NC Cotton Insect Scouting Guide
Written By — Jack S. Bacheler and Dominic D. Reisig
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. Unscouted cotton 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.
Key Cotton Pests
Thrips are a major cotton pest in the Carolinas and Virginia. They invade cotton from crops or weeds that are drying down 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 ar reproducing on the plants. Thrips are usually more of a problem either under dry conditions when plants take up little of an at-planting insecticide, under cool, wet conditions when thrips feed repeatedly on the same tissue in slow-growing cotton, and under conventional tillage. Furthermore, tobacco thrips, the main species infesting North Carolina cotton, are a problem in some areas because of varying localized levels of resistance to insecticidal seed treatments (neonicotinoids). Producers can reduce the chance of early infestations by using treated seed and an in-furrow liquid insecticide contacting the seed through an orifice spray or seed firmer. At recommended rates, this approach is usually effective. However, thrips sometimes require a supplemental foliar insecticide, especially after treated seed alone without an in-furrow insecticide.
Under conditions of high thrips migration into cotton, thrips can sometimes damage cotton seedlings within a week after emergence. To determine the need for a post seedling emergence foliar spray, the initial thrips and seedling damage check should coincide with a stand count and weed survey just after stand emergence and at the emergence of the first true leaf. A minimum of weekly leaf and bud examinations for damage and live immature thrips should be conducted until most cotton plants have reached the thrips-safe 5 true leaf stage.
When scouting for thrips, examine th terminals (including the expanded cotyledons and 1st true leaf at the initial check) of at least 25 individual plants throughout the field. Carefully examine this area 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 injury is found, inspect a portion of these seedlings for live thrips. Note that some types of herbicides can create injury that mimics thrips injury. Therefore it is critical to scout for live insects. A sample of at least 10 plants selected randomly from throughout the field is suggested for finding live thrips, focusing on the injured areas. Both adult larger, with stalk-like hairy wings, and sometimes varied in color) and tiny, immature thrips (pale yellow) may be present, with more adults than immatures 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 at the cotyledon to 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 and spider mites.
The cotton bollworm, also called the corn earworm, can be a significant square and boll-damaging pest of cotton in North Carolina, especially following foliar insecticide sprays for other insects. This species emerges from the soil as a moth in early to mid-May and completes at least two generations, primarily in wild hosts and field corn, before flying to blooming cotton and soybeans in mid-July (southern counties) to late July or early August (northern counties). Regular, systematic scouting for established (1/8-inch or larger) 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.
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 II, WideStrike, WideStrike 3, and TwinLink varieties, with some expressing up to three Bt genes. Bt varieties will not control insect pests other than caterpillars; for instance, 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 zero survival in the fieldwhile other caterpillar pests, such as beet and fall armyworms and cabbage and soybean loopers that may be present at low levels, are no longer economic pests.. In contrast, bollworms can become established, especially if a prior “disruptive” spray has been used that reduces or eliminates beneficial arthropods. Finally, only limited control of cutworms is provided by the Bt toxins, at least in part because cutworms are often partially to fully grown when cotton seedlings are available in the spring.
The presence of eggs or first-stage bollworms can be used as an indication of potential pressure to help gauge scouting frequency. Neonate recently hatched) bollworms must feed on the cotton plant before they ingest a lethal amount of the Bt toxin, so these should never be used as a trigger point to spray. Scouts should direct their attention to detecting and recording square damage, damage to small bolls, and/or second-stage bollworms. Thus, it is essential both to recognize the difference between first and second bollworm stages (second stage bollworms will be greater than 1/8-inch in length) and also to identify the difference between insignificant, superficial square damage and damage that will cause the square to abort. Scouting only large bolls or large-sized bollworms can lead to significant yield losses if these populations exceed threshold and are not sprayed.
When scouting a cotton field for bollworm 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. Pay particular attention to bollworm larvae associated with blooms and bloom tags. The Bt toxin is not well expressed in the pollen anthers and flower tissue. 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. Bloom tags should not be oversampled either, since bollworm larvae can survive here avoiding the Bt toxins produced by the plant. Likewise, do not sample obviously damaged or flared squares.
Again 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 possible damaged fruit (some damage is obvious and some may require a closer inspection at your stop). Nail pouches are available at most hardware stores.
Once-a-week scouting leading up to the major bollworm moth flight is usually adequate. A good indication of when the major flight begins can be 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. At this point, scouting should be done weekly, or more often in a smaller subsample of fields. Scouting frequency should be adjusted according to the moth pressure, the susceptibility of the crop, the variety (and associated Bt toxins produced), environmental conditions, 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 and blooms have become difficult to find (a few squares and blooms will sometimes remain in the terminals, even when cotton has “cut out” and is no longer susceptible to bollworms), 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 and stink bug damage. Spot scouting for late bollworms 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.
Beneficial insect levels can be higher in Bt cotton than in the more heavily treated conventional cotton. These higher beneficial insect numbers may lead to more effective aphid and other insect pest suppression, but only if insecticides are used minimally in Bt cotton.
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 can completely destroy small bolls, causing them to abort. When stink bugs feed on slightly larger to medium-sized bolls (up to about 3.5 weeks), they often introduce hardlock and/or 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. Stink bugs, unlike plant bugs, do not cause square shed.
Externally, boll injury 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 in the absence of stink bug feeding. Internally, the injured 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 injury 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) or where a diamide-class insecticide alone has been used for bollworm. Despite this general rule, significant stink bug damage may occasionally occur before bollworm insecticide applications. 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, though plant bug damage in some years can be significant, especially in eastern counties.
Beginning at anthesis (first bloom), one quarter-sized boll per acre with a minimum of 25 bolls per field should be sampled at least weekly for internal damage to bolls. In cotton fields greater than 10 acres, additional samples should be taken. Bolls should be either hand-crushed or opened with a knife and each lock examined for the presence of stained lint and/or raised warts on the inner surface of the carpal wall. No matter how insignificant, if any stained lint or a wart is found, that boll is counted as injured. Because the small non-raised dark green “pin prick-like” marks on the inner carpal wall surface and external stink bug damage spots are not as well correlated with subsequent boll injury, only warts and stain are counted. 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 injury or field edges. The use of both the dynamic threshold along with the Stink Bug Decision Aid app is advised (see below). The percent damage bolls in the sample is compared to the suggested dynamic threshold based on week of bloom (Table 1).
|Week of bloom||Threshold|
Table 1. Suggested threshold based on week of bloom. Weeks 3 through 5 of the bloom, constitute the period of high boll vulnerability to stink bug injury and yield loss.
Stink Bug Decision Aid Card and Associated Apps
A 3×6-inch plastic stink bug decision aid field card was developed to encourage 1) the increased adoption of scouting for stink bug injury, 2) better field identification of stink bug-induced damage symptoms and 3) use of recommended scouting procedures and thresholds. This stink bug decision aid and its associated web-based and iOS and Android versions (iPhones, Android; based smart phones, iPads, tablets, etc.) provides the end user with the information needed to efficiently scout for stink bug injury to quarter-sized bolls, minimizing both boll damage and unnecessary insecticide use. This app is available as a free download on iTunes and Google Play. The smart phone and other device apps have interactive functions that measure the correct boll size for sampling, capture an image of a stink bug or damage in the field to forward to an agent, specialist or colleague, and calculate if the appropriate threshold has been met by week of bloom based on stakeholder input.
The basic field card layout, which is also contained with the web-based app and electronic device apps, is show below in Figs. 1 and 2.
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. When blooming begins, plant bugs continue to feed not only on the smaller squares, but also on larger squares. Plant bug feeding on large squares just prior to opening cause
“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 a 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 has become more widespread in recent years, particularly in eastern counties. 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 (sometimes called “crazy cotton”).|
|Small Squares||Squares yellowing, turning brown, then black, and finally aborting, leaving a scar. Sometimes square will remain externally yellow (dirty square) from plant bug frass (excrement).|
|Blooming||Various Squares||Plant Bug||Small squares, same as above; larger squares often show internal damage to pollen anthers.|
|White Blooms||Darkened pollen anthers (dirty blooms); petal deformations.|
|Bolls||Plant Bugs &Stink 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 remains high (greater than or equal to 80 percent), further, more comprehensive sampling for live 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 2 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 Irishpotatoes 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. In blooming cotton, the presence of plant bugs and their damage is best assessed by continuing fruit examinations, evaluating dirty blooms, and by the use of a black beat sheet (i.e., ground cloth). Plant bug feeding on large squares damages their pollen anthers which subsequently show up as easy-to-spot brownish to black anthers when the flower opens. Although we do not recommend the use of a dirty bloom threshold, dirty blooms are easy to spot and may indicated plant bug activity.
During the bloom period, a black beat cloth (also called a ground cloth, drop cloth or shake sheet) is probably the best tool to access live plant bug levels. The rolled up cloth is unfurled between two rows to the base of the plants and the cotton plants are firmly beaten onto the area between the rows covered by the cloth. This video below covers the proper use of the black beat cloth. Because plants bug may be active, they should be counted quickly.
Researchers have found that the small plant bug nymphs are much easier to identify on a black beat cloth than older standard white beat cloth. 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.
Miscellaneous Cotton Pests
Aphids are usually considered a minor economic problem in North Carolina. However, from 4 to 12% of the state’s cotton acreage has been treated annually during the past decade. Cotton aphids usually occur in clusters or colonies around the terminal stems and/or under leaves. Their feeding cause 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 and more recently neonicotinoid insecticides has been confirmed in North Carolina, 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 (i.e., lady bird beetles and their larvae or green lacewing larvae), the brownish aphid mummies, or the moldy-looking, parasitic fungus, Neozygites fresenii.
Because economic loss from 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 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 the present. Remember, a particular aphid population may be resistant to all organophosphate and neonicotinoid insecticides.
Two-Spotted Spider Mite
Mites are not insects, but can be scouted as you would other insects. 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 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 throughout a significant portion of the field should be confirmed before treating. A hand lens is very helpful in spotting the small moving adult and nymph stage mites and their very round shiny eggs 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. If significant rainfall is predicted, do not treat but reassess the mite population a few days after the rain. A significant drop in egg levels often indicates a declining mite population.
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 and kill 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. The careful observation of sound economic thresholds, along with assessments of key beneficial insects, offers the producer the best odds of balancing beneficial insect numbers against damaging insects.
Black lights (UV lights) are an important tool for detecting both sexes of bollworm adults as well as some other moths and a few other insect species. The third an largest flight of bollworm moths begins from mid-July to early August in North Carolina. The use of black-light traps provides information on the timing and relative intensity of this flight. Traps should be checked three times a week from early or mid-July through August to keep abreast of the timing and intensity of the bollworm moth flights. 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.
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.
NC Cotton Thresholds
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 injury or 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.
Current thresholds for cotton pests
Cotyledon to the 4 true leaf stage:
- An average of 2 immature thrips per plant.
- Alternatively, an average of 1 immature thrips per plant for each true leaf.
(Timing of thrips applications,especially following seed treatments, is often best targeted to the 1st true leaf stage).
- 3 live second-stage bollworms (1/8 inch or longer) per 100 fruit (Pay particular attention to bollworms in or under yellow, pink, or dried blooms stuck to young bolls.)
- 2 second-stage bollworms on 2 consecutive scouting trips,
- 1 second-stage bollworm on 3 consecutive scouting trips.
Use thresholds of 50, 20, 10, 10, 10, 20, 30 and 50% internal injury to quarter-sized bolls (count warts and stained lint) during weeks 1 to 8 of the bloom period, respectively (see Table 1).
Pre-bloom: Plant bug sweeping advised where retention of young terminal and lateral squares in pre-bloom cotton is less than 80 percent.
- 8* plant bugs per 100 sweeps (from initiation of squaring until the first or second week of blooming).
*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.
- 2-3** adult plus nymph stage plant bugs per 5 row feet taken from 6 to 8 location in the field.
** Use 2.5 foot black beat cloth between two adjacent cotton rows.
- Treat at a rating of 4 (shown below) in opening cotton (15 percent open bolls or greater) or a rating of 5 in pre-opening cotton if plants are under stress and beneficial insects and fungi are at low levels.
Treatment is discouraged under most circumstances if predators and/or parasites appear to be exerting a significant controlling effect on aphid populations, or if the aphid parasitic fungus is present at any level. In opening cotton, treat only if plants are heavily infested and honeydew is detected in significant portions of the field. The aphid rating scale may help define situations where treatment for aphids may be indicated. In North Carolina, cotton aphid populations in a given area may be resistant to most commercially labeled insecticides.
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.
Two-Spotted Spider Mites
- General leaf discoloration (chlorosis, bronzing, or both), plus live mites over most of the field and defoliation from mites in 25 percent or more of the field. (If rain is imminent, delay treatment and reevaluate 3 to 4 days after the rain. If a miticide is used, 2 applications are sometimes necessary.)