The soybean crop in Southwest Louisiana is currently in various stages of development. Some fields are well into the reproductive stages (flowering through pod fill). Other fields are still in the vegetative or growth stages. This variation is due to the bizarre weather that we experienced this early season. Drought caused some fields to be planted late. Others emerged slowly due to the dry soil conditions. Regardless of their stage of development, all are susceptible to insect damage. Now begins the peak season for insect pests. One group, the caterpillars, are all too common in our area.
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 percent defoliation. From bloom through pod set foliage loss should not exceed 20-25 percent.
Probably the most frequently treated foliage feeding pest in our area is the velvetbean caterpillar. The velvetbean caterpillar usually becomes an economic factor around the end of August or the beginning of September. They frequently appear in economically significant numbers around 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. They tend to appear in soybean fields earlier in the season than the velvetbean caterpillar, but can be a problem anytime soybeans have productive leaves. Loopers are often present in fields that were treated for other insect pests during the vegetative stages. Whenever we treat for insect pests, we also reduce the number of beneficial insects that feed on the pests. 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, Belt and Intrepid are recommended for its control.
The corn earworm is another caterpillar pest. This “worm” 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. It made an appearance and caused damage sufficient to require treatment in several soybean fields earlier this year. 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 R 6.5). Before applying any pesticide, always read and then follow all label directions and restrictions.