Illinois Fertilizer
Conference Proceedings
January 26-28, 1998
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M.E. Gray and K.L. Steffey1
This paper was published previously in the 1998 proceedings of the Illinois Agricultural Pesticides Conference under the following citation:
Gray, M.E. and K.L. Steffey. 1998. Soil insecticides and root protection: a sure thing, or a roll of the dice each season. Proc. Illinois Agricultural Pesticides Conference, Jan. 6-8, 1998.
Soil insecticides remain the primary tool that midwestern producers use to minimize yield losses caused by northern and western corn rootworm larval injury to corn roots. Peak use of soil insecticides occurred in the late 1970s and early 1980s when approximately 20 to 30 million acres of corn were treated at planting each spring by producers (Suguiyama and 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 would be targeted at corn rootworm larvae. In Illinois, 2.8 million acres of continuous corn were produced in 1990; 88% of which were treated with a soil insecticide (Pike and Gray 1992). By the early 1990s, soil insecticide use on corn rotated with soybeans (first-year corn) was 13% (Pike and Gray 1992), relatively low as a percentage compared with continuous corn. However, this figure translated into approximately 1 million acres of Illinois farmland that still received an annual application of a soil insecticide. Even though declines in soil insecticide use had occurred by the early 1990s, Illinois producers were spending roughly $52 million each season to protect roots from injurious insect larvae, primarily, northern and western corn rootworms.
Soil insecticide declines of the late 1980s and early 1990s are 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 impressive densities of larvae, a poor growing season, and the general lack of soil insecticide use 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 currently. Suffice it to say, because of the failure of crop rotation to keep western corn rootworms "in check," soil insecticide expenditures will continue to climb to sobering levels.
In 1997, the confidence producers had placed in the efficacy of soil insecticides was shaken in many areas of Illinois. Widespread reports of severe corn rootworm larval injury in continuous and rotated corn acres occurred 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 of the failure of crop rotation in east-central Illinois to afford consistent "rootworm protection" and the poor showing of soil insecticides in 1997, many producers feel their "backs are up against the wall."
An objective of this paper is to offer some possible explanations regarding what may have gone wrong in 1997 with regard to the generally perceived lack of soil insecticide performance. In order to accomplish this goal, we present root rating data (for labeled insecticide rates) from University of Illinois soil insecticide trials collected in DeKalb, Monmouth, and Urbana over the last 10 years (1988-1997). Only those experiments in which root injury (Hills and 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 begin to observe more clearly how soil insecticides perform under intense rootworm pressure in a great variety of environmental circumstances.
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 level of rootworm larvae present. Their experiments involved artificially infesting western corn rootworm eggs at densities of 0, 300, 600, 1,200, and 2,400 eggs per foot of row. 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). 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. Furthermore, Sutter et al. (1989) suggested that very dry soil conditions, particularly in the upper 1 ½ inches, contributed to unsatisfactory levels of soil insecticide performance. If soils were saturated at the time of egg hatch, they indicated that larvae would have difficulty in establishing within a root system. Overall, they concluded that soil moisture is a major factor in determining the dynamics of soil insecticide performance and resulting levels of root protection.
Based upon the observations provided by Sutter et al. (1989), perhaps we should expect that the driest seasons of the past 10 years should have led to reduced levels of insecticide efficacy in our Illinois trials. 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 (1988-1997) from planting to root evaluations, in DeKalb 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 10 years occurred in 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 in Urbana during 1992 (second wettest season); in contrast to 1990, Aztec 2.1G, Counter 15G, Counter 20CR, 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. Heat unit totals from January though 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 untangled on the basis of water solubility properties of products and precipitation amounts. However, root ratings were generally lower in years receiving more precipitation such as 1990 (Urbana), 1991 (DeKalb), 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 suggests 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 Sutter et al. (1989) points out correctly the importance of precipitation and soil moisture in understanding the dynamic nature of soil insecticides and root protection, other environmental and biological variables are involved most assuredly.
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 and 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 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°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 and effectively extend the larval feeding period, perhaps into the last week of July? 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 pushing the planting date back to mid-April, producers should continue to expect performance problems under the environmental parameters described previously.
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 (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 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 University of Illinois experimental trials 87% (40/46) of the time (Table 1). In contrast, Thimet 20G maintained acceptable levels of root protection (rating below 3.0) only 23% 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? 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, producers realize they throw the dice each season as 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? The answer to this question is yes.
Although we know that producers will not alter their planting dates to enhance the peformance of soil insecticides against corn rootworms, they also need to be cognizant 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 scenerario impressive densities of corn rootworms and you have the growing season of 1997.
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 aware inherently 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 are under the control of producers, but other environmental and biological variables never will be.
Table 1. Soil insecticide efficacy data for DeKalb, Monmouth, and Urbana, Illinois, 1988-1997.
1M.E. Gray is an Associate Professor and K.L. Steffey is a Professor of Crop Sciences, University of Illinois, Champaign-Urbana, Illinois.
Bergman, M.K., and F.T. Turpin. 1986. Phenology of field populations of corn rootworms (Coleoptera: Chrysomelidae) relative to calendar date and heat units. Environ. Entomol. 15: 109-112.
Delvo, H.W. 1989. Agricultural resources situation and outlook report. USDA-ERS, Washington, D.C.
Hills, T.M., and D.C. Peters. 1971. A method of evaluating post-plant insecticide treatments for control of western corn rootworm larvae. J. Econ. Entomol. 64: 764-765.
Pike, D.R., and M.E. Gray. 1992. A history of pesticide use in Illinois, pp. 43-52. In Proceedings Eighteenth Annual Illinois Crop Protection Workshop, 1992, University of Illinois, Cooperative Extension Service, Urbana-Champaign.
Suguiyama, L.F., and G.A. Carlson. 1985. Field crop pests: farmers report the severity and intensity. USDA-ERS Agric. Information Bull. 487.
Sutter, G.R. 1982. Comparative toxicity of insecticides for corn rootworm (Coleoptera: Chrysomelidae) larvae in a soil bioassay. J. Econ. Entomol. 75: 489-491.
Sutter, G.R., T.F. Branson, J.R. Fisher, N.C. Elliott, and J.J. Jackson. 1989. Effect of insecticide treatments on root damage ratings of maize in controlled infestations of western corn rootworms. J. Econ. Entomol. 82(6): 1792-1798.
