There has been a lot of talk about yellowing soybeans or potassium deficiency showing up in fields across the Midwest. Recently, a pest has been found to be causing some of this injury in Western Illinois.  In this case, it happens to be a pest that we don’t normally see or talk about and it’s a mealybug.

Photo – Braxton Boyer, Farmer

Mealybugs or trochanter mealybugs (Pseudococcus sp.) were first reported in 2008 as a potential pest of soybeans in the United States.  They are small (< 2mm), flat, white, and wax-covered unarmored, scale insects.  Unfortunately, because they are so small, they often go undetected on roots.  The key here would be to DIG UP PLANTS AND LOOK AT THE ROOTS.

In 2010, Dr. Mike Gray, University of Illinois Extension Entomologist, encouraged soybean producers to examine roots of soybean plants that were exhibiting yellowing leaves because they could be infested with small, white insects that were called trochanter mealybugs.  Dr. Nick Seiter, University of Illinois, says that this can be an “occasional/weird pest of soybean” and it’s not common to see such widespread chlorosis (yellowing) of soybeans as seen in this picture. Mealybugs could cause this damage to soybeans by feeding on roots and removing fluids from plant tissue with their piercing and sucking mouthparts.  It’s interesting to note that fields with reported infestations of mealybugs have been diagnosed as having a potassium deficiency due to the yellowing of leaves and/or stunted growth.  In the past, Purdue Extension has questioned if these pests were the cause of the yellowing or if they were attacking previously stressed plants.

Trochanter mealybugs have a wide host range including many legumes such as alfalfa, red clover, white clover, and soybeans.  They also have been found on corn, johnsongrass, and sorghum.  When mealybugs are young, they only move short distances from one plant to another but have been known to hitch a ride through irrigation water and be able to survive due to their waxy body covering.  Mealybugs can move long distances via calling an Uber by way of ants.  If you see several ants in fields, this could also be a sign of a mealybug infestation!  The ants may eat the honeydew that the mealybugs excrete and in turn, the ants can protect them from predators.

Photo – Braxton Boyer, Farmer

The good news is that mealybugs do have beneficial predators that control them such as the orange Cecidomyiid larva seen in the picture called robberflies.  Be very careful not to confuse these orange flies with soybean gall midge, which have not been reported as of today in Illinois.

As of now, there are no control measures that can be taken for this pest. Controlling ants will not eliminate mealybug populations. Crop rotation may help, but this pest can survive on several crops and weed species.

Sources:

Lookout for Trochanter Mealybugs in Soybean Roots | Farm Progress

Agronomy eUpdate March 16th, 2023 : Issue 947 (ksu.edu)

July 8, 2011 – Issue 14, Pest & Crop Newsletter, Entomology Extension, Purdue University