“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.

cblake@farmpress.com