Plathypena scabra (soybean)
Authors: Buyung Hadi, Jeffrey Bradshaw, Tom Hunt, Jan Knodel, Ken Ostlie, Robert J. Whitworth, J. P. Michaud and Phillip E. Sloderbeck
The green cloverworm, Plathypena scabra, is a migratory moth common in the eastern half of the United States.
Fully grown larvae of the green cloverworm is about 1 inch (25 mm) long, pale green with two horizontal white stripes running along each sides of the body. The larva has three pairs of prolegs (fleshy legs apart from the three pairs of true legs near the head) in the middle of the body and a pair of proleg in the hind end of the body. Unlike other soybean caterpillars, green cloverworm larvae wiggle vigorously when disturbed.
Adult moths of green cloverworm are triangular when at rest. The wingspan is about 1 inch (25 mm) long. The female moths have charcoal-colored wings with brown and silver patches, while the male moths' wings are more uniformly charcoal in color. The male moths have larger body and eyes size than the female moths.
Life Cycle and Seasonal History
The green cloverworm can overwinter as far north as 41°N latitude (roughly around Omaha, NE) but ’reservoir’ populations breed year-round along the Gulf Coast and produce migrants that move northward each spring. In Kansas and southern Nebraska, the green cloverworm may overwinter as a pupa in leaf litter and crop debris.
In northern states, migratory moths usually arrive in early June. Mated females lay eggs singly on the underside of soybean leaves. After 3-4 days, the eggs hatch and the larvae start to feed on the leaves. The larvae develop through 6 instars (stages) in about 14 days, with most leaf area consumed when the larvae are in the final instars. Mature larvae burrow into the soil or plant debris to pupate and emerge as adults in 7-10 days to repeat the cycle. Two generations generally occur in northern states, but more are possible in southern portions of its range .
Populations of green cloverworm are normally regulated by predators, parasitoids and pathogens. In outbreak years, the fungal pathogen Nomuraea rileyi can reach epizootic levels and induce population collapse. Warm and humid conditions favor the development of this fungal disease on grasshoppers.
Plant Injury and Damage
Although the green cloverworm is is ubiquitous and continuously present in soybean fields across the Northern Plains, it only rarely reaches economic threshold. Outbreak years are usually associated with large numbers of migrants from southern states.
The larvae feed on leaf tissues between the main veins. Early instar larvae feed only from the lower layers of cells creating transparent ‘windows’ on the leaf surface, whereas larger larvae produce holes in the leaves. A single larva is estimated to consume 8.5 inch2 (54.9 cm2) of soybean leaf in the course of its development.
Scouting and Threshold
Since green cloverworm is an occasional pest of soybean, regular population monitoring is essential in making management decisions. Two types of economic threshold can be used: one based on a population estimate of larvae and the other based on an estimate of defoliation.
Population sampling is accomplished using the ground cloth technique. The equipment consists of an off-white cloth measuring 36 x 42 inches (0.91 x 1 m) with strips of wood, approximately 1/2 x 1 inch (1.27 x 2.54 cm) wide, stapled to each long side of the cloth. To conduct the sampling, roll the cloth beneath the canopy from one row over to the next row without disturbing the foliage. Vigorously shake the plants from both rows over the cloth for 30 seconds and then count the green cloverworm larvae collected on the cloth. In this manner, two 3-row feet (0.91 row meters)sections (6 feet, or 1.82 m, total) are sampled. Replicate this procedure at four or five sites distributed across the field and calculate the average number of larvae per row-ft. The economic threshold is considered to be 5.2 larvae per row-ft (17 larvae per row-m) between bloom and pod-fill.
Treatment is also recommended if defoliation reaches 40% pre-bloom, 20% during bloom and pod-fill, or 35% from pod-fill to harvest. Defoliation due to green cloverworm may be summed with the damage inflicted by other defoliating caterpillars in making a management decision. Little will be gained from treatment if the majority of larvae are approaching ¾ inch in length as they will be about to pupate.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com