SOYBEAN INSECTS

Insecticide Quick Reference Chart for:  Soybean Insects

Estimating Damage

In soybeans, field scouting to assess insect populations is based on either the number of insects per foot of row, insects per plant, or the level of defoliation.

Insects per foot of row is determined by shaking plants over the inter-row space, on which a strip of cloth has been laid. Count the total number of insect pests per foot of row that fall on the cloth. If sampling a narrow row or drilled soybeans, the use of a "Texas vertical beat sheet" should be considered. The vertical beat sheet is made from a piece of galvanized metal flashing or similar stiff material, 36 inches wide, 32 inches tall and crimped at the bottom to form a collecting trough 4 inches wide. Place the device next to the row and shake the plants against the vertical surface. Insects dislodged from plants collect in the trough where they can be counted or collected.

Percent defoliation is determined by estimating the amount of leaf loss based on visual inspection of randomly selected plants.

The growth stage of the soybean plant is important. Under most conditions, moderate defoliation early in the season has little effect on final bean yield. As plants reach the flowering and pod filling stages, then defoliation poses a greater threat to yield. For example, research indicates that the soybean plant can sustain a 35 percent leaf loss prior to the pre-bloom period. From pod-set to maturity, the plant can tolerate only a 20 percent defoliation level.

ARMYWORMS

Armyworms are greenish-brown with longitudinal stripes. Full grown larvae are smooth, striped and almost hairless. Armyworms feed for three to four weeks. When full grown, larvae are 1� to 2 inches in length. Armyworm larvae have six growth stages, or instars. The armyworms final instar lasts about 10 days and they consume large amounts of plant material during that time.

Armyworms are inactive during the day, resting under plant trash, clumps of grass or lodged plants. They feed at night or on cloudy days, crawling up on plants and consuming foliage. Due to their habit of feeding at night, armyworms may go undetected until significant damage has occurred.

Armyworms do not overwinter in the region. The moths migrate from southern states in late spring and early summer. This helps explain the sporadic infestations that occur. When moths arrive, they prefer to lay their eggs in moist, shady areas, usually where grasses have lodged. Infestations that develop within soybean fields are often due to grassy weed problems.

Armyworms are more of a problem in small grains and corn. Damage to soybeans can occur the armyworms usual host plants become exhausted due to feeding or dry conditions. When their food is depleted in the hatching site, the armyworms may move in large numbers, or "armies", eating and destroying plants or crops in their path.

Threshold:

Control of armyworms is recommended when 25 to 30% of the foliage is destroyed or if significant injury to pods is evident. Most often in soybeans, infestations are due to migrating armyworms. Under these circumstances, treatment of a couple of swaths ahead of the migrating armyworms to establish a barrier strip and prevent further migration and injury.

RUP - Restricted Use Pesticide

CUTWORMS

Several cutworm species affect regional crops. The Dingy cutworm, Feltia jaculifera, overwinters as a partially grown larva and is one of the first cutworm species to cause problems during crop emergence from early to mid-May. The moth of the dingy cutworm is known to lay her eggs on sunflower heads from mid-July through September. Soybeans and other crops following sunflowers in rotation are at greatest risk of injury by this cutworm. Other cutworms, the red-backed, Exoa ochregaster, and the darksided, Exoa messoria, overwinter as eggs which hatch in mid to late May. Eggs are laid in the fall and survive in weedy, wet and reduced tillage areas. Feeding injury by these cutworms normally occurs in late May to early June.

Most damage by cutworms occurs when soybean plants are in the early stage of development. Damage consists of young plants being chewed off slightly below or at ground level. Some cutworm feeding injury may occur on foliage. Cutworms primarily feed at night. When checking soybean fields for cutworms during the day, dig down into soil an inch or two around recently damaged plants; there you can find the gray to gray-brown larva.

Threshold:

Economic thresholds for cutworm treatment decisions are not well established. Treatment guidelines used over the years include when one cutworm or more is found per 3 feet of row and the larvae are small (<3/4 inch long). Another guideline is when 20% of plants are cut or when gaps of 1 foot or more exist in the plant row. When making final decision, consider that surviving soybeans are able to compensate for early stand reductions because of the plants long growth period.

RUP -  Restricted Use Pesticide


FOLIAGE FEEDING CATERPILLARS

Green Cloverworm, Cabbage Looper, Velvetbean Caterpillar, and Thistle Caterpillar

Populations of these caterpillars have been negligible in North Dakota and little treatment to control them has been required. Sampling for these insects is accomplished through the use of a drop cloth or a vertical beat sheet, placed between two rows of plants. The larvae are dislodged from the plants and counted on the cloth or collection tray to arrive at an estimate of the number per row feet.

Green cloverworm: These caterpillars are green with two, narrow, white stripes down the side. When mature, the worms are 1 � inches long. These caterpillars have only three pairs of fleshy prolegs on the abdomen, plus the pair on the back tip. When moving, the worms move by arching the middle of the body, or "looping". Young worms scrape leaf tissue creating a transparent skin, or "window", on the leaf surface. Older clover worms eat holes in the leaves.

Cabbage looper: These caterpillars are light to dark green with lighter colored stripes, along the side and on the top, running the length of the body. When mature, the worms are 1 � inches long. These caterpillars have only two pairs of fleshy prolegs on the abdomen, plus the pair on the back tip. When moving, the caterpillars move by arching the middle of the body, or "looping". These worms feed on leaves on the interior and lower portion of the plant. As defoliation occurs, worms feed higher in the plant. Feeding injury is similar to the cloverworm.

Velvetbean caterpillar: This insect does not overwinter in the region, instead, moths migrate from southern locations. These caterpillars have dark lines bordered by lighter colored, narrower, lines running the length of the body. The background color ranges from a pale yellow-green to brown or black. These larvae have four pairs of fleshy prolegs to distinguish them from the cloverworm and the looper. Young velvetbean caterpillars feed on the underside of leaves in the upper portion of the plant. Older larvae consume the entire leaf, except for the leaf veins.

Thistle caterpillar: This insect is the larva of the butterfly known as the Painted Lady. This butterfly does not overwinter in the region, but migrates from southern locations each spring. These caterpillars are brown to black in color with yellow stripes along each side of the body. They are covered with spiny-hairs that give the caterpillar a prickly appearance. Full grown larvae are about 1 � inches long. The caterpillars feed on the leaves, webbing them together at the feeding site.

Alfalfa webworm:  These larvae are 1 inch when full grown.  They are greenish to nearly black with a light stripe that runs down the middle of the back.  There are three dark spots, each with hairs, on the side of each segment.  These larvae feed for about 3+ weeks.  Infestations are characterized by light webbing over the leaves.  Beneath the web is where the larvae feed, consuming the leaves.  These larvae move very rapidly, forward or backward, when disturbed.

Threshold:

Control of these different caterpillars is normally not warranted until greater than 30% of the foliage is destroyed prior to bloom, or when 20% of the foliage is destroyed after bloom, pod set or fill has been reached. This usually requires an average infestation of 4 to 8 larvae per row foot.

RUP - Restricted Use Pesticide

GRASSHOPPERS

In the northern plains, grasshopper egg hatch normally begins in late April to early May. Most grasshoppers emerge from eggs deposited in uncultivated ground. Soybean growers should expect to find grasshoppers feeding first along bean field margins adjacent to non-crop sites where the nymphs are hatching. Later infestations may develop when grasshopper adults migrate from harvested small grain fields. Grasshoppers will feed upon leaves and pods, chewing holes in them. A result of these migrations is soybean fields becoming sites for significant egg laying.

Threshold:

Grasshopper control is advised whenever 50 or more small nymphs per square yard can be found in adjacent, non-crop areas, or when 30 or more nymphs per square yard can be found within the field. When 20 or more adults per square yard are found in field margins or 8 to 14 adults per square yard are occurring in the crop, treatment would be justified.

Many of the grasshopper infestations in soybeans will be the heaviest on the field margins. Treating these areas may lessen the total numbers of grasshoppers successfully entering a field.

Soybeans are most sensitive to defoliation during pod development (Growth stages R4 to R6). During this time, plants can only tolerate up to 20% defoliation. Of greater concern, would be direct feeding damage to pods and seeds. Grasshoppers are able to chew directly through the pod walls and damage seed directly. If more than 5 to 10% of the pods are injured by grasshoppers, an insecticide application would be recommended.

RUP - Restricted Use Pesticide


BEAN LEAF BEETLE

This beetle can vary in color from yellow to reddish brown, and may have three to four black spots with a black border on the wing covers. Adults emerge from overwintering, moving into bean fields as the seedlings emerge. The white larvae develop in the soil, feeding on the roots and nodules. New adults emerging in August feed on foliage and pods. Feeding injury to leaves appears as small round holes between the leaf veins. Injury to pods appears as lesions similar in size and shape to leaf feeding holes. The injury to pods results in secondary infections by fungi and bacteria, causing rotting and discoloration.

Threshold:

Due to low incidence of this insect in North Dakota, no local control guidelines have been developed. Based on information from other regions where these insects are a common pest, use a sweep net to determine if bean leaf beetles are present. Treatment would be recommended when 3 to 7 beetles per sweep are found.

RUP - Restricted Use Pesticide

POTATO LEAFHOPPER

The adult is wedge shaped and pale green in color. Adults are very active, jumping or flying when disturbed. Nymphs are wingless. Both adults and nymphs run backwards or sideways rapidly when disturbed. Nymphs feed on the underside of the leaf, usually completing their growth on the leaves near where they hatched,. Large numbers of adults may appear early in the season, but there presence is dependent on migration from the eastern US.

Soybeans with moderate to dense pubescence, or plant hairs, are tolerant to leafhopper infestations. The short plant hairs form a barrier that discourages leafhoppers from feeding and ovipositing eggs on plant tissue. When feeding does occur, damage by leafhoppers is referred to as hopper-burn. Foliage becomes dwarfed, crinkled, and curled. Small triangular brown areas appear at the tips of leaves, gradually spreading around the entire leaf margin. Potential damage to soybeans by potato leafhopper is not fully understood. Damage would be more likely when drier growing conditions occur.

Threshold:

The threshold for basing spray decisions is when an average of 5 leafhoppers per plant are found in the vegetative stages, and 9 leafhoppers per plant in early bloom stages.

RUP - Restricted Use Pesticide

SOYBEAN APHID

A new aphid pest feeding on soybeans was found in the midwestern states of Michigan, Illinois, Wisconsin, Iowa, and Minnesota in late July and early August of 2000. It was confirmed that this aphid was the soybean aphid, Aphis glycines, an aphid native to Asia but never reported in the United States prior to this discovery. Soybean aphid was found in North Dakota in August, 2001. In 2002, infestations were found throughout the valley and a limited number of fields were treated in the Red River Valley

The soybean aphid is light yellow with black cornicles ("tail-pipes") and a pale colored cauda (tail projection). As with other aphids, the soybean aphid is small, about the size of a pinhead. Nymphs are smaller.

Aphids suck fluid from plants. When infestations are large, infested leaves are wilted or curled. The aphids excrete honeydew, a sweet substance that accumulates on surfaces of lower leaves and promotes the growth of sooty mold. This aphid colonizes tender leaves and branches from seedling to blooming. Later, as the growing point slows, the aphids slow their reproductive rate, move down to the middle and lower part of the plant, and feed on the undersides of leaves. Towards the end of the season the colonies begin to rapidly increase in number, again. These increases are followed by a migration to the overwintering, alternate host, buckthorn. Future observations will lead to a better understanding of what soybean aphid will do in the US.

Threshold:

RUP - Restricted Use Pesticide

SEEDCORN MAGGOT

Seedcorn maggot attack soybean seed, preventing sprouting or weakening the seedlings. The yellowish white maggot is found burrowing in the seed, emerging stem, or the cotyledon leaves. Damage to the seedlings results in a condition called "snakeheads", or plants without cotyledon leaves.

The adult flies emerge in spring when soil temperatures reach 50^o. They deposit eggs in soil where there is abundant organic matter and decaying crop residue, or on the seed or seedling. Injury from seedcorn maggots is usually most severe during wet, cold springs and in fields with high organic matter soils. When cool, wet conditions occur during planting, the slow emergence of the seedling extends the period of time it is vulnerable to feeding by the maggot.

Threshold:

When conditions are wet and cool, or when planting into high crop residue conditions, seed treatments provide the best defense against injury.

SPIDER MITES

Mites are small and magnification is required to see them. A quick sampling procedure to determine whether mites are present is to hold a piece of white paper below leaves then slap them to dislodge the mites. The mites appear as tiny dust specks, however, they will move after being knocked off the leaf. Feeding damage by mites first appears as small yellow spots ("stipples"). As feeding activity increases, leaves become yellow, bronzed, brown, and eventually shed from the plant.

Mites usually become a problem when hot, dry weather is occurs. These environmental conditions stress the plant, whether mites are present or not. If conditions continue, treating for mites is no guarantee plants will recover. In addition, products labeled for mite control often do not give adequate control and the population of mites may rebound quickly to pretreatment levels or higher. When rain and humidity are present, natural reductions in mite populations occur due to infection by a fungal pathogen. Conditions that are good for the development of the pathogen are temperatures cooler than 85, with at least 90% R.H. for 12 to 24 hours.

Threshold:

Deciding whether to treat is difficult. There is no economic threshold that has clearly been defined. Kansas State University suggests that treatments may be beneficial if significant pod or seed filling remain and leaves have not already yellowed. Plants that lose 50% of their foliage during bloom and pod set will stop producing flowering structures until favorable growing conditions resume or lost leaf area is replaced. If mite injury is evident within the interior of the field, and hot, dry conditions continue, the potential exists for economic populations throughout the field in 1 to 2 weeks.

RUP - Restricted use pesticide