Iowa has reported sightings of Two-Spotted Spider Mite population with feeding on soybean this year. With this hot and dry weather Spider Mites infestations thrive in corn and soybean fields. 80% of Iowa is abnormally dry or in a severe drought as of mid July.
Two-Spotted Spider Mite
Scouting
They generally reach their damaging level in late July or early August when the conditions are ideal for their growth. When out scouting for Spider Mites, it is recommended using a hand magnifying lens as the insect is < 1/60 inches long. To the naked eye this insect could be mistaken for a speck of dirt. Spider Mites will begin feeding on the bottom of the plant and slowly move up the plant and affect the plant's health. Two-spotted Spider Mite larvae will have six legs and the nymphs and adults have eight legs and are typically a cream or green in color. When scouting and collecting the mites, pick a soybean plant and shake the plant onto a white piece of paper and then look for moving mites. They will more likely appear on the field edges where weeds surround the boarder. Wind can spread the insect throughout the entire field.
Symptoms
Initially Two-spotted Spider mites will appear as a small yellow dot or stipples on the lower leaves of the plant. After prolonged feeding on the plants, the leaves will turn completely yellow followed by brown and then will eventually die and fall off the plant.
Economic Threshold
The exact treatment threshold for Spider Mites in corn and soybeans do not exist. Instead, you must look at how long they have infested the field, the location, moisture conditions, and the plants appearance. Spider Mites is capable of reducing soybean yields by 40-60% when left untreated. The University of Minnesota developed a rating scale to help with treatment decisions:
0- no spider mites or injury observed
1- minor stippling on lower leaves and no premature yellowing observed
2- stippling common on lower leaves and small areas on scattered plants with yellowing observed
3- heavy stippling on lower leaves with some stippling progressing into the middle canopy and leaf yellowing and some leaf loss observed; mites scattered in the middle and top canopy. This is the Ecomomic threshold
4- lower leaf yellowing readily apparent and leaf drop common; stippling webbing and mites common in the middle canopy; mites and minor stippling present in upper canopy. This is where Economic injury occurs.
5- lower leaf loss common and yellowing moving to the middle canopy; stippling and distortion of upper leaves common; mites in upper canopy observed.
Treatment of the Twospotted Spider Mites may not be required when temperatures drop below 85 degrees and humidity is greater than 90%. Mites that are infected by the fungus will be brown, and will not move on the piece of paper used when scouting.
Sources: Iowa State University & Google Images
