[Image: Soybeans]Soybean Caterpillar Pests- foliage feeding caterpillars can pose a real threat to soybean plants. They feed on the soybean leaves and reduce the ability of the plant to produce food and fill the pods. Sometimes, they can feed directly on the pods themselves. For maximum production, the soybean plant needs to maintain as many healthy leaves throughout the pod fill period as possible. Prior to bloom, soybean plants can tolerate 30-35% defoliation. From bloom through pod set foliage loss should not exceed 20-25%.
We are approaching the time of season that one of the most common foliage feeding caterpillars reaches its peak populations. The velvetbean caterpillar usually becomes an economic factor around the end of August or the beginning of September. I have often heard that the velvetbean caterpillar likes to come to work on Labor Day. This caterpillar has four sets of fleshy pro-legs located toward the end of its body. It feeds throughout the canopy of the soybean plant. The threshold for treatment for the velvetbean caterpillar is eight worms, ½ inch long or longer, per foot of row or 300 worms per 100 sweeps of a sweep net. That sounds like a lot of worms, and it is. But, this caterpillar tends to be present in very large numbers. There are a large number of effective insecticides labeled for the control of the velvetbean caterpillar. Dimilin is a very effective preventative for velvetbean caterpillars. If applied before the velvetbean caterpillars hatch, (usually when soybean plants are in bloom) it will prevent them from growing and developing. It is not effective on those caterpillars that have reached feeding size. If you are seeing velvetbean caterpillars large enough to feed, you must use another labeled insecticide for their control.
Loopers are another foliage feeding caterpillar. These are less common in southwest Louisiana, but can cause real problems when present. The looper has two sets of fleshy pro-legs and moves in a “looping” gait. They tend to feed low in the canopy. Because of this feeding habit, the threshold for treatment is 150 worms per 100 sweeps of a sweep net. The list of recommended insecticides is much shorter for this pest. Larvin, Lannate, Tracer, Steward and Intrepid are recommended for its control.
I have received reports of corn earworms being present in soybean fields this season. This pest is much less common than velvetbean caterpillars or loopers. It is unique in that it feeds directly on the pod. It eats the part of the plant that the farmer harvests. The corn earworm has four pairs of pro-legs and has tufts of bristles located along its body. The threshold for corn earworm treatment is 38 worms per 100 sweeps of a sweep net.
There are three other species of caterpillars that we sometimes find in soybean fields. The armyworm is most common in seedling soybean plants or along the field margins of maturing fields. This large caterpillar also has four pairs of fleshy pro-legs and it has an inverted Y in its face. Armyworms prefer to feed on grasses, but will eat soybean plants when the grass dies or becomes too mature to be palatable. The green cloverworm has three pairs of pro-legs and is similar to the velvetbean caterpillar in its feeding habits, treatment threshold and recommended insecticides for its control. The salt marsh caterpillar is easy to identify. It is the only worm in this group that is covered with long, thick hair. It comes in a variety of colors, from buff to nearly black. It is also sometimes found in seedling soybeans, but can be present and feeding on foliage of maturing plants.
Continue to scout for caterpillars on a regular basis until the soybean pods have filled and reached maturity (maturity stage R6.5). Before applying any pesticide, always read and then follow all label directions and restrictions.
I’d like to shift gears and talk about a common horticultural problem:
Sooty Mold-is just what it sounds like. It is a black, sooty material that often covers the leaves of plants, especially citrus trees and ornamental species. I have recently seen it growing on shade trees. When rubbed with a cloth or your finger, the sooty material will come off. This is an obvious condition that is easily observed, but doesn’t cause a great deal of harm to the plant. The fungus or mold can block sunlight if it becomes thick enough, but is not parasitic to the plant itself. What causes the sooty mold to grow is harmful to the plant. These are white flies, scale insects, mealy bugs and aphids. All these insects feed on plant juice. They have piercing-sucking mouth parts. They can be present in the tens of thousands on a single plant. They literally suck the juice and life out of the plant. The sooty mold grows on the plant juice that leaks onto the leaf and on the waste of these insects. So, while the sooty mold isn’t very harmful, it does alert you that there is another pest present that can harm your plant. Control the white flies, aphids, mealy bugs, spider mites and scale insects and the sooty mold will, in a short time, disappear from the leaves.
Ultra-fine oil emulsion spray can be used on citrus trees and many ornamental plants for control of these insects. Use care in very hot weather. The possible leaf burn the ultra-fine oil emulsion spray may cause is usually less harmful than the insects that it controls. Leaf burn is not a certainty. If it concerns you, make your application in the late afternoon, when the sun is not directly on the plant. Spring, early summer and fall applications are very effective and these applications seldom produce leaf burn. They also are effective in reducing insect populations before they reach the high levels of late summer (which is now).
Be sure to follow the label directions and restrictions when using any pesticide. Also make sure that you have adequate spray coverage of the entire plant: leaves (top and bottom sides), stems and trunk. The oil emulsion spray only smothers the insects that you cover it with.