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ISSUE 13   August 7, 2008


Late July through late August is the time when growers should monitor for corn rootworm to determine the potential economic injury in next year’s corn fields. This article reviews basic corn rootworm identification, biology, causes of injury, field scouting methods, and control methods for corn rootworm.

Two species of corn rootworm beetles affect corn in North Dakota: western corn rootworm (Diabrotica virgifera virgifera) and northern corn rootworm (D. barberi). Western corn rootworm adults are yellowish orange with three black stripes on the wing covers that do not extend all the way to the tips, while northern corn rootworm is a solid green in color (see photographs). Adults of both species are about 1/4 inch long. Larvae reach a mature length of about 1/4 inch and are creamy white in color (see photograph).

Western corn rootworm adult
Western corn rootworm adult (photo by Whitney
Cranshaw, Colorado State University, Bugwood.org)

Northern corn rootworm adult
Northern corn rootworm adult (photo by Natasha
Wright, Florida Dept. of Agriculture and Consumer
Services, Bugwood.org)

Western corn rootworm larva
Western corn rootworm larva (photo by
Scott Bauer, USDA-ARS, Bugwood.org)

One generation occurs each year and both species have similar life cycles. Both species overwinter as eggs, which hatch in June over a period of a few weeks. Larvae move to nearby corn roots where they begin feeding. Larvae pass through three instars and pupate in early to mid-July. Pupation lasts for five to ten days depending on soil temperature. Adults emerge in late July and begin to reproduce. Females lay 300 to 400 eggs in the upper two to eight inches of the soil.

Corn rootworm injury is caused mainly by larval feeding on the root system. Larvae feed on root hairs and later on main roots, which lessens the plant’s ability to take up water and nutrients. This results in lower yield. In heavily infested fields, root injury may be so severe that the plants lodge at ground level. The plants are still alive and try to grow upright, which results in "gooseneck" plants. Goosenecked corn is difficult to combine and further yield loss results from not being able to harvest ears from these plants. Adults, particularly northern corn rootworm adults, may cause injury by feeding on silks before pollination is complete. This causes reduced seed set, which also lowers yield. However, silk feeding by adults is usually not a problem.

Sampling for corn rootworm adults should be conducted from late July (after pollination is complete) through late August once a week over a three-week period. This will allow the grower to determine the potential infestation in next year’s corn. In continuous corn and first-year corn that will be planted to corn the next year, adult abundance can be determined by direct visual counts of corn rootworm adults on corn plants. For direct visual counts in corn, sample the field in a "W" pattern. Stop periodically and count the number of adults on the foliage and silks of two plants, and continue in this fashion until you have counted 40 sets of two plants per set. Corn rootworm adults may be concentrated in "hot spots" in a field, so be sure to cover a large portion of the field using the "W" pattern. After you have counted 40 sets of two plants per set, simply divide the total number of adults by 80 to obtain an average of adults per plant. For continuous corn using the direct visual observation method, the economic threshold is 1 beetle per plant. For first-year corn using this method, the economic threshold is 0.5 beetle per plant. Be sure to sample fields once a week during August as peak adult emergence may not occur during the first sampling. A common crop rotation in southeastern North Dakota is a corn/soybean rotation. Soybean fields that will be planted to corn the next year can be monitored for corn rootworm adults during August by using yellow sticky traps. Placement of sticky traps in soybean fields should be AT LEAST 100 feet away from any neighboring corn. A minimum of four trap stations should be used, but use more traps in large fields (up to 10 total trapping stations). At least two trap stations should be on a side of the field not adjacent to corn. Traps should be changed once every two weeks. The average number of beetles per trap per day over the entire monitoring period provides an estimate of potential economic injury to corn the next year. This monitoring method has not yet been studied in North Dakota. Research from Illinois, Indiana, and Ohio suggests an economic threshold of 5 beetles per trap per day. Sweep net sampling in soybean for corn rootworm adults is not recommended, as daily weather conditions and beetle flight periods are too variable to give accurate estimates of beetle abundance.

Control of corn rootworm can be achieved by crop rotation, soil insecticides, and by planting transgenic Bt corn labeled for rootworm control. Soil insecticides are typically applied at planting in a 7-inch T-band over the open furrow or directly in-furrow with the seed. Be sure to read and follow insecticide labels. Transgenic Bt corn is a convenient management tool in that it essentially contains a built-in insecticide that offers control of larvae. Be sure to follow the refuge requirement to help prevent the development of Bt resistance in corn rootworm populations! The refuge requirement for corn rootworm is 20% non-Bt corn planted within or directly adjacent to Bt corn.

Patrick Beauzay, Res. Specialist
NDSU Extension Entomology



A Special Local Need (Section 24c) was registered for Mustang Max EC Insecticide® from FMC Corporation for control of grasshoppers and other insects in flax. Active dates are July 24, 2008 through September 1, 2009. The recommended use rate is 4 fl. oz. per acre. Do not apply within 7 days of flax harvest.



Two-spotted spider mites have been observed in about 10% of soybean fields in hot, dry regions of production (Cando, Towner County). Overall, mite populations are low. Mites can be found on the underside of leaves of the lower half of plant. The life cycle of spider mites can be completed in only 5-14 days with fastest development rates occurring above 91F. Each female lives for 30 days and she produces about 300 eggs during her lifetime. In hot, dry weather, natural fungal diseases of mites are slowed and populations can increase from a few individuals to millions within a few generations. Remember, spider mites thrive on the stressed plants that are nutrient rich!

Leaf injury symptoms appear as stippling first and then progresses to yellowing, browning or bronzing as feeding injury increases, and eventually leaf drop (see photograph). Feeding injury causes water loss from the plant and reduces the photosynthetic ability of the plant. In severe cases, premature leaf senescence and pod shattering will occur and even plant death. When a severe mite infestations occur during late vegetative and early reproductive growth, a 40 to 60% yield loss between treated and untreated soybean has been demonstrated in other north central states. Spider mites can cause yield reduction as long as green pods are present.

Spider mite injury
Spider mite injury symptoms in field
(Source: Ken Ostlie - Entomology, UMN)

When scouting for spider mites, look on the underside of leaves and lower foliage for a tiny mite and fine spider-like webbing (see photograph). Adult spider mites are small (< 0.2 inch), greenish-white to orange-red in color, and have 2 dorsal spots and 4 pairs of legs (see photograph). Nymphs are smaller than adults and have 3-4 pairs of legs. By shaking a leaf or plant over a white sheet of paper, you can see these tiny mites (yellow dots), which crawl slowly over paper. Predatory mites will move faster than the two-spotted spider mite. When spider mites need to move due to diminishing food supply, they climb to the tops of plants and are dispersed by the wind "ballooning", so they can spread quickly within a field or to adjacent fields.

Spider mites on leaves
Spider mites on leaves

Spider mite webbing

Be sure to scout during full pod (R4) through beginning seed (R5) stages since these crop stages are the most important contributors to soybean yield. If hot dry conditions exist, mites populations can quickly increase and sprays should not be delayed.

There is no specific threshold that has been developed for two-spotted spider mite in soybean. As a result, several guidelines are available from different states to determine economic infestation of spider mites in soybeans. One of the more commonly used economic threshold is based on different crop stages:

  • before pod development = 20-50% of leaf area is discolored/stippling on plants
  • between R3 (beginning pod) to R5 (beginning seed) = 10 to 15% of leaf area is discolored/stippling on plants
  • R6 (full seed) to R7 (beginning maturity) = 25% of leaf area is discolored/stippling on plants
  • The discoloration is caused from the feeding injury (e.g., leaf stippling) of spider mites, and is difficult to estimate in the field.

    Remember to use an organophosphate insecticide (e.g. Lorsban, Dimethoate) over a pyrethroid insecticide to avoid flaring mite populations. Reasons for the increase in mite populations 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. Current insecticides for soybeans provide short-term protection, maybe 7-10 days, from the pest. Fields will need to be re-monitored continually for resurging populations. The efficacy of an insecticide can be improved significantly with sufficient coverage (>10 GPA of water) and application at high pressure to penetrate foliage.



    Economic levels of soybean aphids have been reported from Barnes, Steele, Stutsman, Foster, Ramsey, and Towner Counties this past week. Recent southernly winds had been favorable for aphid migration into the more northern and central regions of North Dakota. Soybean aphid populations can rebound in later planted soybean fields in late August and early September. Continue scouting soybean fields for aphids until R6 or full seed!

    Soybean aphids map

    Soybean aphids
    Soybean aphids



    A few soybean fields in southeastern North Dakota have reported with low levels of bean leaf beetles (BLB) in soybeans. This is the 2nd generation of BLB, and the most important in terms of potential yield impact due to its pod feeding. This causes pod scarring, pod clipping and decreased seed quality. Fungal pathogens can also infect injured pods causing discolored, shrunken or moldy pods. Adult BLB (see photograph) are 5 mm long with yellow or red wing covers with four black spots and black margin around the edges of the wing covers. The main identification characteristic is the black triangle behind the head. Feeding injury symptoms include round holes between leaf veins (see photograph).

    Adult bean leaf beetle

    It is a difficult insect to scout for, because it often hides or drops to soil when it sees you approaching. Sweep nets can also be used for monitoring or detecting beetles. Treatment would be recommended when 3 to 7 beetles per sweep are found. Economic threshold is 25% defoliation from reproductive pod fill to maturity or 10% pod feeding (or the presence of clipped pods).



    Nuttall blister beetles (see photograph) have been observed feeding on canola pods in Burleigh County.

    Blister beetle
    Blister beetle

    Several species of blister beetle feed on canola including: Lytta nuttalli or Nuttall blister beetle, a large purplish green beetle; Epicauta fabricii or the Ash-gray blister beetle; and Epicauta ferruginea, a smaller rusty color, pubescent beetle. Most species of blister beetle have one generation a year. Adults become active in early to mid summer and lay eggs in the soil. Eggs hatch in about two weeks into a larvae called triungulins, which actively prey on grasshopper egg pods (genus Epicauta) and bee eggs, larvae, and stored food (genus Lytta). Larvae overwinter. Adult blister beetles are attracted to blooming canola fields, where they are ravenous feeders devouring leaves, stems, flowers, and pods. These beetles are mobile and often congregate in certain spots in a field from their gregarious behavior. In some instances, blister beetles feed for a short period of time and then migrate to other plants or fields.

    The presence of large numbers of blister beetles in spots of a canola field has often concerned growers. However, adult feeding is generally not significant enough to warrant an insecticide treatment. The "High Plains Integrated Pest Management Guide" recommends treatment when there are 10 adult blister beetles per plant feeding on the flowers or pods. Spot treatment with foliar insecticides registered in North Dakota is usually recommended when populations are economic. Follow safe pesticides practices when spraying flowering canola to protect honey bees.



    Question: What is this hairy caterpillar? It has been observed feeding on sunflower leaves near Lawton, Ramsey County, North Dakota this year.

    salt marsh caterpillar

    Answer: This is a larvae of the salt marsh caterpillar (Estigmene acrea). The salt marsh moth (Arctiidae) are medium-sized moths with broad wings. It is distributed from Canada to Florida and Texas. Caterpillars (larvae) are large about 2 inches long and can be yellow to black with long setae. Since larvae are hairy, they are often known as woolly bears. The salt marsh caterpillar feeds on a wide variety plants from field crops (sunflowers, clover, corn, peas, potatoes, cabbage) to trees (apples, walnuts). It is not an economic insect pest of sunflower. See Moth of North Dakota website:


    Janet Knodel
    Extension Entomologist

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