The lettuce aphid (also referred to as the red aphid) continues to threaten the marketability of California's billion-dollar lettuce crop. After the aphid made its debut in the state's central coast lettuce fields in the fall of 1998, it spread rapidly throughout other growing areas and is now firmly ensconced.
“The lettuce aphid is more difficult to control than the other species of aphids,” says Bill Chaney, University of California Cooperative Extension Entomologist in Monterey County. “It tends to feed deeper into the heart of the plant where it is more difficult, not only to detect, but also to reach with insecticides. It has a very short life cycle, and populations often build rapidly, so there's a constant battle to keep it under control.”
Tolerance levels for insect contamination in the fresh lettuce market are extremely low.
“When you're talking about a crop like Romaine, it comes down to a supply and demand issue,” says Leslie Woods, Pest Control Adviser with Integrated Crop Management Consultants Inc. (ICMCI) in Greenfield, Calif. “The market has been so soft for Romaine that buyers don't have to take anything but the cleanest product. Just falling within the boundaries of acceptable is not good enough right now.”
There's no such thing as a calendar approach when it comes to insecticide applications for lettuce aphid, according to Woods.
“You have to constantly be on the lookout for it,” she says. “It only takes about three days for an infestation to take hold in a field, and that can turn quickly into a disaster because they are so hard to control once they get down into the inner leaves of the plant. We scout for them constantly and check in with other PCAs up and down the Valley to try to get a feel for how they're moving. Unfortunately, they're very unpredictable. Lettuce aphid tends to have more ‘flyers’ than some of the other species, so they're extremely mobile.”
Fortunately, there are several registered insecticides that provide good control of lettuce aphid if they are applied in a timely fashion. The extremely short life cycle of the lettuce aphid in combination with its dynamics make it a prime candidate for developing resistance. So far, resistance has not been a problem, but researchers are encouraging growers and PCAs to be extremely cautious about the possibility and urging them to practice good resistance management techniques.
“We're fortunate to have quite a few options when it comes to control,” Chaney says. “Some of the newer neonicitinoids do an excellent job, and then we've got some of the older organophosphate materials and other chemistries that can also be used in rotation.”
UC researchers discourage tank mixes within the same class of materials and advocate rotations across the broadest spectrum of registered materials.
“We try to target lettuce aphid from every direction,” Woods says. “When it's a serious situation, I like to use something like Assail because it has multiple modes of action. It works by contact as well as ingestion. Lettuce aphid is not that difficult to kill if you can get to it. That's the beauty of Assail. If you can get the product to the aphid, you can control it.”
If it's a maintenance situation where aphid levels are not too high, Woods will use a neonicitinoid alone. If the threat is more intense, she'll include Diazinon or another material in the tank mix.
“A lot of times we use tank mixes of different products that have different modes of action so that we cover as many bases as possible,” she says. “There's also the consideration of what other species of aphids you might have in the field. You obviously want to use a product that gets as many of them as possible in one shot.”
Assail is effective on the entire aphid spectrum in lettuce, including the green peach aphid, fox glove aphid and black bean aphid. Some of the other products do not control the entire spectrum. It also has an advantage in that it is registered for entire crop groupings such as leafy vegetables, cole crops and fruiting vegetables where other aphid materials are not.
Woods also frequently adds a potassium soap base plant wash to the tank mix to improve coverage. “A lot of times we'll include Green Valley Natural Plant Wash in the tank mix,” she says. “It breaks the surface tension of the water, and gives us better insecticide coverage on the plant. It's approved for use on organic crops, but we'll use it even in conventional crops because we get better overall control from improved coverage when we use it in conjunction with an insecticide.”
If the crop is only a few days from harvest, Woods will opt for the plant wash in a tank mix with Success — a spinosyn class of insecticide which has a three-day pre-harvest interval. Most of the other conventional aphid materials have pre-harvest intervals ranging from 7 days for Assail and Provado to 21 days for Admire.
There are also a few cultural practices that can have somewhat of an impact on lettuce aphid infestations. “We encourage growers to destroy crop residue,” Chaney says. “It's also somewhat beneficial to consider cropping patterns in the area and plant accordingly if possible. Lettuce aphid is very difficult to control in organic production, so conventional growers should stay away from those situations if possible.”
Planting lettuce near crops such as cilantro, anise, and buckwheat can help attract syrphid fly larvae which are naturally occurring predators of lettuce aphid. However, it should be kept in mind that syrphid fly larvae are susceptible to many insecticides, particularly the older, harsher chemistries. Green lacewing larvae are also predators of the lettuce aphid.
Other cultural considerations include maintaining proper nitrogen and irrigation levels. “Basically, you want to keep the plant as healthy as possible,” Woods says. “If you use too much nitrogen, the cell walls become thinner and that creates more susceptibility to a variety of problems.”
Researchers and PCAs have made tremendous strides since 1998 in developing effective pest management strategies for the lettuce aphid. Even though it remains a serious threat to the state's lettuce industry, the newer neonicitinoids have given growers and PCA's an effective new arsenal to combat the problem. With attention to detail, lettuce aphid can be adequately managed with astute field monitoring, timely applications and careful rotation of currently registered insecticides.