April's Ag Advice by April Borders This is a critical time of year for our soybean crops. Insect infestation at this time of year can result in serious yield losses because the soybeans are now in the pod-fill stages. Mike Catangui, SDSU Extension entomologist, said soybean aphids, bean leaf beetles and grasshoppers are showing up in our soybean fields. Catangui said, "Soybean plants are the most sensitive to injury right now."
A new pest to South Dakota since 2001 is the soybean aphid. These insects are very small but can be seen by the naked eye.
They are about one-sixteenth inch long and yellowish-green in color. A magnifying lens will also reveal that they have a pair of black "tail pipes" on the rear. They are found on the growing point, stems and on the underside of leaves. An average of 200 soybean aphids per plant is currently considered economically damaging.
Soybean aphids are tended by ants which "farm" the aphids for their honeydew. Scout for them by watching for ants along the borders of soybean fields as one sign that aphids may be present.
SDSU research in 2002 conducted by Catangui at the Southeast Research Farm near Beresford has shown that spraying for soybean aphids during R5 (beginning seed) stage in early August can improve yields by 2-11 bushels (8-27 percent) per acre depending on the insecticide and rate used.
Insecticides labeled for the soybean aphid on soybeans and their preharvest intervals (PHI) are as follows: Asana XL (21 days PHI), Dimate (21 days PHI), Furadan 4F (21 days PHI), Lorsban 4E (28 days PHI), Mustang (21 days PHI), Mustang MAX (21 days PHI), Penncap-M ( 20 days PHI), Pounce 3.2EC (60 days PHI), and Warrior (45 days PHI). Always read and follow label directions. Consult the label for restricted entry intervals (REI). For specific rates per acre, contact the Extension office.
Bean leaf beetles can be very destructive pests of soybeans because they feed directly on the pods and developing seeds. Whole pods may also fall to the ground as a result of pod clipping by the beetles.
SDSU research in 2002 conducted by Catangui on a grower's field near White has shown that spraying for the bean leaf beetle during the R5 stage in early August can improve yields by 2-9 bushels (7-27 percent) per acre depending on the insecticide and rate used.
Insecticides labeled for use on bean leaf beetle in soybeans are Ambush, Asana XL, Baythroid 2, Chlorpyrifos 4E AG, Dimethoate 400, Lorsban 4E, Mustang, Mustang MAX, Nufos 4E, Penncap-M, Pounce 3.2 EC, Sevin XLR PLUS, and Warrior. Consult your Extension Educator for rates per acre.
Grasshoppers are a concern too. The economic threshold at which it pays to spray for grasshoppers is defoliation, or leaf loss, of 20 percent. Twenty percent leaf loss means that 20 out a 100 of similar-sized leaves are completely eaten by the insects.
SDSU research in 2001 conducted by graduate student Dave Mills at the Southeast Research Farm indicated that a 50 percent defoliation at full pod stage soybean can result in eight bushels per acre (15 percent) yield loss.
Insecticides labeled for use on grasshoppers in soybeans are Asana XL, Baythroid, Dimethoate, Dimilin 2L, Furadan 4F, Lorsban 4E, Mustang, Mustang MAX, Nufos 4E, Penncap-M, Sevin XLR PLUS and Warrior. Rates are available from your Extension Educators.
Two publications are available if you are interested in more details. They are SDSU Extension Fact Sheet 905, "Economic Thresholds in Soybeans: Grasshopper and Bean Leaf Beetle," and SDSU Extension Fact Sheet 914, "Soybean Aphid in South Dakota." Contact your local Extension office for these publications.