Marilyn Monroe and Carol Channing will long be remembered for their infamous vocal renditions of the song “Diamonds are a Girl’s Best Friend” in the movie and later Broadway musical called “Gentlemen Prefer Blondes.”

Far away from Hollywood and theatre are alfalfa farmers in the southern California and Arizona desert who have their own best friends of sorts to help combat plant-chewing, income-stealing insect pests. The group of “friends” includes pest-resistant alfalfa varieties, natural insect enemies, insecticides, and flexible harvest schedules.

Common pests in desert alfalfa production include the spotted alfalfa aphid (Therioaphis maculate), pea aphid (Acyrthosiphon pisum), blue alfalfa aphid (Acyrthosiphon kondoi), and cowpea aphid (Aphis craccivora), and potato leafhopper (E. fabae).

These pests and effective management tools were discussed by University of California (UC) Cooperative Extension farm advisors and entomologists Eric Natwick of Imperial County, Holtville, and Vonny Barlow of Riverside County, Blythe, during the 21st annual Fall Desert Crops Workshop in Imperial, Calif.

“Considerable progress has been achieved toward the control of aphid pests through host plant resistance, but insecticide applications are commonly needed to maintain aphid population densities below damaging levels,” Natwick said.

Aphid problems generally begin in the fall months with the cowpea aphid, followed by the pea and blue alfalfa aphids during the winter.

Natwick said breeding efforts launched by USDA and UC researchers in the 1950s paved the way for alfalfa variety resistance to the spotted alfalfa aphid and other pests.

Pea aphid and blue alfalfa aphid are green in color. A hand lens is recommended to distinguish the two. Antennae of pea aphid have narrow dark bands at the tip of each antenna. Blue alfalfa aphid antennae are uniformly brown.

Spotted alfalfa aphid, yellowish in color, is the only aphid with spots. The adult cowpea aphid is shiny black with whitish legs. The antennae are black at the joints and tips. Cowpea aphid nymphs appear gray due to wax secretions.

Severe aphid infestations can result in economic losses for growers through retarded plant growth, reduced yield, and plant death. Damage can also reduce alfalfa feed value. The number one way to protect plants is to plant aphid-resistant alfalfa varieties.

 “Host plant resistances are the first line of defense,” Natwick said. “However resistance does not mean that growers never have to spray.”

The second line of defense is biological natural enemies, a.k.a. predatory insects including the lady beetle, lace wing, syrphid fly, and others.

“The seven spotted lady beetle is probably our best friend for aphid control in desert-grown alfalfa,” Natwick explained. “If aphid populations build and lady beetles move in then growers may not need to spray.”

Aphid treatment thresholds can depend on the adult lady beetle population per sweep.

“Do not treat if there are four or more adult lady beetles or three or more lady beetle larvae per sweep for every 40 aphids counted per stem,” said Natwick. “On plant stubble the ratio is one lady beetle larva per sweep to every 50 aphids per stem.”

Parasites also offer good aphid control, especially the Lysiphlebus parasite for cowpea aphid control. Entomopathogenic fungi can also reduce the aphid population.

In a 2010 insecticide trial conducted at the UC Desert Research and Extension Center near Holtville, Natwick compared products for blue alfalfa aphid and pea aphid control, including Lorsban 4E, Warrior II, Mustang E W, Stallion (at two application rates), and Avaunt plus Dimethoate.

“These materials provided good aphid control,” Natwick said. “Lorsban Advance is commonly used for aphid and weevil control. I’m a little hesitant to recommend pyrethroids for alfalfa since a beet armyworm resurgence can occur after product use. Avaunt and Dimethoate are also a good choice; Dimethoate for the aphids and Avaunt for weevil larvae.”

Potato leafhopper

Switching gears to Blythe-area alfalfa production, Vonny Barlow joined the UCCE Riverside in 2009 during a heavy potato leafhopper infestation in the Palo Verde Valley.

The potato leafhopper overwinters from northern Florida to Texas. Trade winds transport the pest back to the West annually. The pest has a host range of more than 200 plants. The leafhopper is a mobile insect but is not a persistent alfalfa pest in southern California.

According to the UC Statewide Integrated Pest Management Program, potato leafhopper feeding causes a yellow V-shape on the leaf tip; the leaf margin and tissue surrounding the area turns red. Symptoms can be confused with boron deficiency. The presence or absence of the leafhopper can determine whether the insect is the culprit.

Potato leafhopper feeding causes stunted plants with short internodes. Stunting can occur into the next cutting cycle even after the insect is controlled.

“The plant leaf can die, fall off the plant, and reduce the marketable yield,” Barlow told the crowd. “This is why the potato leafhopper is an issue for some desert alfalfa growers.”

Three-to-four generations of potato leafhopper are common per season. The population peaks from May to September. Three-to-10 days after mating, the female deposits eggs into the stem, petiole, or thick leaf blades. Eggs hatch in 7-to-10 days.

Many wingless immatures are killed during the alfalfa harvest.

“If potato leaf hoppers are at threshold levels (0.2/sweep at 3”, 0.5/sweep at 6”, and 2/sweep >12”) and harvest is near, do not spray,” Barlow said. “Harvest the alfalfa early, bale it, and remove it from the field.”

Barlow conducted an insecticide-potato leafhopper trial from May through September last year in three, 20-acre fields in early spring alfalfa stubble located adjacent to irrigation drainage ditches heavily populated with host plants and the pest. The fields were split down the middle. The half closest to the irrigation ditch was treated with a 2.8 ounce/acre application of Baythroid XL. 

Preliminary results from the first-year study indicate no treatment impact on season-long potato leafhopper populations. An alfalfa stubble application would only indirectly manage leafhopper populations if the pest continuously emerged from protected areas including irrigation ditches.  The study will be continued this year.

Many alfalfa varieties resistant to aphids are also resistant to the potato leafhopper. Varieties with a glandular-haired trait perform as “fences across the fields,” Barlow says, impeding insect movement and causing the insect to move to less “unfriendly” plants.

The Fall Desert Crops Workshop was sponsored by Western Farm Press with commercial sponsor support from: BASF and Bayer CropScience (Platinum sponsors); Dow AgroSciences, Tessenderlo Kerley Inc., and Valent U.S.A. (Gold sponsors); FMC Corporation (Silver sponsor); and Certis U.S.A. and CDMS (Bronze sponsors). 

cblake@farmpress.com