Scouting for Mid -Season Insects
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Leaf Feeding Insects. — Cereal leaf beetle, grasshoppers, fall armyworm, and others may defoliate plants and cause concern to the farmer. In most situations defoliation may appear more serious than it actually is and will often be confined to the field borders. In these circumstances, visual estimates of the percent leaf tissue eaten is made in 10 spots within the affected area and compared to the appropriate value in Table 6-3 to determine the potential yield loss. If yield loss value is suitably high, treatment is warranted. Often only spot treatment is needed.
Second Generation European Corn Borer (Non-Bt refuge corn). This procedure should begin after first generation moths begin to emerge and lay eggs. The moth flight can be determined with the use of pheromone traps or light traps. Commercially available traps and the pheromone bait can be obtained from the suppliers listed at the bottom of this section. At least three pheromone traps of the Heliothis (Hartstack) style should used on a farm since single traps can give misleading information; average trap catches from the three traps should be used to show moth flight trends. Although the traps are moderately expensive they will last several years and can be used to monitor several other moth insects. Generally, traps should be baited with New York strain (E race) pheromone east of Raleigh and Iowa strain (Z race) pheromone west of Raleigh, but in areas along this line both pheromones may be necessary (used in the same trap). Traps should be set into corn fields at ear level and checked at least twice weekly. When the traps show a sudden rise in moth catch number, scouting should be done in four to seven days later in pollinating fields or as soon as pollination begins thereafter. Typically, egg mass laying closely coincides with pollination in normal planted corn and few eggs will be deposited in fields that are not pollinating. Sources of pheromone traps and lures include Great Lakes IPM and Gempler’s.
Scouting for European Corn Borer egg masses— Examine the upper side and underside of the ear leaf, or leaf immediately below, on 25 plants (one leaf/plant) for each sample and search for corn borer egg masses. Egg masses are pearly white to yellowish in color, and contain about 20 flattened, overlapping, fish scale-like eggs. They are mostly found on the underside of leaves within 2/3 of the distance from the corn stalk but may occur anywhere on the leaf. Egg masses are difficult to recognize at first, but with practice can be easily detected by persons with good eyesight. Each field should be sampled using a sequential sampling method where a minimum of 4 samples are taken and then the number of egg masses found compared with the MAX/MIN threshold table listed below. If the egg numbers are equal/greater than the MAX threshold (above threshold) or equal/less than the MIN threshold (below threshold) then sampling is stopped, because a treatment decision has been reached. If egg mass numbers are between the MAX and MIN thresholds, then another sample must be taken and another comparison to the MAX/MIN thresholds is made after each added sample. This procedure is continued until a MAX or MIN threshold is reached or a maximum of 10 samples is taken. Sample sites should be randomly scattered throughout the field. At each scouting, record all egg masses found. If a threshold is reached the first week, discontinue scouting. In the event that fields are scouted a second or third time it should be done in 3-4 day intervals; if properly timed one scouting will usually suffice.
Scouting for European Corn Borer caterpillars — Scouting for caterpillars is recommended in situations where scouting egg masses is done late or after most eggs are hatched. After hatching, corn borer caterpillars are located in the leaf junctions, shucks, and ears for about a week, but they tunnel into the plant soon after. Prior to tunneling, caterpillars can be detected and partially controlled. Scouting involves examining leaf junctions for caterpillars on the leaf below the ear leaf; one leaf per plant. Carefully pull the leaf down and away from the stalk and examine the area where the leaf and stalk meet. Caterpillars will be found in the ligule area (base of leaf), usually among collected pollen, anthers, and other debris. These sites should be examined very carefully for caterpillars about 1/4 inch or less (small caterpillars are not easily seen). Examine 10 leaves per sample, taking 10 samples per field. Also be aware of a oily-bruised appearing area on the outside (underside) of the leaf where the leaf meets the stalk.
Thresholds for Second Generation European Corn Borer
1. Egg mass threshold
* MAX = above threshold
** MIN = below threshold
2. Caterpillar threshold
Treatment is suggested when an average of 15 or more caterpillars per 100 plants (or equivalent) is found in the leaf junctions.