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Crop Sciences

University of Illinois at Urbana-Champaign

Soil Insecticides and Root Protection:
A Sure Thing, or a Roll of the Dice Each Season?

Michael E. Gray and Kevin L. Steffey

Soil insecticides are the primary tool that Midwestern producers use to minimize yield losses caused by northern and western corn root- worm larval injury to corn roots. Peak use of soil insecticides occurred in the late 1970s and early 1980s when approximately 20 million to 30 million acres of corn were treated at planting each spring by producers (Suguiyama & Carlson, 1985). Despite the decline in soil insecticide use during the late 1980s, Delvo (1989) estimated that nearly half of the insecticides used on row crops and small grains in the United States were targeted at corn rootworm larvae. In Illinois, 2.8 million acres of continuous corn were produced in 1990, 88 percent of which were treated with a soil insecticide (Pike & Gray, 1992). By the early 1990s, soil insecticides were applied to 13 percent of the acres of corn rotated with soybeans (first-year corn) (Pike & Gray, 1992), a relatively low percentage compared with the percentage of continuous corn treated. However, 13 percent of first-year corn represented approximately 1 million acres of Illinois farmland that still received an annual application of a soil insecticide. Even though declines in use of soil insecticides had occurred by the early 1990s, Illinois producers were spending roughly $52 million each season to protect corn from injurious insect larvae, primarily northern and western corn rootworms.

The decline in soil insecticide use in the late 1980s and early 1990s is being reversed quickly. Since 1995, western corn rootworms have plagued first-year corn producers in east central Illinois and northwestern Indiana. In 1995, yield losses were particularly acute due to large densities of rootworm larvae, a poor growing season, and the general lack of soil insecticides used on rotated corn acres. Since 1996, producers in eastern Illinois and affected areas of Indiana have increased their use of soil insecticides on rotated corn; however, precise estimates of this escalation are not available. Surveys designed to estimate the extent of soil insecticide use on first-year corn have been conducted (summer 1997) in Illinois and Indiana and are being analyzed. Suffice it to say, because of the failure of crop rotation to keep western corn rootworms "in check," expenditures for soil insecticides will continue to climb to sobering levels.

In 1997, the confidence that producers had placed in the efficacy of soil insecticides was shaken in many areas of Illinois. We received widespread reports of severe corn rootworm larval injury in continuous and rotated corn acres, despite the use of soil insecticides. Also, the performance of several of the most popularly used soil insecticides suffered significantly at University of Illinois experimental plots in Monmouth and Urbana. Because crop rotation in east central Illinois failed to afford consistent "rootworm protection" and the showing of soil insecticides was poor in 1997, many producers believe that their "backs are up against the wall."

An objective of this paper is to offer some possible explanations about what may have "gone wrong" with soil insecticides in 1997. To accomplish this goal, we present root-rating data (for labeled insecticide rates) from University of Illinois soil insecticide trials located near De Kalb, Monmouth, and Urbana during the last ten years (1988-1997). Only those experiments in which root injury (Hills & Peters, 1971) was near or exceeded a root rating of 5.0 (two nodes of roots destroyed) are discussed. By following this approach, we can observe more clearly how soil insecticides perform under intense rootworm pressure in a great variety of environmental circumstances. Finally, we hope to answer the question posed in the title of this paper: Are soil insecticides a sure thing, or a roll of the dice each season?

Soil Moisture and Soil Insecticide Performance

Sutter et al. (1989) concluded that root protection offered by soil insecticides is "highly variable." Factors associated with this variability included soil and environmental conditions, as well as the density of rootworm larvae present. They artificially infested their experiments with western corn rootworm eggs at densities of 300, 600, 1,200, and 2,400 eggs per foot of row. After conducting their studies for five years, Sutter et al. (1989) offered these thoughts on root ratings and water solubilities of soil insecticides: "Root damage ratings appeared to be inversely related to water solubility of the various insecticides. Higher water solubility may have permitted greater vertical and horizontal movement of insecticides in the root zone. The inherent toxicities of these chemicals to larvae in soil (Sutter 1982) did not appear to be related to root damage rating because the two chemicals exhibiting the lowest toxicity to larvae in the same soil had the lowest root ratings." [Water solubilities of the most commonly used soil insecticides are Aztec (5.5 ppm), Counter (15 PPM), Dyfonate (13 PPM), Force (2 PPM), Fortress (3 PPM), Furadan (351 PPM), Lorsban (2 PPM), and Thimet (50 PPM).] Furthermore, Sutter et al. (1989) suggested that very dry soil conditions, particularly in the upper 1'/z inches, contributed to unsatisfactory levels of soil insecticide performance. If soils were saturated at the time of egg hatch, larvae had difficulty establishing within a root system. Overall, Sutter et al. concluded that soil moisture is a major factor in determining the dynamics of soil insecticide performance and resulting levels of root protection.

On the basis of the observations provided by Sutter et al. (1989), perhaps we should have expected that reduced levels of insecticide efficacy in our Illinois trials would have occurred during the driest seasons. In fact, this is precisely what occurred for at least two very water-insoluble compounds (Force and Fortress) in 1988 and 1994, the two driest years (from 1988 through 1997) from planting to root evaluations, in De Kalb and Urbana, respectively (Table 1). The performance of Force 1.5G and 3G was compromised during each of these very dry seasons when the product was applied in-furrow. Surprisingly, Lorsban 15G, also a very water-insoluble compound, kept root injury below a rating of 3.0 in each of these very dry seasons. The wettest season (planting date to root evaluation date) of the last ten years occurred at Urbana during 1990. Even though injury in the check plots was severe (average root rating = 5.10), all soil insecticides kept root ratings below 3.0, the so-called economic root injury index. Nearly 15 inches of rain fell on our experiment at Urbana during 1992 (second wettest season); in contrast to 1990, Aztec 2.1G (furrow), Counter 15G (furrow), Counter 20CR (furrow), Dyfonate II 20G, Lorsban 15G, and Thimet 20G failed to keep root ratings below 3.0. These root-rating results from two very wet seasons are difficult to decipher. Total heat units from January through May and from January through July are similar for 1990 and 1992 in Urbana (Table 1). In addition, the level of rootworm pressure was similar for each year, even slightly less in 1992. Clearly, soil insecticide performance cannot always be determined solely on the basis of water-solubility properties of products and precipitation amounts. However, root ratings in insecticide-treated plots were generally lower in years with more precipitation, such as 1990 (Urbana), 1991 (De Kalb), 1991 (Urbana), and 1993 (Urbana).

Can rainfall totals explain the general lack of soil insecticide performance in 1997? Precipitation data collected from Monmouth and Urbana during 1997 suggest that rainfall was most likely not a factor. Rainfall at both locations was 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 Sutter et al. (1989) correctly identified the importance of precipitation and soil moisture in understanding the dynamic nature of soil insecticides and root protection, other environmental and biological variables are also involved.

Heat Units and 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 (Bergman & Turpin, 1986). 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 corn rootworm egg hatch was delayed considerably, the latest in fifteen years. They attributed the very late hatch to exceptionally cool spring temperatures. In 1997, heat-unit accumulations (base 52 degrees F, air temperature) at Monmouth and Urbana were the lowest for the ten-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. Not surprisingly, similar conditions in producers' fields created unpleasant "rootworm experiences."

Can we generally expect soil insecticides to lose their "edge" if cooler than "normal" spring temperatures contribute to delayed egg hatch and effectively extend the larval feeding period, perhaps into the last week of July? The answer to this question remains somewhat murky. In 1996, cool spring weather contributed to an extended larval feeding period. Roots from our Urbana experiment were evaluated on two different dates, July 15 and July 29. During this two-week interval, root injury in the check plots increased from 4.1 to 5.15. For most of the soil insecticides, average root ratings remained relatively stable from the first to the second evaluation date. However, significant increases in the level of injury occurred in plots treated with in-furrow applications 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 applied in a band or in-furrow at Monmouth was suspect, 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 (similar to 1997). Overall, these data suggest that low to moderate levels of precipitation during a very cool growing season can lead to increased levels of rootworm larval injury. Large densities of corn rootworm larvae and early planting would 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. Nevertheless, persistence of several compounds was evidently a problem by late July. As planting dates become earlier (early to mid-April), producers should expect performance problems when the environmental parameters described previously occur.

  Table 1. Soil insecticide efficacy data for DeKalb, Monmouth, and Urbana, Illinois, 1988-1997.  
   
 
    Locations and years  
   
 
  Soil insecticides D.882 D.893 U.904 D.915 M.916 U.917 M.928 U.929 U.9310 U.9411 M.9512 U.9613 M.9714 U.9715  
 
 
  Aztec 2.1G (band) x.xx16 x.xx 2.50 2.40 2.10 2.40 2.40 2.85 2.25 2.70 2.30 2.90 4.30 3.15  
  Aztec 2.1G (furrow) x.xx x.xx x.xx 2.45 x.xx 2.85 2.55 3.70 2.45 2.80 2.55 3.80 3.80 3.10  
  Counter 15G (band) 2.40 2.60 1.90 2.70 2.10 2.00 2.70 2.30 1.85 2.30 3.40 x.xx x.xx x.xx  
  Counter 15G (furrow) 2.30 2.80 2.00 2.75 2.35 2.15 2.33 3.10 2.05 2.40 3.50 x.xx x.xx x.xx  
  Counter 20CR (band) x.xx x.xx 2.00 2.75 2.05 2.15 2.60 2.25 1.90 2.70 2.40 2.90 4.47 2.85  
  Counter 20CR (furrow) x.xx x.xx 2.20 2.60 2.90 2.55 2.48 3.35 2.15 2.60 2.45 2.68 3.93 2.85  
  Dyfonate 20G (band) 2.85 x.xx x.xx 2.40 2.90 2.80 x.xx x.xx x.xx x.xx x.xx x.xx x.xx x.xx  
  Dyfonate II 20G (band) x.xx 3.25 2.60 x.xx x.xx x.xx 2.90 3.35 2.25 x.xx x.xx x.xx x.xx x.xx  
  Force 1.5G (band) 2.95 2.87 2.10 2.35 2.60 2.70 2.52 2.40 2.60 2.90 x.xx x.xx x.xx x.xx  
  Force 1.5G (furrow) 3.00 3.05 2.20 2.30 2.48 2.95 2.75 2.90 2.40 3.20 x.xx x.xx x.xx x.xx  
  Force 3G (band) x.xx x.xx x.xx x.xx x.xx x.xx x.xx x.xx x.xx 2.80 2.55 2.65 3.60 3.45  
  Force 3G (furrow) x.xx x.xx x.xx x.xx x.xx x.xx x.xx x.xx x.xx 3.20 2.60 3.65 x.xx x.xx  
  Fortress 2.5G (band) x.xx x.xx x.xx x.xx x.xx x.xx x.xx x.xx x.xx 3.10 x.xx 2.05 x.xx x.xx  
  Fortress 5G (band) x.xx 2.45 x.xx x.xx x.xx 3.65 x.xx x.xx x.xx x.xx x.xx 2.45 4.80 3.35  
  Fortress 5G (furrow) x.xx 2.55 x.xx x.xx x.xx x.xx x.xx x.xx x.xx x.xx x.xx 2.34 4.75 2.54  
  Lorsban 15G (band) 2.85 3.20 2.20 2.70 2.65 2.53 x.xx 3.25 2.75 2.80 x.xx 2.70 4.72 2.95  
  Thimet 20G 3.15 3.35 2.90 3.33 4.60 2.20 3.20 3.60 3.15 2.40 4.45 3.10 x.xx 3.60  
  Check 5.30 4.78 5.10 5.03 5.05 5.70 4.95 4.85 5.82 5.50 5.33 5.15 5.18 5.25  
  Rainfall (inches)17 2.89 5.83 18.10 10.16 4.24 12.01 7.03 14.88 11.72 3.90 8.08 12.36 7.27 7.83  
 
 
  Heat Units (base 52° F)18 1863 1552 1667 1836 1934 2249 1449 1630 1673 1849 1491 1779 1429 1437  
  Heat Units (base 52° F)19 598 490 470 602 585 753 390 475 402 430 249 416 222 224  
 
 
  1 Iowa State University 1 to 6 root rating scale (Hills and Peters 1971) 11 Planting date—May 13, 1994; root evaluation date—July 18, 1994, Urbana, IL  
  2 Planting date—May 5, 1988; root evaluation date—July 13, 1988 De Kalb, IL 12 Planting date—May 31, 1995; root evaluation date—July 18, 1995, Monmouth, 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., De Kalb, IL 13 Planting date—May 20, 1996; root evaluation date—August 2, 1996, Urbana, 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 14 Planting date—May 13, 1997; root evaluation date—August 6, 1997, Monmouth, 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., De Kalb, IL 15 Planting date—May 6, 1997; root evaluation date—July 24, 1997, Urbana, 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 16 Root rating data were not collected.  
  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 17 Rainfall total (inches) from planting date through root evaluation date  
  8 Planting date—April 30, 1992; root evaluation date—July 13, 1992, Monmouth, IL 18 Heat unit accumulation (base 52 F air temperature) from January 1 through July 31  
  9 Planting date—May 5, 1992; root evaluation date—July 24, 1992, Urbana, IL 19 Heat unit accumulation (base 52 F air temperature) from January 1 through May 31  
  10 Planting date—May 13, 1993; root evaluation date—July 14, 1993, Urbana, IL    

Soil Products and Consistency of Performance

As Table 1 reveals, some soil insecticides are more consistent performers than others. Consistency should be interpreted as the frequency at which a compound applied during planting keeps root injury below an average root rating of 3.0. A root 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 root rating of 3.0 may or may not result in economic loss. Troubleshooting costs escalate for sales and technical service representatives of insecticide manufacturing companies most often when producers notice lodging in their fields. Typically, lodging of corn is more prevalent when root ratings for a field average 4.0 (one node of roots destroyed) or more.

Will the use of some soil insecticides more likely result in ratings that range from 3.0 to 4.0? Counter (terbufos) and Thimet (phorate), 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 somewhat similar. Inherent toxicities of terbufos and phorate also are similar (acute oral LD50 values: terbufos = 4.5 to 9 mg/kg, phorate = 2 to 4 mg/kg), and both products are systemic. However, consistency of performance of these products is quite different. Formulations of Counter (15G and 20CR) applied in-furrow or as a 7-inch band kept root injury below a rating of 3.0 in University of Illinois experimental trials 87 percent (40/46) of the time (Table 1). In contrast, Thimet 20G provided acceptable levels of root protection (rating below 3.0) only 23 percent of the time (3/ 13). Bottom line: insecticide choice makes a difference when it comes to purchasing consistency of performance.

Concluding Remarks

Is the use of a soil insecticide a "sure thing" when it comes to root protection against corn rootworms? The answer is no. In the "real world," the use of a soil insecticide each spring is like a roll of the dice on a craps table. However, most producers realize that they throw the dice each season when they plant a crop, not much different from an annual trip to Las Vegas. Can the odds of reliable performance of a soil insecticide be improved each year? The answer to this question is yes.

Although we know that producers will not alter their planting dates to enhance the performance of soil insecticides against corn rootworms, producers must be cognizant that applying a soil insecticide at planting time in mid-April increases the odds of a root rating of 3.0 or greater, "rolling out" on the "craps table." If a producer applies a soil insecticide in-furrow in mid-April, root ratings of 3.0 or greater are even more likely. The application, even in mid April, of any product with a history of persistence problems stacks 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 increase the likelihood of performance problems of soil insecticides. Add large densities of rootworm larvae to this scenario and you have the growing season of 1997.

Finally, producers need not abandon their use of soil insecticides. Despite the performance problems that occurred in 1997, several of the soil insecticides have performed consistently in our trials during the past ten years. To be sure, none of the products offers a "sure thing" each season. However, producers inherently are aware of the risks involved each season in providing the world with an abundant supply of food. Producers can manage some of the risks associated with the use of soil insecticides, but they usually cannot manage the many other environmental and biological variables that influence insecticide efficiency.

References