No. 12/June 12, 1998
Corn Rootworm Larvae Common in Many Fields
During the past week, Kevin Steffey and I received quite a few telephone calls concerning corn rootworm larval injury. Many of the reports were from individuals who had scouted first-year cornfields in east-central Illinois. Although it may seem unusually early for rootworm damage already, our records from previous years suggest that this season may be progressing more "normally." The egg hatches and rootworm development of 1996 and 1997 should be viewed as very late and more protracted, respectively. From June 1 to June 8, approximately 150 heat units (4-inch soil, base 52 degrees F) accumulated for southern Illinois. During this same time period, roughly 100 heat units (4-inch soil, base 52 degrees F) accumulated for central and northern Illinois. Figure 2 provided by Bob Scott with the Illinois State Water Survey gives the total heat-unit accumulations (4-inch soil, base 52 degrees F) for the period of January 1 to June 8, 1998. Based upon these heat-unit totals, I would not be surprised to learn that western corn rootworm adults have begun to emerge during the third week of June in central Illinois. By the last week of June, reports of western corn rootworm adults should be common. Projections for the appearance of adult rootworms using air temperatures (from January 1, base 52 degrees F) obtained from Urbana, suggest that 10 percent of the emergence will have occurred when 897 heat units have accumulated. Stay tuned for an interesting rootworm season.
Figure 2. Actual 4-inch soil-temperature heat-unit accumulation (base 52 degrees F), January 1 to June 8, 1998.
Each year in late June and early July, we begin to receive quite a few inquiries concerning the performance of soil insecticides targeted at corn rootworms. In essence, producers would like to know if they've received their money's worth from their investment. The following questions and answers may shed some light on this subject.
I've used a soil insecticide for years on my continuous corn ground. I've never really noticed many beetles. Am I correct in assuming that the soil insecticide worked? No. Soil insecticides provide root protection; that is, they prevent brace roots from being pruned extensively. Research in Illinois and other states clearly indicates that in certain years more corn rootworm beetles may emerge from treated than untreated areas of fields. If you don't find many corn rootworm beetles in your cornfield, it may simply suggest that you didn't have much of a rootworm population in the field to begin with.
If I don't see many lodged corn plants in my field, can I assume that my soil insecticide performed satisfactorily? No. Plants lodge for a variety of reasons that may have nothing to do with corn rootworm larval injury. For instance, plants that are top-heavy (tall with large ears) may topple over due to high winds, wet soil conditions, and shallow root systems. Conversely, if soil conditions are extremely dry, plants with severe corn rootworm pruning may not lodge because the plants are attempting to grow in concretelike conditions.
I've been told corn rootworm larvae are hard to find. If I dig around thebases of some corn plants, will I be able to find some? Finding corn rootwormlarvae in early to mid-June is not easy because the first-instar grubs arevery small (Figure 3). Corn rootworms have three larval instars. An instar isthe stage between molts, the physiological process by which grubs shed theirskins. When rootworms reach the final growth stage (mid-June to late June),they are much easier to find. Another reason that first-instar larvae are hardto find is related to their early season feeding behavior. First-instar larvaedon't spend much time feeding externally on root systems; instead, they tendto burrow into root tissue and feed internally.
Figure 3. Body lengths and head-capsule widths for instars of the western corn rootworm, Diabrotica virgifera LeConte.
If I find rootworm larvae, what, if anything, does it mean? Some producers are surprised to find rootworms on their root systems despite the fact that a soil insecticide was used at planting. Don't be alarmed. Soil insecticides generally do an outstanding job of protecting the root systems and thus preventing plants from lodging. Soil insecticides don't kill every rootworm. In fact, the untreated areas between rows may be viewed as a corn rootworm refuge. Although this may be disturbing, there are advantages in not destroying a high percent of the rootworm population each year. For example, the chance of resistance development to a soil insecticide is rather low under this scenario. If you find three or more larvae per plant (7-inch cube of soil and roots from base of plant) and root injury is evident, a rescue cultivation treatment may be warranted. An insecticide applied during cultivation does not generally perform as well as a planting-time application of a soil insecticide. This is especially true under very dry soil conditions
How should I assess the performance of my soil insecticide for corn rootworm control? To get an accurate picture of how well your soil insecticide worked, you should dig several (5 to 10) plants from about 10 different areas of the field. After you dig the plants, wash off the soil from the roots and look for any general feeding (brown scars) or, more importantly, pruned brace roots. In general, entomologists across much of the Corn Belt suggest that a soilinsecticide has done its job if it keeps root injury below a rating of 3.0 (several roots pruned to within 1.5 inches of the plant; never an entire node pruned) on the Iowa State root-rating scale. The economic root-injury index is static and varies according to the hybrid selected and the amount of precipitation that occurs throughout the growing season. Adequate precipitation following the larval-injury period through mid-August is very important. Root regeneration during this time frame may significantly affect yields.
Let's assume that I follow your suggestions and find very little root injury, can I now assume that I got my money's worth for using a soil insecticide? Perhaps, but only if roots were also removed from a check or an untreated area of the field and you find that they are severely pruned. This assumes that the roots removed from treated areas lack severe pruning. Unfortunately, many producers are unwilling to leave some rows untreated at planting, and because of this, it is impossible to assess accurately the usefulness of a soil insecticide treatment. A point to remember--soil insecticides will appear to work very well when corn rootworms aren't present in a field to begin with. If you don't leave some untreated strips in your cornfield, you'll never be able to estimate the real value of your soil-insecticide purchase.
Mike Gray (firstname.lastname@example.org), Extension Entomology, (217)333-6652