Tulare County

Lygus bugs invade San Joaquin Valley cotton from a wide crop range including alfalfa hay, safflower, sugar beets, and tomatoes, and the weeds often associated with onion, garlic, seed alfalfa, orchards, and vineyards. Lygus migrations into cotton fields can cause economic loss, increase the cost of production through the increase use of insecticides, and cause secondary problems with spider mites, aphids, and worms. Vem Stern and colleagues (Stern et al, 1967) placed Lygus in a regional context and proposed managing surrounding crops to benefit cotton. They recognized the key element in a cotton IPM system is to mitigate the Lygus migration at its source and their advice is still pertinent today:

"The grower, faced with rising production costs at all levels must decide if he will rely completely on insecticides to fight Lygus and accept the financial burden or if he will look for another method of control"

"In attacking the Lygus problem, chemicals are used when absolutely necessary; but first a major change must be made in farm practices to keep Lygus out of cotton"

For most crops, Lygus adults migrate when the crop is no longer a suitable host, usually when the crop approaches maturity and irrigation is no longer provided. In these cases, unless Lygus is prevented from moving, it will move into the next available host. Safflower is a classic example where a Lygus source is managed in order to prevent migration to cotton (Sevacharian and Stern, 1977). Insecticide treatments are scheduled to reduce the population before it matures to adulthood, develops wings and migrates. Population development is estimated through the use of degree days based on Lygus developmental thresholds (52 °F).

However, not all Lygus sources need be managed with insecticides. Alfalfa hay is a unique crop in San Joaquin Valley because it is i) a perennial field crop, ii) a preferred host for Lygus, and iii) Lygus does not cause economic injury to it. Thus, alfalfa has long been recognized as one of the keys to regional Lygus management and is considered to offer advantages to Lygus management in cotton.

To prevent migration, alfalfa must be managed in order to maintain it in a condition favorable to Lygus. The idea of managing alfalfa to prevent Lygus migration goes back 30 years to the work of Vern Stern. He suggested harvesting alfalfa in a manner which left strips of uncut alfalfa equal to about half the field. The strips were about 200 feet wide and every other strip was harvested. A schedule was developed to handle separate harvests and irrigation schedules. Stern provides evidence of the benefit to alfalfa production and pest management through this alternate harvest approach.

This approach is burdensome to alfalfa growers since it requires specific irrigation planning prior to planting. Additionally, the requirement to return every 14 days to the same field is a constraint on those who depend on custom harvesters. For these reasons, strip cutting as described by Stern is not widely practiced. The concept of strip cutting has been adapted to local conditions over the years. In many areas where alfalfa has major presence in the crop landscape, strip cutting is accomplished by the discrete harvest times of individual fields. Because fields are located in close proximity to each other (relative to cotton), there probably are always fields of alfalfa in a suitable condition nearby to attract Lygus coming from a recently harvested field. Thus, Lygus always have an alfalfa field in which to migrate.

In other areas, growers may leave uncut strips on the margins of fields as a place for Lygus to take cover. These fields are cut to "herd" Lygus away from neighboring cotton and toward the uncut strip. There are weaknesses in this approach which include:

• the size of the strip must be of sufficient size to accommodate all the Lygus in the cut portion or they will migrate to neighboring crops.
• if the uncut strip is allowed to slip into water stressed condition, Lygus will migrate
• the strip cannot be located too far away or prevailing winds may point Lygus to neighboring cotton or they may fly to coKon if it is closer than the strip

Dr. Charles Summers (1976) proposed a management method which combines the value of Stern's strip cutting with the convenience of border strips. This involves leaving alternating irrigation levees or berms uncut but maintaining the same cutting schedule (Figure 1). At each harvest, the uncut berms are alternated so the previous uncut berm is mowed and incorporated into the general harvest. No special harvest or irrigation schedule is required. Summers found that Lygus remained in the uncut areas and recolonized the main portion of the field within a week. Leaving uncut strips throughout the field provided easy to find habitat for Lygus, supported a greater number of Lygus than complete harvest, and sheltered many insect natural enemies as well. His results demonstrated no loss in yield or quality of the hay.

Management of alfalfa in this manner could provide valuable assistance in Lygus control for neighboring crops. In areas where few blocks of alfalfa exist, this approach may be critical for downwind cotton. If alfalfa and cotton are not owned by the same grower, then growers should work together to leave uncut strips. Such neighbor to neighbor cooperation has been the foundation of farming and could reduce the need for disruptive insecticide applications in the general neighborhood. If incentives are required, offer to purchase the hay left in the uncut strips. This cost may be many less than the cost of insecticides and potential loss of cotton yield.