Visual Scouting Techniques for Cotton
Visual scouting refers to the examination of plants to determine the presence of pests or their damage. In cotton, this may include the examination of plant terminals, whole-plant examination, or some modification of these approaches. The method and intent of this sampling will vary with the developmental stage of the crop and whether or not it is genetically modified to express Bt toxins (i.e., Bt cotton). The number of plants examined varies with field size but will generally range from 50-100 plants. However, fewer plants can sometimes be sampled if pest densities are well above treatment threshold. A preferred method of sampling is to visually examine 5 consecutive plants in 10-20 representative locations within a field. Counts are typically expressed in insect numbers (or percent) per 100 plants.
Emergence to First Square
Prior to squaring, the scout’s primary responsibility is to monitor general crop health in a field. This includes determining if an adequate stand is established. Environmental conditions such as excessive rainfall and cold weather can reduce cotton emergence and increase the incidence of seed and seedling diseases. Replanting may be necessary for parts or all of some fields. Making a “stand count” may be needed in areas where low plant populations exist. Stand counts are normally done by counting the number of living plants in ten feet of row at several locations within the affected area. Depending on row spacing, the average number of plants per acre can then be calculated. An average of 2-4 healthy plants per foot of row is sufficient to produce optimal cotton yields at typical row spacings (i.e., 30 – 40 inch rows). However, replanting may not be justified even when populations fall below this level depending on the size of the affected area and other factors.
The primary insect pests that threaten seedling cotton plants are thrips and cutworms. Thrips are a common pest of seedling cotton. Severe thrips infestations can kill seedling plants or delay crop maturity and reduce yields. The severity of thrips injury should be determined at several locations within a field. The numbers of thrips per plant should also be recorded. One method of counting thrips is to make a bouquet of 2-3 seedling cotton plants, and while holding them by their base, shake the tops vigorously over an empty cigar box covered with hardware cloth or a Cool Whip® container. Thrips will be visible on the white surface.
Cutworms may only be present in parts of a field such as low, wet or weedy areas. They are normally found underground, near freshly cut plants, during the day. Scouts should walk through representative areas of a field. If “cut” plants are found, the number of cut plants per 100 consecutive plants in a row should be counted at several locations. The presence of other pests or damage that may threaten plant stands should also be recorded.
First Square to First Bloom
When sampling a plant, first examine the terminal for the presence of tarnished plant bugs, particularly adults. Monitoring populations of bollworm and tobacco budworm eggs and small larvae involves the thorough examination the upper 3-4 nodes, known as a “terminal” sample. Check for eggs on leaves, bracts, and stems. Thumb through the terminal and open the bracts of larger squares in search of larvae. If damage is found in the terminal, extra time should be taken to track down the offending larvae and determine its size. Timing of insecticide applications is very important because smaller larvae (1 to 3 days old) in the top of terminals are much easier to control. Also, note what moths (bollworm vs. budworm) are being seen in the field. Species and larval size are two critical components when making an insecticide selection, including rate.
Square retention should be monitored on the same plants. This is done by examining first position fruiting sites in the terminal of the plant and recording the number of missing squares, being sure to count any injured squares that dislodge when touched. Percent square retention is calculated by dividing the number of missing squares by the total number of fruiting sites examined and multiplying time 100. Normally, a sample would consist of first-position square retention on the top five fruiting nodes for a minimum of 20 plants per field. It will be necessary to look at more plants when they have less than five fruiting nodes.
Significant aphid infestations will normally be detected during standard scouting efforts. Aphid populations are characterized has low, medium or high, which corresponds to an average of <10, 10-50, and >50 aphids per leaf, respectively (usually the first fully expanded leaf in the terminal). “High” populations typically result in many plants having an accumulation of honeydew and leaves with the edges curled downward.
Bt cotton: economically damaging infestations of tobacco budworm and bollworm are uncommon prior to bloom in Bt cotton. Thus, scouting efforts should concentrate on monitoring more likely pest problems such as plant bugs. A shift in emphasis toward monitoring square retention will normally help in detecting any injury caused by caterpillar pests.
Non-Bt cotton: Greater emphasis on monitoring bollworm and tobacco budworm populations is required in non-Bt cotton. Scouting for bollworm and tobacco budworm in non-Bt cotton also provides information about what “pressure” the Bt cotton may be experiencing. Significant larval infestations in Bt cotton fields are unlikely if surrounding non-Bt fields have low infestations, assuming fields are in a similar developmental stage.
After First Bloom
Cotton is more tolerant to bollworm and budworm damage prior to bloom. After bloom, greater emphasis should be made in monitoring caterpillar, primarily bollworm and tobacco budworm, and stink bug infestations. Greater reliance on visual scouting occurs once cotton has begun to bloom, and modified “whole-plant” samples are preferred over terminal counts. The intent of a whole-plant count is to find pests that may be occurring relatively low in the canopy, in addition to those in the terminal. This includes detecting 1) bollworm, budworm or fall armyworm or their damage in blooms and small bolls; 2) plant bugs (particularly nymphs) that are often found feeding in blooms and behind the bracts of large squares or small bolls; and 3) stink bugs and their damage. In Bt cotton, initial bollworm infestations are often first associated with white and pink blooms. Eggs are often found on bracts and stuck bloom tags. Bollworm and armyworm larvae may be in blooms and under bloom tags.
A whole-plant count starts by checking the terminal of the plant as previously described. In addition, the scout should also examine behind the bracts of larger squares and small bolls, inside white and pink blooms, and under bloom tags for the presence of plant bugs, stink bugs, caterpillars, and/or their damage. It is generally not necessary to sample every square, bloom or boll on a plant. Besides the terminal, examine at least one large square, one white or pink bloom (bloom tag), and one small to medium sized boll per plant. Bolls should be examined for the presence or injury of caterpillar pests or stink bugs. Counts are expressed in terms of numbers (or percent) per 100 plants or plant parts. For example, 3 bollworm larvae per 100 plants, or 25% of bolls with internal signs of stink bug injury.