2010 Annual Report

Common purslane infestation in lettuce. Photo by J. K. Clark.

Lettuce growers weigh options in face of tight VOC regulation

Clean produce is the goal for California lettuce growers, and they carefully monitor and manage pests to meet high quality standards. Now, UC Cooperative Extension scientists are advising growers on ways to cut use of air pollution-causing chemicals while getting the same or even better control of lettuce insect pests, but the prospect is less promising when it comes to managing problem weeds.

To achieve federal air-quality standards, the California Department of Pesticide Regulation is proposing to strengthen its regulation of pesticides that produce volatile organic compounds, or VOCs. VOCs readily evaporate and interact with other compounds in the air and, in the presence of sunlight, form ozone or smog. To reduce smog, growers can use alternative chemicals or formulations that form fewer VOCs.

Scientists analyzed the alternatives and found that, unfortunately, lettuce growers on the Central Coast have few options when it comes to reducing VOCs while managing tough weeds. Alternative, low-VOC herbicides either aren’t registered for all lettuces or don’t control all the problem weeds, or they have restrictions that interfere with important management practices.

UCCE Specialist Nick Toscano, Entomology Advisor and UC IPM Affiliate Advisor Jianlong Bi, and UCCE Specialist Steve Fennimore recently analyzed options for lettuce growers who need to move away from pesticides that cause air pollution.

The researchers found the VOC-producing herbicide bensulide—which traditionally is used to control grass and broadleaf weeds—is particularly effective on common purslane, pigweeds, and burning nettle. But pronamide, a possible alternative, can’t be used on leaf lettuce, and it doesn’t control pigweed. The only low-VOC alternative available for pigweed control in lettuce is benefin, but rotational crops can’t be planted for 10 months following its use, a problem in coastal areas where carrot, onion, and spinach can be grown in rotation with lettuce.

Without bensulide and without being able to use pronamide in leaf lettuce, some estimate that growers would increase their weeding costs by $300 per acre, since most wouldn’t use benefin due to rotation restrictions and poor weed control.

But the better news for growers is that pesticides that don’t produce smog are available to control aphids and other insect pests, including seedling pests.

Leaf and head lettuces are the third largest contributor of VOCs in agricultural crops. The insecticides dimethoate, permethrin, diazinon, oxydemeton-methyl, and lambda-cyhalothrin are five of the top six VOC-producing pesticides used in lettuce.

Aphids are a major problem in lettuce, and growers traditionally control them with the high emission-potential insecticides dimethoate, diazinon, or oxydemeton-methyl. Fortunately, newer insecticides such as pymetrozine, imidacloprid, and other neonicotinoids have very low emission potentials and are very effective against aphids.

Dimeothoate and permethrin are used against other insect pests such as cutworms, beet armyworm, loopers, tobacco budworm, field crickets, and darkling beetles. Spinosad and the insect growth regulators tebufenozide and methozyfenozide have low emission potential and give better control of these pests than permethrin does.

Growers fight seedling pests such as garden symphylans, springtails, and darkling beetles with diazinon. The wettable powder and granule formulation, with lower emission potentials than the emulsifiable concentrate, are available for use against these insects.

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