SOYBEAN INSECT MANAGEMENT

Variety selection/Cultural Practices

Currently available varieties of soybeans differ in growth characteristics and the time required for maturity. Variety characteristics can affect susceptibility to insect injury. For example, early-maturing varieties are less likely to be seriously damaged by soybean loopers or velvetbean caterpillars because they often mature before late-season generations of the pests occur. Also, varieties with little pubescence (hairs) on the undersides of leaves are susceptible to potato leafhopper infestations.

Maturity differences can be used to manage some insect pests. For example, planting about 5 percent of the soybean acreage in an area 10 to 14 days earlier than the remainder of the crop will concentrate overwintering bean leaf beetles into these earlier plantings. The early-planted soybeans serve as a trap crop for the adults, and a relatively small amount of insecticide can then be used to prevent their spread into later-planted soybeans. If early-maturing varieties are planted as the trap crop, they will also act as a trap crop for stink bugs during pod development.

Soybeans that do not have a closed canopy at the time of bloom, as often occurs in late plantings and wider row spacings, are more susceptible to bollworm infestations. No-till soybeans are at greater risk to cutworm damage than conventionally tilled soybeans.

The performance of many soybean varieties is tested every year in Mississippi at several locations. The information is published annually as a Mississippi Agricultural & Forestry Experiment Station (MAFES) Information Bulletin – Soybean Variety Trials.

Biological Control

Diseases – In mid- to late-season, naturally occurring diseases (fungi, bacteria, and viruses) of soybean insect pests can be important in control. A full leaf canopy, along with certain environmental conditions, apparently produces a microclimate favorable for insect disease development. Diseases often control armyworms, velvetbean caterpillars, green cloverworms, and soybean loopers. After diseased larvae have died, they may have a whitish mold-like growth covering their body surface, a black coloration with their bodies filled with fluid, or a near normal appearance (depending on the disease).

The presence of diseased worms indicates the population is being reduced naturally. When you find diseased larvae, withhold treatment for a few days to see if the disease will spread to a level that can control the population.

Predators and Parasites – Beneficial predators and parasites are very important in reducing the number of early-season insect pests. For this reason you should protect them to have their full benefit. Predators and parasites can often keep pests from reaching treatable levels. Some early-season insecticide applications to soybeans can severely reduce predators and parasites. Regular scouting of fields is essential in detecting insect pests as well as beneficials.

Sampling for soybean Insects

To minimize yield loss from insect pests attacking soybeans, you should sample fields at least once per week from emergence through maturity.

There are several ways to sample soybeans for insect pests. The ground cloth and the sweep net are the two primary tools. Information you get by using either one of these sampling methods should be supplemented by visual examinations of plants for damage or insects.

Ground cloth – The ground cloth is the most accurate method for sampling insect pests in soybeans. A ground cloth is made of heavy white cloth 3 feet long on each side with a half-inch to three-fourths inch dowel rod attached to each side. To use the ground cloth, you unroll it flat be- tween two rows, then bend the plants on either side over the cloth, and shake them vigorously. The dislodged insects fall onto the cloth, where you can easily count them. You should count any insect that has fallen at the base of the plant to the soil surface. This gives the number of insects per 6 feet of row (3 feet on each side of the cloth). Dividing by 6 gives the number of insects per foot of row.

Most soybean producers in Mississippi have changed their production practice from wide-row to narrow-row or drilled soybeans. Soybeans planted on narrow rows are difficult to sample using a ground cloth. In narrow-row soybeans, a sweep net is the preferred method for sampling.
When to Apply Insecticides for stem Feeders

Sweep net – A sweep net is a heavy cloth or canvas net on a strong 15-inch diameter steel hoop attached to a 3-foot wooden handle. To use it, you walk parallel to a row and swing the net briskly through the top third of the foliage. Each pass of the net through the foliage counts as one sweep and should be made 21⁄ to 3 feet apart down the row. Be sure to hold the net at an angle that lets dislodged insects fall into the net bag, and pass the net completely through the row. In soybeans planted on 36-inch rows or wider, sweep only one row. In narrow-row soybeans, let the normal arch of a sweep continue through the adjacent row. Then count insects as they are picked or fly from the net. Counts are usually expressed as number per 25 or 100 sweeps.

The three most common stem-feeding pests are lesser cornstalk borers, cutworms, and three-cornered alfalfa hoppers. Apply insecticide from plant emergence to 10 inches in height when plant stand is being reduced below recommended plant populations. Use Table 1 on page 33 to deter- mine best plant populations for soybeans grown in Mississippi.

Classes of insecticides: Effective resistance management requires rotation among the various classes of available insecticide chemistry. Often when one insecticide in a class fails because of insecticide resistance, other insecticides in the same class will also be ineffective. Selection of an insecticide from a different class will improve the chances of obtaining control. Growers need to be very aware of the type of insecticide chemistry being used. Classes of insecticides recommended in this guide are identified by the following abbreviations:
When to Apply Insecticides for Foliage Feeders

Soybean plants can withstand as much as 35 percent foliage loss up to the blooming period. During blooming and when pods begin to form and fill out, any foliage loss of more than 20 percent will decrease yield. After the soybeans are mature and pods have fully expanded, a 35 percent loss of foliage will not usually reduce yield. Once fruiting begins, the soybean plant does not add new leaves, although existing leaves may expand. If plants are near the fruiting stage, don’t let more foliage be removed if that will cause total defoliation to be more than 20 percent in pod-set or pod-filling.

It requires four or more foliage-feeding larvae one-half inch long or longer per foot of row to cause 20 percent defoliation. It requires eight or more foliage-feeding larvae one-half inch long or longer per foot of row to cause 35 percent defoliation. Apply insecticides when larval populations are at or above the number required to cause defoliation levels listed for the developmental stage of the plants. Apply insecticide if these defoliation levels have already occurred and larvae are still present.

Often several species of foliage-feeding caterpillars will be in a field at the same time. When several species of foliage-feeding caterpillars are present, treatment is necessary if any combination of foliage-feeding caterpillars meets or exceeds the threshold. Foliage-feeding caterpillars such as loopers, velvetbean caterpillars, and green cloverworms consume roughly the same amount of foliage per caterpillar regardless of species. However, the sweep net conversion ratio is about two times higher for velvetbean caterpillars and green cloverworms than for loopers because they are dislodged from the plant easier than loopers, making the catch efficiency of the sweep net greater for these two pests. Be- cause of this, for a complex of foliage-feeding caterpillars, use a threshold of 300 caterpillars/100 sweeps before bloom, counting each looper twice, and 150 caterpillars/100 sweeps after bloom, counting each looper twice.

How to Determine Soybean Growth Stage

Concentrate on the youngest (upper) four nodes to determine soybean growth stage. Estimate the average growth stage for the reproductive growth (flowers and pods) on the youngest four nodes. Begin with the youngest fully expanded leaf (see picture 1 right), and estimate growth stage down the next three nodes (see picture 2 below).

Indeterminate varieties (most group 4 varieties and a few group 5 varieties) start reproductive growth toward the bottom of the plant. The reproductive growth progresses from the bottom of the plant upward as the plant produces more nodes.

Determinate varieties (most of our group 5 varieties) start reproductive growth uniformly up and down the main stem.

Another way of thinking about this is that it is common to see pods in the R5 growth stage at the bottom of plants and new flowers and pods at the top of indeterminate growth plants. Determinate growth plants have same-sized flowers and pods that grow uniformly up and down the stem.