Numerous soybean fields located in southwest Missouri are experiencing problems with green cloverworm larvae. This insect is traditionally a rare to occasional pest in Missouri, where it is usually controlled by the beneficial fungal pathogen Nomuraea rileyi. The moths of this insect may overwinter in more southern areas of the state, but most migrate into Missouri during spring each year. Two generations are produced annually with both generations susceptible to the fungal pathogen if conditions are favorable. Larvae feed on leaf tissue, but not on pods. Larvae killed by the fungal pathogen often take a posture of rearing up off the leaf and then turn white in color as the fungal pathogen produces white fruiting bodies which will blow in the wind to infest additional larvae.
Control is justified when foliar damage reaches or exceeds 20 percent and five larvae or more are present per foot of row. Larvae are most easily monitored using a ground sheet measuring 36” x 42” with a stick wound around each edge on the 42-inch sides. The sticks are used to twist the ground cloth to a size that fits the area between rows. Although you calculate the number of larvae per foot of row, generally one to three feet of row on both sides of the ground cloth are vigorously shaken to dislodge larvae and allow them to drop onto the cloth for collection and counting. Divide the total number of larvae collected per location by the number of feet of row of soybean shaken to determine number of larvae per foot. A minimum of 5 different locations within the field should be sampled to determine a reliable estimate of larvae numbers per foot of row.
Drought may be the contributing factor allowing the high numbers of green cloverworms observed this year. The fungal pathogen is always present at low levels in the soil, but works best when precipitation is present and field conditions are somewhat damp or wet.