Soybean Looper:
Biology and Approaches for Improved Management

The soybean looper, Pseudoplusia includens (Lepidoptera: Noctuidae), is a common pest of soybeans in Central America and most of North and South America. Even though it is found in many states, most economic infestations occur in the Gulf Coast and South Atlantic States. In Mississippi heaviest populations of loopers are most frequently encountered in August and September.

The cabbage looper, Trichoplusia ni, is similar in appearance and may be confused with the soybean looper. The cabbage looper is generally more susceptible to insecticides than is the soybean looper. Surveys in past years in Mississippi indicate more than 90 percent of all loopers attacking soybeans are the soybean looper.

Damage

Soybean looper larvae feed on leaf tissue of the soybean plant. A single larva can consume 22 square inches (145 square centimeters) of leaf tissue, with 90 percent consumed during the last two larval stages. The preferred feeding site is in the lower one-half to two-thirds of the crop canopy although heavy infestations may lead to defoliation of the entire canopy.

Host Plants

The soybean looper has a range of host plants involving members of 28 plant families that include agricultural, vegetable, and floricultural crops. Some of the most common host plants are listed in Table 1.

Table 1. Common Host Plants of the Soybean Looper

Scouting: Base control decisions on random samples to determine the number of loopers per foot of row and foliage loss for a field. The drop cloth is one of two methods used to survey fields; the second method is the sweep net (Figure 3).

Biology

• Overwinters in United States only in southern Florida and southern Texas.
• Each year moths are believed to immigrate to the United States from Central America, South America, and the islands of the Caribbean.
• Female moths deposit an average of 640 eggs; egg production may be enhanced following female moths' feeding in nectaries of cotton.
• The life cycle includes egg, larva, pupa, and adult stages.
• The life cycle is completed in about 28 days. The larval stage requires about 16 days.
• Larvae spin loose cocoons and attach them to host plants for pupation.

Identification

Only the larval stage of the soybean looper is destructive. Larvae are green with longitudinal white stripes, one along each side of the body and two down the back. The body is thick and tapers toward the head. Larvae have three pairs of abdominal prolegs located on segments 5, 6, and 10 (Figure 1) and crawl with a looping motion.

Separating the Larval Stage of Soybean and Cabbage Looper

These two species are identified by examining mandibular characteristics. Soybean looper larvae have mandibles with ribs terminating in an enlargement near the outer margins; in the cabbage looper, the ribs extend to the outer margins of the mandibles (Figure 2).

Tentative field identifications can be made on the basis of color of the true legs and presence of black spots on the body. Soybean looper larvae sometimes have black true legs and often have black spots on the body. Cabbage looper larvae generally have green true legs and few or no black spots (Figure 1).

Control Failures

Soybean producers in several states in the southeastern United States reported insecticide-control failures for the soybean looper during 1986-1987. Damage to the soybean crop by this insect was excessive in many areas in these states, and control failures were frequently reported to be associated with the use of pyrethroid insecticides.

Laboratory studies with populations of insects from areas of known insecticide-control failures confirmed that these populations of soybean loopers contained high frequencies of pyrethroid-resistant individuals and possessed measurable levels of resistance to some of the other recommended insecticides. Again in 1988 and 1989, the soybean looper was an economic pest on soybeans in most of the same areas having experienced insecticide-control failures in 1986 and 1987.

Management Approaches

Scouting

Monitor soybean fields weekly for insect infestations. The drop cloth and/or sweep net can be used to survey fields. When monitoring a field, select samples at random throughout the field (Figure 3). Integrated pest management (IPM) decisions depend on detection and identification of pest species, use of economic thresholds established through research, and, when needed, use of carefully selected insecticides at recommended rates.

Survey Conversions
Drop Cloth
8 loopers ½-inch or longer per foot of row	or	150 loopers in 100 sweeps
4 loopers ½-inch or longer per foot of row	or	75 loopers in 100 sweeps

Factors to consider in control decisions include larval size and number per foot of row, defoliation level, presence of natural enemies, crop maturity, and weather. Small larvae are often killed by predators and parasitoids before economic losses can occur from defoliation. These factors influence product choice and cost of control when an insecticide is required.

Economic Thresholds

Follow economic thresholds established through research. However, management decisions should consider price of the crop, pesticide control cost, maturity of the crop, yield capability, and diversity of pest species.

Plants not blooming or filling pods: If bean plants are not blooming or filling pods and no diseased worms are present, apply insecticide when eight or more worms ½-inch long or longer are present per foot of row or when 35 percent foliage loss has occurred and worms ½-inch long or longer are present.

Plants blooming or filling pods: If bean plants are blooming or filling pods and no diseased worms are present, apply insecticide when four or more worms ½-inch long or longer are present per foot of row, or when 20 percent foliage loss has occurred and worms ½-inch long or longer are present.

Maturity Thresholds

If possible, avoid late planting of some varieties of soybeans. Early plantings may escape high insect infestation and yield better. Generally, maturity groups V, VI, and VII are grown at potentially greater risk for higher insect pest infestations.

Host Plant Resistance

At present no commercial soybean varieties are resistant to soybean looper. However, a new variety, Lamar, has some resistance to this pest and others. Research at the Delta Branch Experiment Station, Stoneville, shows this variety has resistance to soybean looper, velvetbean caterpillar, green cloverworm, armyworms, and podworms. Yield characteristics of this group VI variety are competitive.

Spray Volume Coverage

For improved coverage, the total volume of water applied should be satisfactory for coverage and meet pesticide label requirements. The most common range of volumes is 3 to 5 gallons of water per acre by air and 15 to 18 gallons per acre by ground equipment. Dense crop canopies may require higher water volume. Oil and penetrating compounds may improve performance of some recommended insecticides.

Rate of Insecticide

Use recommended rates of insecticides listed in the Soybean Insect Control Guide (MSU-ES Publication 883). You may need to increase rates in some field situations, but do not exceed the maximum labeled rate. The use of rates higher than recommended may add increased production costs while not significantly improving control and may influence resistance problems.

Timing of Application

Use recommended scouting procedures and economic thresholds to determine when infestations should be sprayed. Spraying too late may lead to poor control and increased production costs when larvae are large and near maturity.

Treatment Follow-up

Allow about 72 hours (or adequate time according to the pesticide label) after treatment before checking a field to determine performance of most insecticides. Early observations may provide inaccurate information that could result in insecticide treatments that may not be needed.

Product Performance

Some products may have special characteristics to consider along with cost versus yield-loss potential. These factors include residual activity period, multiple species control capability, ovicidal activity, speed of action, residue following rainfall, and alternative mode of action for control (e.g., biological and insect growth regulator formulations).

Tolerance to Insecticide

There may be differences in effectiveness of insecticides in some areas compared with other areas. Control of various field populations may be affected by migratory insects or variations of infield, late-season generations. If all management steps discussed are followed and looper control is inadequate, it is advisable to change insecticides or consider best combinations of products.

Table 2. Control recommendations.

Revised and distributed by Dr. Scott Stewart, Entomology Specialist/Scientist, Department of Entomology and Plant Pathology.

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Information Sheet 1400
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

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