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Western Bean Cutworm (WBC) and the potential devastation this pest can inflict on cornfields is becoming a more urgent concern for Western Cornbelt producers. There is also an increased awareness that detection of and protection from this pest is difficult. By knowing more about the WBC’s history, life cycle, physical damage and economic loss, top producers can minimize the impact by this pest.
The WBC is native to the high plains region of the United States, where it was documented as a pest in dry beans years ago. Sometime in the last half-century, the pest developed an appetite for corn and migration to cornfields to the east was inevitable. The WBC adult moths prefers lighter soils when overwintering but have been known to cause economic damage anywhere west of Interstate 35 in Iowa. This leads entomologists to believe the WBC has adapted to laying eggs in finer soils or flight distance is much greater than previously thought. All field corn in the western cornbelt without an in-plant insect-protection trait like Herculex™ I * should be scouted beginning in mid-July. Field corn in the Western Cornbelt has traditionally been treated with a broadcast liquid insecticide.
Depending on the number of heat units accumulated during a given year, the WBC moths can usually be found in light traps in early July. Scouting for egg masses on upper leaves and larvae in the tassels and silks starts in mid-July. The eggs are pumpkin-shaped and become purple when hatch is close. Magnification of the egg mass is necessary for initial identification. Once hatched, the larvae move from the whorl to the tassel. Silks soon follow and the WBC quickly migrates to the ear where insecticides are not effective. The larvae start out dark brown and become lighter with each growth stage until reaching a pinkish-tan color. The larvae are often confused with Corn Earworm that have similar features and occupy corn silks and ears at the same time. Damage and management of these two insects is not the same, so correct identification is important.
In the ear tip, the WBC increases in size over several weeks. Nearly all of the damage is done by the last stages of the larvae by the grain they consume and the diseases they introduce. In the final stage of larvae growth, the larvae are over one inch in length and have an amazing appetite for their size. In addition, the WBC has no problem sharing thesame ear with many other WBC larvae unlike the Corn Earworm which is cannibalistic and generally less destructive.
While WBC is not a serious problem in every field every year, the occurrence of economic loss has been increasing. Warmer, dry winters and/or winters with snow cover keep the over-wintering population high. Documented losses of several bushels per acre have been found when an average of one WBC per ear and it is common to find more than one larvae per ear.
Most University extension departments agree that the threshold level is between five and ten percent egg masses found. However, even professional agronomists will admit to having problems pulling the trigger on treatment. It takes sampling many locations in a field every few days to stay on top of this insect. To make matters worse, insecticides miss those cutworms protected by ear husks and latecomers that have not yet hatched. Herculex™ I * technology mentioned earlier takes care of the timing problems broadcast applications have by providing full season protection of the Western Bean Cutworm. Though the protection is not one-hundred percent, it is high enough to offer a substantial improvement over the downfalls of broadcast insecticide treatments.
*Herculex I Insect Protection technology by Dow AgroScience and Pioneer Hi-Bred. Herculex is a trademark of Dow AgroSciences LLC
THE AUTHOR DOES NOT GUARANTEE THE ACCURACY OF THE INFORMATION CONTAINED IN THIS FEATURE, ALTHOUGH IT IS BELIEVED TO BE ACCURATE. THE AUTHOR ASSUMES NO LIABILTY OR RESPONSIBILITY FOR DIRECT OR INDIRECT, SPECIAL, CONSEQUENTIAL OR INCIDENTAL DAMAGES OR FOR ANY OTHER DAMAGES RELATING OR ARISING OUT OF ANY ACTION TAKEN AS A RESULT OF ANY INFORMATION OR ADVICE CONTAINED IN THIS REPORT. THE AUTHOR DISCLAIMS ANY EXPRESS OR IMPLIED LIABILITY OR RESPONSIBILITY FOR ANY ACTION TAKEN, WHICH IS SOLELY AT THE LIABILITY OF AND RESPONSIBILITY OF THE USER.
CREDITS: University of Nebraska Cooperative Extension, Western Bean Cutworm in Corn and Dry Beans, publication G98-1359-A
Colorado State Universtiy Cooperative Extension, Western bean Cutworm: Characteristics & Management in Corn and Dry Beans, publication 5.538
Where Did the Soybean Aphid Come From?
The soybean aphid is a native of China and Japan.
What Does the Soybean Aphid Look Like and Where Can I Find It?
It is a small yellow insect with distinct black protrusions or “tailpipes” on the tip of its abdomen. Some generations will have wings. The aphid can be found on soybean stems, young leaves of growing soybeans and on the undersides of mature soybean leaves. No other aphid species colonizes on soybean plants so it is probably safe to assume if you find colonies of tiny yellow aphids on your soybean plants, you probably have soybean aphids.
What is the Life Cycle of the Soybean Aphid?
The life cycle of the soybean aphid is complex and can be as many as 15 to 18 generations per growing season. The aphid survives on two host plants during the year. Winter survival for over-wintering eggs is on the buckthorn plant. On the buckthorn, two generations of wingless females and one generation of winged females are produced. The winged females migrate from the buckthorn in search of soybean. The soybean aphid stays on the soybean for the rest of the summer in a repeated series of wingless generations followed by a winged generation. The winged generation is capable of leaving the crowded colony in search of other soybean plants where a new colony is started. In the fall, the winged soybean aphid migrates back to the buckthorn and produces a generation of wingless egg-laying females. The males develop on soybean and also migrate back to buckthorn where they mate with the wingless females which are responsible for laying the eggs that will over-winter on buckthorn twigs.
Are There Other Summer Hosts for the Soybean Aphid?
To date in North America, soybean is the only confirmed summer host for the soybean aphid.
When Do Soybean Aphids Migrate to Soybean Plants?
It is not known exactly when aphids will migrate to soybean plants. Monitoring for aphids at specific growth stages rather than calendar date has been more successful. Populations build and peak starting at the late seedling stage known as V2 (two expanded trifoliate leaves) through the blooming stage known as R1 – R2. Colonies concentrate on new trifoliate leaves and new leaves on side branches. In late July, when the top growing point of the soybean stops growing, the aphids move from the top of the plant to middle or lower areas of the canopy and usually can be found on the undersides of leaves, petioles and pods.
What Symptoms Should I See on Plants Affected By Soybean Aphids?
Aphid infestations that peak at the R1 – R2 growth stage can cause stunted plants and reduced pod and seed counts. Leaves on these plants may be distorted and under very heavy infestations they may be yellowed. Charcoal colored residue can be seen on stems, leaves and pods. This residue is sooty mold which grows on the honeydew excreted by the feeding aphids.
Can the Soybean Aphid Cause Other Problems?
Soybean aphids are capable of transmitting a number of viruses that naturally infect soybean plants. The viruses include alfalfa mosaic, soybean mosaic, bean yellow mosaic, peanut mottle, peanut stunt, and peanut stripe. (Note: It is not possible to prevent these viruses by controlling aphids with insecticides.) The sooty mold residue indirectly can also rob yield by reducing the photosynthetic capacity of the plant. Does the Soybean Aphid Have Any Natural Predators / Diseases? Some natural enemies of the soybean aphid identified so far include lacewings, lady beetles, minute pirate bug, parasitic wasps and some fungal pathogens which turn dead aphids a reddish brown color.
When Should a Soybean Grower Control Soybean Aphids?
Marlin Rice, Professor of Entomology at Iowa State University, thinks it is critical to scout soybeans for aphids starting in early July. A first field check in early to mid August is probably too late. Rice says, “The beginning to second week of August is when growers will get the most benefit out of spraying, because they will have caught the population before it starts to build up. At the end of August is when you get less benefit.” Rice notes that The University of Wisconsin has established thresholds of when spray. The per plant spray thresholds are 200 or more aphids for plants at the R1 or R2 stages and 1000 to 1500 aphids at the R3 and R4 stages respectively.
What is the Bean Leaf Beetle?
The Bean Leaf Beetle is tied for second among all pest species attacking soybeans. They feed on soybean foliage, pods and seeds. The feeding not only causes plant damage but the beetles can also transmit bean pod mottle virus (BPMV).
What is the Life Cycle of the Bean leaf Beetle?
The Bean Leaf Beetle overwinters as adults under leaf litter at or near soybean fields. Once spring temperatures reach 50 – 55 degrees F, adults become active and seek available host plants such as grasses, soybean plants, and other legumes. The coloration varies from red, orange, tan, or gray and the markings (dots, strips, or both) may vary among individual populations. However, all adults have a black triangle at the base of their forewings. Females are capable of producing 130 to 200 reddish colored eggs which they lay in the upper 5 inches of soil adjacent to plant stems. In 5 to 7 days the eggs will hatch and feed on underground plant parts. Depending on the soil temperature, bean leaf beetle larvae may feed 3 to 6 weeks before pupating into earthen cells. Two generations of bean leaf beetles usually occur in the North Central region. On average, the first adult generation peaks around the late vegetative or early reproductive soybean stage (mid July), and the second generation peaks at pod fill time (later August to early September). The second generation becomes the overwintering adults and eventually leave soybeans and feed on alfalfa and other legume hosts before seeking overwintering sites under crop and leaf debris.
What Kind of Damage Does Bean Leaf Beetle Do?
Both the larvae and adults are soybean pests. Larvae attack the roots and root hairs but show a preference for root nodules. Adults feed on foliage and pods. Pod damage by adults is most crucial because it can also lead to secondary disease infections of the pods and seed which lower both seed quality and quantity. Pod feeding may also cause complete pod loss and or pod lesions. Even though soybean plants may sustain more than 50% leaf damage, the soybean plant can compensate, unless damage occurs during the reproductive growth stage. Overall, a reduction of 0.6 pounds per acre can occur when beetles number one or more per foot.
What is the army cutworm?
The army cutworm larvae is an early season pest of several crops including alfalfa. They reach 1.5 to 2 inches in length. They are pale greenish-gray to almost brown in color with no real distinctive markings. They tend to be more of a problem in our western service region but outbreaks east of there are not uncommon. They begin to feed on alfalfa plants soon after plants break dormancy, preventing spring green up.
What is the life cycle?
Only one generation is produced each year. Moths emerge from the soil in late June and fly to mountainous areas and enter a period of inactivity through July and August. In the fall, moths fly back to the plains to lay eggs. Eggs hatch and larvae feed on alfalfa plants until late fall and then overwinter in the soil. Fall soil moisture is required for larvae to survive. When spring arrives and the soil warms, larvae emerge and begin to feed. Once larvae are mature, they pupate 2-3 inches below the soil and emerge as adult moths in June once again.
What damage can army cutworms do?
They feed on stems and leaves of alfalfa plants, preventing spring green-up. Although cutting of plants in established stands is unlikely, new seedings may be prone to cutting by the cutworm. Large numbers of cutworms can be very damaging and consume largeamounts of vegetation. Outbreaks can appear suddenly in the spring and are favored by dry summers followed by a wet fall.
What is the economic threshold?
For established alfalfa stands, 4 to 5 larvae per square foot usually warrants treatment, while new seedings may require treatment at 2 larvae per square foot. Fields should be monitored as soon as conditions allow for alfalfa to break dormancy in the spring. Larvae will likely be just beneath the soil surface during the day as they prefer to feed at night or on overcast days.
How is this pest controlled?
Army cutworm larvae have many natural enemies, including birds, predatory beetles, and wasps which can reduce populations. Although helpful, these beneficial insects cannot be relied upon as a control measure. If the economic threshold is reached and the initial spring growth is being affected, insecticide treatment will provide the best results. The pyrethroids Warrior and Mustang Max are both very effective. Best control will be obtained if applied when the larvae are still small (1” or less).