Aphids appeared earlier than normal this year, beginning in February instead of mid-March, in the alfalfa fields of California's Central Valley.
Part of the reason, according to Charles Summers, University of California entomologist at the Kearney Agricultural Center, Parlier, may have been higher temperatures accelerating development of the insects.
Alfalfa growers are typically concerned about four species: spotted alfalfa aphid typically during the summer, pea aphid during early spring and late fall, cowpea aphid year round, and blue alfalfa aphid during late winter and spring.
Aphids inflict two basic types of injury in alfalfa. First, their direct feeding removes sap from the plant, causing the major portion of damage. Second, and perhaps more importantly, Summers said, aphids inject toxic saliva during the feeding process. Their saliva contains toxins so powerful that, in sufficient quantities and severity, they can kill the plant. The saliva of the spotted alfalfa aphid is the most toxic, while that of cowpea aphid is the least.
Symptoms of aphid feeding include crinkling or curling of leaves, and the effects disappear when the aphids are removed, whereas symptoms caused by a virus continue.
Aphids also secrete honeydew that draws black mold, reducing forage palatability for livestock and market value.
Questions have arisen, Summers said during a recent hay and forage field day at Parlier, why pea aphid and blue alfalfa aphid were earlier than usual, during early to mid-February this year, why spotted alfalfa aphid was heavy this year, and why cowpea aphid is present year-round.
Turning to development periods for each species, Summers said cowpea aphid develops throughout the year. Pea aphid develops during temperatures of 40 to 82 degrees, blue alfalfa aphid during temperatures of 38 to 81 degrees, and the heat-loving, spotted alfalfa aphid develops from 45 to 94 degrees.
Cowpea aphid's summer strain or biotype develops in a minimum of 47 degrees, and the threshold for winter strain has not been determined. “Cowpea aphid is a bit of an enigma,” said Summers. “It's always been in the Central Valley, and we've picked it up in sampling for the past 40 years during the summer. But only recently has it become a pest.”
Two biotypes of cowpea aphid have been identified, and Summers says that's the result of two possibilities: either the native, summer biotype has evolved into a second, winter form, or perhaps a winter form has been somehow introduced from elsewhere. Cold-weather cowpea aphids are known in Europe, particularly in the Balkans.
Alfalfas have been bred to resist aphids and rated according to resistance, but Summers pointed out that these varieties contain differing proportions of resistant plants in a population, or, put another way, not every plant of a variety in a particular field has the same level of resistance.
If zero to 5 percent of the plants are resistant, the variety is considered susceptible. Low resistance is recorded at 6 percent to 14 percent, 15 percent to 30 percent is moderate resistance, 30 percent to 50 percent is resistant, and 50 percent or more is highly resistant.
“You might have a variety that is 51 percent resistant, but remember that the remaining 49 percent is not necessarily resistant. So don't think you are home free because you have a ‘resistant’ variety,” he said.
Exploring potential reasons for the greater pressure from aphids in alfalfa this year, Summers said one possibility is higher temperatures, which averaged from one to six degrees higher than normal during January and February and encouraged pea and blue aphid development earlier than usual.
Summers reiterated that the situation of two biotypes of cowpea aphid is still clouded. “Until we determine if there are indeed two biotypes and determine the origins of both, it will remain a mystery.”
Summers said he suspects outbreaks of spotted alfalfa aphid in the Sacramento Valley were likely due to cultivars having low resistance to that species.
He has not discounted the possibility of resistance to spotted alfalfa aphid being impaired by use of certain insecticides on alfalfa. “Some that were used 20 or 30 years ago might have caused this sort of thing, but they are either no longer registered or not now in use. We've not looked at newer insecticides for this effect, but it is possible.”
It may also be possible, he added, that the spotted alfalfa aphid has also generated a new biotype, which has occurred eight times since resistance to this species was first developed in the late 1950s and early 1960s. “This is evolution in action. These things happen periodically and they will continue to happen.”
So the primary aphid solution for growers, Summers said, is to select a more resistant variety for protection. “Finally, there is always the insecticide solution, if need be.”
Another measure is to encourage natural enemies such as ladybird beetles and parasites of aphids. “Factor the natural enemy populations into the economic thresholds in making a decision to treat.”
In determining action thresholds for treatment of aphids, Summers recommended, for example, with a mixed population of blue alfalfa aphid and cowpea aphid, using the threshold for blue alfalfa aphid, the more manageable species.
Detailed guidelines for dealing with aphids on alfalfa, from identification to economic thresholds and resistant varieties and including recent updates on blue alfalfa aphid and pea aphid, are available at http://www.ipm.ucdavis.edu.
Also on the field day program was Pete Goodell, interim director of the UC Statewide IPM Program, who said the website has been recently expanded and updated to become “an incredibly robust and information-rich” document with “just in time” accuracy. It offers the same convenience, he said, as turning a tap for water or flipping a switch for electric power.
Data and key photographs, complete with links to related topics, are readily delivered to a personal computer. Information on environmental quality issues for air and water has also been introduced.
Even though the object of an inquiry may be narrow and specific, Goodell urged users to take a few moments to explore the diversity of the Web site.