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Alfalfa pests reduce crop yield and quality

Several insect pests injure alfalfa plants and reduce crop yield and quality. The alfalfa weevil complex is the most damaging arthropod to California alfalfa in most parts of the state, according to Larry Godfrey, Extension specialist at UC Davis. It is routine for most fields to be treated once annually for this pest.

“There are two different species of weevils in California — the Egyptian alfalfa weevil in the warm parts of the state and the alfalfa weevil in the cooler parts of the state,” he says. “The larvae are the damaging stage. The larval buildup can occur rapidly and peak or build slowly and persist longer.”

In the future, dealing with these pests is likely to become more challenging due to regulatory issues. There are three primary classes of chemistries commonly used in alfalfa for weevil control. They include carbamates, organophosphates and pyrethroids.

“All three of these classes have some regulatory sensitivities with officials,” Godfrey says. “In terms of surface water hazard, the OP’s are a target, and to a lesser extent, the carbamates. Pyrethroids are presently being re-evaluated by the Department of Pesticide Regulation (DPR) because of aquatic issues. Additionally, air quality concerns in the Central Valley have been an issue the last few years with all the organic compounds. It looks like that has lessened a little bit recently, but certainly there are still issues — especially with the EC formulations of Lorsban, some of the pyrethroids, diomethoate, diabrome, and others.”

UC researchers have long led the charge for IPM solutions that encompass a variety of pest management techniques with “softer” chemistries, pinpoint application timing and other techniques to reduce environmental load.

Researchers have been looking at alternatives such as organic materials for weevil control. In 2007, organic insecticides were tested against weevil larvae in Yolo and Siskiyou counties and the work continued in Yolo County in 2008. Materials such as Pyganic, Ecotrol, GC Mite, CedarGuard, and Entrust were evaluated. At the Siskiyou site, efficacy was compared with a chemical insecticide.

Results ranged from little to no control. Of the organic materials, Entrust provided the best control at 50 percent to 60 percent.

Studies were also conducted with reduced-risk insecticides, registered and experimental, against Egyptian armyworm in 2007 in Yolo County and 2008 in San Joaquin County. Some of the products evaluated included cyazypyr, Movento, and Coragen.

“Work with reduced risk pesticides has shown that they’re not as good as Warrior, but they do have potential,” Godfrey says. “Cyazypyr provided very good weevil larval control and was only slightly less efficacious compared with standards Warrior and Steward.”

Godfrey and his team also evaluated different methods of sampling for alfalfa weevils. “The reality is that if we’re going to use some of these reduced risk materials and even the organic materials, we’re probably going to need to treat when the weevils are smaller,” he says. “If we’re trying to treat the later instars, they’re bigger and more difficult to control. They’re also doing more damage. It may be prudent to try to treat earlier instars if we’re trying to use reduced risk materials.”

The sweep net has been the standard method of sampling for weevils in alfalfa fields for many years, but it has limitations when it comes to sampling for smaller instars. Godfrey and his team are currently working on alternative sampling methods.

“Insecticides are just one of the tools we use in alfalfa to manage weevils and aphids,” Godfrey says. “Early harvest, cultivar resistance and biological control can play a role. As always, natural enemies are important.”

Aphids are another challenge in alfalfa. “We’re dealing with four different species of aphids in California,” Godfrey says. “Work needs to be done on thresholds, especially for the cowpea aphid. It’s really popped up as a pest in the Central Valley in the past few years. Cowpea aphid populations were high in 2008.”

In terms of aphid management, host plant resistance is one way to help minimize aphid populations. “The challenge there is that the resistance breaks down, especially during the cooler times of the year,” Godfrey says. “Additionally, aphids are constantly evolving and breaking through that resistance. There’s not much available currently in terms of host plant resistance for the cowpea aphid.”

Nematodes are another significant problem in California’s alfalfa production.

“Nematodes are more numerous in California alfalfa than almost any other crop except maybe cotton,” says Becky Westerdahl, Extension specialist at UC Davis. “In alfalfa, of the root knot species, northern root knot and southern root knot cause the most damage.”

Although researchers aren’t sure exactly why, stem and bulb nematodes seem to be causing more problems in the past few years.

“Nematodes are basically aquatic organisms,” Westerdahl says “In the soil, they live in a film of moisture that lines the soil pores. If you have moisture in the upper parts of your alfalfa from irrigation or dew, they can move in that moisture and attack the above ground parts of the plant as well.”

The stem and bulb nematode has a life cycle of just 19-23 days. It reproduces in fairly cool temperatures — ideally between 59-68 F. It is most severe in moist cool weather.

There are five species of root knot nematode associated with alfalfa in California. Different species have varying ranges of ideal temperatures, but generally favor warmer temperatures. Root knot nematodes have a wide range of hosts, so rotation with other crops is often problematic.

Both stem and bulb and root knot reproduce very quickly under their optimum temperature range.

Symptoms of infestation are often mistaken for other problems. “Nematodes are plant stressors,” she says. “They don’t typically kill plants. A good parasite is not going to kill its host, but it will take a portion of your yield. If you suspect a problem, you need to sample both soil and plant tissue to determine if you have nematodes.”

Chemically, there aren’t very many economically viable treatments. As such, cultural practices are the primary defense.

“Certified seed that is free of nematodes is very important, because initially the way stem and bulb nematode was transported around the United States was in seed,” Westerdahl says. “If you know you have root knot or stem and bulb nematodes, clean your equipment with water to wash off the soil and plant debris when you go from one field to another. Manage irrigation so that you don’t have as much free water at the top of the plants. That will help reduce the spread of stem and bulb nematode.

Nematode resistant varieties can also help minimize damage, although it will not rid the field of the problem. Rotation is also an option.

“If you can stay out of the field for two to four years it will help,” she says. “There are a lot of non-host crops for stem and bulb, not nearly as many for root knot.”

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