rank crop conditions. The presence of small larvae on bolls or moths in the field or around field borders (and in local light or pheromone traps) in lush cotton fields may justify treatment. If many larvae are present (3 to 5 percent or more of the boll sample) or if many damaged bolls are found (10 percent or more), it indicates that growing a late, rank cotton crop should be avoided in future years.
European corn borers will not likely be an economic pest in transgenic Bt cotton lines until resistance to this technology occurs.
Stink Bugs. In situations of low insecticide use, often the case with Bt cottons, the green stink bug and the brown stink bug have become more abundant and damaging. Stink bugs often invade cotton fields in early to mid July and may reach damaging levels from this time through late August. They damage cotton by puncturing the carpal walls of bolls with their "beaks" and feed primarily on the soft, developing seeds. Heavy feeding often completely destroys small bolls, causing them to abort. When stink bugs feed on slightly larger to medium-sized bolls (up to about 3 to 3.5 weeks), they often introduce bollrot pathogens, resulting in partially or entirely destroyed locks, hard-lock, and a lower grade of harvested cotton. Internal damage to bolls seems to be expressed more in mature opening bolls in wet humid weather.
Externally, boll damage is characterized by small, round, shallow, purplish depressions, usually in the 1/32- to 1/16-inch range. These spots tend to be larger than the tiny spots usually seen on maturing bolls.
Internally, the damaged bolls often have a yellowish to tan to brown stain in the seed areas, which often, but not always, appears under the external feeding spots. Other damage symptoms include small wart-like growths and/or dark "pin prick" spots on the inside of the boll wall. This internal boll damage can be identical to that caused by plant bugs. Internal boll damage may be present without obvious external evidence.
Stink bug damage is more prevalent in fields where bollworm treatments have been minimal (that is, none or one), although significant stink bug damage may occasionally occur before bollworm insecticide applications. Where the bollworm population is high enough so that the field has been treated twice or more (as is often the case with conventional cotton), stink bug numbers usually will be reduced enough to limit damage to low levels. Because stink bug and plant bug damage symptoms are often indistinguishable, damaged bolls may sometimes be the result of feeding by either or both kinds of bugs. At present in most areas, stink bug damage to bolls is more common than plant bug damage.
In Bollgard cotton, or in conventional cotton in areas with a history of stink bug damage or light bollworm pressure, a sample of 50 quarter-sized bolls should be taken specifically at each scouting session, beginning at early boll development. Because stink bugs can be unevenly distributed within a cotton field, it is important to take boll samples from throughout the field and to not be overly influenced by relatively small areas of high stink bug levels.
To determine if a boll has been damaged by stink bugs, the boll should either be crushed by hand or cut open into all locks with a knife to reveal possible internal damage. Count all internal boll damage, including stained or spotted lint and callous growth or warts. If all bolls are less than approximately 3 to 3.5 weeks old (the age at which a boll is no longer susceptible to bug damage), use an initial treatment threshold of 10 percent. As the ratio of larger "safe" bolls to smaller susceptible bolls increases during the remainder of the season, the threshold should be raised.
A BollChecker, any thin, flat piece of material such as thin plywood or Plexiglas, with one hole of about 1.3 inches diameter (for 1st position bolls early in the boll production period- weeks 3 to 5- and/or under conditions of rapid boll growth) and a second hole (for 2nd position bolls, later in the boll production period- weeks 6 to 8 -, and/or under conditions of slower boll growth) can help the scout determine the ratio of larger, safe bolls (which cannot pass through the respective hole in the BollChecker) to smaller, susceptible bolls (which can pass through the BollChecker hole). To use this method, pick a representative sample of 10 various size bolls in 4 or 5 locations in the field to determine the ratio of large to small bolls. Table 1 should serve as a guide in using the appropriate threshold based on boll maturity.
Note: The BollChecker method of adjusting stink bug thresholds is in the first phase of field testing via a Southeastern states grant from Cotton Incorporated. We anticipate that the above BollChecker concept may be modified in the coming years. For the time being, bollsizing should provide a reasonable means of limiting unnecessary treatments as the cotton crop become more immune from stink bug damage.
Table 1. Suggested Damaged Boll Thresholds for Stink Bugs*
(smaller than 1.3 or 1.15 inches in diameter)
Safe (larger than 1.3 or 1.5 inches in diameter)
1 0 10 1 0.5 15 1 1 20 1 1.5 25 1 2 30 1 3 40 1 4 50
* To be used with BollChecker
Once the damaged boll threshold has been met, further scouting may be needed by using general observations for stink bugs while scouting (in the case of high population levels), by sampling with a beat cloth (six 6-row-foot samples or more until adequate stink bug numbers are observed), or by using a sweep net (six 25-sweep samples or more until adequate numbers of stink bugs are observed). These observations or samples are conducted solely to determine if the stink bugs are brown (more difficult to control) or green, and they may be done quickly. For beat cloth sampling, a 3-by-3-foot beat cloth is unfurled between two adjacent rows, and the cotton plants in the two rows (6 row feet) are beaten or shaken over the cloth, causing the large nymphs and adults to fall onto the cloth. Count any adult stink bugs observed flying if the color can be seen. In sweep net sampling, individual sweeps should be made with firm, pendulum-like motions (handle up, net down), swinging down with both hands through the upper middle canopy while walking down the row. If the sweeping motion is correct and vigorous enough, some bolls and leaves usually will be knocked into the net. In either of the above approaches to assessing live stink bugs, count both adults and large nymphs. This quicker sampling may be stopped once an adequate sample (approximately 10 stink bugs) has been counted.
As the proportion of large stink bug safe bolls increases relative to the smaller susceptible bolls during the end of July and throughout August and early September, the threshold may be raised according to the above table. Twice-a-week scouting (or once-a-week scouting coupled with in mid-week scouting in fields showing an increase in stink bug damage) in Bt (or minimally treated) cotton is recommended under heavy stink bug pressure.
Introduction. Several cotton lines that have been genetically altered to express the delta endotoxin of Bacillus thuringiensis (Bt) have been available to North Carolina producers since 1996. These lines are also referred to as Bollgard and Widestrike varieties. Bt varieties will not control insect pests other than caterpillars, for instance, boll weevils, thrips, cotton aphids, plant bugs, and stink bugs. Also, different caterpillar pests are not controlled to the same degree. For example, tobacco budworms attempting to feed on varieties with the Bt endotoxin have shown essentially zero survival in tests, while fall and beet armyworms, and to a lesser extent bollworms, can become established on Bollgard cotton lines, but far less so with Bollgard II and Widestrike cotton varieties.
Because beneficial insect levels are often higher in Bt cotton than in the more heavily treated conventional cotton, higher beneficial insect numbers may lead to more effective aphid suppression. In some situations, beneficial insects are also present at higher levels during the major bollworm and ECB moth flights and augment the efficacy of Bt cottons.
Efficacy. The effectiveness of these genes (1 Bt gene in Bollgard and 2 Bt genes in Bollgard II and Widestrike) against North Carolina's major cotton pests is influenced by a number of factors, including the pest in question, the level of supplemental beneficial insect "help," and the phenology or maturity of the cotton crop. Since supplemental control of bollworms may be needed, and because of potentially higher levels of plant bugs and stink bugs, scouting Bt cotton often must be more intensive than scouting conventional cotton. Under irrigation, or if a beneficial insect-reducing spray is applied just before or early in the bollworm moth flight, moderate to heavy bollworm establishment may occur with associated yield reductions. Less than 5 percent of North Carolina's cotton acreage is presently irrigated, and disruptive insecticide sprays just before the major, mid to late July to mid August bollworm moth flights, thought not common, are becoming more widespread due to the increases in plant bug and stink bug levels.
Scouting Bt cotton for insects is similar to scouting conventional cotton. However, the relative status of a number of these insects (fewer budworms and European corn borers, more stink bugs and plant bugs) has changed, so shifts in scouting emphasis are needed.
Because neonate (recently hatched) bollworms must feed on the cotton plant before they ingest a lethal amount of the Bt endotoxin, scouts no longer use low numbers of eggs or first-stage bollworms as a treatment trigger. Instead, the presence of these stages will only be used as an indication of potential pressure to help gauge scouting frequency. Scouts should direct their attention to detecting and recording square damage and/or second-stage bollworms. Thus, it will be essential both to recognize the difference between first and second bollworm stages and also to identify the difference between insignificant, superficial square damage and damage that will cause the square to abort. However, if egg levels are in the 75- to 100-range per 100 terminals, or 8 to 10 per 100 fruit, an economically damaging infestation of bollworms is likely on Bopllgard cotton, and treatment is advised. Because Widestrike cotton lines, and particularly Bollgard II cotton varieties provide greater bollworm activity than Bollgard cotton, we do not recommend treating these lines on the basis of high egg levels.
Pay particular attention to bollworm larvae associated with blooms and bloom tags. The Bt toxin is not well expressed on the pollen anthers. Bollworm larvae that become established on Bt cotton are often associated with pink blooms and bloom tags. Remember to sample these forms in proportion to their occurrence in the overall boll population. If a higher proportion of pink blooms and bloom tags is sampled, bollworm thresholds must be raised accordingly.
Plant Bugs. Before cotton blooms, plant bugs, or Lygus, damage the crop by feeding in tender terminals and, more commonly, directly on small squares with their needle-like mouthparts, causing the squares to abort. In pre-blooming cotton, plant bugs have required treatment on approximately 5 percent of the cotton acreage averaged over the past six years in North Carolina.
When blooming begins, plant bugs continue to feed on the smaller squares and also on the larger ones. Plant bug feeding on large squares just prior to openong causes "dirty blooms" (white blooms with darkened pollen anthers, and sometimes with small circular deformities on the petals). Additionally, plant bug feeding on small to medium-sized bolls up to approximately 10 to 12 days old may cause stink-bug-like external boll spotting and internal boll damage, such as callous growth (warts), deformed or rotted fruit, or small boll abortion. This boll damage is identical to that caused by stink bugs. Plant bug damage to bolls is more common in untreated, or minimally treated cotton such as Bollgard, Bollgard II, and Widestrike cotton. However, plant bug and stink bug damage can occasionally occur in blooming cotton before the major bollworm moth flight. Plant bugs are capable of causing all of the damage symptoms shown in Table 2.
|Plant Stage||Plant Part||Bug Type||Damage Symptoms|
|Pre-Bloom||Terminals||Plant Bug||With heavy feeding, terminals may be deformed or killed, resulting in a loss of apical dominance (crazy cotton)|
|Small Squares||Squares yellowing, turning brown, then black, and finally aborting, leaving a scar|
|Blooming||Various Squares||Small squares, same as above; larger squares suffer internal damage to pollen anthers|
|White Blooms||Darkened pollen anthers (dirty blooms); small petal deformations|
|Bolls||Plant Bugs &
|Aborted small bolls; external spotting; internal feeding "stings"; wart-like growths; stained lint, often in seed area; damaged seed|
Early season monitoring for plant bug activity is recommended, especially retention counts of small squares (approximately 1/8 to 3/16 inch long, including bracts). If square retention counts remain high (greater than or equal to 80 percent), further sampling for plant bugs is probably unnecessary. If retention rates of small, upper, and other first- or second-position squares drop below this level, further sampling for live plant bugs may be needed. Usually, one terminal square (or its missing position) and one first or second position square (or its missing position) two or three nodes from the top of the plant are inspected per plant from 25 randomly selected plants within a field (50 squares total). Sweep net sampling for plant bug adults and large nymphs typically involves the taking of 25 sweeps at 6 to 10 locations per cotton field.
Be mindful of field edges along ditch banks, adjacent host plants such as weedy flowering fields, or where Irish potatoes or a substantial acreage of corn is present. These areas are often a likely source of migrating adult plant bugs. In taking samples from randomly selected sweeping sites, pay attention to possible differences in plant bug distributions, particularly changes in levels from the edge to the interior of the field. Significantly higher levels around field perimeters may indicate an invading plant bug population.
Once blooming has been under way for 1 to 2 weeks, square retention is a less reliable indicator of possible plant bug feeding, due to natural square loss for mostly weather-related reasons. Late season damage by plant bugs can be effectively assessed by monitoring cotton plants for dirty blooms and for small bolls with signs of internal bug damage (see Table 2 and damage descriptions above). Additionally, live plant bugs may be monitored with either a sweep net, a drop cloth, or by visual inspections of plant terminals, including upper blooms. These latter methods are typically more time-consuming and sometimes less reliable. If both nymphs and adult plant bugs are present, indicating that reproduction has occurred, the population is regarded as potentially more damaging. Scouts should be aware that plant bugs may be more common at field edges or in rank areas. These trends should be noted by scouts, as they are sometimes taken into consideration in making treatment decisions. However, this occurrence also points out the need to sample randomly from throughout the cotton field.
Cotton Aphid. Aphids are usually considered to be a minor economic problem in North Carolina. However, from 4 to 12% of the state's cotton acreage has been treated annually in the past decade. Cotton aphids tend to occur in clusters or colonies around the terminal stems and/or under leaves. Their feeding causes young leaves to curl downward. Aphids excrete a sticky, shiny substance called honeydew, which can cover lower leaves. Later in the season honeydew can fall into opening bolls and support a fungus called sooty mold, which may stain the lint and reduce its grade. Because aphid resistance to organophosphate insecticides was confirmed in North Carolina in 1990 and because beneficial insects and fungi typically hold cotton aphids to low levels, treatment is seldom justified. Spraying for cotton aphids is recommended only when extremely high infestations, coupled with stressed cotton plants, are present throughout much of the field and there is little evidence of predators (such as lady bird beetles or green lacewing larva), the brownish aphid mummies, or the moldy-looking, parasitic fungus.
Because economic loss to aphids is very unusual, scouting for aphids and their symptoms does not take on the same urgency as does scouting for pests such as bollworms. However, because moderate to high populations of cotton aphids can be potentially damaging, routine scouting trips for other pests should include notes on aphid levels.
Based on extensive fall damage surveys, aphid colonies in opening cotton have been almost nonexistent in North Carolina from 1985 to 2004. Remember, a particular aphid population may be resistant to all organophosphate and pyrethroid insecticides.
Two-Spotted Spider Mite. Mite damage can occur almost any time during the season but is usually more prevalent during very dry conditions. It first appears as a slight yellowing of the leaves, which later changes to a reddish to purplish or bronze color, particularly in the interveinal areas of the leaf. Mite damage can also be recognized by the presence of fine webbing on the underside of the affected leaves. This webbing often traps blown sand grains. In severe infestations, spider mites can cause widespread defoliation.
Spot check for mites while scouting for other pests. Even with obvious yellowing and defoliation, the presence of an active mite population in the field should be confirmed before treating. A hand lens is very helpful in spotting the small moving mites and their very round shiny egg on the undersides of speckled and/or bronzy leaves. A fungus that preys upon mites is often present, particularly under rainy or humid conditions, and may greatly reduce mite numbers while the damage symptoms are still present. Mite damage to cotton in North Carolina has been very low for the past 15 years.
Fall and Beet Armyworms. Fall armyworms (FAW) attack blooms and bolls of all sizes. These caterpillars are extremely damaging if present in moderate to high numbers. They can feed on mature bolls normally resistant to bollworm penetration. Because fall armyworms generally occur after the bollworm flight is under way, pyrethroids have often already been applied for bollworms. Under these circumstances, fall armyworms become established more easily behind some pyrethroids than others, although all pyrethroids show some activity against FAW hatching from egg masses. Established medium to large-stage fall armyworms cannot be controlled effectively with insecticides. Because FAW adults migrate into North Carolina from farther south, their numbers vary greatly from year to year and often reach higher levels in our southern counties than farther north. As a general rule, fall armyworms hatching after approximately September 1 in North Carolina appear to have difficulty becoming established and die out. (Bollgard II and Widestrike varietes show high levels of resistance to both fall and beet armyworms)
The presence of FAW and their damage are recorded as part of bollworm scouting. If adequate control of FAW is to be achieved, the larvae must be found and treated either when they are hatching from the egg mass or as very early instars. Early-stage larvae are most often found feeding between the bracts and carpal walls of bolls. This bract feeding can be readily observed on the outside surface. Mid-canopy and lower bolls should be examined for feeding signs and caterpillars. Blooms may also harbor a disproportionally high number of FAW.
Beet armyworms (BAW) are rarely pests of cotton in North Carolina. Larvae of this migratory pest hatch from egg masses usually deposited on the underside of leaves. Early-stage BAWs feed on foliage in groups associated with webbing. Larger BAWs usually disperse and feed on squares, blooms, and bolls if present in higher numbers. A Cotesia wasp parasite is often very effective in reducing beet armyworm populations to very low levels in the absence of insecticides.
Scouting for BAW begins with a visual inspection to confirm the presence of egg masses or young groups of larvae. The light gray egg masses, containing several hundred individual eggs, are often deposited on the undersides of mid-canopy leaves and are approximately the same diameter as a pencil eraser. Open-canopied fields, field edges, gaps in plant stands, and stressed areas are favored sites for egg mass laying.
Initially check 100 feet of row in five randomly selected, widely separated areas in the field for hatching BAW egg masses or "hits.", as more than 5 hits may indicated a potentially damaging beet armyworm population. Inspect squares, blooms, and small bolls for beet armyworms after they disperse and begin to feed on this fruit. Additionally, monitor the level of defoliation, especially of leaves subtending (at the base of) immature bolls.
Check recent recommendations for BAW control. Most bollworm insecticides provide poor control of BAW. Fortunately, treatment for BAW is seldom needed in North Carolina.
Cabbage and Soybean Looper. Loopers rarely damage cotton in North Carolina because they eat only foliage, occur late in the season, are prone to virus attack, and occur sporadically.
Scouting for loopers is done by observing foliage during routine scouting for other pests. However, sometimes soybean looper (more common on cotton in North Carolina in recent years than cabbage loopers) can become established very late in the season, even in opening cotton. If significant leaf feeding is seen, the percentage of defoliation across the entire field should be estimated and recorded. This defoliation, which can occur quickly and is often overlooked, initially occurs on the lower foliage and in the interior of the cotton plants and subsequently develops outward and upward. If the defoliation reaches 25 percent and a significant number of bolls that the producer expects to harvest are still filling out, treatment may be advised. However, remedial sprays may have only marginal effect on the more common soybean looper (see North Carolina Agricultural Chemicals Manual for latest control recommendations).
(As with fall and beet armyworms, Bollgard II and Widestrike varieties show high levels of resistance to loopers)
Black lights (UV lights) are an important tool for detecting green stink bugs and both sexes of bollworm adults as well as some other moths and a few other insect species. The third and largest flight of bollworm moths begins from mid July to early August in North Carolina and almost always results in potentially damaging caterpillar populations on untreated, non-Bt cotton. The use of black-light traps provides information on the timing and relative intensity of this flight. Because this moth activity signals the onset of egg laying, scouting for eggs and small larvae should be intensified (to approximately two times per week) as soon as trap catches increase dramatically. Traps should be checked three times a week from early or mid-July through August. The downside of these traps is their expense, lack of portability, requirement of a source of electricity, and attractiveness to a variety of unwanted insects such as beetles, which can render some of the captured moths unrecognizable. "Hard-shelled" green stink bugs, on the other hand, are easily counted because of their characteristic shape and color and their resistance to breaking down in the traps.
Pheromone traps typically use the synthesized scent of one sex of an insect to attract either both sexes (e.g., boll weevils) or the opposite sex (most moths). Synthesized pheromone of female bollworms, tobacco budworms, European corn borers, other moths, and brown stink bugs, which attracts males of the respective species, is available commercially. These traps have several advantages over light traps-namely lower expense and relative portability. Pheromone traps also lend themselves to placement in remote areas. However, pheromone traps are also more localized in their attractiveness and can be inconsistent in reflecting the adult flights. Average captures from clusters of traps (e.g., 10 or more traps in a cotton production area) may provide a more accurate sample of moth flight activity than individual traps or smaller clusters.
About two dozen beneficial insects and arthropods commonly occur in North Carolina cotton. Ambush bugs, damsel bugs, big-eyed bugs, minute pirate bugs, green lacewings, ladybird beetles, and several types of spiders are examples. They are of two types: predators that prey on an insect pest and parasites that live within the host insect. These insects, particularly the predators, reduce the numbers of eggs and larvae of tobacco budworms, bollworms, and other caterpillars. Aphids in North Carolina are largely held in check by beneficial insects and fungi. Stink bugs in general appear to have fewer natural enemies that the above-mentioned insects. Because these "beneficials" lessen the impact of pest insects, it is common sense for producers to use them as a management tool. Their presence means that growers can sometimes delay, and occasionally eliminate, some insecticide applications.
Many complex factors are involved in determining just how many of each beneficial insect species would be needed to influence a given level of pests. Therefore, it is not possible to assess the value of these insects except in a very general way. If relatively large numbers of beneficial insects are consuming a large proportion of bollworm eggs and larvae, for example, the treatment threshold will be reached later than would otherwise be the case, at times either delaying the initial application or even reducing the number of insecticide applications needed. Alternatively, beneficial insects are often overwhelmed by the third generation of bollworms on conventional cotton, and spraying becomes necessary. The careful observation of sound economic thresholds, along with assessments of key beneficial insects, where appropriate, offers the producer the best odds of balancing beneficial insect numbers against damaging insects.
A threshold is the level of plant damage or the number of insects at which treatment is recommended -- hopefully the point at which the benefits of control will outweigh the costs. Threshold numbers are usually expressed in terms of the percentage or number of insects or instances of damage seen per 100 forms inspected. Based upon years of research, these thresholds form the basis for sound treatment decisions.
Thresholds, however, are only general guidelines applicable to the entire state. A knowledgeable consultant or advisor may be able to modify a threshold, depending on the region of the state, its past history of insect problems, the weather, the amount of risk that the farmer is willing to take, the producer's management capabilities, and other circumstances. Also, these thresholds are periodically refined on the basis of new research results or changes in the status and behavior of the various pests.
On cotton from the cotyledon to the 5 true leaf stage:
Timing of thrips applications,especially following seed treatments, is often best targetd at the 1st true leaf stage.
Pre-bloom: Plant bug sweeping advised where retention of young terminal and lateral squares is less than 80 percent.
*The sweep net threshold may be raised to 10 if fruiting begins on node 4 through 6, or lowered to 6 or 7 if fruiting has begun on node 8 or higher. Thresholds also may be lowered somewhat in stressed cotton.
Post-bloom: Post-bloom thresholds begin approximately 1 to 2 weeks after bloom initiation.
Use sweep net or beat cloth to confirm presence and size (number of adults vs. nymphs) and to determine potential presence of stink bugs.
|Aphid Rating Scale
0. No aphids.
1. Occasional plants with low numbers of aphids.
2. Plants with low numbers common; heavily infested plants rare; honeydew visible occasionally.
3. Most plants with some aphids; occasional plants heavily infested; honeydew visible in spots throughout the field.
4. Heavily infested plants common; aphids clumped on upper leaves; honeydew present in much of the field.
5. Many heavily infested plants and honeydew throughout the field.
Damaged Bolls (primary threshold)
10 percent stink bug internal damage to quarter-sized bolls initially (see Table 1), plus presence of stink bugs. Thresholds should be adjusted upward to reflect advancing boll maturity.
Beat Cloth (shake cloth) and Sweep Net (15 inch diameter).
These devices are most often presently used to help confirm the presence and species of stink bugs. A beat cloth threshold of 1 adult stink bug or large nymph per 6 row feet, or a sweep net threshold of 1 adult stink bug or large nymph per 25 sweeps, may be used. However, these latter thresholds are now regarded as more time consuming and less effective than sampling for damaged bolls.
Bollworm and Tobacco Budworms on Conventional Cotton
Pre-bloom threshold: (Most often reached in southern North Carolina on part of the acreage -- usually tobacco budworm. Only one application of an nonpyrethroid is recommended):
Egg threshold: (at the onset of the major bollworm moth flight):
Post-bloom larval threshold: (usually after the egg threshold has been employed, but may also be reached between bloom initiation and the major bollworm flight):
Note: The above fall armyworm threshold is based on limited information and can probably be at least doubled after September 1. The fall armyworm is primarily a late season pest. Pay particular attention to the small, grayish, fuzzy egg masses deposited on the underside of leaves and to"windowpaning" of the bracts of lower bolls for the presence of larvae. Correct identification is critical; many bollworm insecticides are ineffective against fall armyworms. Control of medium and large larvae often does not exceed 20-40 percent.
Transgenic Bt cotton
After the onset of the major bollworm moth flight:
For all other insects on Bt cotton, use conventional thresholds.
Cabbage and Soybean Loopers