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A rating system was developed to predict the mortality of southern corn rootworm (Diabrotica undecimpunctata howardi Barber) larvae used as test animals to bioasaay soil insecticides. The mortality probabilities, based on behavior of moribund larvae, resulted in data that was more consistent with the assumptions underlying parameteric statistics and provided more precision when compared to the usually accepted alternate method of considering moribund larvae as dead.
The rating system bioassay technique was used to study the effects of different soil factors on the relative decline in biological activity of six soil insecticides: carbofuran, chlorpyrifos, fonofos, isofenphos, phorate, and terbufos. Field studies (including one that altered pH with lime and sulfur) were conducted to measure the effects of soil pH, texture, organic matter, and moisture on insecticide persistence. In the laboratory, the LC90 levels were determined for each insecticide in several soils. Studies measured the effects of moisture and temperature on the change in biological activity of the six insecticides in soils of varying texture, pH, and percent organic matter.
All insecticides were influenced by pH, to varying degrees, in at least one test. Carbofuran persistence was most sensitive to increasing pH. The influence of soil texture was variable. Loss of carbofuran activity in sandy soils was attributed to leaching. Mortality from chlorpyrifos was inversely proportional to percent clay. Soil pH and texture influences on activity of fonofos and isofenphos were inconsistent. The effect of organic matter on biological activity of the insecticides was minimal.
Regression models indicated that time accounted for approximately 55% of variation in the activity of phorate and terbufos with little effect attributed to soil factors. Variation in chlorpyrifos mortality increased an average of 17% (from 25%) by the inclusion of soil factors in the models.
The LC90s revealed that terbufos was the most toxic and fonofos the least toxic insecticide.
Higher moisture levels accelerated the decline of fonofos and terbufos activity. Chlorpyrifos and isofenphos were least influenced by different soil moisture levels. Soil temperature had no effect on insecticide persistence.
All insecticides were affected by some edaphological or environmental factors, but the influences and degrees of response varied.