AGRICULTURAL AND FARM NEWS
Soybean pests have been hard to figure out in 2012. Some pressure was seen towards the end of July into August. Soybean pests, including soybean aphids and two-spotted spider mites are not at consistent levels in all fields this year. Some fields have been sprayed and other fields are not seeing pressure at high enough levels to warrant treatment.
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Farmers are questioning whether or not treatment is necessary at this point. The best answer we can give is that you need to scout your soybean fields to determine the population and pressure of soybean pests. Scouting is the best way to understand where that particular field is at. A neighboring field treated for pests does not necessarily warrant treatment on your field.
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Soybean aphids have become a perennial pest for Minnesota soybean growers since 2002. Aphids are small (1/16? or less) and they are typically light green check for aphids in the soybean canopy. Signs of aphids include white skin casts of molting aphids, honeydew (sap excreted by aphids), or sooty mold (a black mold that grows on honeydew. One method of quickly estimating soybean aphid populations is to employ "Speed Scouting." This is a method of estimating aphid numbers using the presence or absence of aphids on a plant. Details on Speed Scouting are available at http://www.extension.umn.edu/go/1070.?
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Bruce Potter, Integrated Pest Management Specialist based at the University of Minnesota, Lamberton Research and Outreach Center, recently reports that this year is not shaping up to be a high pressure year for soybean aphids from a soybean crop perspective. Winged aphids have been leaving Southwest Minnesota fields since the end of July. Winged aphids may be leaving soybean fields, but there are few aphids around to replace those aphids. Aphid populations might or might not build to economic threshold levels in newly colonized fields. It may be that economic threshold populations could occur later than normal. Look for aphids low in the canopy to get a reliable estimation of infestation level. Use the 250 aphids per plant, with 80 percent or more of the plants in the field harboring aphids until R6 stage soybeans. This stage is known as the ?green bean? stage or beginning full seed stage.
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According to Ken Ostlie, Extension Entomologist at the University of Minnesota and Bruce Potter, prolonged drought raised the threat of the two-spotted spider mite outbreaks in soybeans and corn. Spider mite outbreaks are rare but have occurred more frequently in recent years (1988, 2007, 2009, and 2010). Two-spotted spider mites are minute (<0.002 inch), greenish, yellowish to orange arachnids with two dark spots on their abdomen. Note, that two-spotted spider mites have 8 legs, not 6 as in insects. Spider mite adults are half the size, or less, of the smallest soybean aphid nymph. They attack a variety of plants including soybeans, dry beans, alfalfa, corn, vegetables, ornamentals, and trees. Mites overwinter as eggs and move into crops from permanent vegetation. Hatching mites colonize the undersides of leaves.
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Spider mites injure leaves by piercing cells and sucking out cell contents. The injury produces white or yellow spots or ?stipling? that is the heaviest on the underside of leaves. Infestations typically will be first observed near field edges where soybeans are stressed. If lower leaf loss, yellowed or browning spots are noted at the field edge, it is time for detective work. Examine near roadsides, ditches or alfalfa. Pull plant and examine the leaves from the bottom upwards. Look for stipling or webbing. Tap leaves over a white sheet of paper or utilize a hand lense. Determine how far mite symptoms have progressed up the plant. If observed. Move at least 100 feet into the field to determine presence.
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Ostlie and Potter developed a scale in 1988 to help determine mite infestation. The spray threshold is when there is heavy stipling on lower leaves with some stipling progressing into the middle canopy. Mites will be present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing is common and some lower leaf loss when at the spray threshold. Full pod (R4) and beginning seed (R5) soybean growth stages are critical in determining soybean yield.
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It is important to note that most pyrethroid insecticides, except bifenthrin, are not terribly effective against two-spotted spider mites in Minnesota. Be sure to scout your fields for these pests. Earlier treatment of these pests does not mean that there will not be recolonization. Continue scouting your fields and be sure to contact the McLeod and Meeker County Extension Offices if you are looking for additional information.
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