List of Top 10 Insect Pests and their Management in Beans Production

The most problematic insect pests of dry edible beans are potato leafhoppers, sweetcorn maggots, and grasshoppers. Spider mites also are damaging to dry edible beans during hot, dry conditions, which favor mite populations.

Occasionally, various caterpillars infest beans and can cause severe defoliation. For example, thistle caterpillar infestations defoliate young plants, particularly in areas of fields where Canadian thistle has been treated with herbicides recently. This may prompt an insecticide treatment to reduce the economic densities of caterpillars.

Estimating Insect Feeding Injury

In dry edible beans, field scouting to assess insect populations is based on the number of insects per foot of row, the number of insects per plant, or the level of defoliation. Scout a minimum of five sampling sites per field by walking an “M” pattern throughout the field, and examining 10 or more plants per sampling site for insect pests or defoliation levels.

Insects per foot of row are determined by shaking plants over the row space on which a strip of cloth has been laid. Count the total number of insect pests per foot of row that falls on the cloth.

The percent of defoliation is estimated visually by the amount of leaf loss on the lower, middle, and upper canopy on randomly selected plants throughout the field and by calculating average defoliation from the whole plant.

The growth stage of the 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, defoliation poses a greater threat to yield and seed quality.

Photographs at the end of this publication provide examples to identify insects and their damage.

Table 17. Growing season calendar indicating time of occurrence of dry edible bean insect pests.

List of Top 10 Insect Pests and their Management in Beans Production

 Below is the list of top 10 insect pests and their management in beans production;

1. Bean Aphid

(Hemiptera: Aphidae: Aphis fabae Scopoli)

The bean aphid has not been a major pest in North Dakota and Minnesota. It is nearly black. Bean aphids feed along stems and on the underside of leaves. Aphids feed by sucking sap from plants, causing stunting and curling of leaves.

Infestations may result in a buildup of honeydew on leaf surfaces, promoting the growth of a black, sooty fungus. Bean aphids also vector the bean common mosaic virus and bean yellow mosaic virus.

Threshold

No economic threshold has been established in our area. In other areas of dry bean production, an average of 30 or more aphids per plant is recommended as the action threshold.

2. Armyworm

(Lepidoptera: Noctuidae: Mythimna unipuncta [Haworth])

Armyworms are greenish brown with pale and dark longitudinal stripes. Fully grown larvae are smooth, striped, and almost hairless.

Armyworms feed for three to four weeks. When fully grown, larvae are 1½ to 2 inches long. Armyworm larvae have six growth stages or instars. The 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 and 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. 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 in bean fields often are due to grassy weed problems.

Armyworms are more of a problem in small grains and corn. Damage to dry beans can occur when the armyworms’ usual host plants become exhausted due to feeding or dry conditions. When their food is depleted in the hatching site, 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 a significant injury to pods is evident, or if you find an average of four or more armyworm larvae per row foot.

3. Bean Leaf Beetle

(Coleoptera: Chrysomelidae: Cerotoma trifurcata [Förster])

Bean leaf beetles are uncommon in North Dakota and Minnesota. This beetle can vary from yellow to reddish brown, and usually has four spots with a central triangle and a black border on the wing covers.

Adults emerge from overwintering sites and move into bean fields as the seedlings emerge. The white larvae develop in the soil, feeding on the roots and nodules.

Feeding injury to leaves appears as small, round holes between the leaf veins. New adults emerge in late July and August and feed on foliage and pods causing defoliation.

Defoliation appears as small round holes between the leaf veins. The chewing injury on pods results in scarring and secondary infections by fungi and bacteria, causing rotting and discoloration. Pod injury results in reduced yield and seed quality when populations of bean leaf beetle reach economic thresholds.

Threshold

No local control guidelines have been developed. However, treatment is recommended at 50% defoliation at the seedling stage, 25% defoliation at the pod set/filling stages, or 10% of the pods are damaged.

4. Cutworms

(Lepidoptera: Noctuidae)

Several cutworm species affect regional crops. The dingy cutworm, Feltia jaculifera (Guenée), 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.

Dingy cutworm females lay eggs on sunflower heads from mid-July through September. Dry beans and other crops following sunflower in rotation are at the greatest risk of injury by dingy cutworm.

Red-backed cutworm, Euxoa ochrogaster (Guenée), and the dark-sided cutworm, Euxoa messoria (Harris), overwinter as eggs, which hatch in mid to late May. Eggs are laid in the fall and survive in weedy and reduced-tillage areas. Feeding injury by these cutworms normally occurs in late May to early June.

Most of the damage by cutworms occurs when bean plants are in the early stages of development. Damage consists of young plants being chewed off slightly below or at ground level. Some cutworm feeding injuries may occur on foliage.

Cutworms primarily feed at night. When checking bean fields for cutworms during the day, dig down into soil an inch or two around recently damaged or wilted plants to find the gray to brown larva.

Threshold

Treatment is warranted when one or more cutworms are found per 3 feet of row and the larvae are small (less than ¾ inch long), or 5% of the plants are damaged by cutworm7s.

Read Also: Pests of Stored Products and Damages Caused

5. Foliage-feeding Caterpillars

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

These caterpillars (larvae) are sporadic and populations typically are low in North Dakota and Minnesota, with the minimal treatment necessary. To find these caterpillars that blend in with the bean foliage, place a drop cloth between two rows of plants and vigorously shake plants over the cloth to dislodge caterpillars from the plants. Then count the number of larvae to arrive at an estimate of the number per row of feet.

However, the threshold used for all foliage-feeding caterpillars is based on defoliation. When estimating defoliation, remember to look at the lower, middle, and upper canopy of foliage and calculate an average for the whole plant. Foliage-feeding caterpillars often caused the most damage only to the upper foliage of beans.

■ Green Cloverworm (Lepidoptera: Erebidae: Hypena scabra [Fabricius])

These caterpillars are green with two narrow, white stripes down the side. When mature, the caterpillars are 1¼ inches long. These caterpillars have only three pairs of fleshy prolegs on the abdomen, plus the pair on the posterior end.

Green clover worm caterpillars 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 (Noctuidae: Trichoplusia ni [Hübner])

These caterpillars are light to dark green, with lighter-colored stripes along the sides and top running the length of the body. When mature, the caterpillars are 1½ inches long. These caterpillars have only two pairs of fleshy prolegs on the abdomen, plus the pair on the posterior end.

Cabbage loopers move by arching the middle of the body, or “looping.” Cabbage loopers feed on leaves on the interior and lower portion of the plant. As defoliation occurs, caterpillars feed higher in the plant. Feeding injury is similar to what the green clover worm causes.

■ Velvetbean Caterpillar (Noctuidae: Anticarsia gemmatalis Hübner)

This insect does not overwinter in our area. Instead, moths migrate from southern locations.

These caterpillars have dark lines bordered by light-colored, narrower lines running the length of the body. The background color ranges from pale yellow-green to brown or black. Mature larvae are about 1 inch in length.

Velvet bean caterpillars have four pairs of fleshy prolegs, which help distinguish them from green clover worms and cabbage looper caterpillars. Young velvet bean caterpillars feed on the underside of the leaves in the upper portion of the plant. Older larvae consume the entire leaf except for the leaf veins.

■ Thistle Caterpillar (Nymphalidae: Vanessa cardui [Linnaeus])

This caterpillar is the larva of the Painted Lady butterfly. This butterfly does not overwinter in the region but migrates from southern locations each spring.

Thistle caterpillars are brown to black, with yellow stripes along each side of the body. They are covered with scoli (fleshy structures) that give the caterpillar a prickly appearance. Fully grown larvae are about 1½ inches long. The caterpillars feed on the leaves, webbing them together at the feeding site.

■ Alfalfa Webworm (Crambidae: Loxostege cereralis [Zeller])

These caterpillars are 1 inch long when fully grown and feed for about three weeks. They are greenish to nearly black, with a light stripe that runs down the middle of the back. They have three dark spots, each with hairs, on the side of each segment.

Infestations are characterized by light webbing over the leaves. The larvae feed beneath the web, consuming the leaves. These larvae move very rapidly, forward or backward when disturbed.

The threshold for foliage-feeding caterpillars: Control of these different caterpillars normally is not warranted until greater than 25% to 30% of the foliage is destroyed. This usually requires an average infestation of 10 to 15 larvae per foot of row.

6. Grasshoppers

(Orthoptera: Acrididae)

In the northern Plains, the grasshopper egg hatch normally begins in late April to early May. Most grasshoppers emerge from eggs deposited in the uncultivated ground.

Bean growers should expect to find grasshoppers feeding first along bean field margins adjacent to these sites. Later infestations may develop when grasshopper adults migrate from harvested small-grain fields.

Grasshoppers will attack leaves and pods. Due to these migrations, bean fields become sites for significant egg laying.

Thresholds

“Threatening” is considered the action threshold for grasshoppers. Because estimating the number of grasshoppers per square yard is difficult when population densities are high, pest managers can use four 180-degree sweeps with a 15-inch sweep net, which is equivalent to the number of adults (or nymph) grasshoppers per square yard.

Table 18. Grasshopper rating and number of nymphs and adults.

7. Potato Leafhopper

(Hemiptera: Cicadellidae: Empoasca fabae [Harris])

Potato leafhoppers are a key pest of dry edible beans. They do not overwinter in our area. Adult leafhoppers migrate from southeastern states during the spring and early summer.

Large numbers of adults may appear early in the season if weather conditions favor their migration into North Dakota and Minnesota. Adult potato leafhoppers can move from cut alfalfa fields, a preferred host, to dry bean fields quickly.

The adult is ¹/₈ inch long, wedge-shaped, and pale green. Adults are very active, jumping or flying when disturbed. Nymphs resemble adults but are smaller and wingless. Adults and nymphs will run backward or sideways rapidly.

Females lay eggs inside the stem. Nymphs hatch from the eggs in seven to 10 days and usually complete their growth in two to three weeks on the leaf where they hatched, feeding on the underside of the leaf. Because nymphs are not as mobile as adults, they are regarded as the more damaging life stage of the bean plant.

Adults and nymphs feed on plant sap and inject toxic saliva during feeding. Foliage becomes dwarfed, crinkled, and curled. Small triangular brown areas appear at the tips of leaves, gradually spreading around the entire leaf margin. This feeding injury caused by leafhoppers is referred to as “hopper burn.”

Scouting should begin in early June. Examine five sets of 20 trifoliolate leaves on randomly selected plants throughout the field. Potato leafhoppers typically are found on the underside of leaves.

When counting leafhoppers, cup your hands under the leaves and leafhoppers will move to the upper leaf surface for easier counting. Count and record adults and nymphs per trifoliolate leaf and calculate an average number of leafhoppers (adult and nymph) per trifoliolate leaf.

Threshold

The threshold for spray decisions is an average of 0.5 leafhoppers per plant in unifoliolate stage beans. On plants with one or more fully expanded trifoliate leaves, treat when one leafhopper per trifoliolate leaf is found. Do not let infestations and damage progress to the point that yellowing of foliage is observed easily.

8. Seedcorn Maggot

(Diptera: Anthomyiidae: Delia platura [Meigen])

Larvae of the seedcorn maggot attack bean seeds, which weakens seedlings and even may prevent germination. The yellowish-white maggot is found burrowing in the seed or emerging stem.

Flies emerge in the spring when soil temperatures reach 50 F. They deposit eggs in soil with abundant organic matter and decaying crop residue, or on the seed or seedling.

Seedcorn maggots are usually most severe in wet, cold springs and on high organic-matter soils. Tillage performed just before planting has been shown to attract flies to these sites for egg laying.

Thresholds

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

9. Two-spotted Spider Mites

(Acari: Tetranychidae: Tetranychus urticae Koch)

Spider mites are small, and a hand lens is required to see them. A quick sampling procedure to determine whether mites are present is to hold a piece of white paper below the leaves and gently beat them to dislodge the mites. The mites appear as tiny dust specks; however, they will move after being knocked off the leaf.

Mites feed on the individual plant cells, removing the green chloroplast, which results in small yellow spots (stippling) on leaves. As feeding activity increases, leaves become yellow, bronzed, or brown and eventually shed from the plant. Mite webbing may be present on plants as mites balloon on webs to disperse among plants and between fields.

Mites usually become a problem when hot, dry weather occurs. Infestations typically are noted first near field edges. Dry conditions stress the plants, whether mites are present or not.

If conditions continue, treating 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 fungal pathogens. Conditions that are good for the development of pathogens are temperatures cooler than 85 F, with at least 90% relative humidity for 12 to 24 hours.

Threshold

Deciding whether to treat is difficult. No specific threshold has been developed for two-spotted spider mites in dry edible beans. Sample plants at least 100 feet into the field and walks in a “U” pattern, sampling two plants per location at 20 different locations. A general action threshold is to treat when the lower one-fourth to one-third of the canopy has mite damage symptoms (yellowing canopy) and/or mites present.

Spider mites are difficult to control and may require two applications of insecticide using different modes of action. Remember to use an organophosphate insecticide (for example, Dimethoate) rather than a pyrethroid insecticide to avoid flaring mite populations. However, the only exception is the active ingredient bifenthrin (a pyrethroid), which does not flare mite populations and provides control of mites.

Reasons for the increase in mite populations from some pyrethroids include disruption of the natural enemies that control spider mites (predatory mites), increased movement of mites out of fields, and increased reproductive rates of female mites. Early detection facilitates timely and effective rescue treatments.

Insecticides provide short-term protection, maybe seven days, from spider mites. Fields will need to be monitored continually for resurging populations. The efficacy of an insecticide can be improved significantly with sufficient water coverage (greater than 18 gallons per acre [GPA]) by ground and 3 to 5 GPA by air and application at high pressure to penetrate foliage.

For insecticide-resistance management of mites, do not apply the same class of insecticide (or mode of action) more than twice, and alternate the class of the insecticides (or mode of action) to prevent the buildup of insecticide-resistant mite strains.

10 Wireworms

(Coleoptera: Elateridae)

Wireworms are most likely to be a problem when dry edible beans follow pasture, land in the Conservation Reserve Program, or grassland. Infestations often are found in coarse-textured soils (sandy loam) where moisture is abundant, perhaps in low spots of fields.

Threshold

No easy way exists to estimate wireworm infestations. The following are two methods used:

1) Soil sampling: Sample 20 well-spaced 1-square-foot sites to a depth of 4 to 6 inches for every 40 acres being planted. If an average of one wireworm per square foot is found, treatment would be justified.

2) Solar baiting: In September, establish bait stations for two to three weeks before freeze-up. Place bait stations randomly through the field but representing all areas of the field. You should have 10 to 12 stations per 40 acres.

Place 1 cup of wheat and 1 cup of shelled corn in a 4- to the 6-inch-deep hole. Cover the grain with soil and then an 18-inch-square piece of clear plastic. Dig up the grain and surrounding soil after one to two weeks or leave until spring. If an average of one or more wireworm larvae is found per station, treatment would be justified.

Seed Treatment: Insecticide seed treatments should be applied as a commercial or on-farm application for managing wireworms in dry edible beans.

Read Also: Benefits of Automated Farming