How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Lygus bugs are a serious pest in Central Coast strawberry-growing areas where strawberries are typically grown past May and through the summer months, but they are rarely pests in southern California and the Central Valley where fresh market berry harvest is generally complete by the end of June. However, lygus is an occasional problem in this area on second-year plantings and berries held through the summer.

Adults are about 0.25 inch long, oval, and rather flattened. They are greenish or brownish and have reddish brown markings on their wings. In the center of their back is a distinct, but small, yellow or pale green triangle that helps distinguish them from other insects. The immature forms are pale green and look similar to an aphid. They can be distinguished from aphids by their more rapid movements.

Nymphs of the third and later instars are green and characterized by five black dots on the back - two on the segment immediately behind the head, two on the next segment, and one in the middle of the abdomen. A similar nonpest species that may be confused with lygus, Calocoris, frequently is found when monitoring weed and legume crop hosts for lygus. Calocoris has two prominent black dots on the back, just behind the head, and dark wing tips. Lygus adults have no black dots on the back. Both nymphs and adults of Calocoris are longer and narrower than lygus.

DAMAGE

Lygus bugs are one of the causes of irregularly-shaped, cat-faced strawberries; another cause may be poor pollination, which results in small undeveloped seeds. Lygus bugs damage fruit by puncturing individual seeds; this, in turn, stops development of the berry in the area surrounding the feeding site. Straw-brown seeds that are large and hollow are a good indication of lygus bug damage. Lygus bug damage is more of a problem in strawberry-growing areas where fruit production continues through summer and fall.

MANAGEMENT

Successful management of lygus includes control of weed hosts in winter, monitoring for the appearance of lygus nymphs on weed hosts and adults on strawberries in spring, and timing insecticide sprays to control lygus nymphs before they cause significant damage. Sprays must be timed to kill the earliest instars of nymphs because registered materials are not very effective on adults. It is important to limit the number of treatments for lygus, because most of the materials that are effective against lygus disrupt natural enemies of spider mites. Control actions for lygus in strawberries generally are needed only in growing areas of the Central Coast, and the management activities described below apply to these areas. Once flower development begins in Central Valley strawberries, you can watch for the appearance of lygus adults during other routine monitoring activities.

Biological Control

A parasitic wasp, Anaphes iole, which attacks lygus eggs, is available commercially and can be used for inoculative releases. It can reduce lygus populations in strawberry fields; but because thresholds for this pest are very low and adults moving into the field from external sources are not controlled, economically acceptable results may not be achieved. Naturally occurring predators that feed on the nymphal stages of lygus bug include bigeyed bugs (Geocoris spp.), damsel bugs (Nabis spp.), minute pirate bugs (Orius tristicolor), and several species of spiders.

Cultural Control

Controlling weeds along roadways, ditches, and field borders near strawberry fields to help prevent spring buildup of lygus bugs is fundamental to lygus management in strawberries. Overwintered lygus bugs lay eggs in weeds in January that hatch in March. Carry out weed control measures in March and early April while lygus are still nymphs. Once adults are present on weeds, they will migrate into strawberries when the weeds dry or are removed. Spraying adults or weeds to prevent movement is not very effective. To avoid adult migration in spring, mow or disc under cover crops, especially legumes, before they flower and while lygus are still in the nymphal stages.

One cultural approach is to grow flowering plants in or adjacent to fields to attract lygus bug adults, but this approach requires careful monitoring and management to prevent an even greater problem from occurring. Adult lygus will lay eggs on the flowering plants, and nymphs will emerge from late March through April. The nymphs must be controlled at this time before they become adults and move to the strawberry plantation. Destroying the plants by discing or mowing is the most effective method of removing the infested, flowering plants. It is also possible to apply pesticides registered for use on strawberries for control of nymphs; however, none of the registered pesticides will provide complete control of the nymphs. If the plants are allowed to flower later into the season, carefully monitor the plants for the presence of lygus nymphs and take appropriate actions to prevent their movement into strawberries. This approach generally targets local populations of lygus and does not adequately impact longer range lygus migration from drying foothill weeds.

Growers have experimented with suction devices (bug-vacs) to control lygus bug for many years. Research has shown that an efficient bug-vac can reduce adult populations by 75% and nymphs 9 to 50%, but efficiency can vary considerably depending on the machine. If lygus bug population levels are moderate to heavy, use of vacuum machines alone will not reduce damage to acceptable levels. Vacuums may increase problems with powdery mildew and gray mold by spreading the pathogens that cause these diseases. Additionally, they may remove a disproportionately large portion of the general predator population.

Organically Acceptable Methods

Biological and cultural controls and insecticidal soap sprays are acceptable for use on organically certified strawberries.

Monitoring and Treatment Decisions

In February, monitor for the first appearance of lygus nymphs on plant hosts around the field to determine when these plants should be destroyed and to establish the first biofix for the degree-day model. Important plants and the key times to monitor them are outlined in the following table:

Begin monitoring the strawberry plants in mid-April to detect when adults first appear in the field. Establishing when adults first enter the field also serves as the biofix for part of the degree-day model described later. Continue monitoring the field regularly after this time to establish whether or not lygus densities are economically important and exceed the treatment threshold.

Threshold levels for lygus bugs depend on the monitoring method used. When a beat sheet (12-inch embroidery hoop with muslin or other device of similar size) is used, divide the field into blocks and sample four 200-foot lengths of row in each block. Sample one plant in each 20 feet of row by placing the beating tray under the plant and beating it with your hand. Apply sprays when one lygus nymph is found in 20 plants sampled. The Allen-Vac (a modified leaf blower that sucks lygus from the plant into a screen or net placed within the device) is a more efficient sampling device; the threshold to be used when sampling with it is one lygus per 10 plants. Continue weekly monitoring as long as fruit are being harvested for fresh market or freezer pack.

Currently registered insecticides are most effective against young first- and second-instar nymphs. Insecticides applied to later nymphal stages and adults are not very effective. Adult lygus that are not killed by sprays may migrate from the field to nearby weeds when pesticides are applied, but can return.

Calculating degree-days (DD) is an effective way of determining the time of egg hatch, which occurs just before best treatment times for lygus nymphs. This information can greatly improve the timing of lygus sprays and weed abatement in central coast areas, where damage from lygus is an annual problem.
    Degree-day calculator    Degree-day table

Accumulate degree-days for lygus bug using a lower threshold of 54°F. There are two primary periods when lygus migrate from weeds into strawberries. Use degree-days to determine when peak egg hatch occurs following each migration. The first migration is by the overwintered adults; it usually occurs in April. Not all fields will have damaging levels of lygus at this time. If treatment thresholds are exceeded, apply the first spray 252 DD from the date you find the first adult in the field after April. This will generally be from late May to early June. The second treatment period is at 799 DD (late June/early July) from the date the first nymphs are found in strawberries. A third treatment period corresponds to the emergence of nymphs that come from both adults that have established in the field and those that have migrated to strawberries during the summer; it is about 799 DD (early August) after the first spray. (For assistance in calculating degree-days, see "Degree-days" on the UC IPM Project Web site.)

PUBLICATION

UC IPM Pest Management Guidelines: Strawberry
UC ANR Publication 3468
Insects and Mites
F. G. Zalom, Entomology, UC Davis
P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura Co.
N. C. Toscano, Entomology, UC Riverside
M. Bolda, UC Cooperative Extension, Santa Cruz Co.

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