As reported in last week's (issue no. 20) Bulletin, western corn rootworm densities have reached astonishing levels in many locations of central and northern Illinois. "Swarms" of western corn rootworm adults also have been reported in the gardens of homeowners who reside in metropolitan areas. On August 15, Ellen Phillips, crop systems educator, Countryside Extension Center, reported that western corn rootworms essentially destroyed her 1/8-acre garden. Western corn rootworm adults were observed feeding on a great variety of plants that included ash trees, ornamental cherry trees, roses, marigolds, beets, broccoli, cabbage, black and red raspberries, pumpkins, winter and summer squash, cucumber, redroot pigweed, and velvetleaf. In addition, the western corn rootworm adults showed a remarkable "fondness" for spinach and snap beans. These observations are quite remarkable because western corn rootworms are generally reported to have a much narrower host range (primarily grass-related species). In addition to corn, western corn rootworm larvae are capable of surviving on a number of grass-related species, such as foxtails. Adult western corn rootworms are known to have a strong evolutionary relationship with plants that belong to the family Cucurbitaceae. Western corn rootworm adults have co-evolved with cucurbits and have specific host-plant recognition abilities for the extremely bitter and toxic cucurbitacins (oxygenated tetracyclic triterpenoids, "Cucs"). These compounds generally serve as protectants against other insect herbivores. So, it's not unusual to observe western corn rootworm adults feeding on pumpkin and squash blossoms; however, it is a bit bizarre to witness the consumption of foliage on plants not related to members of this family.|
Why are these unusual observations occurring? At this point we can only speculate. In the 1960s as the resistant strain (to the chlorinated hydrocarbon soil insecticides) of western corn rootworm spread across the Corn Belt, it was suggested that the new strain was more "aggressive" and dispersed more readily than the nonresistant strain. The resistant strain reportedly moved eastward at the rate of about 50 miles per year. Since the mid-1990s, we have observed the loss of crop rotation as an effective pest management tool across many fields in east-central Illinois and northern Indiana. Results from our research efforts clearly indicate that this new strain is capable of laying eggs in a variety of crops that include alfalfa, corn, oat stubble, and soybean. In essence the egg-laying behavior has changed. Indeed, it has broadened considerably to include many crops. Is this new and expanded ovipositional behavior linked to a greater range of acceptable hosts by adult western corn rootworms? Are densities of western corn rootworm adults so great this year that they simply "spill" into suburban gardens and, once there, remain in place because of their "fondness" for cucurbits? After the cucurbit crops are consumed, do they begin to feed on unrelated plants due to hunger? Again, these questions remain unanswered.
We have much to learn about this interesting species of corn rootworm. We also continue to welcome your reports on these peculiar observations.--Mike Gray and Kevin Steffey