We continue to get calls about two-spotted spider mites being found in soybean fields, particularly in the drier areas of the province. Even though this topic was part of our July 30th Bean Report, it’s worth revisiting.
One of the problems with two-spotted spider mites is that they can go unnoticed until fields are really infested. Missing the early signs of plant damage make these pests hard to catch.
Here are a few quick tips:
- Two-spotted spider mites like hot, dry weather and drought stressed plants. Infestations usually begin at field edges and move inward. Examine several areas of the field. We are most concerned about these sap feeders affecting yield from the R1 to the end of the R5 growth stages.
- Early symptoms include white speckling of upper leaf surfaces. The actual spider mites can be found on the undersides of leaves and distinctive webbing will often be present. See Figure 1 below.
- Scout individual plants. Examine leaves closely or shake plants over a white piece of paper to determine the presence of these tiny pests. Bring a magnifying glass. They’re small.
- We don’t count spider mites. Nominal economic thresholds are based on the percentage of leaves infested. Start considering insecticide control if 20% of leaves are infested by spider mites.
- These infestations can occur more readily following a pyrethroid insecticide application. This means that some insecticides applied to control soybean aphids can trigger an increase in spider mite populations. Insecticides can also kill off beneficial predator allies that could have helped us.
- Since infestations often occur in patches, spot treatments, like doing an outside round or two of a field, may be effective. But for that to work, you have to catch the problem early. Which means you must be be scouting!!
- Significant amounts of rainfall on a field, especially if it came down hard, can set back spider mites by knocking them off plants. The increased humidity that follows the storm can also help supress them for awhile. But you have to keep scouting to know if a rainfall event has helped you.
- Unfortunately, two-spotted spider mites are sometimes not noticed until infestations become severe. When such severe infestations of spider mites occur, plants take on a very noticeable bronze to red appearance when viewed from the roadside (see Figure 2 below). At this point, the likelihood that spider mites have migrated deep into the soybean field is significantly increased. Early detection and monitoring is key to determining risk, minimizing yields loss and minimizing control measure costs.
