Persistence of fenvalerate in alfalfa: effect of growth dilution and heat units on residue half-life

Abstract

Fenvalerate [cyano(3-phenoxyphenyl)methyl4-chloro-a-(l-methylethyl)benzeneacetate] was applied to alfalfa at 0.14-0.15 kg/ha in three experiments, and the residues, crop growth, and weather were monitored for 28 days. Initial residues of 22.2-32.5 ppm declined exponentially with a half-life of 9-11 days. When fenvalerate was applied early in the season, the advantage of growth dilution of the residues was negated by cooler temperatures slowing the rate of decline of the chemical itself. The half-life of the chemical per se, with the effect of growth dilution removed, was 11-19 days, depending on the weather after application. Cumulative heat units were used to correlate residue decline to both time and temperature, and the concept of residue half-life in degree-days was introduced. The result was a half-life of 153-189 degree-days above 5 "C for fenvalerate residues in alfalfa.Fenvalerate [cyano(3-phenoxyphenyl)methyl4-chloroa-(l-methylethy1)benzeneacetatel is one of the synthetic pyrethroid insecticides being evaluated at this laboratory for the control of weevils (Hypera postica Gyll.) and Lygus spp. in alfalfa (Charnetski and Schaber, 1980a,b). One concern is that fenvalerate residues are potentially toxic to bees. The National Research Council Canada (1981) rates fenvalerate as highly toxic to honeybees (Apis mellifera L.), LDm = 0.408 pg/bee, and as hazardous to the alfalfa leafcutter bee [Megachile rotundata (F.)] any time application is made during flowering. The leafcutter bee is the primary pollinator of alfalfa grown for seed, and the need to spray insecticides to control high populations of insect pests often occurs close to the start of alfalfa flowering (June 20-30). After flowering, there is little growth of the alfalfa to dilute the insecticide residues, and the amount of insecticide to which the bees are exposed will depend on the persistence of the chemical residue. Thus, information on residue persistence is required to help establish a safety interval between insecticide application and placement of leafcutter bees in the field.There have been few published reports on the persistence of fenvalerate in field crop situations. Harris et al. (1978) applied fenvalerate at 0.14 kg/ha to celery grown in outdoor microplots. Residue levels were 3.3, 1.7,0.45, and 0.36 ppm after 0, 7, 14, and 21 days, respectively. Westcott and Lee (1978) reported a ht-order decline with a half-life of 5 days for fenvalerate residues on wheat foliage. This rate of residue decline included a large growth dilution effect of the crop. Greenberg (1981) described a method for determining fenvalerate in grapes, peppers, and apples and presented residue data from field-treated samples. Although half-lives were not calculated, initial residues declined by half within 14 days. However, when fenvalerate was sprayed on grapes and peppers at higher rates, there was no residue decline within 14 days.The half-life concept, derived from first-order kinetics (Gunther and Blinn, 1955;Gunther, 1969;Hamaker, 1972), is commonly used to d...

Effectiveness of Two Pine Oils for Protecting Lodgepole Pine From Attack by Mountain Pine Beetle (Coleoptera: Scolytidae)

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Residue behavior and removal of iprodione in garlic, green garlic, and garlic shoot

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