Insecticide Choice for Alfalfa May Protect Water Quality I I- I- Rachael Freeman Long Mary Nett Daniel H
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Insecticide choice for alfalfa may protect water quality i I- I- Rachael Freeman Long Mary Nett Daniel H. Putnam Guomin Shan w Jerry Schmierer Barbara Reed n Some insecticides used for control- ling Egyptian alfalfa weevil have been detected in California’s surface waters and are of concern, I due to their impact on water quality and toxicity to some aquatic life. To assess the impact of insec- ticide choice on water quality, we collected tail-water samples from on-farm alfalfa sites in the northern Sacramento Valley over a 3-year period. Samples were To investigate the potential movement of organophosphate and pyrethroid insecticides collected during irrigation after from alfalfa fields, the authors sampled inflows and outflows on alfalfa farms in the organophosphate and pyrethroid northern Sacramento Valley. UC Davis graduate student Corin Pease collects irrigation source water. sprays were applied. We found significant differences between insecticide classes in the mortality lfalfa is grown on over 1 million (Malathion) and dimethoate (Cygon); of Ceriodaphnia dubia (water flea), A acres in California. With an an- the pyrethroids permethrin (Ambush), a test organism used to detect nual value of $873 million in 2001, it lambda-cyhalothrin (Warrior) and pesticides in water. Nearly all sites ranks as one of the state’s top field cyfluthrin (Baythroid);and the car- where organophosphate insec- crops in gross revenue. Alfalfa is pri- bamate carbofuran (Furadan) (Sum- ticides were used resulted in 100% marily used as high-quality feed for mers and Godfrey 2001). An average water flea mortality in a 24-hour dairies, the state’s number one agricul- of 708,600 pounds of these insecticides test of tail-water samples: tural enterprise at about $4.6 billion in was applied to about 1 million acres of ppthroid-treated sections of the 2001. Fields are harvested 6 to 8 times California alfalfa annually from 1998 same fields exhibited insignificant a year in the Central Valley, which through 2000, with multiple applica- flea modality. The pyrethroids we produces more than 60% of the state’s tions on some fields (DPR 2002). The used provided significantly better crop. In this region fields are typically majority of these applications were made for weevil and aphid control in control of Egyptian alfalfa weevil flood irrigated with 4 to 6 inches of water between harvests. late winter/early spring, with a than the organophosphates, with no The major insect pest of alfalfa in smaller number of applications to significant differences in beneficial California is the Egyptian alfalfa wee- control various worms during the insect counts. Although water runoff vil (Hypera brunneipennis). The larvae summer. The use of organophos- does not always occur in alfalfa feed on foliage, causing yield and phates predominated; chlorpyrifos, fields, insecticide choice may be an quality losses to the first and some- phosmet, malathion and dimethoate important tool for prbtecting water times second hay harvests. The most accounted for an average of 87% of the quality. In addition, consideration common strategy for Egyptian alfalfa mass (weight) of total insecticides ap- should be given to the fact that weevil control involves spraying fields plied and 64% of the insecticide appli- pyrethroids, while they proved once or twice each spring. Insecticides cations to alfalfa statewide from 1998 advantageous in these experiments, currently used in California for weevil through 2000. Carbofuran accounted can affect beneficial species and do control include the organophosphates for 9% of the mass applied and 9% of have high toxicity to fish at (OP) chlorpyrifos (Lorsban and Lock- the applications, and the pyrethroids extremely low concentrations. On), phosmet (Imidan), malathion permethrin, lambda-cyhalothrin and I CALIFORNIA AGRICULTURE, SEPTEMBER-OCTOBER 2002 163 cyfluthrin accounted for 4% of the mass applied and 27% of the applica- tions in the same period (DPR 2002). Water quality concerns Irrigation and storm-water runoff from agriculture, including alfalfa, have been implicated as contributing to the presence of chlorpyrifos, mala- (SWRCB). Section 303(d) of the federal control. Although modern alfalfa vari- thion and carbofuran in California sur- Clean Water Act mandates that total eties are resistant to many pests, a cul- face waters at levels that cause maximum daily load (TMDL) restric- tivar has yet to be developed that is mortality to the water flea Ceviodaphnia tions be developed to improve water resistant to the Egyptian alfalfa weevil dubia (SWRCB 2002; deVlaming et al. quality in affected waterways. A (Summers 1998).Planting other for- 2000; Foe and Sheipline 1993). The de- TMDL is the maximum quantity of a ages such as berseem clover or oats tection of these insecticides has caused pollutant that can enter a water body into established alfalfa mitigates wee- several waterways to be classified as without adversely affecting beneficial vil damage, but is limited by market failing to meet water quality stan- uses. The TMDLs for chlorpyrifos, acceptance and management con- dards, as defined by the U.S. Environ- malathion and carbofuran must be es- straints. mental Protection Agency and the tablished between 2005 and 2011 for Most growers continue to use insec- State Water Resources Control Board the Central Valley (Region 5), depend- ticides for compelling economic rea- ing on the waterway. sons. However, all insecticides do not Regulatory implications. An agree- present the same risks to the environ- ment between the California Depart- ment, nor do all insecticide-treated ment of Pesticide Regulation (DPR) fields affect natural waterways. Insec- and the SWRCB outlines a four-stage ticides differ in properties such as process for protecting water quality solubility, adherence to soil particles (see page 148). The first step involves and toxicity to aquatic invertebrates voluntary grower efforts to reduce off- (table 1).Therefore, certain classes of site movement of an insecticide, while insecticides may be better suited to later steps require further restrictions prevent off-site pesticide movement or regulation by DPR or the regional from farms where irrigation water water quality boards. At this writing, drains into natural waterways. regulators were encouraging volun- Measuring water quality. Aquatic tary steps to reduce the presence of biologists commonly use C. dubia as a these compounds in natural water- test organism to help characterize the ways. However, further restrictions risk of toxins to aquatic invertebrates and regulations may be forthcoming (devlaming and Norberg-King 1999). for some insecticides such as chlor- Although the use of this invertebrate pyrifos. as the key predictor of the environ- Alfalfa management limitations. mental impact of pesticides has been There are few nonchemical manage- the subject of debate, it continues to be ment practices available to control used to detect pesticides in water due Egyptian alfalfa weevil. While early to its extreme sensitivity to toxins Alfalfa, one of California's top field crops, harvest can reduce feeding damage, present at very low (parts per billion is used primarily as feed for the state's this option is often not economically [ppb])concentrations (table 1). dairy industry. The federal Clean Water viable or is restricted by spring rains. The objective of this study was to Act will require Central Valley growers, including alfalfa farmers, to limit Beneficial insects such as parasitic evaluate the insect control efficacy of insecticide pollution in waterways in wasps are natural weevil enemies but organophosphates and pyrethroids, as coming years. generally do not provide sufficient well as their effect on the mortality of 164 CALIFORNIA AGRICULTURE, VOLUME 56. NUMBER 5 C. dubia in tail-water samples obtained from alfalfa production fields. Field studies Commercial alfalfa fields (22 in 1999,9 in 2000 and 10 in 2001) were se- lected as study sites in the northern Sacramento Valley (table 2). In 1999 and 2000, nearly all fields were di- vided into two distinct test areas: one received an organophosphate insecti- cide for weevil control and the other received a pyrethroid treatment. Each test area was a 20- to 60-acre block and each farm was considered a replica- tion. In 2001, monitoring was re- stricted to seven alfalfa fields treated with lambda-cyhalothrin (a pyre- throid) at labeled rates. Each year, all fields were treated with insecticides once in the late winter/early spring when weevils reached economic threshold levels of 15 to 20 larvae per sweep, or when sig- nificant damage to the alfalfa was ob- which occurred 22 to 62 days after the its the toxicity of metabolically acti- served (Summers and Godfrey 2001). fields were sprayed. During our study, vated organophosphate insecticides Application dates ranged from March in 1999 there were 10 rainfall events such as chlorpyrifos (Bailey et al. 10 to April 9 in all 3 years. Insecticides with a total of 2.7 inches; in 2000, 1996).If the addition of PBO renders a were applied by air or ground at 10 to 8 events with 2.8 inches; and in 2001, toxic sample nontoxic, organophos- 15 gallons per acre within each field. 7 events with 1.5 inches. Rainfall was phate insecticides are indicated as a There was a range of days between ap- not sufficient to cause runoff from the likely source of C. dubia mortality. The plication and water runoff sampling, treatment fields. Tests for mortality of other set of serial dilutions was used due to irrigation differences. C. dubia were done according to stan- to determine the dilution necessary to Toxicity testing. Water samples dardized 24-hour and 96-hour toxicity render the original field sample non- were collected from each test area us- testing procedures. toxic. ing standard methods employed by Dilution tests. During both 1999 Chemical analyses of pyrethroids. the UC Davis Aquatic Toxicology and 2000, all field samples that re- In 2001, chemical analyses were per- Laboratory (Emanuel and Cabugao sulted in 100% mortality within formed on tail-water samples from the 2000).