Soil Insecticide Performance for 2000: Will Anticipated Dry Weather Have an Effect?

April 21, 2000
Soil insecticides remain the primary management approach for corn rootworms throughout much of the Corn Belt in continuous corn. With the loss of crop rotation as an effective cultural pest management approach for western corn rootworms, soil insecticide usage also has skyrocketed in rotated cornfields in east-central Illinois and northern Indiana. Because of the current dry soil conditions and the potential for a dry summer, questions are being raised about soil insecticide performance and rootworm control. In addition, many producers are seeking answers concerning the optimum placement of soil insecticides during planting. To provide some information for these questions, we present root-rating data (for labeled insecticide rates) from University of Illinois soil insecticide trials for DeKalb, Monmouth, and Urbana for a 10-year period (1988­1997). Only data from experiments in which root injury was near or exceeded a root rating of 5.0 (two nodes of roots destroyed) are presented. By taking this approach, we can begin to observe more clearly how soil insecticides perform under intense rootworm pressure in a great variety of environmental circumstances. Please note that root-rating results were not presented for any location during 1998 and 1999. Because rootworm injury in our check plots (no soil insecticide used) was so low, these data were considered to be of little use. The following questions and answers are designed to shed some light on many of the soil insecticide performance issues that are now surfacing.

Does soil moisture make a difference regarding soil insecticide performance? Water solubilities of the most commonly used soil insecticides are Aztec (5.5 ppm), Counter (15 ppm), Force (2 ppm), Fortress (3 ppm), Furadan (351 ppm), Lorsban (2 ppm), Regent (2.4 ppm), and Thimet (50 ppm). After conducting studies for 5 years, researchers in South Dakota offered the following observations 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 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." Additional observations by entomologists in South Dakota suggested that very dry soil conditions, particularly in the upper 1 1/2 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.

Have dry soil conditions in Illinois resulted in poor performance of soil insecticides? Based on the studies in South Dakota, perhaps we should expect that dry summers should have led to poor insecticide performance 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 (1988­1997) 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 this 10-year period 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 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, 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).

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. Based on some observations during the past 4 to 5 years, this contention has been challenged. For instance, 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.

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 on 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).

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 need to recognize that delivering a soil insecticide at planting in mid-April increases the odds of potential economic root damage, especially if a product is applied in-furrow, and lower rates are used. The exception to this insecticide-placement advice is Fortress, which protects roots from corn rootworm larval injury better when applied in-furrow due to its volatility. The application of a soil insecticide in early April, followed by a late egg hatch and a dry summer, would suggest that producers are really beginning to stack the odds in favor of the rootworm.--Mike Gray and Kevin Steffey

Author: Mike Gray Kevin Steffey