Efficacy, soil insecticides

Table VII. Effect of first-year use, multi-year use and rotations on efficacy and persistence of soil insecticides during 1987 at Monmouth, IL...

G formulation. In related work with aldicarb, Coppedge et al. (21) concluded that slow-release formulations may extend bioactivity over a longer time but not improve efficacy of compounds with moderate or low insecticidal activity at normal application rates. Pertinently, of the currently registered soil insecticides, carbofuran is the least toxic to CRW (22.), and therefore, rate of release would be critical to its bioactivity. 

A number of studies have examined the role of various factors such as volatility and solubility on the efficacy of soil insecticides.(14) While the soil itself affects the efficacy of soil insecticides, the major determinate of biological activity is the amount of organic material in the soil.(15) Simmons, Lew, Silverman and Ali studied the effect of pyrethroids and some commercial insecticides on 3 instar southern com rootworm larva (Diabrotica undecimpunctata howardii).(16) They found that a combination of calculated lipophilicity and calculated volatility could predict soil pLCso based on the topical pLDso. The volatility was expressed as the log of the vapor pressure in nun Hg. We re-plotted the difference of the topical pLDso - soil pLCso with the calculated logP and calculated log volatility in a 3D graph shown in Figure 9. It can be seen as the compounds become more volatile (logVp < 5) that is a marked increase in soil toxicity. As the compounds become less lipophilic (more hydrophilic), they also become more toxic in the soil. 

Synthetic pyrethroids now account for at least 30% of the world insecticide market and are rapidly replacing other agricultural chemicals for control of insect pests. Fenvalerate is one of the more widely used synthetic pyrethroid insecticides. It is derived from a combination of a-cyano-3-phenoxybenzyl alcohol and a-isopropyl phenylacetate ester. Technical fenvalerate is a mixture of four optical isomers, each occurring in equal amounts but with different efficacies against insect pests. Fenvalerate does not usually persist in the environment for >10 weeks, and it does not accumulate readily in the biosphere. Time for 50% loss (Tb 1/2) in fenvalerate-exposed amphibians, birds, and mammals was 6 to 14 h for reptiles, terrestrial insects, aquatic snails, and fish it was >14 h to <2 days and for various species of crop plants, it was 2 to 28 days. Fenvalerate degradation in water is due primarily to photoactivity, and in soils to microbial activity. Half-time persistence in nonbiological materials is variable, but may range up to 6 days in freshwater, 34 days in seawater, 6 weeks in estuarine sediments, and 9 weeks in soils. 

In the first year, the maximum concentrations of sulfoxide and sulfone in soil, seed potatoes, and foliage were approximately 2, 2, and 6 times, respectively, the concentrations of those metabolites measured in the second and third year treatments. These results demonstrated that enhanced microbial degradation of relatively minor insecticidal compounds in the soil can significantly affect insecticide levels in the plant (when these degradation products are the major insecticidal component accumulated). As the sulfoxide and the sulfone metabolites are the major toxicants in the foliage of potato plants grown in disulfoton-treated soil, this reduction in toxicant residues overtime can be expected to reduce insecticide efficacy. 

This work reports on the effect of Incorporation of a nitrile group In to terbufos and what effect this group had on Its laboratory Insecticidal activity and field performance. With respect to the question regarding efficacy, we demonstrated that the addition of a nitrile moiety did not markedly reduce the field performance as compared to terbufos. This was despite a significantly shorter soil half-life as estimated from the laboratory data In the Dlabrotlca jar test. With respect to the question of the effect on the vapor pressure of incorporation of a nitrile Into terbufos, the compound (C2HgO) P(S)SCH(CH2)SC(CH-) Cl, had an estimated vapor pressure which Is ID-fold lower (3.0xft) mm) than the measured vapor pressure of terbufos. With respect to alteration of Insect spectra, especially those Insects Injurious to Midwest field corn, laboratory studies Indicated that like terbufos, only corn root-... 

Toxicity to house fly is obviously quite good. Moderate efficacy is demonstrated against the corn rootworm beetles and mosquito larvae. For an insecticide of the prolan/DDT class, the potency demonstrated against the wild strain German cockroach is quite remarkable. Preliminary tests on the larval stage corn rootworm revealed soil activity of a monochloro compound, unlike most previously reported chemicals in this class. Overall, the spectrum of activity is quite broad, although other categories of insect pests must still be tested (e.g., lepidopteran larvae). 

In an effort to determine the criteria that should be used to invoke cases of enhanced degradation, an experimental approach for Its study was developed that focused on laboratory investigations with field-collected soils. It was obvious that Insecticide control failures were common occurrences and certainly not all due to enhanced degradation, as Investigations of faulty application methods and unusual environmental conditions have shown (18). The ideal approach to the study of enhanced degradation would Involve controlled field research in which pesticide persistence and control efficacy were both measured at many locations over a number of years. However, the tremendous cost In time and effort and confounding of results by environmental variables make a controlled laboratory approach desirable. The limitation of laboratory efforts focused exclusively on the soil-lnsecticlde Interaction is that they cannot fully address the additional insect-insecticide and Insect-crop interactions present in the field. This means that caution must be excercised when proof of enhanced degradation is discovered In the laboratory, for this does not necessarily mean that Insect control and crop yield will be adversely affected under field conditions. 

The effect of formulation and spray adjuvants on insecticide efficacy has received considerable attention from the pesticide industry. However, few detailed mechanistic studies on the role these additives play in environmental fate processes have appeared in the open literature. Application of laboratory-derived process information to field scenarios is hindered by the fact that most laboratory investigations have used technically pure (unformulated) organophosphorus insecticides. Including the effects of formulation ingredients on such processes as volatilization and sorption to soil solids would allow laboratory studies to better predict the environmental behavior of these compounds. 

---