University of Illinois
1 Iowa State University 1-to-6 root-rating scale (Hills and Peters 1971)
2 Planting date--May 5, 1988; root evaluation date--July 13, 1988, DeKalb, IL
3 Planting date--May 8, 1989; root evaluation date--July 12, 1989; Fortress 5G applied at 6.1 oz product/1,000 row ft, DeKalb, IL
4 Planting date--May 8, 1990; root evaluation date--July 12, 1990; Aztec 2.1G applied at 7.0 oz product/1,000 row ft, Urbana, IL
5 Planting date--May 10, 1991; root evaluation date--July 22, 1991; Aztec 2.1G applied at 7.0 oz product/1,000 row ft, DeKalb, IL
6 Planting date--May 9, 1991; root evaluation date--July 15, 1991; Aztec 2.1G applied at 7.0 oz product/1,000 row ft, Monmouth, IL
7 Planting date--May 2, 1991; root evaluation date--July 11, 1991; Aztec 2.1G and Fortress 5G applied at 7.0 oz and 3.0 oz product/1,000 row ft, respectively, Urbana, IL
8 Planting date--April 30, 1992; root evaluation date--July 13, 1992, Monmouth, IL
9 Planting date--May 5, 1992; root evaluation date--July 24, 1992, Urbana, IL
10 Planting date--May 13, 1993; root evaluation date--July 14, 1993, Urbana, IL
11 Planting date--May 13, 1994; root evaluation date--July 18, 1994, Urbana, IL
12 Planting date--May 31, 1995; root evaluation date--July 18, 1995, Monmouth, IL
13 Planting date--May 20, 1996; root evaluation date--August 2, 1996, Urbana, IL
14 Planting date--May 13, 1997; root evaluation date--August 6, 1997, Monmouth, IL
15 Planting date--May 6, 1997; root evaluation date--July 24, 1997, Urbana, IL
16 Root-rating data were not collected.
17 Rainfall total (inches) from planting date through root evaluation date
18 Heat unit accumulation (base 52 degrees F) using air temperatures from January 1 to July 31
19 Heat unit accumulation (base 52 degrees F) using air temperatures from January 1 to May 31
Can rainfall totals explain the general lack of soil insecticide performance in Illinois during 1997? Precipitation data collected from Monmouth and Urbana suggest that rainfall was most likely not a factor. Rainfall at both locations was well between the wet and dry extremes described previously. Despite moderate precipitation levels, root ratings for many soil insecticides were well above 3.0 and 4.0 at Urbana and Monmouth, respectively, in 1997. Although the importance of precipitation and soil moisture can't be downplayed in understanding the dynamic nature of soil insecticides and root protection, other environmental and biological variables are involved.
Does the accumulation of heat units during the spring affect soil insecticide performance? Corn rootworm development is paced by the accumulation of heat units. However, some entomologists have contended that predicting corn rootworm phenology could be accomplished just as effectively with a calendar. Observations during the past several years have led us to believe that corn rootworm egg hatch and root injury are affected profoundly by seasonal temperatures. In 1997, Purdue University entomologists (Pest and Crop Newsletter, no. 13) reported that the corn rootworm egg hatch was delayed considerably, the latest in 15 years. They attributed the very late hatch to the exceptionally cool spring temperatures. In 1997, heat-unit accumulations (base 52 degrees F, air temperatures) in Monmouth and Urbana were the lowest for the 10-year period being discussed. Under conditions of moderate rainfall, below-average heat accumulations, and intense rootworm pressure, the stage was set for the very poor performance of most soil insecticides at these two locations in 1997. Conditions in producers' fields similar to these, not unexpectedly, created unpleasant "rootworm experiences."
Can we generally expect to see soil insecticides lose their "edge" if cooler-than-normal spring temperatures contribute to a delayed egg hatch? The answer to this question remains somewhat murky. In 1996, another cool spring contributed to an extended larval feeding period. Roots from our Urbana experiment were evaluated on two dates, July 15 and July 29. During this 2-week interval, root injury in the check plots increased from 4.1 to 5.15. For most of the soil insecticides, average ratings remained relatively stable; however, significant increases in the level of injury occurred for in-furrow treatments of Aztec 2.1G and Force 3G (from 2.95 to 3.80 for Aztec, and from 2.65 to 3.65 for Force). In 1995, the performance of Counter 15G at Monmouth was suspect when applied in a band or in-furrow, resulting in root ratings of 3.40 and 3.50, respectively. Monmouth received moderate levels of precipitation during 1995, and the spring was very cool (almost identical to 1997). Overall these data seem to suggest that low to moderate levels of precipitation in a very cool growing season can lead to increased levels of rootworm larval injury. Impressive densities of corn rootworm larvae and early planting would only worsen the severity of economic losses. During 1997, our trials at Monmouth and Urbana were planted on May 13 and May 6, respectively, late by today's standards. Yet, persistence of several compounds was evidently a problem by late July. By planting in mid-April, producers should continue to expect performance problems under the environmental parameters described previously.
Are some soil insecticides more consistent performers? Table 1 reveals that some soil insecticides are more consistent performers than others. Consistency should be interpreted as how often a compound applied during planting keeps root injury below an average rating of 3.0. A rating of 3.0 on the Iowa State root-rating scale is still considered by many entomologists as the economic-injury index. However, depending upon a range of variables (including environmental conditions, planting date, hybrid, cost of an insecticide, and the market price of corn), a rating of 3.0 may or may not lead to economic losses. Troubleshooting costs begin to escalate for sales and technical service representatives for insecticide-manufacturing companies most often when producers notice lodging in their fields. Typically, lodging problems begin to escalate when root ratings for a field average 4.0 (one node of roots destroyed).
Is the use of certain soil insecticides more likely to result in ratings that range from 3.0 to 4.0? Counter and Thimet, both manufactured by American Cyanamid Co., Princeton, New Jersey, differ considerably in their consistency of performance. Water solubilities of terbufos (15 ppm) and phorate (50 ppm) are similar. Inherent toxicities of each compound also are in a similar range (acute oral LD50 values; terbufos = 4.5 to 9 mg/kg, phorate = 2 to 4 mg/kg), and both products are systemic.
Consistency of performance between these products could not be more different. Formulations of Counter (15G and 20CR) applied in-furrow or as a band (7-inch) kept root injury below a rating of 3.0 in U of I experimental trials 87 percent of the time (40/46; Table 1). In contrast, Thimet 20G maintained acceptable levels of root protection (rating below 3.0) only 23 percent of the time (3/13). Bottom line--insecticide choice does make a difference when it comes to purchasing consistency of performance.
Is the use of a soil insecticide a "sure thing" when it comes to root protection against corn rootworms? No. In the real world, the use of a soil insecticide each spring is like a roll of the dice. However, producers realize they throw the dice each season they plant a crop--not much different than an annual trip to Las Vegas.
Can the odds be improved regarding the performance of a soil insecticide each year? Yes. Although we know that producers will not alter their planting dates to enhance the performance of soil insecticides against corn rootworms, they also need to be aware that delivering a soil insecticide at planting in mid-April increases the odds of a root rating of 3.0 or beyond "rolling out" on the "craps table." In mid-April, apply a soil insecticide in furrow, and you're even more likely to roll dice with root ratings of 3.0 or beyond. The application of a product, as early as mid-April, with a history of persistence problems, would suggest that producers are really beginning to stack the odds in favor of the rootworm. Environmental conditions such as a very cool spring followed by low to moderate rainfall throughout the growing season will only exacerbate performance problems of soil insecticides. Add to this scenario impressive densities of corn rootworms, and you have the 1977 growing season.
Finally, producers should not abandon their use of soil insecticides. Despite the general performance problems associated with 1997, several of the soil insecticides have performed consistently in our trials during the past 10 years. To be sure, none of the products offer a sure thing each season. But producers are inherently aware of the risks involved each season in providing the world with an abundant supply of food. Managing some of the risks associated with the use of soil insecticides is under the control of producers; other environmental and biological variables will never be.