No. 13/June 19, 1998
Excessive Soil Moisture and Rootworm Survival
Rain, rain, rain. As you might suspect, we've received a number of inquiries concerning the potential effect of saturated soils on corn rootworm survival and the performance of soil insecticides (Table 14). To date, we continue to receive scattered reports of larval injury from east-central Illinois producers. Most recently, Kevin Black, Cargill, indicated that corn rootworm larvae (1/4- to 3/8-inch long) had been discovered in northern McLean County on June 15. Don Rhoads, Burrus Hybrids, also confirmed that some fields near Chenoa (located in northern McLean County) had corn rootworm larvae about 1/4 inch in length. Apparently, in at least one of the fields, the producer was reported to have used a soil insecticide in strips (6 rows treated, 6 rows untreated), and the untreated rows were lodged. In our own research plots, we have observed (June 16) severe rootworm injury in first-year corn. Not unlike many cornfields in east-central Illinois, our plots also were under-water this spring. Not good news as we look ahead to more rootworm feeding in June. These observations support the fact that some pockets of rootworms are alive and well. However, it remains clear from many published reports that saturated soils, in general, work against the establishment of corn rootworm larvae soon after egg hatch occurs.
Table 14. Precipitation totals for April, May, and through June 15, provided by Bob Scott, Illinois State Water Survey
|St. Charles||2.89||2.12||2.04|| 7.1|
Research conducted over a 5-year period (1980 to 1982 and 1984 to 1985) in Brookings, South Dakota, by entomologists with the Northern Grain Insects Research Lab showed that soil moisture can exert a significant influence on the level of root protection afforded by soil insecticides. They reported that root-damage ratings were inversely related to the water solubilities of the various soil insecticides. That is, those insecticides with high water solubilities were found to provide the greatest root protection (lower root ratings. They speculated that those products with high water solubilities may have greater vertical and horizontal movement through the root zone. They further concluded that the inherent toxicity of the soil insecticide was of less importance in limiting root damage than the actual water-solubility characteristics of the compound. The relative water solubilities of soil insecticides from least to most water soluble are Force (less than 2 parts per million), Lorsban (2 ppm), Regent (2 ppm), Fortress (3 ppm), Aztec (5.5 ppm), Dyfonate (13 ppm), Counter (45 ppm), Thimet (50 ppm), and Furadan (700 ppm). Keep in mind, the water-solubility characteristic of the main active ingredient (parent compound) may differ from that of metabolites. Also, the toxicity, and hence, the potential effectiveness of a soil insecticide is closely related to the properties of a given soil type. Variability in soil insecticide performance can often be related to the influence of soil organic matter on the movement of an insecticide off soil particles, into the soil solution (water between soil particles), and ultimately through the insect cuticle (skin). Dry soils tend to "hold" or adsorb soil insecticides more tightly. Wetter soils allow greater movement of a soil insecticide away from a soil particle and into solution. The dynamics of this relationship vary according to organic matter. Soils with greater organic-matter concentrations tend to adsorb soil insecticides more "tightly."
The timing of precipitation and duration of saturated soil conditions also can significantly affect corn rootworm survival and root damage. Research conducted at the Brookings, South Dakota, laboratory revealed that if the soil was saturated during egg hatch and for roughly 2 weeks beyond the egg-hatch period, corn rootworm establishment was significantly reduced. Standing water in cornfields during early to mid-June doesn't always spell the end for corn rootworms. If areas of a given field are "ponded" for only a few days, enough corn rootworms may survive and still inflict serious root damage. In 1991, a corn rootworm experiment near Urbana was severely flooded due to heavy rains soon after planting. Root-damage ratings were taken in July for this experiment and revealed that the corn rootworms survived in sufficient numbers to inflict very impressive damage. Due to the more "normal" egg hatch period this year (late May), first-instar corn rootworm larvae may have had sufficient time to locate roots prior to the deluge of rain in many fields.
In 1983, one of our experimental plots received over 9 inches of rain throughout June. Western corn rootworm seasonal emergence was greater in the insecticide-treated plots than in the control (no insecticide applied at planting). We suspect that soil insecticides moved through the root zone very well and killed a high percentage of larvae directly beneath the insecticide band. Roots were allowed to proliferate very efficiently between the corn rows, allowing greater overall rootworm survival and beetle emergence. In 1984, June was very dry, with slightly over 1 inch of precipitation falling on our plots. Seasonal beetle emergence was reduced in those plots receiving a soil insecticide at planting. Our suspicion--soil insecticides did not adequately reduce larval densities directly below the soil-insecticide band due to drier soil conditions. This may have caused greater overall larval competition and ultimately greater starvation and reduced adult emergence. Results from our investigations were obtained by a reliance on natural infestations of corn rootworms; whereas, those reported from South Dakota were obtained using artificial infestations of corn rootworm eggs. The relationship between soil-insecticide efficacy, root damage, beetle emergence, soil types, and soil moisture is complicated. One would hope that this discussion will shed a little light on the interrelationships between these factors.
Mike Gray (firstname.lastname@example.org), and Kevin Steffey (email@example.com), Extension Entomology, (217)333-6652