Scout corn and soybeans for spider mites

Twospotted spider mites (left) and Banks grass mites (right). Notice the difference in the pigment patterns between the two insects.
Banks grass mites, Oligonychus pratensis.
Twospotted spider mites, Tetranychus urticae.

Reports of spider mites in corn and soybeans have begun to come in from across Nebraska, although we haven't heard of any economically damaging infestations to date. Current and projected weather conditions favor an increase in spider mite populations, so farmers and field scouts should keep a close eye on fields.

Two species of spider mites, the Banks grass mite and twospotted spider mite, commonly feed on Nebraska corn. Banks grass mites feed almost exclusively on grasses, including corn and sorghum. Twospotted spider mites not only feed on many species of grasses, but also on soybeans, fruit trees and a variety of vegetables and ornamental plants. Although these two species are somewhat similar in appearance, they differ in several biological characteristics and in their susceptibility to pesticides.

Banks grass mites usually appear earlier in the season, feed mostly on the lower leaves of the corn plant, and in Nebraska are moderately susceptible to many of the commonly used miticides. On the other hand, twospotted spider mites tend to appear in mid to late season, increase rapidly, feed over the entire plant, and often are not consistently controlled by available pesticides.

The most useful characteristics for identification are the overall shape of the body and the pattern of pigmentation spots on the back (see figure). The dark green spots on both species are caused by food particles that accumulate in their gut. Because of differences in gut structure, these pigment spots accumulate in slightly different patterns. In Banks grass mites the pigments accumulate along both edges of the body near the rear and along the sides of the body. In twospotted spider mites, the pigments accumulate along the sides of the body in two distinct spots and do not extend back more than halfway on the body. The Banks grass mite is also slightly less robust than the twospotted spider mite and is slightly flatter from top to bottom. Mites damage crops by piercing plant cells with their mouthparts and sucking the plant juices.

The first evidence of mite feeding, which can usually be seen on the top of the leaf, is a yellow or whitish spotting of the leaf tissues in areas where the mites are feeding on the lower leaf surface. Because many other things can cause similar discoloration, it is important to check leaves closely to make sure mites are actually causing the damage. Leaf discoloration caused by mite feeding can be easily identified by checking the undersurface of leaves for the presence of mites, eggs and webbing. Both Banks grass mites and twospotted spider mites produce webbing, and a fine network of silken webs will likely be associated with mite colonies. A magnifying glass or 10X hand lens is helpful in examining plants for mites.

The economic injury level indicated in the table provides a method for deciding when to treat, taking into account the value of corn. This table works for both twospotted spider mites and Banks grass mites.

The first row refers to the expected value of the crop ($/acre), determined by multiplying the expected yield (bu/acre), by the expected crop price ($/bu). For example, if the expected yield is 200 bu/acre and the expected price is $1.50/bu, then the value per acre is $300.

Deciding whether to treat involves two steps. First, determine the percentage of leaves infested with mites (an infested leaf has one or more live mites). Compare that number with the first number in the table. If the field average is less than the table value, you don't need to treat, but do continue to monitor the field. If the field average exceeds the table value, then estimate the percentage of total leaf area damaged by mites. If the field average exceeds the table value, it is likely that treating for spider mites will increase yield above the cost of treatment.

Table I. Economic injury level for the Banks grass mite or twospotted spider mite on corn, based on the percentage of infested leaves per plant and percentage of total leaf area damaged. Control cost/acre Market value per acre ($) 200 250 300 350 400 450 500 550 600 650 700 Percent infested leaves per plant/Percent of total leaf area damaged $5 15/8 12/6 10/5 8/5 7/4 7/3 6/3 5/6 5/3 5/2 4/2 $10 29/16 24/13 20/10 17/9 15/8 13/7 12/6 11/6 10/5 9/5 8/4 $15 44/23 35/19 29/16 25/13 22/12 20/10 18/9 16/9 15/8 14/7 13/7 $20 59/31 47/25 39/21 34/18 29/16 26/14 24/13 21/11 20/10 18/10 17/9 $25 74/39 59/31 49/26 42/22 37/20 33/17 29/16 27/14 25/13 23/12 21/11

Also, note that control costs are a factor in this table. Depending on the product chosen, the critical values may change greatly. For example, under the column $300 market value, the critical value for percent infested leaves varies from 20%, if control costs are $10, to 49% if control costs are $25.

Labeled products for spider mite control on corn include dimethoate (several formulations), Comite 6.55EC and Capture 2EC. Dimethoate has performed reasonably well in Nebraska against Banks grass mites, but not twospotted spider mites. If twospotted spider mites are present, either Comite or Capture would provide better control. See the Department of Entomology web site on spider mite control or product labels for specific rates and restrictions. With the exception of Comite, pesticides do not kill mite eggs, and there is a possibility of re-infestation of the fields as eggs hatch out. Since many of their natural enemies were probably killed by the initial pesticide application, these populations may build up rather quickly and should be monitored carefully. Corn is unlikely to benefit from treatment for spider mites after the dent stage.

Twospotted spider mites also may develop on soybeans. No research has been conducted that would allow calculation of an economic injury level for twospotted spider mites on soybeans. Iowa State University Extension specialists have suggested that control may be warranted when infested plants have substantial spotting or leaf yellowing and live mites, but before mites cause browning and leaf drop. Damage from mites may be confused with that caused by drought and several foliar diseases, so be sure to base treatment decisions on the presence of mites, rather than just apparent injury symptoms.

Fields may be spot treated if the infestation is localized, but check other areas for mites (especially downwind of infestation) and extend treatments into these areas if large numbers of mites are found. Although late season infestations may accelerate soybean senescence and increase pod shattering, use caution when evaluating whether to treat with pesticides because many of the pesticides used for mite control have 21-28 day preharvest intervals. Products to use in soybeans include dimethoate (several formulations) and Lorsban 4E at ˝ to 1 pt per acre. See the Entomology web site or product labels for specific rates and restrictions.

Answer
Extension Entomologist Gary Hein: I do not think that it will have much impact as we have never seen irrigated pivots as a barrier to grasshopper infestations. The only advantage that I would see is that if the environmental conditions are right for disease development, the increased humidities in the irrigated fields may enhance disease development by a few days at best.

As reported in the April 19, 2002 issue of CropWatch, this insect originated in Asia and was first reported in North America in 2000. It quickly spread to 13 states and Canada, so soybean aphid movement into Nebraska was not unexpected. NU researchers and specialists had been surveying specifically for the pest this summer.

Extremely low numbers of aphids were found by entomologists Tom Hunt, Leon Higley, and Steve Spomer. It seems unlikely that economic problems will occur this season; however, fields will be monitored and surveys will continue throughout the growing season.

This small, yellow aphid has black cornicles ("tail-pipes") and can be found on soybean from seedling through reproductive stages. It feeds on new growth or the undersides of mature leaves. Heavy feeding may produce wilting, leaf distortion and yellowing.

Further information on the aphid and current management strategies will be discussed in subsequent issues of CropWatch.

Hagel will discuss the availability of federal crop disaster and livestock assistance programs in a taped interview with Roy Frederick, University of Nebraska ag policy specialist. "Market Journal," sponsored by NU Cooperative Extension, will be available on the Web at 8 p.m. Central Time (7 p.m. Mountain Time).

"Market Journal" also will feature a discussion of strategies livestock producers can use to minimize the impact of drought on the operations. Other guests scheduled to appear include Rick Rasby, NU beef specialist; Bruce Anderson, NU forage specialist; and Darrell Mark, NU agribusiness management expert. Topics to be discussed include developing feed rations for dry-lotted cows, forage management and the July Livestock Inventory Report.

Also on the July 25 "Market Journal," Mike Briggs, Seward cattle producer, will assess current beef market conditions; Al Prosch, NU Pork Central director, will analyze pork market trends; and Al Dutcher, state climatologist, will review the most recent Drought Monitor and offer an ag weather forecast.

"Market Journal" will be available on the Web at http://marketjournal.unl.edu. To view the program, follow links to either audio/video or audio-only streaming media. All programs are archived for later viewing online.

The July 25 "Market Journal" also will be available via satellite July 26 at county extension offices from 9-10 a.m. Central Time (8-9 a.m. Mountain Time.) The program can be viewed at county extension offices across Nebraska. Contact your local county extension educator for program availability in your area.

"Market Journal" is presented by NU Cooperative Extension and the Department of Agricultural Economics.

The old adage, “Store a small crop, sell a large crop,” might not hold true this year, according to Dillon Feuz, ag economist at the University of Nebraska Panhandle Research and Extension Center in Scottsbluff.

With the recent rally, cash prices have increased $0.30 to $0.40 at most elevators. The KCBT September wheat contract increased $0.35 per bushel. From mid May prices have rallied $0.60 to $0.70 per bushel. Basis levels in Western Nebraska at -$0.15 to -$0.20 per bushel are about $0.30 per bushel stronger than last year and are $0.15 per bushel stronger than the five-year average. “Those basis levels make it very attractive to sell now,” Feuz said.

“The market is not providing many incentives to store. KCBT December and March contracts are only priced about a nickel more than the September contract.” If the market doesn’t continue to rally, a producer would lose storage costs. Typical storage costs, including interest on the grain are in the range of $0.02 to $0.04 per bushel per month for on-farm and off-farm storage, respectively.

“If exports pick up from the present dismal rate and if the spring wheat crop continues to deteriorate, we may still see rallies in the market,” Feuz says. If you want to be a part of this potential market rally, but feel like you should sell your wheat now with the positive basis, then you might consider buying a call option. A $3.50 KCBT Mar call is currently priced around $0.25 per bushel. If you buy the call and the market does not rally, you will lose your $0.25 per bushel. However, if the market rallies to $4.00 per bushel, you could sell the Mar call for $0.50 per bushel and you would net a $0.25 per bushel gain.

Feuz encourages producers to review all market options. “If you don’t have much wheat to sell, you sure need to sell that at a favorable price.”

Except for narrow pockets of heavy rainfall earlier this month in Keith, Gage, Lancaster, Saline, and Saunders counties, most of Nebraska continues to miss the precipitation it desperately needs. Last week's cold snap was short-lived as significantly warmer temperatures returned to the central United States this week.

Corn and soybean conditions continue to deteriorate across the 10 leading grain producing states. As of July 14, only 48% of the U.S. corn crop was rated in the good to excellent category, representing a drop of 14 percentage points from June 23. Fifty percent of U.S. soybeans were reported in the good to excellent category, down from 62% June 23.

If there is any consolation for Nebraska producers, dry conditions are beginning to have a major impact on the eastern corn belt. Southwestern Iowa has now been upgraded to moderate drought status, while abnormally dry conditions are being reported over the southern two-thirds of Illinois, as well as portions of Indiana, Ohio, and Michigan. Another week of heat and dry conditions will warrant further upgrades to these states, as well as eastern Nebraska.

Drought info A number of Cooperative Extension resources are available to address various aspects of the developing drought, from crop and livestock management to landscape care and personal health. Check the new IANR Drought Resources and Information Web site, developed by the NU Institute of Agriculture and Natural Resources. It includes news releases, audio and video interviews, and of course, links to CropWatch.

Colleagues from the Midwestern Climate Center have indicated that Champaign, Illinois had received only 0.07 inches of rain for the 32 days preceding July 15. The extremely wet conditions that materialized across Illinois during April and May are now reversed. Because significant crop planting delays occurred across the eastern corn belt, corn fields have very shallow rooting depths and have undergone severe stress with the recent dryness. Estimated rooting depths for the corn crop are less than 20 inches at many Illinois locations.

In Nebraska, indications are that some surface irrigation deliveries will be shut down by this weekend. In addition, some of the shallower irrigation wells in northeastern Nebraska are running dry.

I wish I could provide promising news as far as the forecast was concerned. Unfortunately, the models don't indicate any significant precipitation in the immediate future. The models have consistently shown a precipitation field generated at the end of each 10-day run for the last week. This is typical output for weather models trying to break down a dominant ridge.

The best hope for western Nebraska will be for the monsoon season in the desert southwest to become more active than usual over the next few weeks. This could be possible if a tropical storm comes ashore on the Baha Peninsula of Mexico. For eastern Nebraska, we will need a frontal system to move southward out of the Dakota's to increase our rain chances. However, neither of these options looks very promising for the next 10 days.

The presence of adult beetles or rootworms in a field is not necessarily an indication of insecticide failure. Soil insecticides are applied in a narrow band or infurrow to the soil, or as a seed treatment, and corn roots grow beyond the treated zone where rootworm larvae may survive. Also, plant lodging may occur without significant rootworm feeding. Dig and wash some roots to check for rootworm injury before assuming that rootworm damage is responsible for lodging.

Rootworm insecticide efficacy can only be evaluated reliably if replicated, untreated check strips are left in the same field as the treatment. Without check strips, you won't know whether the absence of injury is due to insecticide efficacy or the absence of rootworms.

On Monday, July 22, meetings will be held at 10 a.m. at the Holt County Extension Office in O'Neill and at 2 p.m. at the Knox County Extension Office at Center to address drought concerns for dryland crops. Topics will include nitrates, prussic acid, grazing, silage, hay and alternative feeds.

In August meetings will be held to address winter preparations following a drought. The Aug. 26 meeting will be at the Holt County Extension Office in O'Neill and the Aug. 27 meeting will be in the Bloomfield Community Center. Both will be held from 1 p.m. to 5:30 p.m.

Speakers will include Al Dutcher, NU state climatologist; Bruce Anderson, NU extension forage specialist; Tom Hunt, NU entomologist, and three Extension educators, Gary Stauffer, Ralph Kulm and Terry Gompert. They will address insect considerations; climate probabilities, feeding cows whole shelled corn; grazing standing corn; purchased protein sources; and planning winter feed.

Extension educators Terry Gompert and Ralph Kulm also hope to mix in a little humor by encouraging farmers to participate in two drought-related contests: the Best Clean Drought Joke and the Largest Drought Crack in Holt County or Knox County.

For those interested in entering the contests, the largest crack in the earth from the drought will be judged on a point system:

• width (10 points per 1/4 inch at the widest part),
• depth (10 points per inch at the deepest part), and
• length (1 point per foot at the longest part).

The dry bean crop is our lowest water user and shortest season crop of the traditional spring planted crops. The outlook for economically viable yield levels is most likely found with the dry bean crop in western Nebraska, if the beans have been adequately irrigated up to this time. Use of the remaining short supply or irrigation water will be critical, and can be very beneficial to the production of dry beans.

With the bulk of the national supply of Great Northern beans being grown in western Nebraska, there is some potential for price increases. A very short crop in this area could have a significant influence on the Great Northern bean price, making even limited yields valuable. With the short irrigation water supply, our dry bean crop may be the most profitable of our valley crops if the water has been applied in a timely manner. Any further irrigations and rainfall will only solidify the profitability of the crop.

The grant program began in 2000 and has provided approximately $1.7 million in funding for 31 Nebraska agriculture projects. The $238,000 remaining in the program will be awarded in August.

To be eligible, applicants must be involved in collaborative efforts such as agricultural cooperatives, agricultural nonprofit corporations, economic development providers, or farming and ranching operations in collaborative arrangements with other operations. Grant money is used to support projects in research, training, market development, feasibility studies, cooperative development, and other value-added efforts.

All applications must be submitted to the Nebraska Department of Agriculture (NDA) by August 1. An application form is available on the NDA web site at www.agr.state.ne.us. For more information, contact NDA at 800-422-6692.