 |
ProCrop |
Hot Weather Brings on Spider Mite Problems in Soybean Fields
Spider mites can be found in soybean fields in areas of the state when rainfall has been lacking and temperatures are high. Infestations may be concentrated on field margins where mites migrate into fields from border vegetation, or there can be small areas scattered through the field where the populations are increasing and spreading.
Damage by mites first appears as small yellow spots ("stipples"). As feeding activity increases, leaves become yellow, bronzed, brown, and eventually drop off the plant.
Mites are small and magnification is required to see them. A quick sampling procedure to determine whether mites are present is to hold a white cloth or paper below leaves then tap them to dislodge the mites. The mites appear as tiny dust specks; however, they will move after being knocked off the leaf.
Deciding whether to treat is difficult. Mites usually become a problem when hot, dry weather is occurring. Insecticide applications that kill predators also contribute to increasing populations of mites. The hot, dry conditions stress the plant whether mites are present or not. If environmental conditions continue, treating for mites is no guarantee plants will recover. In addition, products labeled for mite control often do not produce adequate results and the population of mites may rebound quickly to pretreatment levels or higher.
At what level do you treat? There is no economic threshold that has clearly been defined. Kansas State University suggests that treatments may be beneficial if significant pod or seed filling remain and leaves have not already yellowed. Plants that lose 50% of their foliage during bloom and pod set will stop producing these structures until favorable growing conditions resume or lost leaf area is replaced. If mite injury is evident within the interior of the field, and hot, dry conditions continue, the potential exists for economic populations throughout the field in 1 to 2 weeks. These infestations normally develop late in the season and would not be expected to impact yield as greatly as mite infestations that develop in July, as they did in 1988.
Products recommended for spider mite control in soybeans are Lorsban (0.5 lbs ai/a or 1 pint/a, 28 days to harvest) and dimethoate (Cygon 4E at 0.5 lbs ai/a or 1 pint/a, 21 days to harvest).
When rain and humidity are present, natural reductions in mite populations occur due to infection by a fungal pathogen. Conditions that are good for the pathogen are temperatures cooler than 85, with at least 90% R.H. for 12 to 24 hours.
Back to Soybeans Menu
Back to Main ProCrop Index
