Newsletters

Print this newsletter (pdf)
water use/GDD tables

Soybean Aphid Update

Celebrating 50 Years

The University of Nebraska-Lincoln , Northeast Research and Extension Center , Haskell Agricultural Laboratory at Concord , will hold a public field day on Tuesday, August 14, 2007, from 8:30 A.M. – 4:00 P.M. A flyer on the events of the day are at the end of this page. Hope to see you there.

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 any reports of 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 with insecticides. A recent practice that has become all too common in soybeans is the tendency to want to “clean up” a field with an insecticide mixed in with the last application of glyphosate, just to kill whatever may be there. Usually, this backfires because beneficial insects like lady beetles, pirate bugs, and other predators are removed, allowing insects like soybean aphids and spider mites that migrate into crops easy access.

Two species of spider mites, the Bank's grass mite and two-spotted spider mite, commonly feed on Nebraska corn. Bank's grass mites feed almost exclusively on grasses, including corn and sorghum. Two-spotted 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 – Two-spotted spider mite; Right – Bank's grass mite

Bank's 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, two-spotted 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 Bank's grass mites the pigments accumulate along both edges of the body near the rear and along the sides of the body. In two-spotted 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 Bank's grass mite is also slightly less robust than the two-spotted 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 Bank's grass mites and two-spotted 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 two-spotted spider mites and Bank's 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 $3.50/bu, then the value per acre is $700.

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 Bank's grass mites, but not two-spotted spider mites. If two-spotted 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.

Two-spotted spider mites also may develop on soybeans. No research has been conducted that would allow calculation of an economic injury level for two-spotted spider mites on soybeans. Midwest entomologists suggest 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.

Disease Mimic Present in Corn

The most common symptom on corn samples being received at the UNL Plant and Pest Diagnostic Clinic lately is not likely caused by a disease, but instead a genetic disorder that causes lesions under stressful conditions. The symptom has been reported previously in Nebraska, particularly in 1999 and 2005 and across multiple hybrids from several companies. The symptoms behave and appear similar to a residue-borne foliar disease because they tend to begin on the lower leaves and spread up the plant, but no pathogen has been confirmed in samples. A likely cause of this symptom is a genetic mutation called a disease lesion mimic. It has been confused mostly with grey leaf spot. This mimic is showing up in northeast Nebraska and to confirm it samples should be sent to the UNL Plant and Pest Diagnostic Clinic. Contact your extension educator for more information.

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 may leave a window of opportunity for the aphids to rebound. Do not let down your guard and continue to scout for them.

Green Cloverworm Counts Up, Watch Soybeans

Light trap counts at the Haskell Ag Lab have shown a large increase in the flight of green cloverworm moths. These moths will lay eggs that will produce caterpillar larvae. These larvae will defoliate soybeans. Larvae can be identified by a combination of characters. They are pale green with one or two white stripes extending down each side of the body, and three pairs of prolegs in the middle of the body. Larvae also wiggle violently when squeezed; no other caterpillar in soybeans exhibits this behavior. The eggs of this insect hatch in 4 days and the larvae feed for 17-23 days. They develop through six instars (stages) and consume most of their food during stages 4-6. Young larvae will feed anywhere throughout the soybean plant, but older larvae confine their feeding to the upper one-third of the soybean canopy. Diseases frequently suppress green cloverworm populations; however, this may not occur until after a substantial amount of defoliation has occurred.

Researchers at Iowa State have developed economic thresholds for this insect that are based on number of larvae per foot of row. Since most scouts or farmers don't use a drop cloth, use the 20% defoliation level to help you decide whether you need to treat. Lorsban and pyrethroids such as Warrior will give good control of cloverworms.

David P. Shelton
Extension Agricultural Engineer

Newsletters
ipm042307
ipm050907
ipm051607
ipm052307
ipm053007
ipm060607
ipm061307
ipm062007
ipm062707
ipm071107
ipm071807