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water use/GDD tables

Soybean Rust Update
Spider Mites

Entomologists Out of Town Next Week

There will be no entomologists at Norfolk or the Haskell Ag Lab next week as Keith is going on vacation and Tom is going to a meeting in Brazil . If you need entomology help contact the Entomology Department at 402-472-2123.

Soybean Management Field Days

The eighth annual Soybean Management Field Days on Aug. 15-18 at four locations across the state will offer producers unbiased and research-based information to improve their soybean profitability. Topics include: Management for Improving Crop Water Use, Marketing/Management/Government Programs, Managing New and Emerging Disease, Insect and Weed Problems, and Ten Steps to Finding Answers to Your Soybean Production Questions. The field days begin at 9 a.m. and conclude at 2:30 p.m. Free registration is available the day of the event. Field days will be held August 15 near West Point, August 16 near Hazard, August 17 near Dorchester , and August 18 near Wahoo. Contact your nearest extension office for more information.

Soybean Cyst Nematode Field Days Near Neligh and North Bend

Soybean cyst nematodes cost Nebraska soybean producers millions of dollars in reduced yields each year, said John Wilson, extension educator in Burt County . The problem is that soybean cyst nematodes often have no visible symptoms and go undetected.

"Yield losses of 20 to 30 percent have been documented in the state with no above ground symptoms on the plant," he said. "The first indication of the problem is soybean yields that have leveled off or even started to drop while corn and other crop yields in the same field continue to improve."

Twenty years ago, soybean cyst nematodes were only found in eastern Nebraska along the Missouri River . Today, the nematodes have been identified in 34 Nebraska counties with high counts being reported as far west as Kearney , Wilson said.

The field days are designed to teach producers to identify and manage soybean cyst nematodes infestations, said Loren Giesler, UNL plant pathologist in the university's Institute of Agriculture and Natural Resources. Participants will see soybean cyst nematode-resistant and susceptible soybean varieties and examine cysts on infested soybean plant roots. They also will receive a free kit for one soybean cyst nematodes analysis.

"If SCN caused holes, lesions, spots or other plant abnormalities, it would be much easier to convince producers to test for and manage it," Giesler said. "However, infested plants often look healthy. Soybean cyst nematodes have earned the title of the silent yield robber."

Dates, times and locations for the field days include:

Aug. 9 – 6:30 p.m., Kenny Reinke farm, from Neligh go 5.5 miles north on Highway 14, turn into farmstead on west side of highway.

Aug. 10 – 10 a.m., Vaughn Gross-Rhode farm, from North Bend go 2.5 miles north on Highway 79, 2.5 miles east on County Road Q

Soybean Aphid Update

Soybean aphids are largely absent or holding steady in most fields, and the hot temperatures have limited populations to scattered areas in fields that have them. More moderate temperatures forecast for the next couple of weeks leave a window of opportunity for the aphids to rebound. Do not let down your guard and continue to scout for them.

Soybean Rust Update

What's soybean rust?

Spider Mites

The lack of rain and hot temperatures has made conditions favorable for spider mites in corn and soybeans. We have not seen or received many reports or mites, but they are almost certainly present in some fields. Growers and field scouts should keep a close eye on fields, especially those that have been treated for western bean cutworms or other pests.

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.

Fig. 1. Left - twospotted spider mite; Right - banks grass mite

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 1 ). 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. 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, 4E rate 2/3 - 1 pt/acre), Comite 6.55EC (2-3 pts/acre), Capture 2EC (5.12 - 6.4 oz/acre), and Oberon 4SC (2.85 – 4.25 oz/acre). Oberon is a new product recently registered by Bayer. We do not have our own data but Kansas data has shown it to be an acceptable alternative. Dimethoate has performed reasonably well in Nebraska against Banks grass mites, but not twospotted spider mites. If twospotted spider mites are present, either Comite, Capture or Oberon would provide better control. Comite and Oberon affect all stages of mites and are reported to be less harmful to beneficials. The other pesticides do not kill mite eggs, and there is a slight possibility of re-infestation of the fields as eggs hatch out, in combination with removal of natural enemies. Corn is unlikely to benefit from treatment for spider mites after the dent stage. See the Department of Entomology web (entomology.unl.edu) site on spider mite control or product labels for restrictions.

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 ½ to 1 pt per acre. See the Entomology web site or product labels for specific rates and restrictions. For more information see the UNL Cooperative Extension publication, Spider Mite Management in Corn and Soybeans, G1167.

David P. Shelton
Extension Agricultural Engineer

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