Lygus is the single worst insect when it comes to unraveling an alfalfa seed grower's profit potential. In California's San Joaquin Valley, it's a season-long pest, moving between crops as plants become unsuitable hosts due to maturity or harvest.
“Lygus is the key insect pest, and by far the most difficult to manage, in alfalfa seed fields,” according to University of California Cooperative Extension Farm Advisor Shannon Mueller of Fresno County. “When present in high numbers, lygus can completely destroy the crop.”
Two lygus species threaten seed alfalfa — Lygus herperus and Lygus elisus. Adults and nymphs feed on the alfalfa plant by attacking reproductive parts, causing the premature drop of buds and flowers (stripping), seed deformation, and reduced seed viability.
Mueller addressed “Managing Lygus in Seed Alfalfa” at the Second International Lygus Bug Symposium at Pacific Grove, Calif. She and UC Statewide Integrated Pest Management Program Interim Director Peter Goodell compiled the presentation.
“What we really do is climb on to the carnival ride at the beginning of the season and just hang on until it's over — there's not very much management of lygus involved,” Mueller quipped. Then she laid out the framework of chemical, cultural, and biological management strategies that can help reduce growers' lygus losses.
Chemical control is the most often-used and effective lygus control strategy in seed alfalfa; unfortunately, limited materials are available.
“If you're not growing corn, cotton, wheat, or soybeans, it's hard to get the attention of many chemical companies for seed alfalfa,” Mueller said. “The companies can't justify spending the money needed to register a product for such a small commodity nationwide. Plus, trying to get a product registered in California is even more difficult.”
Since chemicals are limited and a high potential for resistance exists, it's critical to maintain and preserve the efficacy of current registered chemicals, she noted.
Lygus are more resistance-prone due to their short life cycle, with many generations per year, a wide host range, and the exposure to many insecticide applications annually in seed alfalfa and other crops.
“The best insurance against insecticide resistance is rotating chemical controls and maintaining the insect's natural enemies in the field,” Mueller said.
“The most common biological control strategies are degree day models to forecast lygus development,” she said. “Models utilize weather averages to predict the beginning of lygus hatch or migration.”
Most models have been used to increase lygus-scouting efficiencies in strawberries and safflower in the Pacific Northwest. But Mueller said, similar efforts by several growers in California's San Joaquin Valley to predict first hatch and first emergence of lygus has yielded minimal success.
“If there's going to be success, it will be early in the season,” she explained. “After that, we have multiple overlapping generations and models will not provide help in forecasting or predicting damage.”
Naturally-occurring or released predators and parasites also offer biological control.
“Seed alfalfa growers should not assume this will solve their lygus problems, but it is something to be incorporated into an overall pest management strategy,” Mueller said.
“Perhaps parasites or predators released in the foothills or some other unmanaged or uncultivated areas may allow the population of parasites and predators to build up and possibly prevent mass or early season migrations into crops.”
Some California alfalfa seed growers have heard success stories from the Pacific Northwest and the Northeast with the parasite Peristenus, which attacks the lygus nymph and prevents the egg from developing into an adult.
Is the Peristenus located in the SJV? Unfortunately not, Mueller said. Millions of parasite samples taken in the valley have not confirmed a single positive find.
“In cultural control, we're hedging our bets on host plant resistance,” she said. “The Seed Board for many years has funded the research work of UC Davis Alfalfa Geneticist Larry Teuber.
“It appears progress is being made toward developing alfalfa with resistance to damage from lygus feeding. We feel this is a direction that may benefit the seed industry.”
A successful cultural strategy occasionally used in the SJV involves planting crops in large blocks.
“The growers separate alfalfa seed production from other crops with a wheat or small grain planting and put cotton elsewhere.”
But Mueller said, few growers in the valley have that type of production system.
Strip or trap cropping won't work, since seed alfalfa is the most preferred crop for lygus, and this would attract lygus into the field. It's a good idea for seed alfalfa growers to be aware of the impact neighboring crops can have on lygus, she said.
Historically California has been the major seed producer in the U.S., but in recent years, production has shifted to the Pacific Northwest and to other countries.
Today, California's two major production areas are Fresno and Kings Counties in the central SJV and the Imperial Valley.
California farmers grow about 35,600 acres of seed alfalfa annually, yielding about 1.5 million pounds. Most is non-dormant seed and is exported, Mueller noted.
The seed alfalfa industry follows a general trend — a good year followed by an oversupply of seed, with price and acreage reductions, and then the cycle is repeated.
Lygus is a pest throughout the seed alfalfa season. “Once we clip back fields in early April to start the season, we start looking for lygus, and from May until the August harvest the pest is found,” Mueller said.
In seed alfalfa, lygus adults and nymphs feeding on the plants affects the reproductive structure. When the plant is in the bud stage and early bloom stage, lygus can cause bud blasting or bud abortion.
“They also feed when the plants are in bloom, creating a stripping effect,” Mueller said. The damage continues throughout the season.
“Even at the seed pod stage, the lygus probe through the developing seed pod into the seed and suck out the juices. The end result is a shriveled, dried seed. The grower may not even initially realize the extent of the damage.”
Pesticide applications should coincide with the hatching of lygus broods. Treatment can be delayed until the egg hatch is complete, but shouldn't occur before the nymphs reach the fourth or fifth instar. Older instars and adults are more difficult to control with insecticides than younger instars, Mueller said.
Sponsors of the lygus symposium included the University of California Statewide Pest Management Program, the Arizona Pest Management Center, FMC, and the U.S. Department of Agriculture's Risk Avoidance and Mitigation Program.