Two-spotted spider mites (Tetranychus urticae) can be a sporadic pest of soybeans and dry beans in Manitoba. They prefer hot, dry weather and may appear from July to August, if conditions are favourable. Infestations usually begin at field edges and move inward. Examine several areas of the field to determine the level of infestation.
White speckling may be seen on upper leaf surfaces, but the actual spider mites can be found on the undersides of leaves among their webbing (Figure 1).
These infestations can occur following a pyrethroid insecticide application. So, insecticides applied for control of soybean aphids or another type of insect pest can trigger an increase in spider mite populations. Insecticides can also kill off predatory mite populations and other beneficial mite predators.
Scout for spider mites by examining individual plants. Examine leaves closely or shake plants over a white piece of paper to determine the presence of these tiny pests. You may need a magnifying glass to see them clearly.
Spider mites are difficult to count, so economic thresholds are based on the percentage of leaves infested. Consider insecticidal control if 20% of the leaves are infested by spider mites.
Since infestation often occurs in patches, spot treatment may be effective for this pest. Organophosphates can be used for spider mite control, including Malathion registered in dry beans and Lagon/Cygon registered in soybeans.
Have you seen these symptoms?
This soybean discolouration that appears bronze/red from afar is a severe infestation of spider mites. These symptoms were found only in small, isolated patches near field edges in Manitoba in 2017.