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Improved hopper insect management in desert alfalfa

Improved hopper insect management in desert alfalfa

The potato leaf hopper (PLH) and the threecornered alfalfa hopper (TCAH) can be found year round in the Southern California desert. Effective PLH management is important to reduce the loss of alfalfa quality and yield. The PLH moves into alfalfa when other hosts become unavailable. TCAH adults and nymphs cause plant girdling (outer tissue loss) 3-5 inches above the soil..

The potato leafhopper insect is certainly not the Easter Bunny. Yet the one-eighth-inch-long, lime green-colored pest hops into Western alfalfa fields each year delivering eggs which once mature threaten crop yield and quality.

Improving the management of hopper insects including the potato leafhopper (Empoasca-fabae) and the threecornered alfalfa hopper (Spissistilus festinus) in low desert alfalfa production is the goal of Vonny Barlow, University of California Cooperative Extension (UCCE) entomologist and crop production specialist based in Riverside County’s Palo Verde Valley.

The potato leaf hopper (PLH) and the threecornered alfalfa hopper (TCAH) can be found year round in the Southern California desert, Barlow says. The PLH is found in the Palo Verde Valley from March through September. The insect can invade Central Valley alfalfa fields from April through July.

The TCAH is an occasional pest in both regions.

Barlow discussed alfalfa pest management efforts with farmers, pest control advisers, and industry representatives during the 2011 Fall Desert Crops Workshop in El Centro, Calif. The workshop, sponsored by Western Farm Press, was conducted by UCCE Imperial County and the University of Arizona Cooperative Extension, Yuma County.

Commercial sponsors of the workshop included: Platinum Level – BASF, Bayer CropScience, and Syngenta; Gold Level - Dow AgroSciences and Valent U.S.A.; Silver Level - FMC; and Bronze Level - Westbridge Agricultural Products.

The PLH normally dies during the winter months from the extreme cold. A new infestation occurs in the early spring when PLH populations originating in the Gulf Coast states migrate westward.

Effective management of PLH is important to reduce the loss of alfalfa quality and yield from the insect. Barlow says the recommended pest threshold in the Palo Verde Valley is .2 leafhoppers when alfalfa plants reach three inches in height, .5 leafhoppers at 6 inches, and one-to-two insects on a 1-foot-tall plant.

“The potato leafhopper feeds on the plant and injects toxic saliva,” Barlow said. “If the saliva is injected high in the plant the result can be reduced growth at the plant’s top. If the saliva is injected at the plant base, severe stunting can occur.”

Early insect control is important.

“With a 1-foot plant, the loss of an inch is not a big deal,” Barlow said. “A 1-inch plant which loses a foot of growth potential is a big deal. That’s why the recommended threshold is lower when the plant is smaller.”

Insecticides can provide good hopper control in desert-grown alfalfa. One to two sprays are usually required annually for PLH control, but the number can increase based on larger populations. 

Reducing indiscriminate insecticide use lowers secondary pest outbreaks and pest resurgence (bounce back effect). It also reduces hazards to honeybees and other desirable insects and lowers crop phytotoxicity.

Barlow says integrated pest management (IPM) is a crucial tool to reduce control costs and maximize plant quality and yield. Since alfalfa is a perennial crop, insect management strategies generally last longer than with an annual crop.

Crucial IPM

“A large number of beneficial insects are found in alfalfa,” Barlow said. “If we can tailor our strategy to manage the potato leafhopper and the threecornered alfalfa hopper then we can conserve the natural enemies.”

The PLH family includes 200-plus species in North America. Adults and immature both damage alfalfa. The female deposits eggs on the plant 10 days after mating. The first through fifth instars are identical in appearance; the only difference is the insect size.

The PLH moves into alfalfa when other hosts become unavailable. More than 200 plants host PLH.

PLH adults feed excessively and extract plant sap. The insect injects phytotoxic saliva which results in ‘hopper burn,’ a v-notch located at the tip of the leaf which becomes yellow. The leaf then dies and falls off the plant. Feeding typically occurs on the harvestable part of the plant so plant protein and yield are lost.

The TCAH, also called the buffalo bug, is a triangular, heavy-sclerotized insect one-quarter-inch long and stands taller than its width. The mature adult is almost immune from natural enemies due to its heavily sclerotized surface. Immatures are prone to natural predators including the big-eyed bug.

“When sweeping for insects, adult threecornered alfalfa hoppers are easily caught since it is found in the plant crown and upper canopy,” Barlow said. “Immatures are almost exclusively in the crown so sweeping will not recover those.”

The sweeping problem with immatures makes it difficult to develop an economic injury level.

TCAH adults and nymphs cause plant girdling (outer tissue loss) 3-5 inches above the soil. Girdling reduces the flow of nutrients and water from the roots into the plant resulting in breakage and lodging. The stem color can change from green to purple. Plant stunting can occur.

“Stem breakage at the crown increases the introduction of Fusarium crown rot fungus,” Barlow said. “The fungus can enter the plant via irrigation or rain water through the open plant wound.”

The two population peaks for TCAH adults are late July to early August and September to early October. The most critical period to monitor for TCAH is the first three weeks of July.

“Threecornered alfalfa hoppers rarely cause economic damage but there is a working economic threshold of two treehoppers per net sweep,” Barlow said. “Natural enemy suppression is the only effective control in early instars.”

The TCAH host-plant range includes alfalfa, clover, cowpea, grasses, small grains (barley, oats, and wheat), soybeans, sunflowers, tomatoes, vetch, and weeds.

Unmanaged areas, including drainage ditches near alfalfa fields, are a source of arthropods, weeds, and plant pathogens. In Barlow’s field research over the last two years, a single application of the insecticide Cyfluthrin was applied to spring stubble alfalfa to test for season-long management of PLH and TCAH.

“The hopper populations were not significantly affected by stubble alfalfa treated with Cyfluthrin after the first spring cutting versus the untreated controls,” Barlow said.

The alfalfa plant with its dense upper canopy acts as a barrier to insecticides penetrating the plant crown where immatures feed. This basically creates a pesticide-free space allowing the TCAH population to build.

“The development of an IPM strategy to manage TCAH and PLH in alfalfa could potentially reduce insecticide applications to manage these pests,” Barlow said.

A possible alternative to direct hopper management in the alfalfa field could be hopper management in drainage ditches to disrupt the overwintering-reproductive areas.

Alfalfa production covered about 48,000 acres in Riverside County in 2009. Riverside was the ninth largest alfalfa-producing county in California, according to the National Agricultural Statistics Service in Sacramento, Calif.

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