SCOUT NOW AND WATCH FOR SPIDER MITES, APHIDS
Jul 27, 2021
“It’s that time of year again. Weeds are hopefully suppressed, beans are closing in the rows, the corn is pollinating, and scouting for bugs is now in full swing,” said Mike Slater, Federated agronomy sales rep at the Isanti location.
Growers are “always battling something – the weather, weeds, bugs, etc,” Slater mused, and the primary battle at this point in the season is over spider mites and aphids on soybeans.
Spider mites are “extremely small – only 0.05 cm long, about a 1/3 of the size of a soybean aphid,” said Slater. (See photo. Spider mites appear to look like grains of sand.) “Their webbing and damage to leaves are easier to spot than the pest themselves,” he added. Spider mites feed on the plant leaves and “suck out the contents of the leaf cells.”
Spider mite damage typically appears as white or yellow markings on plant leaves, especially those near the field edges. Once the mites invade the perimeter, they “move across the fields in prevailing winds,” he said.
When should you scout for spider mites? Now. “Spider mites overwinter in plant residue with multiple generations occurring per year,” said Slater. High temperatures and dry conditions – such as that in Federated’s sandier regions lately, Slater noted – can increase infestations extremely quickly.
The “spray threshold, per the University of Minnesota, is typically when heavy spots of yellow and white (stippling) on the lower leaves are progressing into the middle of the canopy of soybeans,” said Slater. Leaf loss is also an indicator that it’s time to spray.
Soybean aphids are “little ‘pear-shaped’ bugs that feed/suck on the sap inside of the plant leaves; they can reduce yields and transmit diseases between soybeans,” said Slater. Since aphids overwinter in buckthorn, fields with buckthorn on the perimeter deserve additional attention. Once the winged aphids form, they head for the soybean fields.
Scouting is the only way to determine whether aphids have reached the economic threshold for spraying. “When scouting,” said Slater, “20-30 plants should be sampled per field.” If an average of 250+ aphids per plant can be found, it’s time to consider spraying insecticides.
When should you scout for soybean aphids? “From R1 (beginning flower) all the way through R6 (seed set in a pod on one of the top four nodes),” said Slater. This article from the U of MN Extension offers further specifics.